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A Brief Treatise on Eutrophication

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

May 03, 2008      Limnology

Shallow lakes; and the indicator thresholds for anthropogenic stressors in Nova Scotia

Note: Following are mostly excerpts from leading literature in limnology and we salute the dedicated scientific authors!


(cf. Report# XVI)

Contents:


It is generally found that the more eutrophic a waterbody is the greater its tendency to experience water quality problems that impair its use as a domestic or industrial water supply, or for contact recreation. Because of the association of the process of eutrophication with water quality impacts, and because increased aquatic plant growth is associated with increased input of aquatic plant nutrients, the term "eutrophication" is synonymous with "fertilization".

.................. Lee and Jones, OECD



Introduction

Eutrophication is the response in water due to overenrichment by nutrients, primarily phosphorus and nitrogen, and can occur under natural or manmade (anthropogenic) conditions. Manmade (or cultural) eutrophication, in the absence of control measures, proceeds at an accelerated rate compared to the natural phenomenon and is one of the main forms of water pollution. The resultant increase in fertility of affected lakes, reservoirs, slow-flowing rivers and certain coastal waters causes symptoms such as algal blooms, heavy growth of rooted aquatic plants (macrophytes), algal mats, deoxygenation and, in some cases, unpleasant odour, which often affects most of the vital uses of the water such as water supply, recreation, fisheries (both commercial and recreational), or aesthetics. In addition, lakes become unattractive for bathing, boating and other water oriented recreations. Most often economically and socially important species, such as salmonids decline or disappear and are replaced by coarser fish of reduced economic/social value.

Ecosystem

Ecosystem is the unit of natural organization in which all living organisms interact collectively with the physical chemical environment as one physical system. Lakes are living ecosystems. Trophy refers to the rate of supply of organic matter. Lake ecosystems are complex, involving both terrestrial and aquatic photosynthesis, external and internal nutrients, grazer and detrital food webs, and aerobic and anaerobic metabolism. Lake ecosystem consists of two major components: the "aquatic" component which is the waterbody itself, and the "paralimnetic" component which consists of the drainage basin or watershed. The paralimnetic component could be divided into a variety of land-use fractions (urban, agricultural, and wooded/wetland), soil groupings, slope classes, or other categories. Likewise, the aquatic component could be divided into littoral zone, pelagic zone, benthic (profundal zone) boundary layer, sediments, and during summer stratification into epilimnion, metalimnion, and hypolimnion.

Most energy enters a small lake through terrestrial photosynthesis in the watershed (paralimnion). About one-half of the incident PAR (photosynthetically active radiation) is reflected or refracted at the lake surface, and much of the rest may be absorbed by lake water and organic matter dissolved in it. Autochthonous production by aquatic macrophytes (littoral zone photosynthesis) and phytoplankton (pelagic photosynthesis) is grazed by littoral invertebrates and pelagic zooplankton, then by forage fish preying on zooplankton, and finally by predatory fish (piscivores) on forage fish. This trophic dynamic structure prevails in the littoral zone and trophogenic pelagic zone of mesotrophic and eutrophic lakes.


Eutrophication

Eutrophication is an ecosystem response to increasing nutrient availability. The response not only involves increased autochthonous primary productivity, but also all other aspects of lake ecosystems, biotic and abiotic, autotrophic and heterotrophic, autochthonous and allochthonous. Trophic level energy exchange operates at a transfer efficiency of approximately 10-15%. Ecological efficiencies are low because the denominator of the efficiency ratio (predator/prey) contains much organic matter (nonpredatory losses) not assimilated by predators. But this low efficiency does not reflect true ecosystem energetic efficiency (predatory/prey-nonpredatory losses). Nonpredatory losses from all trophic levels enter the detrital system supporting a large biomass of heterotrophic microflora.

Respiration

"Life is energy", all living organisms burn organic matter in a slow, controlled way.

Respiration is an "oxidation-reduction" reaction where organic matter (fuel) is "oxidized" to CO2 and another substance "X" is "reduced". All organisms do this. The only differentiating feature is the substance (X) used to accept transferred energy ("terminal electron acceptor", [TEA]).

Organisms that require oxygen to combust organic matter (such as humans) perform aerobic respiration. Many organisms do not require oxygen to combust organic matter. Anaerobic respiration is the process by which organic matter is combusted (oxidized) using an alternate TEA (to oxygen). The alternate can be a variety of substances (X) which become reduced, and these alternates in the sequence in which they are used after oxygen is depleted are first, nitrate reduction (@Eh=220mv), manganese reduction (@Eh=200mv), iron reduction (@Eh=120mv), sulfur reduction (@Eh<-75mv), and fermentation (@Eh<-75mv).


