The Lakes Handbook, Volume 1 || Aquatic Plants and Lake Ecosystems

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  • 11.1 INTRODUCTION

    Aquatic macrophytes are those water plants whichcan be viewed without a microscope. They com-prise vascular plants, mosses and the larger filamentous algae. The term freshwater macro-phytes is to be interpreted as including the Charophyta (stoneworts), Bryophyta (mosses andliverworts), Pteridophyta (ferns and their allies)and some Spermatophyta (seed-bearing plants),whose photosynthetically active parts are perma-nently, or for some months of each year, sub-merged in freshwater, or float on the water surfaceor emerge above it. The term is also taken to in-clude certain macrophytic taxa of Chlorophyta(green algae), which therefore are known asmacroalgae.

    All aquatic vascular plants are descended fromterrestrial ancestors and have returned secondari-ly to the water. Many of them exhibit some reduc-tion in their ancestral features, often possessingthin cuticles, functionless stomata and weak ligni-fication of the xylem elements. They have not,however, reverted to fertilisation by motilesperms: most aquatic angiosperms still presenttheir flowers above the water surface, to be polli-nated aerially by insects or by wind. A few specieshave developed submerged flowers, but fertilisa-tion continues to involve gamete and pollen-tubestructures analogous to those of terrestrial plants(Sculthorpe 1985).

    Aquatic macrophytes are able to colonise stand-ing and flowing waters in all climatic zones. Al-though most are rooted, some species float freelyin the water and a few are epiphytic. There is some

    considerable plasticity in somatic organisation,which often creates problems for the taxonomist,particularly when diagnostic flowers are absent.Indeed, many macrophytes are able to reproduceand spread rapidly by vegetative means. For exam-ple, Elodea canadensis Michx. was first recordedin Europe in 1836; by the end of the century it hadbecome well-established in Scandinavia, centralEurope and east to Russia and Hungary. During the 20th century it has become equally commonthroughout eastern and southeastern Europe andwestern Siberia. The remarkable fact is that thisinvasion has been accomplished exclusively byvegetative propagules (Sculthorpe 1985).

    The sweet flag (Acorus calamus L.) is also a neo-phyte in Europe. It is probably derived from EastAsia, and it is very likely that, at least in southernEurope, the present populations are descendedfrom a single ancestral rhizome. The plant was in-troduced to Europe at an early but unknown time(Hendrych 2003), ostensibly for its medicinalvalue (it has been used in the treatment of oph-thalmic and stomach disorders, hysteria, epilepsyand chronic rheumatism; its fragrance has led to itsuse in perfumery and brewing). Sweet flag flowersinfrequently, and fertile seeds are unknown, but ithas spread throughout central Europe, Belgium,France, Germany and, later, to the British Isles(Wein 1939; Casper & Krausch 1980; Weber &Brndle 1996). In tropical and subtropical regions,the South American Salvinia molesta (giantsalvinia) has invaded large water bodies in Africa,Asia and Australia and caused serious manage-ment problems. By 1962, shortly after the creationof the lake, Salvinia covered 100,000ha of Lake

    11 Aquatic Plants and Lake Ecosystms

    JAN POKORNY AND JAN KVET

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    The Lakes Handbook: Limnology and Limnetic Ecology, Volume 1Edited by P.E. OSullivan, C.S. Reynolds

    Copyright 2004 by Blackwell Publishing Ltd

  • 310 j. pokorny and j. kvet

    Kariba (Zambia/Zimbabwe). The invasion hassince been successfully contained, thanks to bio-logical control (Mitchell 1976).

    On the other hand, stands of macrophytes maypersist for centuries, with unchanged species com-position. This is especially so for plant associa-tions in lakes of low trophic status, such as those ofthe Bohemian Forest, Czech Republic, with theirenduring stands of Isotes lacustris L. and I. echi-nospora Durieu.

    Macrophytes respond to changes in water qual-ity, to water-level fluctuations and to other envi-ronmental factors. They are a natural componentof lake ecosystems, where they provide habitat andfood for animals, concentrate nutrients and releaseoxygen during photosynthesis. In general, the influence of macrophytes on lake ecosystems isstabilising but, with increasing nutrient status,their development sometimes causes its own prob-lems. Thus, they sometimes also earn the name ofaquatic weeds.

    11.2 LIFE-FORMS OF AQUATICMACROPHYTES

    The morphology, anatomy, life-forms and physio-logical features of water macrophytes are de-scribed in several well known monographs(Gessner 1955, 1959; Hutchinson 1977; Sculthorpe 1985). Detailed descriptions of individ-ual macrophyte species, including their biology,distribution (especially in relation to water chem-istry and human impact) and their indicator value,are widely available. A relevant bibliography isthat by Hejny & Sytnik (1993). A widespread fea-ture of water macrophytes is the presence ofaerenchyma, a loose tissue with large air spaces, orlacunae. The lacunal system within the plants fa-cilitates the internal transport of gases (air, carbondioxide, etc.) to roots, stems and leaves.

    Aquatic macrophytes are usefully classified bytheir life-forms (Fig. 11.1), which distinguishesemergent macrophytes (such as Phragmites,Typha), surface-floating macrophytes (such asEichhornia, Lemna), the euhydrophytes withfloating leaves (such as Nymphaea, Potamogeton

    natans) and the fully submersed euhydrophytes(such as Najas, Ceratophyllum, Lemna trisulca;see Denny 1987). More elaborate systems of classi-fying the life- and growth-forms of aquatic macro-phytes have been devised (see, e.g., Tansley 1939;Hejny 1957, 1960; den Hartog & Segal 1964;Spence 1964; Hejny et al. 1998).

    Among cold and temperate water bodies, wherefactors such as littoral topography, bottom condi-tions, exposure to wind and waves, light penetra-tion in the water and presence of grazers variouslyfavour development of a constellation of life-forms, the distribution of aquatic macrophytes isessentially littoral, conforming to the schematicillustration in Fig. 11.2. The littoral zone is divi-sible, however, into a series of sub-zones based onthe occurrence of different life-forms. The zona-tion system is useful for the characterisation of thelittoral vegetation and its various adaptive traits.The groups of macrophytic life-forms comprise:1 The hyperhydates (helophytes) emergentplants such as the graminoids Phragmites andTypha, and the herbids Alisma and Cicuta.2 The ephydates floating-leaved (natant) plantsrepresented by spirodelids (Spirodela, Lemna) andnymphaeids such as Nymphaea and the species ofPotamogeton with floating leaves.3 The hyphydates the group of submergedplants including riccids (Riccia), elodeids (i.e.plants with long shoots such as Elodea, Myrio-phyllum and Potamogeton species without float-ing leaves), isotids (submerged plants with shortshoots, exemplified by Isotes and Littorella) andmuscids (submerged mosses).

    The uppermost part of the littoral zone, the eulittoral, is delimited by the extreme highest(generally spring) and lowest (usually summer)water levels. Differentiation of permanently sub-merged sublittoral zones is based on the verticaldistributions of the hyperhydate, nymphaeid,elodeid and isoetid plant types. Thus, the lakewardlimit of the upper sublittoral zone coincides withthe extent of the emergent hyperhydates (helo-phytes), and that of the lower sublittoral zone withthe deepest occurrence of nymphaeids. In the sameway, the upper elittoral zone ends with the deepestelodeids, and the lower elittoral zone with that of

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  • Aquatic Plants and Lake Ecosystems 311

    Lemna trisulca

    Cyperus papyrus

    Typha

    Phragmites

    EmergentsSurface-floating

    Eichhornia

    Hydromystria

    Lemna minor

    Euhydrophytes

    Nymphaea Potamogeton

    Najas Ceratophyllum

    Fig. 11.1 Wetland plant life-forms: emergent, surface-floating and euhydrophytes. (From Denny 1987.)

    Isoetids

    Supra-littoral

    Eulittoral

    Sublittoral

    Elittoral

    Profundal

    Upper

    Lower

    Upper

    Lower

    Spring high-water level

    Summer low-water level

    Helophytes

    Nymphaeids

    Elodeids

    Fig. 11.2 Schematic illustration ofthe littoral zonation of macrophyticvegetation in an oligotrophic lake.(From Pokorny 1994.)

