RESTORATION AND MANAGEMENT OF LAKES AND RESERVOIRS - CHAPTER 2 ppt

While both lakes and reservoirs are subject tosilt, organic, and nutrient loadings, reservoirs are more likely to have water quality problems due to their usually large watersheds and th

Readers familiar with the fundamentals of limnology could go directly to sections on restorationmethods While these next two chapters cannot substitute for the in-depth understanding of lim-nology required to make competent and effective decisions, they do provide a review or guide tosome basic principles The reader is referred to Hutchinson (1957, 1967, 1975), Cole (1994), Horneand Goldman (1994), Lampert and Sommer (1997) and Scheffer (1998) for thorough discussions

of limnology Welch and Jacoby (2004), Wetzel (2001), and Kalff (2002) are especially useful fortheir holistic viewpoints, and for their coverage of macrophyte biology and stream and reservoirecology

2.2 LAKES AND RESERVOIRS

The physics, chemistry, and biology of dimictic (deeper lakes that thermally stratify in summerand winter) natural lakes have dominated limnological literature and the training of many limnol-ogists This bias reflects the fact that there are many of these lakes in North America and Europe

It is also a result of the emergence of “limnology schools” located primarily in North Americanand European areas dominated by deep lakes But, shallow lakes are far more common than deeplakes (Wetzel, 1992), and limnology programs emphasizing them are now emerging, particularly

in Europe

Reservoirs are as important as natural lakes for recreation, but have additional values for floodcontrol, hydropower generation, and water supply While both lakes and reservoirs are subject tosilt, organic, and nutrient loadings, reservoirs are more likely to have water quality problems due

to their usually large watersheds and their morphometric configurations Reservoirs are a vital part

of the economy of many nations The U.S Army Corps of Engineers (USCOE) manages imately 783 reservoirs with a combined surface area of 27,000 km2 (Kennedy and Gaugush, 1988).Despite their abundance and importance, most limnology texts only mention them, or incorrectlyimply that they are functionally equivalent to natural lakes and that no distinction is necessary.While natural lakes and reservoirs have biotic and abiotic processes in common, they haveimportant differences Both have similar habitats (pelagic, benthic, profundal, and littoral zones),organisms, and processes, but it is their differences, summarized by Thornton et al (1980), Walker(1981), Kennedy et al (1982, 1985), Søballe and Kimmel (1987), Thornton et al (1990), andKennedy (1999, 2001) (Table 2.1), that also must be understood to successfully manage them.These fundamental reports are important supplements to most texts in limnology A brief compar-ison of lakes and reservoirs is presented here

approx-Reservoirs differ from lakes in their geologic history and setting, basin morphology, andhydrologic factors (Kennedy et al., 1985; Kennedy, 2001) When natural lakes and USCOE reser-voirs are compared, it is apparent that reservoirs are located primarily where flooding may occur

or where water shortages require water storage Reservoirs thus dominate the middle latitudes ofthe U.S (Walker, 1981) Reservoirs are also used for hydropower generation Very small reservoirsfor recreation and farming operations are found at all latitudes

Lakes of North America are also located in distinct regions They are: (1) the continental glaciallakes in the mesic northeast, Canada and upper midwest, (2) the mostly alpine glacial lakes inAlaska and the mountainous west, (3) the coastal plain and karst (solution) lakes of the southeast,especially Florida, and (4) scattered small regions of playas, potholes and sandhill lakes in aridand semi arid areas (J.M Omernik, USEPA, personal communication) Further discussions of theselake distributions are found in Hutchinson (1957) and Frey (1966)

Latitudinal differences in climate and geology have a major influence on the quality and rates

of materials loaded to lakes and reservoirs, and on their degree of thermal stratification and mixing.The average reservoir watershed area is nearly an order of magnitude greater than the averagelake’s watershed, a factor accounting for the much higher average areal water (and contaminant)loadings to reservoirs (Table 2.1) Some lakes also have large watersheds and thus, like reservoirs,have high water loads Reservoirs can become distinctly “lake-like“ during summer low flowperiods Therefore, it should be noted that the values in Table 2.1 are averages, and that the ranges

of lake and reservoir characteristics overlap

Natural lakes are more likely to be located centrally in a fairly symmetrical drainage area,whereas reservoirs are elongated and dendritic, and usually at the downstream boundary of thewatershed The deep zone of a reservoir is normally at the dam; in lakes there may be several

“deep holes.”