Production (or Synthesis)

Production (or Synthesis) refers to new organic matter formed over a period of time `plus' losses to respiration, excretion, secretion, mortality, grazing, and predation. "Autotrophs" capture solar energy radiating through air or water and store ("fix") captured energy as environmental redox potential ("Eh") between the photosynthetic products, oxygen and organic matter. The photosynthetic process (phototrophy) is also an "oxidation-reduction" reaction, but uses solar energy to reduce CO2 to organic matter. In photosynthesis, "X" is oxygen, and water is oxidized to oxygen. Photosynthesis is not the only process which produces organic matter. Chemolithotrophy synthesizes organic matter in the absence of light, and where for example X is sulfur, H2S is oxidized.

Standing Crop, Biomass and Productivity

Standing crop refers to the above-ground weight of organic matter which can be sampled or harvested at any one time from an area.

Biomass is the weight of all living material in a unit area at a given time. Biomass should be used for ecosystem analyses.

Productivity is the rate of production per unit time. Biomass can be low, while productivity is high (e.g. when grazing and predation rates are high). Likewise, biomass can be large, while productivity is low (e.g. when grazing and predation rates are low).


Trophic Classification

Lakes in which most of the organic matter is from autochthonous sources are referred to as "autotrophic", whereas those dominated by the input of paralimnetic particulate organic matter (POM) and dissolved organic matter (DOM) are termed "allotrophic".

Rodhe's scheme included Oligotrophic (low in both auto- and allotrophic organic sources), eutrophic (dominated by autotrophy), dystrophic (dominated by allotrophy, brown coloured water), and mixotrophy (high in both auto- and allotrophic organic source).

It has been pointed out that the "low productivity of dystrophic lakes" refers to planktonic productivity, and that littoral plants completely dominate as sources of dissolved and particulate organic carbon.

The standards that many lake users desire of their lakes usually imply the need for `oligotrophic' lakes with `mesotrophic' lakes being tolerable, though long term residents in the Metro area have observed suttle changes over the years (prior and post development). Eutrophic and the extreme condition of eutrophy, hypereutrophic lakes are not desired by most citizens, except that they provide excellent cases for scientific research into productivity.

Eutrophication and Fisheries

Eutrophication is both beneficial and detrimental to fisheries. Increasing the primary production of a waterbody will generally increase overall fish yield. However, changes in the quality of the fishery to favor those species that are generally less desirable in the North American culture may also be expected to accompany this increase in yield, especially at high trophic levels.

One of the most dramatic effects of this type is the loss of coldwater fish associated with deoxygenation of colder, hypolimnetic waters due to bacterial decomposition of algae. Literature also cites reduced grazing ability of carnivorous fish brought about by increased turbidity from increased amounts of phytoplankton as well as suspended sediment. Some highly eutrophic waterbodies also tend to produce large populations of stunted pan fish, which may be the result of inadequate predation on these fish arising from the inability of predators to see them due to increased turbidity from planktonic algae and suspended sediment.


Phosphorus-overview

Phosphorus may enter a water body through the inflows, precipitation, dry fallout and from sediments, and it may be removed by sedimentation and through the outflow.

Nitrogen has a more complex pathway. In addition to the inputs and outputs described for phosphorus, nitrogen can enter and leave a water body in the form of free nitrogen gas through atmospheric exchange. Carbon has been shown to diffuse into the water column at rates sufficient to meet the needs of photosynthesizing cells. Phosphorus, on the other hand, cycles between living and nonliving particulate forms and the dissolved form.

The different pathways of phosphorus, nitrogen and carbon in lake metabolism make phosphorus the obvious choice for eutrophication control. A certain reduction of phosphorus input will generally result in a greater reduction in algal biomass compared with the same reduction of nitrogen. Furthermore, the reduction of nitrogen input without a proportional reduction in phosphorus, creates low N/P ratio which favors nitrogen fixing nuisance algae, without any reduction in algal biomass.

