encyclopedia of inland waters || sediments of aquatic ecosystems

Sediments of Aquatic EcosystemsJ Bloesch, Swiss Federal Institute of Aquatic Science and Technology (Eawag), Dubendorf, Switzerland

ã 2009 Elsevier Inc. All rights reserved.

Introduction

Particulate material of all sizes within aquatic eco-systems is referred to collectively as ‘sediment’ or‘sediments’. The lower end of the size spectrum forsediment is defined operationally by the pore sizeof commonly used filters, which has been arbi-trarily accepted at 0.45 mm but in practice rangesbetween 0.45 and 1.0 mm. The upper end of the sizerange could include boulders, which have dimen-sions of 1m or larger. Although the largest particlesseem immobile, under certain circumstances theymay move.There are three general classes of sediments,

according to mobility. Sediments suspended in waterare referred to as ‘total suspended solids’ (TSS) or‘suspended particulate matter’ (SPM). A standardmeasurement of total suspended solids, as obtainedby drying a filtered sample, is dry mass per unitvolume of water (most commonly mg l�1). Becauselarge particles within aquatic environments are notsuspended, suspended solids typically are made up ofsmall particles including minerals (silt, clay), fineorganic particles in various stages of decompositionand inorganic components of biogenic origin such asdiatom frustules. A special component is providedby living organisms suspended in the water (e.g., bac-terioplankton, phytoplankton). At high flows foundin some rivers, but not in lakes, sand also may besuspended in substantial quantities.A second category of sediments consists of particles

that are not suspended, but rather tumble along thebottom of a stream or river in response to high cur-rent velocities. These sediments are called ‘bed load’.For any given stream or river, the bed load varies withcurrent velocity. At points within the channel wherevelocity is low, there may be no movement of bedload, whereas a location with higher flow velocitymay show movement of bed load. Also, movementof bed load varies greatly with time. Bed load move-ment is maximum at the highest flow velocity, whenthe entire channel surface of a stream may show bedload movement. Bed load typically includes sand,where it is present, but also gravel and even muchlarger particles, even boulders at the highest flow.The final category of sediment in aquatic ecosys-

tems consists of stable particle accumulations occupy-ing fixed positions for extended periods. In lake basins,particles continually settle to the bottom. Except inthe near shore area where waves are influential, these

particles are not subject to substantial movement.They may be disturbed over small distances by movingor burrowing organisms, but they typically move verylittle, and are essentially permanent after being buriedby newly accumulating sediments. In the littoral zoneof lakes and in shallow systems, bottom sedimentsmay be mobilized temporarily by dispersion of wave-generated energy, thus joining the suspended solidsuntil they again sink to the bottom.

Within streams and rivers, the stability of bed sedi-ments is not so great as it is in lakes. The uppermostsediments (1–10 cm) in streams and rivers becomebed load at high flow. Deeper bed sediments may bestable for extended intervals, but are subject to dis-ruption during extraordinary flow events that changethe configuration of channels, excavating and rede-positing even coarse sediments to considerable widthand depth. The beds of streams in some cases consistof sediments that are cemented together (armored),which inhibits the suspension of these bed sediments,even at high flow.

The three sediment compartments (total suspendedsolids, bed load, and stable bed sediments) are inter-connected. Suspended sediments in a lake, for example,often become a component of stable bed sediments ofthe lake. Suspended sediments and bed load in riversmay join stable bed sediments for weeks or months oflow flow, only to be mobilized again with high flow, ormay even be buried to such a depth that they cease to bemobilized annually.

Origin of Sediments

Sediments in aquatic ecosystems are classified accord-ing to their origin (Figure 1). Particles that derivefrom sources external to the aquatic ecosystem aredesignated as ‘allochthonous’ particles. Particles thatoriginate within aquatic ecosystems are designated as‘autochthonous.’ Examples of allochthonous parti-cles within streams and rivers include the byproductsof rock weathering, i.e., clay, silt, sand, and coarseinorganic sediments. In addition, substantial amountsof particulate organic matter may enter streams andrivers in the form of leaf litter, organic particles fromsoils, or fecal material from animals. In most lakes,allochthonous material is less dominant than in riv-ers, but some component of suspended solids withinlakes can be traced to the watershed, which provides

479

Pollutants

Atmosphericparticles

Vulcano

Biogenicparticles

Settlements

Soil

Riverine output

Sediment column with layers (varves)

Dissolutionand release

Clasticparticles

Resuspension

Bioturbation

TributaryVegetation

Erosion

Erosion

Industries1

1

1

1

1

1

1

1

1

2

2

2 2

2

2

Sedimentation

Upwelling by eddiesand horizontaltransportation

Sliding, slumping,turbidity currents

Sediment

3

3

Shoreline erosionand resuspension

Sedimentfocusing

2Sedimentation

3

Figure 1 The influence of external and internal environmental factors on suspended sediments, sedimentation, and sedimentformation in a lake basin. 1: sources of suspended particulate matter (SPM); 2: transport and transformation of SPM; 3: removal of SPM.

Clastic particles have a grain size of >2mm. Turbidity currents originate from slope slides on steep shores. Biogenic particles and

varves (sediment layers) refer to particles produced by combined chemical and biological processes. Modified from Bloesch (2004),

Fig. 8.1; with permission from Blackwell Publishing.

480 Biological Integration _ Sediments of Aquatic Ecosystems

particles that pass into the lake by way of streamsor rivers.Autochthonous particles within streams and rivers

are accounted for by the growth of algae or aquaticvascular plants or mosses within the aquatic ecosys-tem, as well as bacteria and animals that use the plantmatter. Thus, autochthonous particles in the streammay consist of living or nonliving organic matterof plant, animal, or microbial origin. Within lakes,autochthonous particulate matter originates fromsimilar sources, i.e., growth of algae or aquatic vas-cular plants, plus bacteria and animals. As in the caseof streams and rivers, autochthonous organic mattermay be living or nonliving. In addition, new inorganicautochthonous particles can form in streams and riv-ers under some circumstances (see below).Organic sediments, whether allochthonous or au-

tochthonous, are referred to collectively as particu-late organic matter (POM). About 50% of POM isparticulate organic carbon (POC). Stream ecologistsalso make a distinction between large and smallorganic particles. Large organic particles, such as

leaves or wood, are referred to as coarse particulateorganic matter (CPOM), and smaller particles arereferred to as fine particulate organic matter (FPOM).In general, CPOM is processed biologically by chew-ing processes of invertebrates, whereas FPOM is pro-cessed as food through the collection of particles onfilters that are attached to the mandibles of inverte-brates specializing in eating these kinds of foods, oreven on silken nets that are held in currents.