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  • 312 j. pokorny and j. kvet

    isoetids and mosses. Beyond the littoral zones isthe profundal zone, which is typically outside therange in which rooted photoautotrophs are able tofunction.

    Depending on environmental conditions, thetypes of plants present in lake systems can varytremendously. Similarly, there is a considerablerange of depth distributions. Thus, in the clear-water lakes of southern Scandinavia, macrophytesoccur down to a depth of c.5m. In some of the Eifelmaar lakes of Germany, Nitella flexilis grows at adepth of more than 20m (Melzer 1992), whilst inBavarian lakes, meadows of Chara have been ob-served at depths of around 15m (Melzer 1976). Themost diverse vegetation is found in the upper lit-toral sub-zones, where there is typically a mixtureof life-forms. With increasing water depth, first thehyperhydates, then the nymphaeids, the elodeidsand the isoetids are excluded sequentially, as theirrespective ecological limits are exceeded.

    The full spectrum of macrophyte life-forms, asrepresented in Fig. 11.2, may be encountered inclear, oligotrophic lakes (Lobelia lakes), in whichdense eulittoral carpets of isoetids are able to de-velop. In humic, brown-water lakes, the sub-zonesare compressed and displaced upwards. With in-creased turbidity, including that attributable tothe abundance of phytoplankton (stimulated bygreater availability of dissolved nutrients, or withenhanced overgrowth of periphytic algae), first theisoetids and then the elodeids, are excluded: i.e.turbid lakes lose their lower and upper elittoralzones. At the same time, the biomass and densityof emergent and floating-leaved plants usually increases.

    With advanced nutrient enrichment ecosys-tems are increasingly characterised by instability,being subject to abrupt changes and populationcollapses. Such systems are exemplified by theheavily fertilised fishponds of Bohemia (Czech Re-public; see below).

    11.3 PRACTICAL ASPECTS

    11.3.1 Species identification

    The identification of aquatic macrophytes is partic-ularly difficult. Many are quite plastic in their fea-

    tures, and easily modified in response to environ-mental conditions. They are frequently found with-out reproductive structures. As the present systemof classification, and most keys to the identity ofaquatic macrophytes, are based largely on repro-ductive anatomy, correct identification requiresconsiderable practical experience. Besides, manymacrophytes are very mobile, appearing at or disap-pearing from new locations, apparently quite spon-taneously, so there is a risk that local floras quicklybecome out of date. Moreover, many botanists areapparently reluctant to get their feet wet; thus thestate of knowledge on aquatic plants is weak incomparison to that for terrestrial species.

    It is recommended that, as part of any site sur-vey, specimens of macrophytes are collected andtheir identities checked against herbarium ma-terial. Each specimen needs a correct label. Thefollowing manuals and keys for determination ofwater macrophytes are recommended: Hotchkiss(1972), Haslam et al. (1975), Casper & Krausch(1980), Clapham et al. (1981), Spencer-Jones &Wade (1986), Sainty & Jacobs (1994) and Rothmaler(1995).

    11.3.2 Biomass determination

    For assessing the role of macrophytes in aquaticand wetland ecosystems, it is often necessary toknow their aggregate biomass, net production andchemical composition. It is valuable to be able toexpress the chemical content of plants in termsboth of concentration per unit plant dry mass andof standing stock, i.e. amount per unit area. Theprecision required depends upon the level of the as-sessment. Such assessments can be made either di-rectly, by harvesting plants from a certain area, orindirectly, by calculating biomass from estimatesof stand density and measurements of those mor-phological characters which are correlated withthe dry mass of individual plants or shoots.

    11.3.3 Sampling aquatic macrophytes

    Examples of devices for the direct quantitativesampling of aquatic macrophytes are illustrated inFig. 11.3. Metallic frames, enclosing a square or cir-cular area (usually) of 0.25m2, are used to sample

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  • stands in relatively deep water. A diver places theframe on the vegetated bottom and all plants root-ed within the frame are collected. Such devices aresuitable for sampling rooted macrophytes whichare short and not too dense. For sampling in shal-low water, a wooden or metallic cage with wire-mesh sides (up to 0.8m in height) and enclosing aknown area (between 0.25 and 1m2) is more appro-priate. All plants rooted or floating within the cageare sampled by hand. A sieve may be used to sam-ple small free-floating plants (Azolla, Lemna, etc.).Even dense aquatic vegetation can be sampled inthis way.

    The rotary sampler described by Howard-Williams & Longman (1976) cuts plants at theirbase, and then removes them from a circular areadetermined by the length of the rotating knifeblade. A blade of 35.68cm in diameter harvestsplants from an area of 0.1m2. Extension poles enable sampling from depths between 1 and 4m.The apparatus is suitable for sampling loose tomedium-density rooted aquatic vegetation. Lossesof sampled plants must be checked, for over-estimates of biomass are possible in dense stands.

    It is usual to collect several replicate samples inorder to obtain reliable average data. Random vari-ation in macrophyte biomass occurs even under

    identical habitat conditions. Systematic vari-ations in macrophyte biomass may correlate withthe presence of environmental gradients (e.g. ofwater depth). The statistical validity of any deduc-tions based upon biomass data needs to be borne inmind in the initial sampling design.

    Sampling of the aerial parts of emergent vegeta-tion (helophytes) growing in very shallow water (toabout 0.5m depth), or in wet mud, is accomplishedin a manner similar to that for terrestrial vegeta-tion, i.e., by cutting off the shoots at ground (bottom) level. Underground biomass may be estimated by direct coring (where a large number of samples may be needed in order to detect non-random variation) or from correlations betweenabove-ground and underground biomass (see, e.g.,Hejny et al. 1981; Kvet et al. 1998).

    11.4 PRODUCTIVITY

    As a general rule, net primary production (P) over acertain time interval (t2 - t1) is estimated from theequation:

    (11.1)P W W L T= - + -2 1

    Aquatic Plants and Lake Ecosystems 313

    Fig. 11.3 Photograph of varioussamplers, especially of a rotarysampler.

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  • 314 j. pokorny and j. kvet

    where W1,2 is biomass assessed at times t1,2, re-spectively, and L denotes loss of biomass due tomortality or consumption of plants or plant partsover the interval (t2 - t1). The facultative term T isused only when partial (above-ground or under-ground) production is considered: T then denotestransport of assimilates to or from the plant partsconsidered.

    The assumptions that P = W2 - W1 and that Lmay be neglected are valid only over very shorttime intervals, or over longer intervals if thegrowth in the plant stands is well synchronised.For this reason, seasonal maximum biomass,Wmax, approaches above-ground production inmacrophyte communities growing in seasonal cli-mates (e.g., temperate, subarctic). The biomassturnover factor used for estimating annual net production of such communities from Wmax usual-ly varies between 1.05 and 1.5 in well synchronisedstands of rooted macrophytes. The turnover factor,calculated with respect to average biomass record-ed, cannot be ignored and acquires much highervalues over longer time intervals in communitiesof short-lived plants (e.g. Lemna, Azolla), or incommunities where plant growth is unsynchro-nised. This is the case in most tropical and sub-tropical macrophyte communities. A correct assessment of biomass turnover is necessary formaking reliable estimates of annual net primaryproduction of macrophyte stands from their aver-age biomass (W), i.e. for estimating the P/W ratio.The most reliable estimates of biomass turnoverare those based on observation of growth, produc-tion, development and mortality in carefully se-lected (either individually or by cohorts, i.e., sets ofmuch the same age) plants, shoots or otherwise de-fined population or community elements (rametsof clonal plants). For correct estimation of the organic matter production of macrophytes, the ash content must be deducted from the dryweight. This is particularly important in many euhydrophytes, in which the ash content can be high (up to some 50% of dry mass in someCharophytes).

    In temperate zones, the annual net productionof submerged water macrophytes usually reachesnot more than 0.5kg of ash-free dry mass m-2. The

    annual above-ground production of emergentmacrophytes can reach 1 to 2kgm-2. When an opti-mised harvest regime is applied, annual net pro-duction by macrophytes may reach as much as 3kgm-2 in eutrophic habitats.