Total Phosphorus (TP) and not other phosphorus species, is considered the key variable for practical rather than theoretical reasons. TP includes some or all of the following fractions: crystalline, occluded, absorbed, particulate organic, soluble organic and soluble inorganic phosphorus. Out of these fractions, the three biologically available phosphorus fractions listed in order of decreasing availability are soluble reactive phosphorus (a mixture of dissolved inorganic and organic species), soluble unreactive phosphorus (some include dissolved phosphorus fed by pesulfate oxidation, and is available for phytoplankton by enzymatic hydralisation which frees organically bound fractions), and labile phosphorus (associated with soil particles).

However the term biologically available phosphorus still remains somewhat vague because it describes a mixture of phosphorus fractions of different availability. Vollenweider (1979) described the following sources which should be considered as priorities in nutrient control measures in order of decreasing biological availability of phosphorus as:-

Urban sewage + certain industrial effluents ---> Erosional runoff and leaching from forests and agricultural areas.

Internal loading of Phosphorus:

Where suitable conditions develop at the water sediment interface, substances contained in the sediments, including nutrients, are released into the water column. Below compensation depth (in the tropholytic zone), net oxygen consumption occurs in a eutrophic lake. As alternate TEAs (terminal electron acceptors) are consumed, Eh (redox potential) decreases. Eh tends to decrease with greater depth in the water column and in sediments. Once the Eh of the ferric-ferrous iron couple is reached (@ approx.120 mv, Kortmann & Rich 1994), both soluble ferrous iron and soluble phosphate accumulate. If Eh continues to decrease, sulfate is reduced to sulfide (@ <-75mv, Kortmann & Rich 1994), which can remove iron and permanently reduce phosphate binding capacity, by interacting readily with ferrous iron to produce ferrous sulfide (FeS). If FeS precipitates to form pyrite (FeS2), ferrous iron is no longer susceptible to oxidation to ferrric iron with the return of aerobic conditions.

The relationships among sulfur, iron, and phosphorus binding capacity raises questions about potential impacts from increased sulfate loading by algicide applications (copper sulfate), alum treatments (aluminum sulfate), and acid rain (sulfuric acid).

Holdren and Armstrong (1980) per Fricker (1981) quoted literature values of sediment phosphorus release rates from several lakes in the U.S. for aerobic (0 to 13 mg P/sq.m./day) and anaerobic conditions (0 to 50 [max. 150] mg P/sq.m./day).


Nitrogen Transformation

Although availability of phosphorus is most often limiting to aquatic plants, quantities and forms of nitrogen can influence phosphorus availability and the type of biotic response to a given phosphorus level. Transformations between various nitrogen compounds in the nitrogen cycle of aquatic ecosystems offer significant management potential for lakes. Most phytoplankton which create nuisance bloom conditions are capable of nitrogen fixation and are not dependant on dissolved combined forms of nitrogen. Nitrogen fixation occurs only in bacterial cells (bluegreen algae are prokaryotic, unlike other phytoplankton which are eukaryotic), however nitrogen fixation is inhibited by high cellular ammonia content.

Of the combined forms of nitrogen the most important are ammonia and nitrate. The reactant (ammonia) is not derived from a respiration process. Decomposition of organic matter results in release and accumulation of ammonia. Ultimate sources of ammonia include nitrogen fixation and assimilation in the aquatic and paralimnetic ecosystem components. Under aerobic conditions, ammonia is oxidized in a two step process called nitrification, first to nitrite, then to nitrate. Under anaerobic conditions nitrification of ammonia to nitrate does not occur, and ammonia accumulates often at the bottom of lakes. Much of the historic difficulty with quantifying total oxygen demand (and sizing of aeration systems) can be attributed to this "ammonia anomaly". Total oxygen demand includes respiratory demand and nonrespiratory demand (e.g. chemosynthesis).


Chlorophylla

Chlorophylla is considered the principal variable to use as a trophic state indicator. There is generally a good agreement between planktonic primary production and algal biomass, and algal biomass is an excellent trophic state indicator.

Furthermore, algal biomass is associated with the visible symptoms of eutrophication, and it is usually the cause of the practical problems resulting from eutrophication. Chlorophylla is relatively easy to measure compared to algal biomass.

One serious weakness of the use of Chlorophylla is the great variability of cellular chlorophyll content (0.1 to 9.7% of fresh algal weight)depending on algal species. A great variability in individual cases can be expected, either seasonally or on an annual basis due to a species composition, light conditions and nutrient (particularly nitrogen) availability.

