Encyclopedia of Inland Waters || Dystrophy

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  • matiodiv

    dystrophic, which can be translated as defectively

    The concentration of humic matter in lake water is

    tter). Such lakes have low biomass and produc-n of phytoplankton as well as low biomass andersity of benthic fauna. These are characteristics

    usually assessed by measuring the absorbance of thewater (commonly at around 430 nm), or by com-paring the color of the lake water sample with asurrounding watershed (i.e., allochthonous organic

    centration of dissolved organic substances from the

    Occurrence of Humic WatersDystrophyL J Tranvik, Uppsala University, Uppsala, Sweden

    2009 Elsevier Inc. All rights reserved.

    Introduction and Historical Outlook

    Dystrophy refers to the limnological conditions ofwater in lacustrine and riverine systems with highconcentrations of colored dissolved organic matterderived from the watershed, giving the water a yellowor brown color. Because of the dark appearance whenviewed from the surface, dystrophic waters some-times are referred to as blackwater lakes and rivers.The colored organic matter of external origin(allochthonous) is largely composed of humic sub-stances (humic and fulvic acids), in contrast to theorganic matter derived from the indigenous (autoch-thonous) primary production of the aquatic system.The development of classification schemes for lakes

    was an important theme for limnologists throughoutmuch of the twentieth century. Dystrophic lakes wereearly recognized as a special case, particularly whenlakes were classified according to their productivity,and received considerable attention from some ofthe most influential of the early limnologists. EinarNaumann described this lake type from the borealforest region of southern Sweden in several publica-tions around 1920. August Thienemann originallycoined the term dystrophic in a publication from1921, and further discussed it in Die BinnengewasserMitteleuropas (Inland Waters of Central Europe) in1925. Thus, the dystrophic lake type stood separatelyfrom the eutrophic (nutrient rich) and oligotrophic(nutrient poor) lake types. Although Thienemannpointed out early that there may be gradients betweenall normal or ideal lakes described in his lake typol-ogy, limnologists still often treat dystrophic lakes as aspecial case peripheral to the major pattern of lakesbeing distributed along a continuum from oligotro-phy to eutrophy. However, as will be demonstratedlater, dystrophy can be regarded a common and gen-eral feature of inland waters. Lakes with little, inter-mediate, or large influence from terrestrially derivedhumic substances can be denoted as oligo-, meso-,and polyhumic.Important characteristics of the dystrophic lake

    type, as identified by the earliest investigators, includebrown to yellow water color caused by a high con-nourished. The conventional view of the primaryproduction of the phytoplankton as the critical stepof energy mobilization into the food web is in accor-dance with the perception of these lakes as poorhabitats. Other characteristics of dystrophic lakes,identified already by Naumann and Thienemann,include low oxygen concentrations (in particular inthe hypolimnion during stratification), due to bacte-rial degradation of imported organic carbon, andhigh zooplankton production. The low oxygen con-centration is very typical of dystrophic lakes, whilehigh zooplankton production has not been unequivo-cally supported in later studies of specific lakes.These traits indicate substantial microbial meta-

    bolism, as well as a considerable resource base forzooplankton. The possible significance of the alloch-thonous organic carbon for the production of thedystrophic lakes led G. E. Hutchinson, in the secondvolume of ATreatise on Limnology (1967), to specu-late that the term dystrophic suggests a more patho-logical condition than perhaps exists.Limnological research during the last decades dem-

    onstrate that dystrophic lakes are sites of intensemineralization (decomposition) of organic matterwhere allochthonous organic matter constitutes a ter-restrial subsidy to the aquatic food web. Although theterm dystrophy is unfortunate, it is established inthe limnological literature and also among research-ers who are aware of the substantial flow of energyoccurring in these lakes. A useful alternative to dys-trophic is humic, which characterizes the chemistryof the water with no implications regarding biologicalconditions. The sediments of dystrophic lakes aredominated by organic matter with a prevalence ofhumic particles originating from terrestrial sources(dy). The term dystrophic can be more easily accom-modated with our current understanding of theselakes if associated with dy, rather than with defectivenourishment.of low productivity, a plausible reason for the term405