    The relationship between production and drybiomass of a submersed plant stand is shown inFig. 11.4. The continuous line describes the seasonal course of biomass in a stand of Elodeacanadensis (Pokorny et al. 1984). The course of cu-mulative net production in the same stand is cal-culated by adding the cumulative biomass lossesdue to death or consumption of certain plants orplant parts (as well as the increments recordedafter the seasonal peak was reached) to the biomasscourse. Thus, the dashed line describes seasonalcumulative net production. Total annual net pro-duction is represented by the ordinate of thiscurve, when biomass has fallen to its minimumvalue, at the end of the growing season.

    11.5 DECOMPOSITION

    The assessment of macrophyte decomposition isimportant for an evaluation of their role in aquaticor wetland ecosystems, particularly with respectto the budgets for carbon, oxygen, nitrogen and

    Estimating net primary production from macrophyte biomass

    W NPP = Wmax TF

    Turnover factor (TF):Phragmites = 1.05Typha = 1.1Elodea = 1.11.2

    Wmax(Biomass)

    NPP

    A M J J A S O N

    Fig. 11.4 Fitted growth curve of the total stand biomass(W), net primary production (NPP) and turnover factor(TF). (According to data from Pokorny et al. 1984.)

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  • other nutrients in these systems. The rate of re-lease of nutrients contained in the macrophytesand their detritus is proportional to the decompo-sition rate. As a rule, algal macrophytes decom-pose most readily, followed by submerged andfloating-leaved vascular plants (especially theirleaves and stems). The half-lives of their decompo-sition (the time needed for the decomposition of50% of the initial biomass) usually range betweenseveral days and a few weeks. On the other hand,emergent macrophytes decompose more slowly,with half-lives ranging from a few months to a few years, depending on the nature of the plant ma-terial and the climatic and hydrological condi-tions. Decomposition of detritus may cease at thestage of humic substance formation: the result isan accumulation of peat (nutrient-poor in bogsand nutrient-rich in fens). In this way, peat layersseveral metres thick are formed in bogs underhumid and cool climatic conditions.

    Rapid decomposition rates often occur in eu-trophic waters, where a surplus of nutrients stimu-lates the development of microbes. Detritusaccumulates at the lake bottom, and decomposeswith an equivalent consumption of oxygen. Con-sequently, anaerobic conditions and low redox potential are created. Black sediments releasemethane, hydrogen sulphide and also phosphorus.Usually, organic matter does not simply accumu-late in situ.

    11.6 NUTRIENT CONTENT

    Aquatic macrophytes take up nutrients boththrough their leaves and via their roots. The oldcontroversy over the role of leaves and roots in nu-trient uptake by submersed macrophytes has beenended by experimental studies (see literature re-viewed by Bristow 1975; Hutchinson 1975; Denny1980; Dykyjov & lehlov 1998). Helophytessuch as Phragmites, Typha, Scirpus and Carex takeup mineral nutrients not only via their edaphicroots (which penetrate the bottom mud or the sub-soil), but also via their accessory roots, which growdirectly into the water, or into detritus. Rootedplants do not need to compete with non-rooted

    macrophytes or phytoplankton for nutrients dis-solved in water. Generally, concentrations of bothmajor and minor elements are higher in the bio-mass than in the environment.

    The three different kinds of vascular waterplants (emergent, floating and submerged) differ intheir ash and mineral contents. Submersed plantscontain a higher water and ash content than theemergent vegetation, whilst floating species fallbetween the two (Westlake 1965; Dykyjov &lehlov 1998). Concentration factors (concentra-tion of an element in dried plant/concentration ofthe same element in water) mostly exceed 1000and over 10,000 for P, K and Mn (Boyd 1969;Dykyjov & lehlov 1998).

    The accumulation of minerals in individual or-gans varies with metabolic activity. The greatestnutrient content is generally found in the stemsand leaves; phosphorus and magnesium often ac-cumulate in flowers and fruits. In the aerial parts ofthe plant, concentrations of most nutrients otherthan calcium tend to decline during the growingseason. Those aerial and submerged organs whichdecompose each season release their nutrients intothe environment. The large underground storageorgans of perennial macrophytes (rhizomes, rootsand tubers) retain a substantial portion of the nu-trients accumulated by the parent plants, and do sofor several years. Return of nutrients to the bottomsubsoil occurs mainly only after decomposition.Vymazal (1995) gives a large literature review onnutrient and heavy metal contents of the biomassof algae and water macrophytes. The data includeinformation on nutrient concentrations and stand-ing stocks, rates of daily production, daily nutrientuptake and decomposition rates of macrophytes.Figure 11.5 presents an example of the seasonalcourse of nutrient standing stock in a submersedmacrophyte stand (Elodea canadensis).

    Besides the natural sources of variability inchemical composition of aquatic plants, many dis-crepancies exist in the analytical techniques used.Even sets of data on water content are often notcomparable with each other, as drying in ovensmay be carried out at 60, 70, 80 or 105C. Some-times, only air drying is carried out. For a rough es-timation of amounts of nutrients bound in the

    Aquatic Plants and Lake Ecosystems 315

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    biomass of macrophytes (the standing stock) thefollowing approximate ranges of concentrations(% dry mass) may be used (as a rule of thumb): nitrogen 14%, phosphorus 0.10.5%, sulphur0.10.7%, potassium 15%, magnesium0.10.6%, calcium 0.14% (variability caused byprecipitated calcium carbonate), iron 0.025%,manganese 0.010.5% in submersed macrophytesand 0.010.1 for helophytes.

    11.7 THE ECOPHYSIOLOGICALROLE OF MACROPHYTES IN LAKE

    ECOSYSTEMS

    The lives of macrophytes in lakewater or on flood-ed soil are conditioned by the solubility and slowrate of movement of gases. Concentrations of gasesare different in air and soil. Moreover, as a result ofthe metabolic activities of plants and other biota,concentrations of oxygen and carbon dioxide inwater may change markedly during the course ofthe day, whereas atmospheric concentrations areeffectively constant. In this section, we considerthe impact of gases exchanged in photosynthesisand respiration.

    11.7.1 Oxygen in water and in flooded soil,and its estimation

    The solubility of oxygen in water is weak: at 20C air-equilibrated water contains over thirtytimes less oxygen than the same volume of air (9mg versus 285mg). Its solubility is also temperature sensitive, saturation at 30C beingreached at half the concentration (7.4mg O2 L

    -1) atwhich it occurs at 10C (14.2mg O2 L

    -1). Diffusionof gases is also slower in water than in air (coefficients of molecular diffusion in the order 10-9 m2 s-1) by about an order of magnitude. Un-replenished oxygen demand in water quickly leads to exhaustion.

    The greater the plant biomass, the higher thepotential photosynthetic release of oxygen and thegreater the net respiratory oxygen consumption inthe dark. Large diel amplitudes in the concentra-tion of oxygen are thus typical of dense, activemacrophyte stands. Dissolved oxygen concentra-tions may be restored by daytime production andrelease of the gas during plant photosynthesis.Oxygen content of the water increases around theleaves of submersed macrophytes, and may wellrise above the value of air saturation. Under sunnyconditions, water with abundant macrophytesmay well become considerably supersaturatedwith oxygen (concentrations corresponding to150200% of air saturation are not unknown). Incontrast, the oxygen content of clear, unpolluted,

    2

    0

    2

    0

    4

    8

    0

    4

    8

    0

    4

    8

    0

    2

    0

    June July August September

    Ca

    Ca

    K

    P

    N

    Mg

    g m

    2

    Fig. 11.5 Seasonal course of the mineral nutrientcontents in Elodea canadensis biomass per unit area ofthe stand (standing stock [gm-2 dry mass]). (FromPokorny et al. 1984.)

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  • plant-free water varies little from the air-saturation concentration.