Organic particles of any kind are subject to changeby consumption or decomposition by bacteria. Smallparticles may be harvested from the water by filterfeeding insect larvae, and part of the undigested massmay be returned to the water as fecal pellets. Bacteriadissolve particles by releasing enzymes onto theorganic particles, reducing the mass of organic mat-ter. Thus, the size spectrum and total mass of particlesmay change over time in response to biotic processes.

Not all autochthonous particles are organic. HighpH caused by rapid photosynthesis often is accompa-nied by the formation of calcium carbonate thatforms the mineral calcite (CaCO3). These fine

Biological Integration _ Sediments of Aquatic Ecosystems 481

particles collect in large amounts in the sediments ofsome lakes. In addition, processes occurring in thedeepest part of the water column of lakes (hypolim-netic zone) may lead to the formation of inorganicparticles. Under anoxic conditions, which occur inmany lakes during summer stratification, iron andmanganese within the water or sediment surfacemay be converted to their reduced forms (lower oxi-dation states: Mn2þ, Fe2þ). These forms, unlike thehigher oxidation states (Mn4þ, Fe3þ) that are foundin oxidized waters, are quite soluble. Therefore, thedeep water of lakes may show an accumulation ofreduced forms of iron and manganese under anoxicconditions during summer. When the lake mixes inthe fall, however, these soluble forms are reconvertedto insoluble oxidized forms, i.e., manganic oxide(MnO2) or ferric hydroxide (FeOH3). The conversionprocess may be spontaneous or it may be facilitatedby specialized microbes. An example is the conver-sion of Mn2þ to Mn4þ by the bacterial genus Meta-logenium. Hence, the oxidized forms of manganeseand iron precipitate, forming flocculent masses thatsink slowly back to the sediment surface of the lake.Small particles often are hybrids of organic and

inorganic forms. For example, organic matter maybe adsorbed onto the surface of particles. Some bac-teria attached to small particles form polysaccharidefibrils, thus enhancing the aggregation of particles. Ingeneral, small particles are influenced by electrostaticforces because of their high ratio of surface area tovolume, and the tendency of surfaces to carry anelectrical charge. Thus, particle aggregation is com-mon in the aquatic environment.

Ecological Significance of Sediment inAquatic Ecosystems

Suspended solids in water reduce transparencybecause they adsorb or scatter irradiance within thewater column. The optical effect of suspended sedi-ments in water is obvious to the human eye, and isreferred to as ‘turbidity’. Optical effects are veryminor at concentrations below 10mg l�1, which aretypical of most oligotrophic lakes as well as streamsand rivers that drain undisturbed watersheds wherethe natural erosion rates are low. In contrast, sus-pended particles have a dominant effect on opticalproperties of water within upper mesotrophic toeutrophic lakes, where the particles consist mostlyof phytoplankton, and within rivers that carry largeinorganic sediment loads (e.g., 100–1000mg l�1)because of watershed disturbance or naturally higherosion rates, as in the case, for example, of manylarge Asian rivers such as the Ganges or Mekong.

Darkening of the water by suspended solids has anegative effect on photosynthetic potential becauseof its suppression of light penetration.

Suspended sediments provide biotic habitat formicrobes that can grow on surfaces. Thus, all parti-cles should be viewed as potentially metabolicallyactive sites within the water column, although therelative level of activity depends on the chemicaland physical nature of the particle.

Suspended organic particles provide food for filterfeeding organisms in both flowing waters and lakes.Because of the common abundance of nutritious par-ticles in water, numerous groups of organisms suchas aquatic insect larvae or zooplankton have becomeadapted to collecting particles from water as a majorfood source.

One final significance of particles suspended inwater is as an exchange site for substances dissolvedin water. Metals in particular are readily bound byparticles through electrical attraction. Thus, the pres-ence of particlesmay reduce the toxicity ofmetals undersome circumstances, and may bind and carry metals tolocations beyond their origin. Dissolved organic matteroften is electrically attracted to particles aswell, and thisattraction leads to the coating of inorganic particlessuch as clay or silt with organic molecules.

Stable surface sediments in streams and rivers, ifexposed to light, typically support the growth of algaldominated biofilms (periphyton), which may gener-ate substantial photosynthesis at low to moderateflows. At high flows, however, a substantial portionof the surface sediment in streams and rivers becomesbed load. The tumbling action of particles duringtransport as bed load detaches the algal biofilms.When the particles come to rest following a highflow event, photosynthetic potential is very lowbecause the particles have been scoured and some ofthem have been buried and replaced by other particlesfrom deeper within the channel sediments. In addi-tion to algal biofilm, the surface sediments carrymicrobes and small invertebrates or, in the case ofparticles that are as large as gravel or larger, inverte-brates of many kinds, some of which are attached tothe particles and some of which simply live beneaththem. These life forms also are disturbed and to someextent are suppressed by high flow events that convertsurface sediments into bed load.

Stable bed sediments in streams and rivers thatoccupy the hyporheic zone (the zone that is infiltratedby slowly moving water from the channel) can sup-port a rich diversity of animal life and microbial activ-ity if the zone remains well-oxygenated. Microbesthrive in this environment whether oxygen is presentor absent. The contrast with the surface sediments,however, is the absence of any significant amount of


irradiance, whichmeans the growth of photosyntheticorganisms (algae) is not possible throughout most ofthe large column of stable sediments that underliesmost streams and rivers.In lakes, periphyton is also abundant on rocky lit-

toral substrate. Stable sediments, which dominate thebottom of the lake beyond the near shore area, are richin microbes but, except in very shallow lakes, do notsupport photosynthesis because they are not exposedto light. Animal life of many types also is characteristicof these stable sediments, provided that they have atleast some oxygen above them. The sediment faunamay live on the surface of the sediment or burrow intoit, seeking protection from bottom dwelling fishes.Because the bottom sediments of lakes are very calm,they may accumulate an abundance of fine organicmaterial that provides an excellent substrate formicro-bial respiration, thus generating a substantial amountof bacterial production but also demanding oxygenfrom the water column. Organic sediments are themain factor causing depletion of oxygen from thedeep water of lakes. In general, more productivelakes (eutrophic lakes) have a higher probability ofdeveloping anoxia over sediments, as compared withunproductive lakes (oligotrophic lakes).