Average nutrient and sediment loads are much higher for reservoirs and this material may haveundergone a far longer period of in-stream processing than material loaded to natural lakes Wateroften enters lakes via smaller streams that are likely to traverse wetland or littoral areas, whereasreservoirs may have characteristics of a river for long distances into the reservoir

While natural lake outflows are at the surface, or occasionally through the ground, reservoirsusually have multiple depth, constructed outlets, leading to in-reservoir mixing processes and todischarge of water that might be anoxic, enriched with soluble nutrients, or high in hydrogen sulfide,

TABLE 2.1

Comparison of Geometric Means (Probability That Means for

Each Comparison Are < 0.0001) of Selected Variables of Natural

Lakes and Army Corps of Engineers Reservoirs

Variable Natural Lakes (N = 309) Reservoirs (N = 107)

Source: Modified from Thornton, K.W et al., 1980 Symposium on Surface Water

Impoundments Proceedings Am Soc Civil Eng pp 654–661 With permission.

methane, and reduced metals Lake levels vary with precipitation, evaporation, and surface outflows,but it is uncommon for the amplitude to be large or to change quickly An exception is the wind-induced displacement of some of the water mass, creating a to and fro “sloshing” of water in thelake basin, sometimes with amplitudes of one meter or more (surface and internal seiches) A well-known example is the occasional seiche in Lake Erie (U.S.-Canada) Reservoirs, however, can haverapid and significant changes in levels due to management decisions and these changes in levelmay eliminate or greatly reduce the littoral community of rooted aquatic plants.

Unlike lakes, reservoirs are operated to store and release water, and these operations profoundlyinfluence their limnological characteristics (Kennedy, 2001; Cooke and Kennedy, 2001) For exam-ple, when deep waters are released, heat is stored When surface waters are released heat isdissipated These actions greatly alter thermal structure, including depth of the metalimnion (layer

of water with a sharp thermal gradient) and retention or loss of materials

Lakes and reservoirs represent a continuum of ecological conditions (Canfield and Bachmann,1981) Kimmel and Groeger (1984) and Søballe and Kimmel (1987) viewed this continuum as oneordered by water residence time (volume divided by outflow rate), and indicated that reservoirsand natural lakes with similar residence times have similar ecological attributes In rapidly flushedsystems, for example, algal abundance is less likely to depend on nutrient concentrations than onflushing rate (Chapter 6) Therefore, despite features that might separate lakes and reservoirs asclasses of aquatic habitats, convergence can occur when water residence times are similar.Geographic location of a reservoir determines the quantity and timing of inflow For example,inflows in some areas of California are in spring to mid-summer, whereas peak inflows in thePacific Northwest and southeastern U.S are in winter to early spring Similarities and differencesbetween lakes and reservoirs based on water residence times are thus modified by location (Kennedy,1999)

Figure 2.1 illustrates the expected gradient in reservoir characteristics of a main-stem reservoir(dam on the stream) from the river entrance to the dam Unlike many natural lakes where waterenters from several smaller tributaries draining comparatively small sub-watersheds, reservoirs have

a distinct riverine zone dominated by flow and mixing, followed by a transition zone where inflowvelocity slows, rapid sedimentation begins, and water clarity increases When inflowing river water