    Accumulation and decomposition of deadmacrophyte biomass places extra consumptive de-mand on oxygen exchanges, to the extent of localexhaustion, both in deep water and in flooded soils. Environments in which the oxygen concen-trations are too low to be detected by standardmethods (Winkler, electrochemical Clark typeoxygen sensor) are described as being anoxic, oranaerobic. In practice, the label applies to waterwhere the concentration of dissolved oxygen is

  • 318 j. pokorny and j. kvet

    however, the oxygen regime represents a systemdisturbed by biotic processes in which equilibriumis steadily re-established by physical processes ofaeration and diffusion (possibly also by advection).Periodicity in system behaviour appears to be a re-sult of dependence of biotic processes, which arethemselves periodic, on solar radiation input. Theamplitude of diurnal (diel) changes in oxygen con-centration shows how strongly biotic activity af-fects the dynamics of oxygen regime in the habitat.By way of an illustration, Fig. 11.7 tracks diurnalchanges of oxygen concentration in a sublittoralstand of Utricularia vulgaris. The difference between the maximum oxygen concentration (at about 1800 hours) and the minimum (at about 0500 hours) was 11mgL-1. High daytimeirradiance (photosynthetically active radiation(PAR) of 350Wm-2 at midday) was favourable forphotosynthesis.

    A further feature of the oxygen regime in standsof submersed macrophytes, illustrated in Fig. 11.7,is the vertical stratification of oxygen concentra-tion. This may be generated via the occurrence ofdiffering rates of photosynthetic production andrespiratory consumption of oxygen at differentdepths in the stand. As the plant biomass is not dis-tributed uniformly, each layer may contain con-

    trasting amounts of biomass and receive differentfluxes of PAR. Thus gross rates of photosynthesismay vary considerably. Usually, oxygen is pro-duced at a faster rate in the near-surface layers(which support a dense plant biomass) than inshaded or deeper stand layers. Characteristic pro-files of oxygen concentration may develop withinthe submersed stands.

    Another source of variation in oxygen concen-tration is the flux of the element into the bottomsediments, where it is consumed by the respirationof benthic organisms, and by chemical oxidationprocesses. In phytoplankton communities, strati-fication of oxygen is smoothed out by the relative-ly homogeneous biomass distribution of thephytoplankton and by turbulent diffusion, whichcompensates for stratification of oxygen concen-tration more effectively than in stands of sub-mersed macrophytes where the turbulentdiffusion may be weak (see Chapter 10). However,stratification of oxygen near the lake bottom issimilar in phytoplankton- and in macrophyte-dominated systems.

    Differentiation of the water column in terms ofdissolved oxygen concentration mirrors the typi-cal course of diurnal density stratification, stabil-ising during the morning, when irradiance is

    12 14 16 18 2220 2 4 6 8 10 12 140

    2

    4

    6

    8

    10

    12

    Oxy

    gen

    conc

    entr

    atio

    n (g

    m3

    )

    Day hours

    Fig. 11.7 Diel changes in oxygen concentrations measured at three depths: 0.05m (o), 0.2m () and 0.4m (D) in a standof Utricularia vulgaris (Neusiedlersee Austria). (Redrawn from Pokorny & Ondok 1991.)

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  • increasing in intensity, and weakening throughthe afternoon. During the night, when depletion ofoxygen is proportional to the density of the respir-ing biomass, convection and turbulent diffusivityoverride some of the differences in oxygen concen-tration between various individual layers. In shal-low waters, stratification may almost disappear,except in a thin layer just above the lake bottom,where the flux of oxygen to the sediments perma-nently diminishes its concentration.

    Other oxygen sinks, such as bacterial respira-tion, and consumption in oxidation processes, gen-erally play a proportionately lesser role in theoxygen balance of stands of submersed macro-phytes than they do in planktonic communities.Because separate measurement of individual com-ponents of the oxygen balance is difficult, they areusually determined as BOD (biological oxygen de-mand) and COD (chemical oxygen demand). In themodel of oxygen dynamics of submerged vegeta-tion of Ondok & Pokorny (1987) they are treated asa single sink and simply compounded with theother main sinks identified.

    11.7.3 Carbon dioxide, carbonateequilibrium and photosynthesis of water

    plants

    Carbon dioxide (CO2) is more soluble in water thanoxygen. Although air contains several orders ofmagnitude less of carbon dioxide (about 0.03%)than of oxygen (21%), water may be supersaturatedwith carbon dioxide above the air-equilibratedconcentration in pure water (0.51.0mg CO2 L

    -1).Aqueous concentrations of carbon dioxide are alsoinfluenced by equilibria involving various forms of inorganic carbon, Ct found in water, which in-clude: (i) free carbon dioxide (CO2), (ii) the bicar-bonate (HCO3

    -) and carbonate (CO32-) ions and (iii)

    carbonic acid (H2CO3). Concentrations (C) of eachform tend towards equilibrium, as described in thefollowing equations for total inorganic (Ct) and freeCO2:

    (11.2)

    C TA OH Ht = - [ ] + [ ]( ) [ ] + [ ] +[ ] +- +

    + +

    +

    H K H K K

    K H K K

    21 1 2

    1 1 22

    (11.3)

    (11.4)

    (11.5)

    (11.6)

    where TA is total alkalinity (see below) and K1, K2,. . . are the reaction constants given.

    The carbonate equilibrium system regulatesthe pH of the oceans, of most freshwaters, of mostsoils and even of vertebrate blood. This makes itone of the most ubiquitous, and one of the mostimportant, buffering agents on the planet. Whenfree carbon dioxide (CO2) is added to water (e.g.when released during respiration), pH falls becauseCO2 forms a weak carbonic acid with water (CO2 +H2O = H2CO3). When free carbon dioxide is re-moved from water (e.g. taken up during photo-synthesis), then pH rises. The well-known relationship shown in Fig. 11.8 plots the distribu-tion of free carbon dioxide, bicarbonate and car-bonate as a function of pH. It is evident that at pH4.3, all inorganic carbon is present as free CO2. AtpH 8.3, bicarbonate (HCO3

    -) predominates, whilstat pH 12.3 all inorganic carbon is in the form of car-bonate ions (CO3

    2-).Whilst photosynthesising, aquatic plants take

    up carbon dioxide from the water around them.When respiration exceeds photosynthesis, there isnet release of carbon dioxide, and pH falls. Therange of pH variation depends on the amount ofcarbon dioxide released or consumed, and on thebuffering capacity of the water. The latter is deter-mined by its alkalinity, which is the sum of HCO3

    -

    + 2CO2 + OH- - H+ concentrations. The total al-kalinity of water is relatively easily determined: agiven volume of the test water is titrated with hy-drochloric acid (HCl) of known strength to pH 4.3,at which point, the HCl consumed is equivalent tothe original alkalinity of the water. At pH 4.3, all

    lnK T Tw = - + --13847 26 148 9802 23 65211. . . ln

    lnK T T2 111843 79 207 6548 33 6484= - + --. . . ln

    lnK T T1 114554 21 290 9097 45 0575= - + --. . . ln

    Free CO2

    21

    21 1 2

    =[ ]

    [ ] + [ ] ++

    + +

    H C

    H K H K K

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  • 320 j. pokorny and j. kvet

    carbonate and bicarbonate ions have been convert-ed to free carbon dioxide. At very low total alkalin-ity, a correction of end-point should be madeowing to interaction with carbon dioxide from air(the Gran titration: see Talling 1973).

    The alkalinity of water emanating from nutrient-poor soil is low (typically

  • Aquatic Plants and Lake Ecosystems 321

    Bottom

    Air

    f1

    f6f3

    f2

    f4

    f7

    f5

    Free CO2

    HCO3

    Sediments

    pH

    Primary producers

    Atmosphere

    Temperature

    Water

    PAR

    Benthos

    Fig. 11.9 Scheme of inorganic carbon regime in water inhabited by submersed vegetation: PAR, photosyntheticallyactive radiation; f1f7, reaction functions.

    0 2 4 6 8

    0

    2

    4

    6

    8

    10

    12

    10 12 14 16 18 20 22

    Free

    [CO

    2] (g

    m3

    ); pH

    Day hours

    120

    100

    80

    60

    40

    [HCO

    3] (g

    m3

    )

    20

    0

    Fig. 11.10 Diurnal changes in free CO2 () and HCO3- () concentrations, and pH (D) in a stand of Elodea canadensis,

    calculated from measured pH and alkalinity.

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  • 322 j. pokorny and j. kvet

    roughly equivalent to the amount of solar energyreaching 1m2 on a single clear day. The total annualsolar energy flux amounts to 10001200kWhm-2

    ( 36004300MJm-2), while the amount of solarenergy bound in 1kg of dry biomass is about 5kWh( 18MJ), i.e. less than 0.5% of annual incidentsolar energy. One may well enquire as to the fate of the more than 99% of incoming solar energy notrealised in photoautotrophy. In fact, plants whichare well supplied with water are engaged in the dissipation of solar energy in other important ways, which are relevant to the local and regionalclimate.