Erosion

Cla

y

Silt

Fin

esa

nd

Coa

rse

sand

Bou

lder

Rub

ble

Gra

vel

and

pebb

les

Erosionvelocity

Fall velocity

SedimentationTransportation

Vel

ocity

(cm

s−1

)

1000

100

10

1.0

00.001 0.01 0.1 1.0

Size (mm)10 100 1000

Figure 2 Relation of mean current velocity in water at least

1m deep to the size of mineral grains that can be eroded from abed of material of a given size. Below the velocity sufficient for

erosion of grains of a given size (shown as a band), grains can

continue to be transported. Deposition occurs at lower velocities

than required for erosion of a particle of a given size. From Allan(1995), Fig. 1.7; with permission from Chapman & Hall.

Sediments in Rivers

Inorganic particles are produced by weathering ofrock. Weathering involves chemical and physical pro-cesses that lead to the conversion of large masses ofrock to a mixed size spectrum of smaller rocks or rockparticles, some of which can be transported by flowingwater. If not moved by water, inorganic weatheringproducts may be stored within soils, where they maychange in size or chemical composition over geologictime scales. Weathering products within soil or directlyfrom the source rock cause the formation of clay,silt, sand, gravel, cobble, and boulders in drainagenetworks.The rate at which a given watershed produces and

transports inorganic particles is controlled partly bylithology (rock type), as the weathering rate of rocksvaries greatly. In addition, slope is an importantfactor controlling transport, as water removes parti-cles most easily from steep slopes. In contrast,particles may tend to accumulate on the more gradualslopes that are characteristic of the lowland reachesof rivers. Other geologic factors such as topographyalso are important.Climate has effects on both weathering and trans-

port processes. The hydrograph (change in flowthrough time) for any given reach of stream controlsthe ability of the stream to transport particles down

the gradient. Brief high flows are especially importantin moving particles downstream.

Human actions have greatly affected the transportof particles of all kinds through drainage networks, aswell as the production of particles within watersheds.Factors that affect the transport of particles includeland use, which often involves disturbances thatmobi-lize particles, channelization of rivers or streams,dams, clear-cutting of forests, removal of riparianvegetation, installation of levees that disconnect riversfrom floodplains, and removal of gravel or water fromthe river channel.

In general, the mean grain size for inorganic parti-cles decreases in the downgradient direction withindrainage networks. Steep channels, which often arefound within the headwaters, rapidly remove thesmaller particles because these waters have highenergy (high current velocity), whereas streams orrivers of lower gradient, which most often are foundin lowland areas distant from the headwaters, aredominated by fine sediments (Figure 2). Fine sedi-ments accumulate at low flow velocities becausethere is not enough energy to move them downstreamefficiently. For example, sand particles can be movedby currents of approximately 20 cm s�1, which almostall streams and rivers experience at some time duringthe annual hydrologic cycle. In contrast, coarse gravelmay require 1m s�1 to become mobile, and currentsof this magnitude are much more common in theheadwaters than in lowland streams or rivers.


For any stream or river, high flows are likely tomove the largest amount of bed material per unittime (Figure 3), and also are most likely to changethe shape of the channel by eroding banks or deposit-ing large amounts of sediment. These processes leadto the formation and movement of braids, islands andmeanders, the isolation of channel segments thatbecome oxbows, and the creation of off-channeldepressions that are often important for aquatic life.River sediments are the basis for important habitat

components occupied by aquatic plants and animals.Surface sediments within channels are covered withbiofilms consisting of bacteria, attached algae, andsome small invertebrates. Biofilms may be disruptedor buried during high flow events, especially instreams that have fine sediments. Below the surfaceof the channel bed, where water is moving slowly asthe channel waters infiltrate the sediment, is thehyporheic zone. The sediments within the hyporheiczone consistently are occupied by bacteria, which areattached to particles. In addition, if the particles arelarge (gravel, cobble), the hyporheic zone may havelarge spaces between particles (interstitial spaces) thatare suitable for occupation by invertebrates such asinsect larvae, worms, or mollusks. Where sedimentsare coarse, the hyporheic zone may extend to a meteror more of depth below the water surface andhundreds of meters laterally from the river. In con-trast, some streams and rivers have very limitedhyporheic zones because of the very small particlesizes of their bed sediments. Coarse sediments arealso required spawning habitat for some importantfishes, especially salmonids. One undesirable effect ofsediment mobilization or reduction of flow velocitiesby human actions is to allow clogging of the inter-stitial spaces in streams and rivers by fine particles.

Discharge

1

2

Freq

uenc

y or

mag

nitu

de

Qd(a)

Figure 3 The relationship between frequency and magnitude of dis

(b) bedload. Curve 1 depicts the increase of sediment load with incre

frequency of discharge events of a given magnitude. Their product (d

discharge, is approximately bankfull flow for suspended sediments, aFrom Allan 1995, Fig. 1.8; with permission from Chapman & Hall.

The transport of particulate material in streams orrivers at any given point within the drainage networkis the ‘sediment load’ at that point. For total sus-pended solids, annual loads often range between200 and 800 metric tons per km2 of watershed (bedload typically is smaller than suspended load). Parti-cle transport also can be estimated for any particlecomponent that is of interest. For example, phospho-rus, a major plant nutrient in aquatic ecosystems,often is transported predominantly in particulateform (44–90%); typically, less than half is trans-ported in soluble form. Similarly, organic carbon istransported predominately in particulate form (e.g.,50–80%), and secondarily in dissolved form. In con-trast, nitrogen shows a much smaller contribution ofparticulate forms (<20%). Total annual transport isconcentrated within brief intervals of high flow, whenmore than half of the annual load occurs.