is colder than surface water of the reservoir, a “plunge point” is found where the colder, heavierwater loses velocity and descends to a depth equal to its density, creating a distinct inter- orunderflow (Figure 2.2) Unlike lakes, where it is often assumed that nutrient loads are completelymixed with lake waters, loading to a reservoir might not mix with upper waters at all, but insteadmight be carried through the reservoir via an inter- or underflow, greatly altering standard loadingmodel assumptions (Chapter 3) (Kimmel and Groeger, 1984; Gaugush, 1986; Walker, 1987) Thelacustrine zone near the dam is the most lake-like, with thermal stratification and a higher probabilitythat algae growth is nutrient limited Some natural lakes in narrow valleys, with large inflow riversand low water residence times, have many reservoir characteristics This gradient of conditionsalong the length of a reservoir means that reservoir characterization requires multiple samplingstations The same is true for large natural lakes, and lakes with distinct pelagic and littoral zones.Reservoir basin design also influences hydrodynamic features For example, a tributary reservoirand a main stem reservoir receiving identical water loads and having identical basin volumes arelikely to have different responses Main stem reservoirs have low capacity to store excess volumeand thus have water residence times that fluctuate with water loading events, whereas tributaryreservoirs have much higher storage capacity and are used for flood control In these reservoirs,the hypolimnion may be large whereas the main stem reservoir may be longer and shallower andgreatly influenced by interactions between sediments and overlying water (Kennedy, 1999).Reservoirs are important sources of fresh water for potable, irrigation, and industrial purposes.Their protection and management requires that more traditional views of sampling, correlationsbetween loading and responses of biota, and choice of restoration techniques be modified to takethese, and the basic differences between natural lakes and reservoirs, into account

2.3 BASIC LIMNOLOGY

2.3.1 PHYSICAL–CHEMICAL LIMNOLOGY

Some lakes and reservoirs stratify thermally during summer months into an upper warm, mixed zone termed the epilimnion Below this is a zone of rapidly decreasing temperature withdepth, the metalimnion, followed by a deep, colder, often dark bottom layer, the hypolimnion Thisphenomenon, brought about by wind mixing, solar input, and by large differences in water densitybetween cold and warm waters, is a primary determinant of summer physical, chemical, andbiological interactions During ice cover lake water temperature inversely stratifies, with colderwater at the surface This happens because water’s maximum density is at 4°C, and water colder

well-than this temperature, including ice, is lighter and floats above this slightly warmer layer Lakeswith two mixing periods (spring and fall) and two stratified periods (summer, winter) are dimicticand are typical of deep lakes and reservoirs of north temperate latitudes (Wetzel, 2001) Details ofmechanisms leading to this and other types of thermal stratification are found in all basic limnologytexts Figure 2.3 illustrates the characteristics of the three thermal layers in a dimictic lake, or inthe lacustrine zone (Figure 2.1) of a reservoir during summer months The figure also illustratestypical summer temperature and dissolved oxygen (DO) profiles with depth in a stratified eutrophiclake or reservoir

Polymictic lakes are more common than dimictic lakes Because polymictic lakes are shallow,they may mix continuously, or stratify briefly (hours, days) in calm, hot weather, followed byrenewed complete mixing Polymictic lakes are found at all latitudes

FIGURE 2.1 Longitudinal zonation in environmental factors that control primary productivity, phytoplankton

biomass, and trophic state within reservoir basins Changes in shading indicate decline in turbidity (From

Kimmel, B.C and A.W Groeger, 1984 Lake and Reservoir Management USEPA 440/5-84-001 pp 277–281

Riverine zone Transitional zone

• Broader, deeper basin

• Algal cell loss by grazing

• Organic matter supply autochthonous

• More “oligotrophic”