Chlorophylla is to be measured within the euphotic zone. Simply, the euphotic zone is defined as the depth at which the light intensity of the photosynthetically active spectrum (400-700 nm) equals 1% of the subsurface light intensity. It is desirable to use a spherical quantum sensor (4π type). Where this information is not available, a Secchi disc reading in which Ze = 2.5xSecchi may be used (OECD 1982). In dystrophic lakes, use Ze = Secchi (Kerekes, pers. comm. 1991).


Secchi disk

Secchi disk (20 cm disc with alternate balck and white quadrants) transparency measurements is perhaps one of the oldest and simplest of all measurements. But there is grave danger of errors in such measurements where a water telescope is not utilized, as well as in the presence of water color and inorganic turbidity.

Planktonic primary production and Hypolimnetic Oxygen Depletion Rates

In addition to Chlorophylla, planktonic primary production and hypolimnetic oxygen depletion rates are desirable as trophic state indicators. In contrast to daily rates of primary production which have a very high short-term variability and are difficult to measure, hypolimnetic oxygen depletion has a low short-term variability and is relatively easy to measure.

However, the oxygen depletion measurements can be obtained in deep lakes only, which eliminates a large number of shallow lakes from consideration. Anaerobic hypolimnetic conditions caused by overfertilisation are one of the undesirable effects of eutrophication.

To avoid erroneous conclusions concerning trophic state, the precedent setting international OECD studies caution the following: lakes with high inputs of allochthonous organic matter or lakes where water color is over 10 pt. units, should not be used for oxygen deficit calculations.

In addition, only lakes with a well-defined thermocline (>1 °C/m) at the end of the summer stratification are to be considered, and the hypolimnium was defined as beginning downwards from the depth of the inflection point during the two months preceding the onset of the fall overturn. In addition, only lakes where the hypolimnetic to epilimnetic volume ratio is atleast 1.5 were considered.


Compensation Depth

Compensation Depth is the depth at which photosynthetic oxygen production by phytoplankton is balanced with respiratory demand for oxygen. The depth to which 1% incident PAR (photosynthetically active radiation) penetrates approximates the compensation depth in a eutrophic lake, which is the boundary between the trophogenic (above) and tropholytic (below) zones. Compensation depth can be estimated my multiplying Secchi disk depth by between 1.6 and 2.4 (Kortmann and Rich, 1994), depending on light attenuation in lake water due to color, dissolved organic matter, etc.

The ascent of Compensation Depth:

As eutrophication advances, transparency declines, and compensation depth ascends, which leads to process changes. When anoxia reaches the upper metalimnetic boundary, and the trophogenic zone becomes more shallow than the epilimnetic mixing depth, abrupt shifts in the phytoplankton community occur. Epilimnetic cyanobacteria become dominant.

Epilimnetic loading of bottom-generated constituents increases, and critical zooplankton refuge habitat is lost. A shift from metalimnetic communities (e.g. Oscillatoria sp. which can perform phototrophy, chemotrophy, and heterotrophy) to epilimnetic Cyanobacteria blooms (e.g. Anabaeba sp.) may occur as eutrophication advances.

As compensation depth ascends above the thermocline in a eutrophic lake, internal structure shifts from "control by diffusion" to "control by light penetration". Photosynthetic oxygen production occurs only in more shallow waters, nitrification and subsequent denitrification in deeper strata declines; ammonia accumulation intensifies.

As autochthonous production intensifies, the organic load to the detrital dynamic structure increases, favoring bactivory (e.g. by Bosmina sp.) over phytoplankton grazing (e.g. by Daphnia sp.). The shift in dominance from trophic to detrital components may become more pronounced due to a decline in suitable habitat for piscivorous fish, an overabundance of zooplanktivorous fish, and decline in grazer refuge habitat. Watershed nutrient loading affects the entire structure and function of the lake ecosystem, not simply increased primary production.



Toxic and potentially hazardous substances

While the aforementioned sections clarify the role of nutrients in eutrophication, it is recognized, however, that along with an increased trophic response, other harmful effects of certain substances are part of the overall problem of man-made (cultural) eutrophication. Some of these substances such as trace elements were always present in low quantities in aquatic systems supplied in the basic natural load, but with accelerated eutrophication, the increased amounts supplied, accumulated and recycled in the aquatic system cause problems.

Other substances, mainly organic compounds of an anthropogenic nature, originating from pesticides, paints and other chemicals, also enter into watercourses and add to the problem. These substances are usually found in very low concentrations in water but they can accumulate in animal tissues and persist in a water body.