Deltas: Sediments at the Junctionbetween Flowing and Standing Waters

Particles carried by rivers or streams settle from thewater column upon entering lakes, oceans, or wet-lands, where current velocities usually are muchlower than in streams and rivers. An accumulationof particle mass at the junction between flowing andstanding waters is a delta. Under natural conditions,the sediments of deltas may be sorted by size becausethe largest particles are most quickly removed bygravity, whereas finer particles may be depositedover a greater distance along the delta. When deltasaccumulate sufficient sediment to reach the level ofthe entering river or stream, they may developbraided channels that carry the incoming river water

Discharge

1

2

Freq

uenc

y or

mag

nitu

de

Qd(b)

charge events causing sediment transport. (a) Suspended load,

asing magnitude of discharge, and curve 2 describes the

ashed curve) is the total sediment transported. Qd, the dominant

nd is in the range of 1.5 year to 10 year peak flow for bedload.


out into the standing water. Deltas often are altered,however, by human actions, including excavation forgravel or channelization of the flow path for theentering water. In these cases, the gradual distributionof incoming particles may be disrupted, leading tocanyon-like structures within the formerly naturaldelta. Channelization may change the pathway offiner particles, which may reach the deeper watersof lakes in greater quantity through energetic channe-lized flows. Under some conditions, very fine particlesmay be distributed widely over a lake. This is espe-cially the case in reservoirs, which may have currentvelocities under some conditions that are intermedi-ate between those of natural lakes and rivers, and arecapable of maintaining fine particles in suspension.Flowing water that enters standing water will

penetrate the water column to a depth of matchingdensity. Density is affected by temperature, amount oftotal dissolved solids (salinity), and amount of sus-pended solids. Thus, river water of higher densitythan the surface water of a lake into which it flowswill plunge toward deeper water where the lake waterdensity is highest. In contrast, river water of low den-sity will tend to move over the surface, often carryingthe finest particles great distances over the surface.

Sediments of Lakes

Sedimentation of Particles

Bottom sediments beneath the pelagic zone (openwater) of lakes are formed by the settling of sus-pended particles whose specific gravity is greaterthan that of the surrounding water. The sedimenta-tion rate of particulate matter is governed by particlesize, shape, and specific gravity. Viscosity and densityof water, both of which vary with temperature, alsohave effects, but they are small. Sedimentation can bepredicted for particles on the basis of these factorsthrough the use of the ‘Stokes equation’.Because size is a major determinant of settling rate,

organic particles less than 50 mm typically sink at ratesbelow 1mday�1, while the largest algal cells, fecalpellets, and other organic matter sink at rates between5 and more than 100mday�1, depending upon size.For a given size, mineral particles settle more rapidlythan biomass particles because of their greater density.Sinking rates are calculated or they are measured

in still water. Because lakes have persistent, weakcurrents (typically between 1 and 10 cm s�1), theexpected sedimentation rate, when derived fromcalculations based on a completely calm water body,cannot be easily extrapolated to lakes. At present,there is no simple quantitative treatment of therelationship between particle sinking rate and the

amount of water movement in a lake. It is possible,however, to measure the sinking rate of particles (mday�1) in a lake by using a sediment trap that isdeployed and moored within the water column. Anestimate of the sinking rate of particles (mday�1) isobtained from the settling flux of particles (mgm�2

day�1) divided by the concentration of particles(mgm�3) in the water column above.

Because the vertical sinking velocity of particles inlakes is generally 1–6 orders of magnitude less thanthe velocity of horizontal water currents, the move-ment of individual particles is not correctly viewed asa constant unidirectional downward movement, butrather as a weak downward movement coupled tomuch more substantial movements in the horizontaldirection under the influence of currents. In fact,individual particles also are carried by the swirlingcurrents (eddies) characteristic of the turbulentmotion of water in large waterbodies. Thus, thedownward movement is not steady, and is scarcelydetectable. Large particles of low density (e.g., floc-culent organic matter) may behave much as smallparticles do, whereas large particles of high densitymay have a steadier movement downward.

For phytoplankton, vertical sedimentation is sig-nificant in removing algal biomass from the uppermixed layer, where there is sufficient light for phyto-plankton growth. Thus, high sinking rates can bedetrimental to phytoplankton within stratified lakes.Some phytoplankton have developed mechanisms tooffset sinking by buoyancy control. The best docu-mented example occurs in some genera of the cyano-bacteria (bluegreen algae) that have the capacity toform pseudovacuoles filled with gas that imparts aneutral or negative buoyancy to the cells. Buoyancyprevents their loss from the illuminated zone, thusincreasing their solar exposure in calm water.

Resuspension of Sediments

High current velocities involving strong turbulentmotion are found in the littoral zones of lakes andmay be found over the entire bottom of the shallowestlakes (e.g., 1–2m depth). Strong currents and turbu-lent motion lead to resuspension of sediments thathave previously accumulated on the lake bottom.The energy source for resuspension is wind; the effectof wind is related to lake size. Fetch is the distancethat wind blows over a water surface. Therefore, around lake with a diameter of 1 km will have a fetchof 1 km, provided that the surrounding land is flat.If the lake is elongate, the fetch will differ accordingto whether it is blowing across the short axis or thelong axis of the lake. In general, lakes with largerfetch have stronger wind-generated currents for a


given wind velocity. Therefore, the formation of cur-rents that are capable of causing resuspension is mostlikely in lakes that have large fetch (Figure 4).The critical velocities required to resuspend the

least dense particles in lakes is 0.5–1.7 cm s�1, asmeasured at the sediment surface, if the particlesare unconsolidated. For consolidated sediments, thethreshold is closer to 5 cm s�1. Most resuspension iscaused by wind-generated currents or traveling waves,although resuspension can also be induced by periodicoscillations (‘seiches’) in deep stratified lakes.Resuspension of sediments in lakes may enhance

the release of nutrients from sediments, thus stimulat-ing algal production. In addition, however, vigorousresuspension of sediments, as may occur duringstorms in lakes, may create a sufficient mass of parti-cles to suppress light penetration in the nearshoreareas of lakes, thus temporarily suppressing the abil-ity of algae or aquatic vascular plants to grow.Resuspension of sediments in lakes leads to

lateral transport of particles. Sediments that movelaterally are most likely to be redeposited wherethe water movement is weakest. Because the strengthof water movement declines with water depth, lateralmovement tends to transfer particles toward dee-per water. This phenomenon is called ‘sediment

00 10 20

Erosion (winnowing)

Transportation

Water content (WO−1)

Accumulation

30

Effective fetch [km]

Wat

er d

epth

[m]

40 50

DE−T

DT−AWT−A= WK−10

60

10

20

30

40

50

WT−A

Figure 4 Diagram showing areas of sediment erosion,

transport, and accumulation in lakes, as a function of wind fetch

and water depth.