There can be significant modifications of temperature regimes, particularly in reservoirs Riverinflows to reservoirs may have very different temperatures than reservoir waters, producing under-inter-, or overflows of incoming water (Figure 2.2) The upper reaches of the reservoir, like thewave washed littoral zone of a lake, may exhibit little thermal stratification except during hot, calm,low-flow periods In the transition zone (Figure 2.1), where mixing and sedimentation processesare dominant, the volume of a reservoir’s hypolimnion may be small Only in the deep lacustrinezone is the temperature stratification similar to natural lakes, though the hypolimnion is likely to

be less stable due to underflows and withdrawals of deep water at the dam

The shape of a reservoir or lake’s basin affects its productivity, kinds of organisms, waterchemistry, and the choices available to manage and restore it Most natural lakes are small in areaand shallow (mean depth ≤ 3 m) Rooted plants, and algae associated with leaf and sediment

surfaces, can have very high primary productivity, biomass, and areal distribution, unless the lake

is turbid from silt loading, wind mixing, or algal blooms that cause rooted plants to be light-limited.Also, as noted in later sections and other chapters, the large area of shallow, warm sediments andthe small hypolimnetic volumes associated with (mainly) polymictic shallow lakes and reservoirsprovide ideal circumstances for processes that allow sediment nutrient release (actually recycling

or “internal loading”) and transport to the water column This can greatly stimulate algal tivity Internal loading processes may be biological (e.g., microbial activities and temporary anoxia,and sediment disturbance by methane release or by burrowing animals), chemical (e.g., high pHfrom photosynthesis), and physical (e.g., turbulence from the wind) in nature (Chapter 3) Because

produc-of these processes, lake productivity is produc-often negatively correlated with mean depth (Wetzel, 2001)and with the ratio of mean to maximum depth (Carpenter, 1983) Therefore, many shallow waterbodies will have more algae or rooted plants than the less common steep-sided, deep lakes andreservoirs

High macrophyte growth is to be expected in shallow lakes A hypsograph (a representationdescribing the relationship between lake area and depth) is useful in explaining this Figure 2.4compares the area–depth relationship for two hypothetical lakes with different areas of shallow

FIGURE 2.2 Density flows in reservoirs The upper panel illustrates an “overflow“ of warm incoming waters,

the middle panel illustrates an “interflow,“ and the bottom panel shows an “underflow.“ (From Moore, L and

K Thornton (Eds.) 1988 Lake and Reservoir Restoration Guidance Manual USEPA 440/5-88-002

Outflow

Dam

Outflow

Dam Outflow

Plunge point

Underflow

water Both lakes could have nuisance algal blooms if nutrient concentrations were high Only theshallow one has the potential to have a large area with rooted plants because of the extensiveshallow, well-lighted sediment area Physical factors, particularly waves, transparency, and the slope

of the littoral zone (amount of stable sediment area exposed to light) are among the determinants

of maximum macrophyte biomass and maximum depth of plant colonization (Canfield et al., 1985;Duarte and Kalff, 1986, 1988) (Chapter 11) The development of a hypsograph is an important firststep in lake and reservoir problem diagnosis

2.4 BIOLOGICAL LIMNOLOGY

Lakes and reservoirs have three distinct and interacting biotic communities (Figure 2.5): (1) thewetland-littoral zone, and its sediments, (2) the open water pelagic zone, and (3) the benthic ordeep water (profundal) zone and sediments Problems or characteristics appearing in one zone (e.g.,deep water oxygen depletions, littoral zone aquatic plants, pelagic zone algal blooms) directly orindirectly affect other zones, meaning that successful lake restoration requires a holistic view oflake and watershed processes For example, nutrients causing algal blooms may come from lakesediments and decomposition of littoral plants, as well as from external loading All sources mightrequire attention to solve the problem

FIGURE 2.3 Cross section of a thermally stratified reservoir indicating location and characteristics of the

epilimnion, metalimnion, and hypolimnion, and typical summer temperature-dissolved oxygen distributions

in the lacustrine zone of a eutrophic reservoir (From Gunnison, D and J.M Brannon 1981 Characterization

of Anaerobic Chemical Processes in Reservoirs: Problem Description and Conceptual Model Formulation.