Trace Elements:

Mercury, lead, arsenic, cadmium, selenium, copper, zinc, chromium, and vanadium could cause serious local problems near point sources of industrial releases. The additive and synergistic effects of the mixture of heavy metals can further increase the hazard to aquatic life.

Mercury and lead rank highest with respect to real or anticipated environmental hazard. Both of these elements can be converted by the process of methylation by microorganisms into methyl mercury and methyl lead, which are strong human nerve poisons.

Organic Compounds:

Organochlorine pesticides such as DDT, aldrin-dieldrin, chlordene, polychlorinated biphenyls (PCBs) are extremely persistent chemicals and have the ability to bioaccumulate. These substances are known to cause reproductive failure in fish-eating birds, either by failure of eggs to hatch or by the production of non-viable offspring.

Micro-organisms:

Pathogenic organisms can enter water systems from direct sewage discharge, sewer overflows and septic system failures. Depending on the size of the waterbody, they can cause health hazards in nearshore regions or they can affect the whole waterbody.



Effects of Climate Warming and Acid Deposition on Lakes

The effects of climatic change on freshwaters have been largely disregarded in major global change programs. Obviously, they must be included, because freshwaters are already scarce in many regions of the world, and they are a key element in the maintenance of nonmarine organisms, including man (Schindler et al., 1990). Algal communities of small lakes and ponds in alpine and arctic environments may be sensitive indicators of climatic variation (Vinebrooke and Leavitt, 1999).

Consequences of climate warming and lake acidification for UV-B penetration in North American boreal lakes:

In the study area in northwestern Ontario, both climate warming and lake acidification led to declines in the dissolved organic (DOC) content of lake waters, allowing increased penetration of solar radiation. Some of the changes in aquatic ecosystems that have been attributed to lake acidification may in fact have involved increased exposure to ultraviolet light. Moreover, it seems that-- particularly in clear, shallow lakes and streams-- climate warming and/or acidification can be more effective than stratospheric ozone depletion in increasing the exposure of aquatic organisms to biologically effective UV-B radiation (Schindler et al., 1996).

Dramatic changes have been shown to occur in aquatic communities exposed to realistic intensities of UV-B, and photoinhibition of phytoplankton can occur to depths of several metres. In most boreal lakes, DOC concentrations of several milligrams per litre are sufficient to provide an effective shield against ultraviolet radiation for aquatic organisms, restricting penetration of UV-B to a few decimetres. But, UV-B penetration increases exponentially as DOC declines. Two factors are responsible for the relationship: the proportion of colourless DOC produced in the lake increases in relative importance as DOC declines, and photobleaching and photodegradation of coloured DOC compounds increase as a function of residence time in the lake.

Depth of the 1% UV-B isopleth = 5.173(DOC)-0.706 - 1.029, r2 = 0.98 fitted to 18 lakes of boreal and northern Canada including lakes in ELA (1970-90) (Schindler et al., 1996)

Low-DOC lakes are not rare in the boreal zone of North America. In boreal regions of Ontario, lakes with less than 3.6 mg/l DOC are about 20% of the total. The number of low-DOC lakes is higher in Quebec, but lower in the maritime provinces. Overall, about 140,000 of the nearly 700,000 lakes in eastern Canada may have DOC concentrations low enough for UV-B penetration to be of concern. Low-DOC lakes are even more common in arctic, alpine and subalpine regions, where concentrations less than 1 mg/l are common. The highest concern must be for clear, shallow lakes, streams and ponds, where even modest declines in DOC may eliminate the small regions that are deep enough to provide refuges from damaging UV-B radiation. High altitude species of trout have been shown to suffer sunburn patterns, increased fungal infections and higher mortalities at environmentally realistic exposures of UV-B.

In clear oligotrophic lakes, the decreases in DOC caused by climate warming, drought and acidification should be of much more concern with respect to UV-B exposure than depletion of stratospheric ozone. In addition, the decline in DOC has other important effects. Increased penetration of total solar radiation causes thermocline deepening in small lakes. DOC is also important in chelation, flocculation and changes in mobility of trace metals and other chemicals (Schindler et al., 1996).

Acid deposition:

Acid deposition, caused by man-made emissions of oxides of sulphur and nitrogen, is probably the greatest threat of small boreal lakes in Canada and Eurasia. Although acidifying sulphur oxide emissions have been reduced by over 50% in Canada, and legislation is in place to compel similar reductions in the USA by early in the 21st century, these measures are estimated to have reduced the potential effect of acid precipitation on Canadian lakes by only about half Schindler et al., 1996).