    Most of the solar energy incident on the watersurface, and of the irrigated surfaces of aquaticplants, is consumed in evaporation. The amount

    needed for evaporation of 1L of water is 2.5MJ =0.7kWh. This is a relatively large amount, whichis invested in water vapour and only released again(as the latent heat of evaporation) when it con-denses, in clouds or as dew. Evaporation fromplants is called transpiration, therefore evapora-tion of water from plant stands (plants and soil orwater surface) is called evapotranspiration (seealso Chapter 3). This process damps the potential local temperature fluctuations in space (cooler and hotter places) and time (day and night). Bydraining land, wetlands, lakes and their littoralzones, steppe-like conditions are created. If plantsare unable to dissipate heat by vaporising water,solar energy is converted directly to heat (see Fig.11.11).

    Reflection515%

    Reflection510%

    Heat flux510%

    Evapotranspiration1020%

    Evapotranspiration7080%

    Heat510%

    Heat6070%

    Heatflux

    510%

    Drained field Lake, meadow, forest,landscape saturated with water

    Daily input of solar energy6 kWh m2

    max energy flux8001000 Wm2

    Fig. 11.11 Dissipation of solar energy in a drained field and in a landscape saturated with water.

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  • 11.7.5 Solar radiation in water

    Solar radiation reaching the water surface is partlyreflected (Fig. 11.11). Reflection (albedo) increaseswith decreasing declination (or angle of incidence),and is also increased by surface wave formation.Ultraviolet radiation and infrared wavelength aremore easily absorbed than visible light, of whichthe blue wavelengths naturally penetrate the furthest in pure water, though in lakes they aresubject to absorbance by dissolved substances.Relative penetration of light determines the trans-parency of the water, which is found from the ratiobetween the irradiance observed at a given depthand that recorded at the water surface. Transparen-cy is commonly estimated using a Secchi disc, adisc about 25cm in diameter, divided into twowhite and two black equal quarters. This is low-ered into the water to the depth where the blackand white sectors become indistinguishable. Thisis the Secchi-disc depth, which is a reasonableguide to the euphotic depth that marks the lowerbounds of the layer in which net photosyntheticproduction is possible and where the growth ofwater plants becomes limited by lack of light (seeChapter 10). Roughly, it corresponds with about1% of full daylight. Euphotic depth is usually sup-posed to approximate to 2.02.5 Secchi-discdepth (see Chapter 5). In the most transparent, oligotrophic lakes, the euphotic zone may extendto 30m depth or more 50m in extreme cases and certain macrophytes (e.g. Isotes lacustris,Characeae) may grow in such lakes to depths of1520m. In contrast, transparency in dense macro-phyte stands may be a few decimetres only.

    Transparency is roughly reciprocal to the under-water attenuation of photosynthetically active ra-diation (PAR). Light energy penetrating the watersurface, I0, is progressively extinguished with depth;Iz, plots of which generally conform to an exponen-tial equation (the LambertBeer law) of the form,

    (11.7a)

    or

    (11.7b)ln lnI I zz - = -0 e

    I I zz = -0 exp e

    where e is the coefficient of vertical light attenua-tion. Typical light attenuation plots in macro-phyte beds conform to this general equation, butthe attenuation coefficients vary with plant den-sity. The plots shown in Fig. 11.12 have been standardised to a common base of percentage penetration, (Iz/I0) 100%.

    There are significant differences between thespectral composition of light reaching submersedvegetation and that received by terrestrial stands.For instance, almost all infrared radiation is ab-sorbed during the first 20mm of its passagethrough water; thus the radiant energy penetratinginto water is diminished by approximately 50%.On the other hand, the reflectance, or albedo, of thewater surface, defined as the percentage of incidentglobal radiation reflected, is significantly less(410%) than that of terrestrial vegetation(2030%). Reflectance of light by water varies withthe angle of incidence, and is modified by wave ruffling and by turbulence (Bykovskij 1980). Moreover, spectral composition of radiation penetrating natural waters varies more rapidlywith depth than does that of light penetrating into

    Aquatic Plants and Lake Ecosystems 323

    0.0 0.2 0.4 0.60

    10

    20

    30

    40

    50

    Depth (m)

    (I (z)/

    I 0) 1

    00

    Fig. 11.12 Examples of vertical profiles ofphotosynthetically active radiation (PAR) extinction instands of Potamogeton lucens (), Ceratophyllumdemersum (o), Nuphar lutea (D) and in a mixed stand ofCeratophyllum demersum and Lemna sp. (): I0, Iz, PARirradiance above water surface and at the depth z,respectively.

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  • 324 j. pokorny and j. kvet

    terrestrial vegetation. This is principally due topreferential absorption by water of certain spectralbands, but behaviour attributable to pure water ismodified by dissolved material (especially humicand fulvic acids), and by selective dispersal by or-ganic and inorganic particles suspended in water(Wetzel 1975). In phytoplankton, the fraction ofthe radiation thus dispersed may be c.20% (Ondok1977), whereas in emergent vegetation the value isusually less than 5%.

    Radiation not absorbed or dispersed in water isavailable to plants, although the amount whichthey absorb is related to the density of the sub-mersed vegetation and to the extent of its own self-shading. Many species of macrophytes, especiallythose of eutrophic habitats, accumulate biomassin the upper layers where radiation is mostfavourable for photosynthesis. Vertical biomassdistribution is then heterogeneous, and the bio-mass accumulated in the upper layers causes rapidradiation extinction in water. In such cases, lightextinction with depth through dense plant standsis distorted with respect to the orderedLambertBeer expression (Monsi & Saeki 1953;Anderson 1971; Pokorny & Ondok 1991). In shal-low lakes, macrophytes occupy the entire watercolumn, but in deeper lakes with clear water theymay grow within a depth range of 26m or more.The upper parts of the stand may then be within a metre or so of the water surface: this was the case in a stand of Potamogeton lucens rooted inStechlinsee (Germany) at a depth of 2m below thewater surface but with a stand height of about 1m,for which a representative light-extinction profileis shown in Fig. 11.13. Ikushima (1970) suggestedthat for such cases an extinction formula that sepa-rated the two coefficients be used, as in:

    (11.8)

    where a and h are the respective extinction coeffi-cients in the upper zone (void of submersed vegeta-tion), and z expresses the distance from 0 to z0 andh that from z0 to h0 (see Fig. 11.13). If the stand bio-mass is homogeneously distributed the formuladescribes the actual extinction profile.

    In shallow ponds with macrophyte biomass dis-

    I I z hz 0 = - -( )exp a

    tributed unevenly in the water column, the fit ofextinction data to the LambertBeer law is usuallynot satisfactory. Ondok & Pokorny (1982) intro-duced a further modification to the formulation, towhich actual PAR profiles fit well:

    (11.9)

    where Iz and I0 are PAR irradiances at the depth z[m] below the water surface and above the water,respectively, e is the extinction coefficient and a isa parameter expressing the degree of heterogeneityin the spatial distribution of plant biomass. Figure11.14 shows an example of fitted PAR extinctionprofiles in a stand of Elodea canadensis on two separate occasions. The coefficients e and a wereestimated by a standard linear regression tech-nique. Their values were approximated to be, res-pectively, 4.01 and 0.73 for the June profile, and8.31 and 0.57 for August.

    I I zz = -( )0 exp e a

    0.0 0.4 0.8 1.2 1.6 2.00

    10

    20

    30

    40

    50

    60

    70

    80< 8.3 almost no free carbondioxide is available. At pH > 8.3, bicarbonate re-acts with hydroxyl ions to form carbonate. AbovepH 10, there is more carbonate than bicarbonate(see Fig. 11.8). The carbonates of calcium and

    magnesium are much less soluble than their bicarbonates. Thus, rapid photosynthetic uptakeof carbon dioxide by water plants in calcareous waters may eventually lead to precipitation of car-bonate (marl deposition) on the surfaces of theplants.