Tech Rept E-81-6 U.S Army Corps Engineers, Vicksburg, MS

*

Epilimnion

Metalimnion

Hypolimnion

Sediment – region of material

adsorption and release

• Oxygen low or absent-increased concentrations

of soluble forms of contaminants and nutrients

• Coldwater fishery if oxygen adequate

*Typical vertical temperature and do

distributions during stratification:

Bottom Hypolimnion Metalimnion Epilimnion Temperature DO

Rooted emergent, floating, and submersed vascular plants, collectively called macrophytes, andtheir attached flora and fauna, dominate the wetland-littoral These plants are distinctly differentfrom the microscopic, floating (planktonic) cells, colonies, and filaments of algae, often seen assurface “scums” in some eutrophic systems Macrophytes are usually vascular plants and are found

in shallow water They may have large masses of filamentous (string or hair-like) algae attached

to them as thick mats Shallow, lighted sediments often have a highly productive epilithic, epipelic,and epiphytic flora (algae growing on surfaces of rocks, sediments and vascular plants) Macrophytebiology is described in Chapter 11

The littoral zone often has high species diversity, and is commonly the site where fish duction and development occurs It is also an important waterfowl habitat Littoral zone plant

repro-FIGURE 2.4 Depth-area hypsographs Solid line illustrates the less common deep lake with a small littoral

zone; dotted line illustrates the more common shallow lake with extensive littoral area and volume (From

Cooke, G.D., E.B Welch, S.A Petersen, and P.R Newroth 1993 Restoration and Management of Lakes and Reservoirs, 2nd Edition Lewis Publishers and CRC Press, Boca Raton, FL.

FIGURE 2.5 Biotic communities in lakes and reservoirs.(From Cooke, G.D., E.B Welch, S.A Petersen, and

P.R Newroth 1993 Restoration and Management of Lakes and Reservoirs, 2nd Edition Lewis Publishers

and CRC Press, Boca Raton, FL.)

Surf.

Wetland-littoral

Profundal zone Pelagic

biomass replaces itself two or more times per summer in productive lakes, leading to inputs ofnon-living dissolved and particulate organic matter, termed “detritus,” to the water column andsediments Detritus, whether from watershed drainage or from in-lake productivity, is a stableenergy and nutrient source to the lake’s food webs, especially to microbial flora and plankton(Wetzel, 1992, 1995) Many lakes, especially those surrounded by dense forest, are actually het-erotrophic (photosynthetic rate is less than total respiration rate), and depend upon organic carbonfrom terrestrial sources to subsidize their food webs (Cole, 1999) Therefore lakes are stronglylinked to the land, not only through nutrient and silt loading, but through detritus imports.Macrophytes, in addition to being a significant energy source and habitat, stabilize littoral zonesediments from the impacts of wind and boat-generated waves, thus reducing internal P loadingand sediment resuspension (Bachmann et al., 2000; Anthony and Downing, 2003; Horppila andNurminen, 2003).

Macro- and microplankton, and the fish and invertebrates grazing on them, dominate the pelagiczone The plankton includes algae that produce unsightly “blooms” and low water clarity, and

bacteria, fungi, Protozoa, and filter-feeding crustaceans like Bosmina and Daphnia The pelagic

community obtains energy from sunlight and from detritus transported to it from stream inflowsand the littoral zone The plankton of most enriched lakes and reservoirs is dominated by one or

a few species of highly adapted algae and bacteria, particularly nuisance blue-green algae

(cyano-bacteria) Bosmina, Daphnia, and other planktonic microcrustacea are significant grazers of detritus,

bacteria, and some algae species, though their abundance may be regulated by complex interactionswith predators such as fish and insects (Chapter 9)

The profundal benthic community receives nutrients and energy from organic matter loaded to

or produced in the lake or reservoir and deposited on the sediments Inorganic forms of nutrientsmay be added to the sediments in the form of precipitates This pelagic-benthic coupling is afundamental feature of lakes (Vadeboncoeur et al., 2002) In productive lakes and reservoirs, largeareas of the sediment community in deep water are continuously anoxic during thermal stratificationdue to intense microbial respiration that is stimulated by deposits of detritus Anoxic conditionsprovide conditions favoring high rates of nutrient release to the water column (Figure 2.3)