DE�T ¼ ð30:4 � LfÞ=ðLf þ 34:2ÞD T�A ¼ ð45:7 � LfÞ=ðLf þ 21:4Þ

where DE�T and DT�A ¼ ‘critical’ water depth [m]; Lf ¼ effectivefetch, i.e., the potential maximum effective fetch [km]. The rough

general distinction between erosion and transport is based upon

the water contentW0–1 ¼ 50%. The ‘critical’ limit, between areasof transportation and accumulation, may be given by WT�A ¼Wk-10, where Wk is the characteristic water content and WT�A is

the ‘critical’ water content of surficial lake sediments (0–1 cm).

From Hakanson & Jansson 1983, Fig. 7.26; with permission fromSpringer.

focusing’. A parallel phenomenon is ‘sediment win-nowing’. Because small particles move farther thanlarge particles, sediments may show a gradient fromcoarser near shore to very fine sediments toward thecenter of a lake.

Because of lateral movement of particles, the col-lection of particles by use of sediment traps in a watercolumn may not provide an accurate indication ofsedimentation rate of particles from the upper watercolumn if they are deployed too close to the lakebottom. A portion of the particles that are capturedby a sediment trap will have originated from resus-pension processes in the shallow-water sediments of alake, rather than the water column above the sedi-ment trap. Because of the effect of resuspension onsedimentation, sedimentation rate estimated nearshore may be 2–3 times the sedimentation over thedeepest water of the pelagic zone, reflecting theweaker effect of resuspension on sedimentation inthe middle of a deep lake. For shallow lakes, higherratios are possible.

Spatial and Temporal Variation of Sedimentation

Although resuspension can account for a proportionof particles reaching the sediment surface beneath thepelagic zone of a lake, the offshore waters of lakesoften show a predominance of particles that areaccounted for by a production of particles withinthe lake (autochthonous particles), and especially bythe growth of biomass. Because production of livingparticles (mostly phytoplankton) varies seasonally,sedimentation rates in the pelagic zone show strongseasonal fluctuations. A burst of growth in springmay produce a particularly high sedimentation rate,reflecting the movement of organic particles from themixed layer toward deeper water. During midsum-mer stratification, production and sedimentationrates may be low if nutrients are exhausted, as isoften the case. In fall, a second pulse may beobserved, when nutrients from deep water are gradu-ally brought to the surface through thickening of themixed layer. In winter, sedimentation rates are lowbecause biomass production is low and because pos-sible ice cover may block resuspension. For example,winter sedimentation may be less than 1 g dry massm�2 d�1, whereas summer sedimentation may be10–30 g dry mass m�2 day�1. Differences acrossyears are generally smaller than differences acrossseasons within a given year.

Because the productivity of lakes depends on theirtrophic state, the production of particles also isreflected by trophic state (Table 1). Thus, highly pro-ductive (eutrophic) lakes transfer many more parti-cles to the sediment surface over the course of a

Table 1 Primary production, SPM concentration, primary settling flux (export production), and accumulation rates in lakes of the

temperate zone, with different trophic state and tributary input

Parameter Oligotrophic lakes Mesotrophic lakes Eutrophic lakes

Allochthonous input Allochthonous input Allochthonous input

Small Large Small Large Small Large

Primary production (g C assimilated m�2�a�1) <150 150–350 >350–700

SPM concentrationa

Dry weight (g�m�3) 1.3–2.3 10–25 2.2 2.9–21 2.5–3.2 >2.5

Particulate organic carbon (mg�m�3) 435–700 230 500 448 1150 3100

Particulate nitrogen (mg�m�3) 32–53 20 – 43 142 536

Particulate phosphorus (mg�m�3) 2–6.6 4 5–12 6.3 18 39–64Settling fluxa

Dry weight (g�m�2�day�1) 0.1–1.2 7.4–30 0.3–2 2–6.5 2–10 5–31

Particulate organic carbon (mg�m�2�day�1) 41–120 250 160–390 268 180–920 380–1300

Particulate nitrogen (mg�m�2�day�1) 4.8–12 9 45 20 45–53 131Particulate phosphorus (mg�m�2�day�1) 1.6 13 6–7 5.2 6–13 26

Accumulation rates (mm�a�1) 4 >20 3.5–4 4.5–5 3.7–10 13–31

The given average figures (yearly means per lake) represent typical order of magnitudes, and specific sites and seasonal patterns may differ significantly.

From Bloesch J (2004) Sedimentation and lake sediment formation. In: O’Sullivan PE and Reynolds CS (eds.) The Lakes Handbook, Vol. 1. Limnology and

Limnetic Ecology. Oxford, UK: Blackwell, pp. 197–229, where original data sources are quoted. With permission from Blackwell Publishing.aData indicating significant resuspension (turbidity, secondary flux) in shallow lakes are excluded.


growing season than do unproductive (oligotrophic)lakes. For this reason, the range of mass to the lakebottom is very broad across lakes (0.1–30 g dry massm�2 day�1). The yearly accumulation of sediment,measured as sediment thickness, ranges from 1mmyear�1 to as much as 1 cm year�1 or, in lakes thatreceive especially large amounts of inorganic sedi-ments from streams or rivers, several cm year�1.The accumulation of sediments, measured as thick-ness, is the rate at which lakes become shallower, thusdetermining their lifespan, following which theybecome wetlands and subsequently even dry land.