    Whereas all water plants readily utilise freelyassimilable carbon dioxide, the ability to use bicar-bonate is not universal, and none are able to takeup carbon from carbonate ions. The ability of waterplants to use bicarbonate, and to photosynthesiseat high pH, is tested by enclosing plants in glassbottles of lakewater, and exposing them to lightuntil a constant pH is reached. The higher the finalpH, the greater the affinity of the plants for inor-ganic carbon. Macrophytes adapted to utilise bi-carbonate are able to raise pH to 10, a value which ahandful of planktonic algae and cyanobacteria arealso able to reach. Typical bicarbonate users in-clude small Potamogeton species, Elodea andNajas. In contrast, most mosses, Utricularia, andthe water plants of acidic or peaty waters (e.g. Hot-tonia) are unable to use bicarbonate, and their photosynthesis ceases at pH 8.59.

    11.8.5 Photosynthetic adaptation to highpH and high oxygen

    It may be deduced that photosynthesis in waterplants involves two negative feedbacks: the sup-pression caused by pH increase and the effect of in-organic carbon depletion. In addition, the excess ofoxygen produced is also eventually inhibitory tophotosynthetic carbon reduction. Water plants,however, possess an elegant strategy for minimis-ing inhibition at high pH. Broad leaves (for exam-ple, those of Potamogeton lucens) take upinorganic carbon (carbon dioxide and bicarbonate)through their lower (abaxial) surfaces and releasehydroxyl ions at the upper (adaxial) surface. In thisway, pH on the lower side of the leaf increases lessthan it does on the upper side (Raven 1984). Acidicand alkaline zones have also been found in plantsof Chara sp., indicating areas of preferential bicar-bonate uptake and hydroxyl release, distinguishedby differences in pH of up to two units (Lucas1975). As a result of this polarity, calcium carbon-

    TLH11 10/14/03 10:40 PM Page 328

  • ate is precipitated only in regions of hydroxyl release.

    Some macrophytes (e.g. Isotes) have developedthe adaptation of crassulacean acid metabolism(CAM). They take up carbon dioxide during thenight, when its concentration is relatively high,through the action of phosphoenol pyruvate (PEP)carboxylase. The product malate is accumulatedin the cells. By day, malate is metabolised to re-lease a dedicated supply of carbon dioxide for conventional fixation by ribulose biphosphate carboxylase (Keeley & Morton 1982). These plantscan also use carbon dioxide produced by root respi-ration, which passes to the assimilatory tissuesthrough the lacunal system in the aerenchyma.

    The amount of calcium carbonate precipitatedon leaves of water plants, and on filamentousalgae, can reach values of tens of grams per squaremetre, i.e. several hundred kilograms per hectare.In this way, the concentration of dissociated cal-cium is reduced, phosphorus can be bound in theprecipitate, and total alkalinity of water decreasesowing to precipitation of carbonate. The pH of thewater in macrophyte stands rich in calcium car-bonate precipitate is high and stable, even duringthe night, because carbon dioxide released at thattime reacts with calcium carbonate. The pH doesnot change but total alkalinity increases owing tothe release of bicarbonate.

    The problem with excess oxygen is manifest inan effect on the compensation level of carbon up-take. When oxygen concentration increases from 8to 40% air saturation, the carbon compensationpoint in low-pH-adapted plants of Myriophyllumsalsugineum rises from 5.3 to 8.9mmolL-1, butfrom 1.8 to 2.8mmolL-1 in M. salsugineum adaptedto high pH. However, some aquatic macrophytesexhibit some of the features of C4 plants, in thatthey possess low photorespiration levels and lowCO2 compensation points, which are relatively insensitive to oxygen concentration (Bowes &Salvucci 1989; Lambers et al. 1998). These featuresmay be attributed to high PEP carboxylase activity,which has been demonstrated in Hydrilla verticil-lata adapted to a high temperature and low free car-bon dioxide concentration (Holaday et al. 1983).Internal accumulation of inorganic carbon in ex-

    cess of the concentration in the external mediumsuppresses photorespiration in most submersedmacrophytes with high ribulose biphosphate car-boxylase activity. Most are able to acclimatise tosummer conditions of high temperature, high oxy-gen and low carbon dioxide concentrations.

    11.8.6 Dark respiration andphotorespiration

    The ratio of dark respiration rate to net photosyn-thesis in submerged macrophytes has been report-ed as being three to five times greater than interrestrial plants (e.g., Van et al. 1976; Sndergaard& Wetzel 1980; Westlake 1980). Van et al. (1976)reported a dark respiration rate in three species ofsubmersed macrophytes (Myriophyllum spica-tum, Ceratophyllum demersum and Hydrilla verticillata) which constituted over 50% of netphotosynthesis. In terrestrial plants, dark respira-tion is typically only 510% of the net photosyn-thesis. Pokorny & Ondok (1991) measured thedark respiration rate of several species of sub-merged macrophytes, which ranged between 0.6and 1.8gO2 kg

    -1 dry matter h-1. In Utricularia vulgaris this corresponds to a range of 620% ofmaximum net photosynthesis. Dark respiration inElodea densa was 79% of the maximum photo-synthesis; Myriophyllum salsugineum, La-garosiphon major, Cabomba caroliniana, Egeriadensa and Nitella sp. possessed an even smallerratio. The most important factor controlling thisratio is plant age and periphyton density on theplant surface. Plants from eutrophic water bodiesovergrown by periphytic organisms exhibit thehighest respiration rate at the end of the growingseason. This also leads to oxygen deficits in standsof water macrophytes.

    According to several authors (Hough & Wetzel1972; Prins & Wolf 1974; Hough 1979) photorespi-ration occurring in submersed macrophytes is lessintense than in terrestrial plants. Salvucci &Bowes (1982) found photorespiration to be about15% of the net photosynthetic rate in Myriophyl-lum brasiliense. Jana & Choudhuri (1971) studiedphotorespiration in Hydrilla verticillata, Valisne-ria spiralis and Potamogeton pectinatus. Both dark

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  • 330 j. pokorny and j. kvet

    respiration and photorespiration were highest in P. pectinatus. Photorespiration of the abovethree species was about 50% of net photosynthe-sis. In contrast to the results of Hough (1979), pho-torespiration exceeded dark respiration: the ratioof light/dark respiration was 1.62, 1.6 and 1.23, re-spectively, for Valisneria spiralis, Potamogetonpectinatus and Hydrilla verticillata. The ratios oflight/dark respiration measured by Sndergaard &Wetzel (1980) in Elodea canadensis, Lobelia dort-mana, Fontinalis antipyretica and Littorella uni-flora were in agreement with the data reported byHough (1979), ranging between 0.07 and 0.64, de-pending upon the oxygen concentration. The ratioincreased when oxygen concentration rose. Theratio of photosynthesis to photorespiration is simi-lar to that found in C3 plants, but it is recognisedthat precise practical measurement of the refixa-tion of carbon dioxide from the internal lacunalsystem is difficult to achieve.

    11.9 MACROPHYTES ANDASSOCIATED BIOTA

    11.9.1 Invertebrates associated with aerialparts of emergent and floating vegetation

    Grazing invertebrates directly impinge on the de-velopment of water macrophytes by damagingtheir stems and meristems. Dvork et al. (1998) re-viewed the results of the International BiologicalProgramme and other findings on the role of ani-mals and animal communities in wetlands, withparticular emphasis on the role of macroinverte-brates. They distinguished between those groupsof invertebrates which use the emergent vegeta-tion only as a refuge or shelter (adults of Chirono-midae, Culicidae, Ephemeroptera, Trichoptera,etc.), and several groups of organisms which areplant feeders, or which have adopted special feed-ing mechanisms prejudicial to the plants, many ofwhich figure prominently in the energy budget ofthe ecosystem. Gaevskaya (1966, 1969) showedthat, when they were at their most abundant, adultpopulations of two coleopteran species, Donaciadentata and Hydronomus alismatis, together con-

    sumed up to 6% of leaf biomass of Sagittaria sagit-tifolia each day. Donacia dentata and D. crassipesgrazing on Nymphaea candida were found to con-sume 2.6% of the biomass per day. The feeding intensity of individual invertebrates depends ontheir age, and on the suitability of the plant speciesas a food. In some species, daily consumption bylarvae is equivalent to 50% of body mass. The olderand larger the larvae, however, the smaller themean daily consumption per unit body weight.