2.5 LIMITING FACTORS

Nuisance densities of algae or macrophytes, and associated water quality problems, are conditionsmanaged by manipulating or altering their biomass or by manipulating one or more of the factorscontrolling their abundance Macrophyte density, while in part related to sediment type and com-position, and to nutrient factors, is often determined by light availability (Duarte and Kalff, 1986;Canfield et al., 1985; Barko et al., 1986; Smith and Barko, 1990) Long-term control of algalbiomass requires significant water column nutrient reduction Phosphorus (P) is most frequentlytargeted because it is usually the nutrient in shortest supply relative to demands by algae (thelimiting nutrient) Phosphorus does not have a gaseous phase so the atmosphere is not a significantsource, unlike nitrogen or carbon Lake P concentration, therefore, can be lowered significantly byreducing loading from land and in-lake sources

A significant reduction in external nutrient loading is an essential, but not necessarily sufficient,step toward reducing lake P concentrations Internal loading from aerobic and anaerobic sediments,groundwater seepage, decomposing macrophytes, sediment resuspension, and organism activitiesmight add more nutrients to the lake than external loading during some times of the year.The shape of a lake’s basin (Figure 2.4) has an important bearing on the amount of internalloading Most of the variance in algal productivity among some Ontario lakes was explained bythe ratio of sediment area in contact with the epilimnion to epilimnion volume Steep-sided, deeplakes have a low ratio, producing less influence on overlying water (Fee, 1979) Epilimneticsediments are warm, leading to increased microbial decomposition rates and to nutrient release(Jensen and Andersen, 1992) Extensive littoral areas, typical of shallower lakes, may have distinct

day-night cycles of high and low DO concentrations that stimulate nighttime P releases, especiallyunder dense macrophyte beds (Frodge et al., 1991) Wind mixing and convective currents mayscour sediments or entrain nutrient-rich littoral or bottom waters of shallow lakes, especially thosewith low macrophyte density, thus transporting nutrients to the pelagic zone.

The hypolimnion may or may not be a P source to the epilimnion When thermal stratificationoccurs, hypolimnetic waters are isolated from the atmosphere and are usually too deep to permitsufficient light penetration for photosynthetic oxygen generation Respiration in deep waters leads

to DO depletion or elimination, to reducing conditions, and to the associated release of P fromsediment iron complexes High sulfate concentrations may lead to ferrous sulfide (FeS) productionunder reducing conditions, and loss of Fe control of sediment P (Caraco et al., 1989; Golterman,1995; Gächter and Müller, 2003) In stratified lakes with low resistance to mixing (large surfacearea relative to depth), summer winds either briefly destratify the lake (polymixis), or force verticalentrainment of P-rich hypolimnetic water to the epilimnion In either case, surface water P con-centration increases, stimulating an algal bloom For example, Stauffer and Lee (1973) calculatedthat all of the summer algal blooms in Lake Mendota, Wisconsin could be accounted for by transport

of P from the metalimnion to the epilimnion

This internal P source to the epilimnion may not be significant in lakes that are deep relative

to area of lake surface exposed to wind mixing This type of lake offers greater resistance to theforce of summer wind (Osgood, 1988) The best predictor of vertical P transport to the epilimnionappears to be the vertical gradient of P concentration, not lake morphometry (Mataraza and Cooke,1997) These ideas are explored in Chapters 3 and 4 with respect to model predictions, and inChapter 8 where sediment treatment with P inactivating chemicals is discussed

Macroscopic animals play major roles in nutrient releases from lake sediments Common carpdigestive activities release P at rates similar to external loading (La Marra, 1975) Bioturbation(sediment disturbance) by fish and insects and high rates of sloughing of vascular plant tissues arealso nutrient sources to the epilimnion Reviews of internal recycling include Carlton and Wetzel(1988), Marsden (1989), Welch and Cooke (1995), Pettersson (1998), and Søndergaard et al (2001).These characteristics of littoral and pelagic zones mean that expensive nutrient diversion projectsmay not meet expectations for reduced algal biomass until internal nutrient sources are addressed(Chapters 4 and 8)