Pelagic–Benthic Coupling

Particles that reach the lake bottom provide food forbacteria and invertebrates that live within or on thesediment surface. Thus, particulate organic matterthat reaches the bottom of a lake is subject to decom-position or ingestion. Even so, a great deal of the con-sumption and decomposition of particles typicallyoccurs within the water column of a lake, prior to thedeposition of particles on the sediment surface.The ratio of particles produced to particles that reachthe sediment surface lieswithin the range 3–7, and doesnot seem to vary greatly between unproductive andhighly productive lakes. Because of the loss of particleswithin the upper layer, in addition to further loss afterparticles have reached the sediment surface (asmuch as20–60% in1–2 years), only a small proportion actuallyis incorporated into buried sediments that are not sub-ject to significant further metabolism. In Lake Zug,Switzerland, for example, only 19% of the particles

deposited were actually buried over the long term,and the total amount of the material buried corre-sponded only to 4% of particles produced throughprimary production (Figure 5).

Aside from organic matter, sediment particles con-tain the skeletal material (frustules) of diatoms, pol-len grains, volcanic ash, dust, and some invertebrateremains. These particles often can be used in deter-mining the conditions within a watershed or lake overgeologic time.

The Sediment–Water Interface

The sediment–water interface of lakes is a zone ofintensive organic matter decomposition caused bybacteria. The decomposition process leads to miner-alization of organic particles within top few milli-meters of bottom sediments. Decomposition alsouses up oxidants in the process of bacterial respira-tion and enhances the release of nutrients, such asinorganic forms of phosphorus and nitrogen. Whenfree oxygen is exhausted at the sediment–water inter-face, the potential for support of invertebrates isreduced, and most of the biotic action from thatpoint forward is caused by bacteria, which can with-stand loss of oxygen.

Diagenetic Processes

Chemical and physical changes that occur inlake-bottom sediments are called ‘diagenesis’. Dia-genesis of a given particle begins when the particle isdeposited, and continues indefinitely, even after it

Trophogeniczone

Tropholyticzone

Sediment-water-interface

17% CO2 CH4

12%

79%CO2

Air

Production 100%440 g C m−2 a−1

21%Corg

9%

5%4%

Sediment

Net sedimentation18 g C m−2 a−1

Figure 5 Production, sedimentation, and mineralization of particulate organic carbon (POC) in 200 m deep Lake Zug, Switzerland.The processes in the water column (during settling) and in the upper 15 cm of the bottom sediments (after burial) are represented

schematically. The deep hypolimnion of Lake Zug is anaerobic during stratification. From Bloesch 2004, Fig. 8.11; with permission from

Blackwell Publishing.


is deeply buried. The rate of diagenetic processesgenerally is much higher in recently deposited parti-cles than it is in older particles that are deeplyburied. Diagenetic processes often are mediated bymicrobes.Diagenesis also can be influenced by invertebrates

that live within or upon the sediment surface. Theirdisturbance of the sediment is referred to as ‘biotur-bation’, which can only occur when there is someoxygen available above the sediment, as invertebratesrequire oxygen for life. When oxygen is available,however, they may change both the physical andchemical properties of sediment.Water containing oxygen has an abundance of

oxidized chemical substances. First is oxygen itself,which is the most powerful chemical oxidant in anatural aquatic environment. Other oxidized sub-stances include nitrate, nitrite, oxidized forms of ironand manganese, sulfate, and organic matter contain-ing oxygen. Within a hypolimnion, where there is norenewal of oxygen because of the absence of photo-synthesis and lack of contact with the atmosphere, thesupply of oxidized chemical species declines steadily.The most powerful oxidizing agents are used up first,followed by progressively weaker oxidizing agents.

The cause of depletion is respiration by microbes,which must use an oxidizing agent in order to conductrespiration (the oxidation of organic matter to pro-duce energy). The byproduct of this oxidation is theconversion of oxidizing agents to a nonoxidized(reduced) form. Common conversions are as follows:nitrate to nitrogen gas; oxidized manganese or iron toreduced manganese or iron; sulfate to sulfide, organicmatter to methane. In the absence of oxygen, thesediagenetic conversions occur in lake sediments.

As a sediment surface in a lake loses its oxygen anddepletes oxidized forms of common ions, there is achange in the chemistry of the water surroundingthe sediment, and also a strong release of dissolvedsubstances from the sediment. Released substancesinclude the following reduced chemical species: Mn2þ

(reduced manganese), Fe2þ (reduced ferrous iron),NHþ

4 (ammonium), CH4 (methane). Under reducingconditions, other ionic substances also are mobilizedin the sediments, including Ca2þ (calcium), HCO�

3

(bicarbonate), and PO�34 (ortho-phosphate). The loss

of these substances from the sediment enriches theoverlying hypolimnetic water with a variety of ions,including especially phosphorus, a growth-limitingplant nutrient.


Paleolimnology

Because they store particles in a physically stableenvironment, sediments of the deep waters of lakesare archives of environmental history. The sedimentscontain information on the history of the watershed,as determined by pollen analysis and inorganic sedi-ment components, and also about the limnologicalconditions in the lake itself, as determined frommicrofossils of both algae or invertebrates as pigmentremnants or chemical indicators of past dissolvedsubstances in water.Paleolimnological studies of old lakes may extend

over thousands of years, as determined from coresthat are many meters long. Examples of studiesbased on long sediment cores include those of LakeWindermere, UK, and Lake Biwa, Japan. Interpreta-tions include climatic change, changes in watershedvegetation, early human effects on lakes, and moderneutrophication.Some deepwater sediments have annual layers,

which may be referred to as ‘laminations’ or ‘varves’.Such layers favor exact resolution of a progression of

Table 2 Range of heavy metal and organic contaminants concentr

Heavy metalsa Concentration invarious lake andriverine sediments

Geochemicastandard inclaysc

Organic contaminantsb (withnumber of single compoundsinvestigated)

Zn (mg�g�1 dry weight) 81–543 95Cu (mg�g�1 dry weight) 11–56 45

Cr (mg�g�1 dry weight) 10–51 90

Ni (mg�g�1 dry weight) 33–76 68Cd (mg�g�1 dry weight) 0.4–1.7 0.3

Pb (mg�g�1 dry weight) 11–90 20

Hg (mg�g�1 dry weight) 0.1–0.3 0.4

Pesticides, 21 (mg�kg�1 dryweight)