    Such consumption data indicate intensity offeeding on plant biomass at selected times, butgive little clue as to annual food consumption, orthe loss of macrophyte biomass sustained. Thesequantities are determined by a complex of factors,including the seasonal dynamics of recruitmentand growth of consumers, seasonal changes in thequality and abundance of the plant parts con-sumed, and temperature. Smirnov (1961) estimat-ed the consumption of Alisma plantago-aquaticaand Oenanthe aquatica by Donacia dentata to be,respectively, 3 and 6.1% of their annual produc-tion. Imhof (1973) expressed the plant biomassconsumed by all phytophagous insects present in astand of Phragmites in terms of energy intake the total of which, 170250kJm-2 yr-1, representedsome 0.30.4% of annual net production.

    The complex impacts of invertebrates onmacrophyte growth in terms of biomass consumedor potential energy sequestered cannot always begauged. For instance, the stem-boring lepidopter-an, Phragmataecia, takes up 6.3kJ per Phragmitesstem per year without visibly affecting its growth,whereas the gall-forming fly, Lipara, taking only0.25kJ, inhibits stem growth and prevents flower-ing (Dvork et al. 1998).

    11.9.2 Invertebrate macrofauna associatedwith the submerged parts of the vegetation

    Studies of invertebrates associated with macro-phyte stands are somewhat inconsistent, someconcentrating on benthos but not plankton, otherson submerged plants, but ignoring free-living bot-tom fauna. Sampling equipment is non-standard,and sieves are of differing mesh sizes (see alsoChapter 12). Few studies include an attempt to

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  • quantify plant biomass. Thus, comparison is limit-ed mainly to generalisations. Dvork et al. (1998)reviewed data on invertebrate biomass found indifferent types of vegetation from various northerntemperate and tropical wetlands, within the con-text of a derived unit of vegetated area. The highestnon-benthic macrofaunal biomass concentrations(3.210gm-2 drymass) were found among Lemnatrisulca, Potamogeton crispus, Schoenoplectus la-custris and on Ranunculus aquatilis and Elodeacanadensis. Other plant species supported 0.21.9gm-2. There are often no apparent differencesbetween colonisation of submerged plants andemergent species.

    It generally may be assumed that detritus andperiphyton are the prime trophic links governingthe productivity of macrophyteinvertebrate com-munities. Although these invertebrates are oftencalled phytophilous, they survive mostly as con-sumers of detritus or of periphyton (aufwuchs), for which the living plants provide suitable envi-ronments for accumulation, namely shelter andsubstrate. Some invertebrate species directly consume the living tissues of submerged plants(Acentropus niveus, Elophila nymphaeata, Phry-ganea grandis; Dvork et al. 1998) but these aregenerally distinguished by low biomass and bytemporal variability, their feeding periods being re-stricted to certain, often relatively short, periods.In contrast, soft, decaying and bacteria-rich planttissues are consumed for longer periods. Entrap-ment of foods of allochthonous origin, such as terrestrial leaves and detritus, can contribute animportant component of available resources.

    11.9.3 Role of filamentous algae indevelopment of macrophyte stands

    Owing to their fast rates of growth, filamentousalgae are able rapidly to overwhelm vascularplants, shading them and lowering the availabilityof carbon dioxide, sometimes below their compen-sation points. In this case, vascular plants becomelittle more than substrata for filamentous algae.

    The development of filamentous algae is affect-ed by concentrations of phosphorus and nitrogenin water. It is at high concentrations of nutrients,

    and low feeding pressures, that filamentous algaegrow most rapidly. When phosphorus concentra-tion in the algal dry mass exceeds 0.25%, growth ofCladophora is considered not to be phosphoruslimited (Auer & Canale 1982). Phosphorus con-centrations in Cladophora from the fishpondsstudied in the Trebon Basin Biosphere Reserve ex-ceeded this value, and the nitrogen content rangedbetween 1.3 and 4.3% of dry mass (Eiseltov &Pokorny 1994). Ettl et al. (1973) estimated themaximum rate of biomass doubling of Cladophorato be about 20 hours: maximum oxygen produc-tion reached 400% saturation and pH rose to 11.6.

    These observations of the conditions that de-velop in dense algal mats exceed those cataloguedby Hillebrand (1983). The expectation is that de-pletion of carbon dioxide and high pH restrict thegrowth of most other autotrophs. Simultaneous re-lease of free ammonia is toxic to other organisms,causing, inter alia, gill necrosis in fish. High pHalso slows microbial activity, including decompo-sition. Decay of Cladophora fracta can be as slow (90% dry-weight loss in 95 days; Pieczynska1972) as reported for Cladophora albida in tidal estuaries (Birch et al. 1983).

    Light sensitivity of photosynthesis inCladophora fracta is demonstrable (authors data): faster rates, equivalent to 6.5mgO2 g

    -1 h-1,have been measured at the water surface (PAR irradiance >100Wm-2) than at the lake bottom (40Wm-2). The estimated compensating light in-tensity, about 6Wm-2, does not suggest that thereis much adaptation to low irradiance in this alga.The photosynthetic activity of filamentous algaemay lead to precipitation of calcium carbonate onfilaments. For example, 32mg of CaCO3 were de-posited on 1g of Cladophora fracta dry mass (i.e.,134kgCaCO3ha

    -1at biomass of 420g dry matter m-2).

    11.10 DEVELOPMENT OFMACROPHYTE STANDS IN

    RELATION TO ANTHROPOGENICIMPACTS

    The various responses of limnetic macrophyticvegetation to nutrient load, and to management of

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  • 332 j. pokorny and j. kvet

    land fertility, drainage, fish stocking, applicationof lime and organic fertiliser (manure), are demon-strable among the fishponds of southern Bohemia.During the early decades of the twentieth century,fishponds were mostly slightly acidic (pH 66.5),with alkalinity generally

  • Aquatic Plants and Lake Ecosystems 333

    1 2 3 4 5

    0

    0.4

    0.8

    0

    0.4

    0.8

    0

    0.4

    0.8

    0

    0.4

    0.8

    0

    0.4

    0.8

    0

    0.4

    0.8

    0

    0.4

    0.8

    Dep

    th (m

    )

    Dry mass(g m2)

    EZ/E0

    Oscillatoria

    Chlorococcalalgae +dense fishstock

    Dissolved oxygen(% of air saturation)

    pH0 0 0.2 0.6 6 8 0 100 200100 200

    A

    B

    C

    D

    E

    F

    G

    Fig. 11.18 Development of aquaticvegetation in ponds of increasingtrophic status (1) from oligotrophic(A) over mildly eutrophic (B),eutrophic (C, D) to hypertrophic (E,F, G) stages; (2) biomass distributionin vertical profile; (3) extinctionprofile, E0 = photosyntheticallyactive radiation (PAR) irradiance atwater surface, EZ = PAR irradianceat the depth z; (4) pH; (5) oxygenconcentrations in the verticalprofile. Dashed line, nightminimum values; solid line, daymaximum values. (From Pokorny &Ondok 1991.)

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  • 334 j. pokorny and j. kvet

    and an oxygen deficit often develops near the lakebottom. Here, decomposition of organic matterleads to low oxygen concentration, and release ofphosphorus, ammonium and ferrous iron mayoccur. Internal nutrient cycling therefore now be-comes important.

    11.10.4 The progression to hypertrophy

    In the stages represented by Fig. 11.18DF, nutri-ents have ceased to be a controlling factor in com-munity structure. Instead, the fish stock itself mayexert a crucial role in the extent of the submersedvegetation. At a low fish stock (seasonal mean livebiomass of fish 11 (Eiseltov &Pokorny 1994), and the macrophytes die back. Development of blue-green algae is particularlytolerant of a low availability of inorganic carbon,but is favoured by the rapid cycling of phosphorusand ammonium nitrogen (Pokorny et al. 1999).Furthermore, blue-greens are less intensely con-sumed by zooplanktonic filter-feeders than arechlorococcal algae.