  • color scale, such as the platinum units (expressed asmg Pt l1), through the use of a solution containing abrown platinum salt. The two measurements corre-late closely, and they also correlate with the totalconcentration of dissolved organic carbon (DOC) inthe water, at least where the DOC concentration oflakes is under strong control from the watershed(Figure 1). They can thus be used interchangeably asindicators of dystrophic conditions. The ratio of colorto the amount of organic carbon reflects the relativeimportance of humic substances in the pool of totaldissolved organic carbon (organic compounds otherthan humic substances are not typically colored).The DOC concentration of lakes ranges from less

    than 1mg l1 in ultraoligotrophic lakes at high alti-tudes and high latitudes to several 100mg l1 but thedistribution is highly skewed towards lower values.Only few lakes have concentrations above about30mg l1. In a global analysis of dissolved organiccarbon (DOC) in 7500 lakes, the median concentra-tion of DOC was close to 6mg l1, a concentration atwhich the brown color of the water is often visible.Assuming that the surveyed lakes are representative

    ern boreal zone is rich in dystrophic lakes. In particu-lar, flat topography favors accumulation of organic

    406 Biological Integration _ Dystrophyof lakes on earth, a globally typical lake has weakbut detectable dystrophic traits. In a recent nationalanalysis of about 3000 lakes in Sweden, the averageDOC concentration was 11mg l1, and a survey ofabout 1000 Finnish lakes in 1987 showed a medianconcentration of 12mg l1. These are concentrationsat which the water is markedly brown, and theecological conditions typical for dystrophic lakes pre-vail. The classical dystrophic lakes of Fennoscandia

    0

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    Total organic carbon, mg C liter 10 20 40 60 80

    Figure 1 The concentration of total organic carbon (TOC) andhumic color, measured as absorbance at 420 nm, in the water ofabout 2800 Swedish lakes (from http://info1.ma.slu.se/db.html).

    The concentration of organic carbon in the water is correlated to

    the concentration of humic, colored organic substances. The

    relationship reflects that terrestrially derived humic substancesare a major component of the organic carbon in lake water.layers and peat, because of thicker soil layers andhigher extent of water-logged soils. This explains theabundance of humic lakes in some northern borealforest regions, e.g., Finland and Sweden. Also in trop-ical climates, the concentration of humic matter insurface waters may be high, as exemplified by theAmazon basin, which has some lakes and rivers(blackwater rivers), such as Rio Negro, highlystained by humic substances.

    Chemical Effects of Humic Substancesin Lake Water

    Humic substances are complex mixtures of heteroge-neous organic compounds of biotic origin that haveundergone extensive transformations since they werefirst produced by plants. Lignin is probably an impor-tant precursor, and based on solubility, humic sub-stances can be classified into humic acids (insolublebelow pH 2), fulvic acids (soluble at any pH), andhumin (insoluble in water). Because of the complexityand irregularity of humic substances, and of the path-ways of their formation, they should not be consid-ered as strictly defined molecules, but can rather becharacterized by average properties. Among the mostimportant of these properties are (1) prevalence ofaromatic structures, which absorb light, induce avariety of photochemical reactions, and are involvedin adsorption and aggregation; and (2) presence ofionic structures, including carboxylic and phenolicgroups, which affect solubility of humic matter, andare acidic, and have low buffering capacity and a lowcontent of inorganic ions. This is probably a commonfeature of dystrophic waters worldwide, but not aprerequisite. There are also examples of dystrophiclakes with high concentrations of inorganic dissolvedsolids, e.g., in calcareous watersheds.The global data on lakes mentioned reveals that

    properties of the drainage area (watershed) of lakesare important regulators of DOC concentration in thelake water. Thus, high amount of soil carbon andratio of carbon to nitrogen in soils are positivelycorrelated to DOC in lakes. In the Arctic, permafrostand barren watersheds limit the export of DOC fromsoils, but large areas with extensive peat layers harborhighly humic lakes and ponds.In cold and temperate climates, soils and wetlands

    accumulate large amounts of organic matter, becauseof high input of detritus from the vegetation in com-bination with relatively slow microbial degradation.These habitats are an important source of DOC in thesurface waters. Accordingly, the circumpolar north-

  • light at short wavelengths affects the underwater

    with the high concentration of organic matter suscep-tible to bacterial degradation and concomitant con-

    biopolymers such as carbohydrates, the overalldegradability of dissolved organic matter in lakewater appears to be weakly coupled to the relativecontribution of humic matter to the total dissolvedorganic matter. About 10% of the dissolved organicmatter is degradable by bacteria in bioassay experi-ments lasting about one week, regardless of the rela-tive contribution of humic substances. Accordingly,the high concentration of dissolved organic carbon inhumic waters is a source of energy for bacteria.Shortly after the first use of epifluorescence micros-

    copy to count bacteria in samples taken from lakes,cross-system comparisons of lakes demonstrated apositive correlation between the color and the bacte-rial abundance of lake water. It was suggested that thehigher bacterial abundance in humic lakes was due to