N.D.–390

Low molecular PAHs,

8 (mg�kg�1 dry weight)

N.D.–151 500

High molecular PAHs, 12(mg�kg�1 dry weight)

N.D.–290 000

Dioxins, 5 (ng�kg�1 dry weight) N.D.–46 000

Furans, 5 (ng�kg�1 dry weight) N.D.–22 000PCBs, 7 (mg�kg�1 dry weight) N.D.–60 000

From Bloesch J (2004) Sedimentation and lake sediment formation. In: O’Sulliv

Limnetic Ecology, pp. 197–229. Oxford, UK: Blackwell, with permission from BaFrom various Swiss Lakes: Bloesch J, Hohmann D, and Leemann A (1995) D

Planktons, der Sedimentation und der Schwermetallbelastung. Mittg. Natf. GebFrom Laurentian Great Lakes: Papoulias DM and Buckler DR (1996) Mut

22: 591–601.cFrom Forstner U and Muller G (1974) Schwermetalle in Flussen und Seen alsdFrom Baudo R and Muntau H (1990) Lesser known in-place pollutants and

Sediments: Chemistry and Toxicity of In-Place Pollutants, pp. 1–14. Boston: LeeFrom Giesy JP and Hoke RA (1990) Freshwater sediment quality criteria: Toxic

Chemistry and Toxicity of In-Place Pollutants, pp. 265–348. Boston: Lewis Pub

LJ (1996) A preliminary evaluation of sediment quality assessment values for f

change from one year or one decade to the next, e.g.,the transition from oligotrophic to eutrophic state. Insediments that are not laminated, the use of isotopedating (mostly 210Pb and 137Cs, for deep cores also14C, radiocarbon) is the only means of judging theage of a given portion of a sediment core, and thetemporal resolution is weaker.

Chemical Contamination in Relation toSediments

Because sediments, and fine sediments in particular,offer very large amounts of surface area, they mayadsorb nutrients or toxins (Table 2). Contaminants ofthis type also may be released from sediments undersome circumstances. Thus, sediments may be animportant consideration in the transport, storage,and release of a wide variety of contaminants, includ-ing such substances as phosphorus (a nutrient), heavymetals (potential toxins), or organic chemicals ofindustrial origin (typically toxins).

ation in settling SPM and bottom sediments of various lakes

l Meanconcentrationin unpollutedsoil d

Canadian standardse Tolerance limitsevere/toxiceffect level

No pollutionminimum/lowestlimit no effect level

1–900 65–110 8002–250 15–25 114

5–1500 22–31 111

2–750 15–31 900.01–2 0.6–1.0 10.0

2–300 23–40 250

0.01–0.5 0.1–1.0 2.0

2–10 9–1300

400–560 800–9500

320–750 500–14800

– –

– –70–200 1000–5300

an PE and Reynolds CS (eds.) The Lakes Handbook, vol. 1. Limnology and

lackwell Publishing.

ie Limnologie des Oeschinensees, mit besonderer Berucksichtigung des

sellschaft Bern, N.F. 52: 121–145.

agenicity of Great Lakes sediments. Journal of Great Lakes Research

Ausdruck der Umweltverschmutzung, 225 pp. Berlin: Springer-Verlag.

diffuse source problems. In: Baudo R, Giesy JP and Muntau H (eds.),

wis Publ.

ity bioassessment. In: Baudo R, Giesy JP, and Muntau H (eds.) Sediments:

l. and Smith SL, MacDonald DD, Keenleyside KA, Ingersoll CG., and Field

reshwater ecosystems. Journal of Great Lakes Research 22: 624–638.


Contaminants carried by sediments may be storedin river beds, deltas, or lake beds. Stable sedimentaccumulations may show varied concentrations ofspecific contaminants with depth, reflecting contami-nation incidents or changes in regulations that havereduced contamination sources. When the sedimentaccumulations are disturbed, contaminants may bemobilized unexpectedly.Sediments include particles, particularly in lakes

and lowland rivers, that may contain contaminants.Living organisms may concentrate contaminants(bioaccumulation or biomagnification), thus creatingthe potential for high concentrations within organic-rich sediments. Disturbance of these sediments maymobilize the contaminants.Contaminated sediments may be removed by

dredging, but this process carries the risk of mobiliz-ing contaminants that otherwise would remainstably stored during burial. In general, the worsetypes of sediment contamination are now preventedby environmental regulations in developed countries,although uncontrolled contamination can occurthrough spills, and may be common in parts of theworld that do not yet have well developed environ-mental regulations.

Nomenclature

– automated electron probe X-ray microanalysis(EPXMA)

– bicarbonate (HCO3�)

– contaminants concentration: pesticides, PAHs,dioxins, furans, PCBs [ng or mg�kg�1 dry weight]

– ‘Critical’ lake water depth (DE-T and DT-A) [m]– heavy metal concentration: Zn, Cu, Cr, Ni, Cd, Pb,

and Hg [mg g�1 dry weight]– laser ablation-inductively coupled plasma-mass

spectrometry (LA-ICP-MS)– Lf, effective fetch [km]– mean SPM (particle) concentration [mgm�3] or

[mg l�1]– particulate inorganic matter (PIM)– particulate nitrogen (PN)– particulate organic carbon (POC)– particulate phosphorus (PP)– particulate organic matter (POM)– p/s, production/sedimentation ratio [�]– polycyclic aromatic hydrocarbons (PAHs)– polychlorinated biphenyls (PCBs)– primary production [g C’assimilated m�2 a�1]r [kg dm�3], densityQ, discharge [m�3 s�1]– Release fluxes of Mn2þ, HPO4

2�, Fe2þ, NH4þ, CH4,

Ca2þ, HCO3� [mmol m�2 day�1]

S [g kg�1], salinity

– scanning electron microscopy (SEM)– sediment accumulation rates [mma�1]– sediment water interface (SWI)– sinking velocity of SPM (particles) [mday�1]– settling flux of SPM (particles) [mgm�2 day�1]– suspended particulate matter (SPM)T [�C], temperatureWk, characteristic water content of sediments [%]W0–1, ‘critical’ water content of surficial lake sedi-

ments 0–1 cm [%]

Glossary

Allochthonous particulate matter – Sediments/parti-cles originating from a lake’s catchment and aretransported through rivers or air into a lake.