    At high fish-stocking densities (seasonal meanbiomass of fish 400kgha-1), large Daphnia spp.are likely to be eliminated. This situation allowsfast-growing chlorococcal algae to develop densepopulations of several hundred mgL-1 of chloro-phyll a. The dense algal populations (Fig. 11.18F)

    create low transparency (less than 0.4m), whichprevents the development of blue-green algae suchas Aphanizomenon and Microcystis. Other, low-light tolerant blue-greens (Limnothrix, Plank-tothrix) may occur in the phytoplankton andpersist (see Chapter 10). Dense stocking of plank-tivorous fish thus prevents the development ofmacrophytes in very eutrophicated water bodies.A factor leading to positive feedback is that largefish searching for food in the bottom sediments in-terfere with establishment of rooted macrophytes.

    11.10.5 Development of water plants onbottom sediments in spring and on fish-kills

    Spring development of filamentous blue-greenalgae (such as Oscillatoria spp., Fig. 11.18G) at thesediment surface leads to high pH in the wholewater column. High pH in the entire water columnmay also be caused by Chara sp., and by otherspecies of fast growing macrophytes. However, thesubsequent conversion of ammonium ions to am-monia gas is lethal to fish. Fish-kills are frequent infishponds at times when the water is cold, respira-tion is slow and net photosynthesis is strong. Gen-eration of ammonia is represented thus:

    (11.10)

    (11.11)

    (11.12)

    Concentrations of NH3 in excess of 0.01mgL-1 are

    toxic to most fish species, especially their fry(Schperclaus 1979). As a result, stocking of fish in ammonium-rich fishponds is delayed during periods of high pH.

    11.11 AQUATIC WEEDS

    Macrophytes become weeds when their vigor-ous development causes problems by:

    NH

    NH OH pH pH4

    3

    4 8 4 8

    14

    9 210 1010 10

    1010

    + -

    -

    -

    -[ ][ ] = [ ] = ( )( ) =

    . . .

    NH OH

    NHK4

    31

    4 810+ -

    -[ ][ ][ ] = =

    .

    NH OH NH H O24 3+ -+ +

    TLH11 10/14/03 10:40 PM Page 334

  • interference with flow in irrigation systems andin flood mitigation drains, overgrowing of waterbodies, etc.; promotion of certain diseases, such as thoseborne by mosquitoes, or molluscs living in associa-tion with aquatic macrophytes (bilharsiosis), etc.; interference with recreation; interference with fish production, the densestands of aquatic weeds preventing movements offish. Anaerobic conditions develop at the bottomand high assimilatory pH at the water surface. Softmacrophyte biomass may rapidly decompose andconsume oxygen; aesthetic degradation of the environment;

    Management of aquatic weeds involves a num-ber of control techniques. Physical removal, byhand or with a machine, is effective and immediatebut slow and expensive, and requires sites for dis-posal of the vegetation removed. In principle, com-posting or fermentation is the best way of dealingwith the material, as its nutrient content may berecovered. Chemical controls are still widelyfavoured, often without due attention to their pol-luting effects on the water (killing of non-target,sometimes remote organisms) or to deleterioussecondary effects when the killed material decom-poses, including nutrient release and oxygen consumption. Acquired resistance may make herbicide application selectively favourable to tar-get species. Many herbicides indirectly stimulategrowth of algae, through release of nutrients fromdead macrophytes.

    11.11.1 Application of methods of aquaticweed control: biological methods

    Tree shading is an important means of controllingmacrophytes at the waters edge, stunting but noteliminating submersed plants. Tree litter in thewater (fallen leaves, shed branches), increasing detrital material at the bottom and releasinghumic and other organic substances may becounter-productive.

    Herbivorous, weed-eating fish (such as the Chinese grass carp Ctenopharyngodon idella) caneffectively control growth and propagation ofmacrophytes, or even eliminate them. Enhanced

    nutrient turnover is again the drawback, riskingthe development of dense phytoplankton, perhapscyanobacteria. Selective effects are also apparentin that grass carp prefers soft submersed vegetation(such as Elodea sp.) to hard macrophytes such asNajas marina or Myriophyllum sp. Grass carp doesnot reproduce naturally in cooler regions of thetemperate zone, where the species has been used tocontrol macrophytes without risk to the existingfish fauna.

    Insects or other invertebrates can also be usedto control aquatic weeds. Examples of success-ful application of this technique are provided bythe use of the curculionid beetles Neochetlinaeichhorniae and Cytobagous salviniae, againstEichhornia crassipes and Salvinia molesta, re-spectively. The control effect of insects or other animals can be augmented by that of fungal plant pathogens, which may be inoculated on theanimal-damaged plants (e.g., Cercospora rodmaniion Eichhornia in the USA; Pieterse & Murphy1990).

    11.11.2 Mechanical methods

    Laborious hand cutting or chain scything tech-niques have been all but finally superseded by spe-cialised mechanical cutters: flails, weed-cuttingboats and dredges, of which there are many differ-ent types available. In order to avoid release of nu-trients when decomposing, plants are harvestedand removed from the shores or other sites fromwhich nutrients might leach back into the waterbody. Removal of the harvested material alsoavoids oxygen deficit which may develop whenplants are rapidly decomposing in a relativelywarm water rich in nutrients. The harvestedplants can be used as animal feed or can be com-posted, provided that they are substantially free (orcontain only permitted concentrations) of toxicsubstances such as heavy metals, pesticideresidues or PCBs.

    11.11.3 Chemical methods

    There is now available a sufficient range of aquaticherbicides to permit chemical control of almost all

    Aquatic Plants and Lake Ecosystems 335

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  • 336 j. pokorny and j. kvet

    aquatic plants. Each should be subjected to ex-haustive toxicological and environmental testingand, as a rule, only those compounds which pre-cisely satisfy the registration authorities should beavailable for commercial use. Nevertheless, atten-tion must always be paid to the manufacturers in-structions, and warnings about the harmful effectsof over-application Even then, the fate of the herbi-cide in the food web should be understood if del-eterious consequences of use are to be avoided.

    11.12 COMMERCIAL USES OFAQUATIC PLANTS

    There are also positive commercial benefits ofaquatic macrophyte growth (National Academy ofSciences 1976). Among the economic uses towhich aquatic plants are put are: energy generation: fermentation to yieldmethane gas, direct burning after drying (especial-ly emergent life-forms with the ash, if it is nottoxically contaminated, providing fertiliser). waste-water treatment and nutrient stripping:living natural and artificial macrophyte beds areused to treat waste waters by absorbing and incor-porating dissolved compounds of nitrogen andphosphorus into their biomass. Emergent macro-phytes (reed, cattail, etc.) are now widely used inconstructed wetlands to treat waste waters fromsmall to medium rural settlements (artificial reedbed treatment systems; RBTS). Some plants, forexample, the water hyacinth, also accumulatephenols, heavy metals or other toxic substances,thereby reducing the concentrations of noxiouschemicals in the water. For this treatment to bepersistently effective, macrophytes must continueto be harvested and removed. pulp, paper and fibre: fibrous, reed-like plants can be processed in cellulose and paper manufacture. food: paddy rice (Oryza sativa), Chinese waterchestnut (Eleocharis dulcis), watercress (Nastur-tium), water spinach (Ipomea aquatica), wild rice(Zizania aquatica), lotus (Nelumbo nucifera), taro(Colocasia), swamp taro (Cyrtosperma), arrow-

    head (Sagittaria trifolia) are among the macro-phytes raised deliberately for their yields of edibleproducts. animal feed: this applies to a great many speciesof macrophytes. Quite often, however, the freshplants must be thoroughly dried before they are fed to domestic animals as a precaution againstparasites or toxicity of the fresh plants.

    11.13 CONCLUDING REMARKS

    This chapter has explored the enormous phyloge-netic and adaptive diversity among water plants,as well as their ecology and their contribution toecological functioning and the metabolism of lakeecosystems. They are prominent producers andprocessors of carbon in shallow lakes, and aroundthe margins of larger bodies. Their direct impactson lake-wide carbon cycling, not least throughgenerating the habitat of an important part of itsanimal biomass, are clearly significant, continu-ing to contribute to and influence carbon cyclinginto the very largest lakes on the planet. Macro-phytes also influence our perception of lake beau-ty, and impinge upon the societal judgement oftheir value: interest in their well-being is not sole-ly the concern of the limnetic ecologist.

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