Other factors affecting algal biomass include flushing rate, light availability, pH, and ton grazing These factors can be manipulated as part of a management plan, though significantreduction of external and internal nutrient loading remains the central part of plans for long termimprovement of excessive algae problems

zooplank-2.6 THE EUTROPHICATION PROCESS

A eutrophic lake or reservoir is rich in nutrients and organic materials, and those enriched by humanactivities are said to be culturally eutrophic We have expanded the definition of the eutrophicationprocess to include the loading of silt and organic matter, as well as nutrients Thus, we define theeutrophication process as the loading of inorganic and organic dissolved and particulate matter tolakes and reservoirs at rates sufficient to increase the potential for high biological production,decrease basin volume, and deplete DO This concept of eutrophication is more complete because

it includes all materials that produce the eutrophic condition The eutrophication process andassociated major in-lake interactions are summarized in Figure 2.6

Traditionally, eutrophication referred only to nutrient loading, its eventual high concentrations

in the water column, and the high productivity and biomass of algae that could occur Organicmatter loading may lead to sediment enrichment and loss of volume Organic matter, whether added

to the water column from external or internal sources, also leads to increased nutrient availabilityvia direct mineralization, or through release from sediments when respiration is stimulated by thisorganic matter and DO is depleted Net internal P loading appears to increase exponentially with

increasing dissolved organic carbon content of the lake (Ryding, 1985) Allochthonous organicmatter contains molecules producing changes in algal and microbial metabolism independently ofeffects of added nutrients (e.g., Franko and Wetzel, 1981) Finally, organic matter added to a lake

or reservoir contains energy that is incorporated, in both dissolved and particulate forms, into plantand animal biomass, leading directly to increased living biomass (the microbial loop) Dissolvedand particulate organic matter entering the lake or reservoir from streams, wetlands, and frommacrophytes, is of great significance to lake metabolism These ideas are developed in Wetzel(1995, 2001) and Cole (1999)

Silt may be rich in organic matter and in nutrients sorbed to surfaces of particulate matter Thesemay become available to algae or macrophytes immediately or at some later time Silt loading alsocontributes directly to volume loss and to an increase in shallow sediment area Whether volumeloss is produced by silt deposition or by the build-up of refractory organic matter from terrestrialand aquatic sources, the development of shallow areas fosters further spread of macrophytes andtheir attendant epiphytic algae Ultimately these plants promote further losses of DO and release oforganic molecules and nutrients as they decay (Carpenter, 1980, 1981, 1983) (Figure 2.6).Thus, silt and organic loadings have effects on lakes that are additional to their nutrient content,and cannot be excluded when defining the eutrophication process This view is not meant todownplay or negate the fundamental importance of high nutrient loading in stimulating lakeproductivity Instead, following Odum’s (1971) holistic view, it is meant as a more completedescription of the process

Excessive nutrient loading creates potential for eutrophic conditions but does not guaranteeincreased productivity Figure 2.6 does not account for the “oligotrophication” effects of high rates

of lake flushing and dilution, the effects of organisms in stimulating nutrient release from sediments,

or the effects of grazing (or lack of grazing) on algae biomass

Lakes and reservoirs that are naturally eutrophic, or have become so, have characteristicsseparating them from less enriched and oligotrophic (“poorly nourished”) water bodies Eutrophiclakes have algal “blooms,” often of monospecific blue-green (cyanobacteria) populations Some

FIGURE 2.6 Loadings and primary interactions in lakes and reservoirs (From Cooke, G.D., E.B Welch,

S.A Petersen, and P.R Newroth 1993 Restoration and Management of Lakes and Reservoirs, 2nd Edition.

Lewis Publishers and CRC Press, Boca Raton, FL.)