Autochthonous particulate matter – Sediments orparticles produced or formed within a lake.

Bedload sediments – Sediments transported along thechannel surface by rivers at high flow.

Biogenic calcite precipitation – Formation of calcitecrystal particles triggered by supersaturation of bi-carbonate (HCO3

�).

Bioturbation – Burrowing activities of worms,insects, amphipods, clams, and other benthic ani-mals in lake bottom sediments that change thephysical and chemical properties of sediments andhence early sediment diagenesis.

Bioaccumulation – A concentration of contaminantswithin a food chain.

Export production – Sedimentation flux of particu-late matter out of the productive zone of a lake.

Hyporheic zone – Hollow space in bed sediments ofrunning waters, acting as boundary between surfaceand groundwater (phreatic zone) and habitat forbenthos.

Floc (marine snow) – Bulk sedimentation of fluffyorganic detritus, as first observed in oceans butalso occurring in lakes.

Redox processes – Electron transfer between inorgan-ic salts (nutrients) triggered by chemical oxidation(e.g., photosynthesis) and reduction (e.g., respira-tion) and resulting in the subsequent formation ofaerobic and anaerobic zones.

Scavenging – Adsorptive process attaching contami-nants, radionuclides, heavy metals, and phosphorusto settling particles.

Sediment focusing – Concentration of particulatematter in the deepest parts of deep lakes by lateralsediment transport.


Sediment resuspension – Bottom sediments beingbrought into suspension again by wind inducedcurrents in shallow (parts of) standing waters(lakes); defined as secondary settling flux whenresettling.

Sediment traps – Cylindrical samplers deployed instanding waters (lakes, ponds) to collect settlingparticles and measure downward settling flux.

Sediment winnowing – Changes in sediment grainsize mainly observed in shallow zones owing todifferent pathways of small/light and coarse/denseparticles during lateral sediment transport.

Sediment–water interface – Very active zone of bacte-rial metabolism in the top few cm of lake bottomsediments.

Shear stress – Friction force needed to resuspend sedi-ment particles at the benthic boundary layer instanding waters and to transport bed load sedi-ments in running waters.

Suspended solids – Fine grained particulate matterkept in suspension by water flow.

Stokes law – Fundamental equation for spherical andnonspherical particles for calculating the sinkingvelocity of particles (by gravity), in function of par-ticle size, shape, and specific weight, and watertemperature, viscosity, and density.

See also: Bacteria, Attached to Surfaces; The BenthicBoundary Layer (in Rivers, Lakes and Reservoirs);Benthic Invertebrate Fauna; Benthic Invertebrate Fauna,Lakes and Reservoirs; Benthic Invertebrate Fauna, Riverand Floodplain Ecosystems; Biogeochemistry of TraceMetals and Mettaloids; Carbon, Unifying Currency;Climate and Rivers; Currents in Stratified Water Bodies1: Density-Driven Flows; Currents in the Upper MixedLayer and in Unstratified Water Bodies; Flood Plains;Fluvial Export; Fluvial Transport of Suspended Solids;Geomorphology of Streams and Rivers; Ground Waterand Surface Water Interaction; Hydrological Cycle andWater Budgets; Iron and Manganese; Methane;

Paleolimnology; Phosphorus; Phytoplankton Productivity;Redox Potential; Small-scale Turbulence and Mixing:Energy Fluxes in Stratified Lakes; Sulfur Bacteria;Turbidity.

Further Reading

Allan JD (1995) Stream ecology. In: Structure and Function ofRunning Waters, 388 pp. London: Chapman & Hall.

Bloesch J (1995) Mechanisms, measurement and importance of

sediment resuspension in lakes. Marine & Freshwater Research46: 295–304.

Bloesch J (1996) Towards a new generation of sediment traps and a

better measurement/understanding of settling particle flux in

lakes and oceans: A hydrodynamical protocol. Aquatic Science58: 283–296.

Bloesch J (2004) Sedimentation and lake sediment formation. In:

O’Sullivan PE and Reynolds CS (eds.) The Lakes Handbook,Vol. 1. Limnology and Limnetic Ecology, pp. 197–229. Oxford,UK: Blackwell.

Forstner U (1987) Sediment-associated contamination – An over-

view of scientific bases for developing remedial options. In:

Thomas R, Evans R, Hamilton A, Munawar M, Reynoldson T,and Sadar H (eds.) Ecological Effects of In Situ Sediment Con-tamination, Hydrobiologia, 149: 221–246.

Hakanson L and Jansson M (1983) Principles of Lake Sedimentol-ogy. 316 pp. Berlin: Springer.

Hilton J, Lishman JP, and Allen PV (1986) The dominant processes

of sediment distribution and focusing in a small, eutrophic,

monomictic lake. Limnology and Oceanography 31: 125–133.Lerman A (1979) Geochemical Processes. Water and Sediment

Environments. 481 pp. New York: Wiley-Interscience.

Lewin J (1992) Floodplain construction and erosion. In: Calow P

and Petts GE (eds.) The Rivers Handbook, Vol. 1. Hydrologicaland Ecological Principles, pp. 144–162. Oxford, UK: Blackwell

Scientific.

Murdoch A and MacKnight SD (1994) Handbook of Techniquesfor Aquatic Sediments Sampling, 2nd edn, 236 pp. Boca Raton,FL: CRC Press.

Petts GE and Amoros C (eds.) (1996) Fluvial Hydrosystems,322 pp. London: Chapman & Hall.

Reynolds CS (1984) The Ecology of Freshwater Phytoplankton,384 pp. Cambridge, UK: Cambridge University Press.

Skei JM (1992) A review of assessment and remediation strategies

for hot spot sediments. Hydrobiologia 235/236: 629–638.Smith IR (1975) Turbulence in Lakes and Rivers. Freshwater

Biological Association, Scientific Publication 29: 79.

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