    Biological Integration _ Dystrophy 407light climate; light of long wavelength (orange, red)penetrates more deeply than light of shorter wave-length (violet, blue), than would be the case in lakeswith low concentrations of dissolved humic matter.The rapid attenuation of light in the water column ofdystrophic lakes may lead to decreased primary pro-duction, when compared with corresponding lakeswith lower concentrations of humic substances. Onthe other hand, since the incoming solar radiation isefficiently absorbed in the upper layers of the watercolumn of a dystrophic lake, the surface water warmsup rapidly and a stable stratification may be formedwith a shallower epilimnion than would be formed inan oligohumic lake, in particular in small, shelteredhumic lakes (Figure 2). A shallow epilimnion maycreate a favorable light climate for algae in theupper mixed layer. Also, flagellated algae that canuse locomotion to actively stay in the photic zone(in particular chrysophytes and cryptophytes) arecharacteristic of humic lakes.The strong thermal stratification of dystrophic

    lakes creates an effective barrier against movementcause complexation of metals and other substances.Humic substances are weak buffers, which take overfrom the carbonate system between pH 4 and 5.Hence, dystrophic waters typically are naturallyacidic, but less sensitive to anthropogenic acidifica-tion than oligohumic waters.The complexation properties of humic substances

    are important in several ways. Binding of persistenthydrophobic organic contaminants to humic sub-stances decreases their bioavailability, and thus theirtoxicity. Chelation of toxic metals (e.g., copper)greatly reduces their toxicity. Chelating by humiccompounds can also enhance the availability of ironto phytoplankton by preventing the iron from preci-pitating as iron oxides or it being adsorbed on particlesurfaces, which would make it unavailable for uptakeby phytoplankton. At high concentrations of dis-solved humic matter, however, overchelation mayoccur, hampering bioavailability of metals. Likewise,free phosphate can become immobilized by humicsubstances and thereby become unavailable tophytoplankton.

    Light, Thermal Stratification, andPhytoplankton

    A striking characteristic of dystrophic lakes is theirbrown water color, which reduces light penetration.The dissolved humic substances not only absorbstrongly the UV-B and UV-A, but also compete withphytoplankton for the photosynthetically availablelight (400700 nm). The selective attenuation ofsumption of oxygen, the strong stratification ofdystrophic lakes frequently results in oxygen deple-tion in the hypolimnion and sediments, both duringsummer stratification and winter stratification.

    Bacterioplankton and Food WebStructure

    Humic substances are traditionally regarded highlyresistant to biological degradation, which impliesthat they can reside in the environment for longperiods of time. Although they are clearly more re-calcitrant than monomers (e.g., glucose) or simpleof nutrients and gases from deeper layers. Thus, keynutrients (N, P) may become scarce in the epilimnion.Flagellated algae (e.g., Cryptomonas) may circumventnutrient scarcity in the epilimnion by crossing the ther-mocline during diel migration, which allows them toretrieve nutrients in the hypolimnion at night, and tophotosynthesize in the epilimnion during the day. Also,

    Dep

    th

    Dystrophic lake Clearwater lake

    Light Temperature

    Figure 2 Light penetration and thermal stratification indystrophic and clear, nondystrophic waters. The colored water of

    dystrophic lakes attenuates incident radiation more strongly thanthe water of clear lakes, resulting in rapid heat accumulation near

    the lake surface and a more stable and shallower thermal

    stratification, when compared with clearwater lakes.

  • matter. This was an extension of the previous view

    sedimentwater interface. The sediment is dominatedby humic colloids derived from terrestrial vegetation,with only limited contribution from autochthonousorganic matter. Rates of microbial metabolism in the

    Phytoplankton

    Bacterioplankton Protozooplankton

    Metazooplankton Fish

    Metazooplankton Fish

    DOC

    Bacterioplankton Protozooplankton

    Bacterioplankton Protozooplankton

    DOCPhytoplankton

    Methanotrophs

    CH4Methanogens

    Oxic water

    Anoxic water

    (a)

    (b)

    (c)Figure 3 Schematic view of energy transfer in lacustrine foodwebs (a) with no dissolved organic carbon (DOC) subsidy from the

    watershed, and (b) with substantial DOC originating from the

    watershed andusedbybacteria (dystrophic lake). In thed...