External-internal nutrient load

Sediment nutrient release

Macroscopic animal grazing

Streams, overland flows, wetland discharge

Dissolved & particulate organic matter +

“microbial loop”

also have macrophytes, though exotic macrophyte infestations are not a symptom of the eutrophiccondition because large populations can develop in oligotrophic waters Eutrophic lakes and res-ervoirs also have colored water (green/brown), and low or zero DO levels in the deepest areas(Figure 2.3) Warmwater fish production is likely to be high (Jones and Hoyer, 1982) Fish can belimited by low DO and high pH (Welch and Jacoby, 2004), and lakes may be dominated by lessdesirable fish species or stunted fish populations.

An oligotrophic lake or reservoir is low in nutrients and productivity because organic matterand nutrient loadings are low or large basin water volumes and short water residence times dilute

or pass material through the lake In addition, high water hardness may foster co-precipitation ofcalcium carbonate (e.g., marl lakes) and essential nutrients, rendering them unavailable to algae.Oligotrophic lakes are often deep and steep-sided, with nutrient-poor sediments, few macrophytes,usually no nuisance cyanobacteria, and large amounts of DO in deep water Water clarity is high,

as is phytoplankton diversity, but total algal biomass is low

Low biological productivity is not always perceived as a benefit when, for example, a sportsfishery is desired Some lakes, Lake Mead, Nevada, for example (Axler et al., 1988), have beenfertilized in an attempt to develop more fish biomass The words “eutrophic” and “oligotrophic”therefore do not represent “bad” and “good,” but are only descriptive of the state or condition of

a lake or reservoir Perceived quality is a judgment based upon needs and expectations

2.7 CHARACTERISTICS OF SHALLOW AND DEEP LAKES

Shallow lakes and reservoirs (< 3 m mean depth) are more common than larger, deeper ones, andmany are eutrophic or heavily impacted by siltation and high turbidity Their problems, and solutions

to those problems, are reflected in their characteristics Most lake and reservoir restoration niques and paradigms were developed from research and testing on less common deep lakes andmay not be entirely suitable for shallow lakes Throughout this text, we attempt to emphasizeapplicability of methods to both classes of lakes Table 2.2 is a comparison of the characteristics

tech-of deep and shallow lakes, primarily based on European research (e.g., Moss et al., 1996; Jeppesen,1998; Scheffer, 1998; Havens et al., 1999; Cooke et al., 2001; NALMS, 2003)

Shallow lakes are less sensitive to significant reductions in external nutrient loading becausebenthic-pelagic interactions tend to maintain high nutrient levels Nutrients released from bottomsediments of shallow lakes affect the entire water column, in contrast to stratified, deep lakes Inshallow lakes, nutrient release may be very high from bioturbation, wind disturbance, the effects

of gas bubbles, high pH from intense photosynthesis, and from DO deficits at the sediment-waterinterface Diversion of external nutrient loading, while necessary, may not be sufficient to rehabil-itate a shallow lake and a sediment treatment may be necessary

Shallow lakes are more likely to exist in one of two alternative and often stable states (Chapter

9) The algae-dominated turbid state is almost a certainty at high nutrient concentrations, whereasthe clear water state, possibly with macrophytes across the well-lighted sediments, will occur atlow concentrations Between these extremes, either the clear or turbid water state can exist, largelybased on biotic interactions Lakes with dense populations of planktivorous and benthivorous fish(e.g., grass carp, common carp, shad), and lakes with large populations of herbivorous birds, arelikely to have few phytoplankton grazers (large-bodied zooplankton), high internal P loading, turbidwater, and little chance of extensive establishment of native submersed plants Canopy-forming

plants such as Eurasian watermilfoil (Myriophyllum spicatum) may be successful in these lakes.

In contrast, shallow lakes with dominance by piscivorous fish and birds (e.g., largemouth bass,northern pike, Great Blue Heron) may have abundant algae grazers, stable sediments, clear water,and populations of submersed plants, even at nutrient concentrations identical to the algae-domi-nated lake (Moss et al., 1996) In most cases, a shallow lake will have either a community ofmacrophytes or turbid water with phytoplankton A shallow lake that is free of both aquatic plants