The Influence of Ultraviolet Radiation on Alkaline Phosphatase (ELA):

Although ultraviolet radiation (UVR) has been shown to influence primary production, little is known about the influence of UVR on alkaline phosphatase, an extracellular enzyme that cleaves inorganic phosphorus from dissolved organic matter. The impact of UV-A and -B radiation was assessed on alkaline phosphatase activity (APA) in two boreal lakes of differing light and chemical characteristics at the Experimental Lakes Area. Both unfiltered water samples, and samples filtered through a 0.2 micron filter were measured for APA. Further analysis will be undertaken to correct samples for the level of biomass present in them, and assess the nutrient status of the organisms under differing radiation treatments.

Decreases of APA in the presence of ultraviolet radiation could increase P-stress in low nutrient aquatic environments.



Carbon

Storage of terrestrial carbon in boreal lake sediments (Molot and Dillon, 1996):

Seemingly independent human activities such as global warming, acidification, and ozone depletion are interconnected, although the linkages and effects are still only vaguely understood. Increasing acidification, climate change, and perhaps stratospheric ozone depletion may reduce biospheric carbon sinks by causing higher rates of lake evasion and lower storage rates in lake sediments.

The annual amount of CO2 produced by land use change and fossil fuel combustion is currently thought to be about 1 Pg (1 Pg = 1015 g) greater per year than the known atmospheric, terrestrial, and marine carbon sinks, although there is much uncertainty in the flux estimates.

As future disturbance could begin to release this stored carbon, it is important to understand not only the magnitude of current carbon fluxes and pools but also how the fluxes and pool sizes are regulated. The study of carbon fluxes is also salient because of the role of dissolved organic carbon in regulating water quality and light transparency lakes.

Long term (June 1980 to May 1992) average DOC stream export from forested catchments ranged ninefold from 1.0 to 9.1 g C m-2 yr-1. DOC export was highly correlated with the percent area covered with peat (r = 0.88), DOC = 2.39 + 0.26 %Peat

TC (DOC + DIC) = 1.25 (2.39 + 0.26 %Peat), since particulate organic carbon export was negligible in the Dorset streams.

The partitioning of retained carbon between the sediments and the atmosphere appeared to be a function of lake alkalinity with,

Evaded/sediment C = 2.29 - 1.43 ln (alkalinity), where alkalinity is measured by Gran titration (in µeq/liter).

The increase in evasion with lower alkalinity may be due, in part, to lower equilibrium DIC levels, however, another mechanism is clearly involved. The increase in evasion with lower alkalinity is also inconsistent with the conventional wisdom that DOC precipitation is enhanced in acidified lakes, perhaps by complexation with Al. While lake acidity is known to result in lower DOC levels (e.g., when pH is less than 5) it appears that lake acidification may enhance oxidation of DOC more so than chemical precipitation.

Photolytic regulation of DOC (Molot and Dillon, 1997):

It was concluded that photodecay was potentially large enough in situ to account for all of the DOC losses to the atmosphere and sediments in the low DOC lakes (< 4 mg L-1) but could not account for all of the DOC lost in the high DOC lakes (> 4 mg L-1).

Several in-lake mechanisms may be responsible for conversion of stream DOC to DIC and particulate organic C (POC). Coagulation/flocculation will account for removal of some DOC to bottom sediments particularly in lakes that have been acidified by atmospheric deposition of S and N oxides with concurrent elevation of aluminum levels, or in lakes with high ionic strength; however, the mechanism alone cannot account for C losses to the atmosphere. It has been suggested in literature that organic rich sediments can absorb high molecular weight DOC and release low molecular weight (LMW) DOC compounds, depending on the type of sediment and pH. Other plausible removal mechanisms are DOC oxidation by heterotrophic microbes and photolytic decomposition (i.e., photodecay). Heterotrophic production in lakes sometimes exceeds primary production, a phenomenon which requires inputs of carbon/energy in a form other than primary production.

Bacteria can metabolize humic substances although the consumption rate is likely quite small because of the refractory nature of much of the DOC. There is substantial evidence, however, that DOC becomes less biologically refractory, that is, more biologically available, after exposure to solar UV radiation (UVB 280-320 nm, UVA 320-400 nm).

Visible light (400-700 nm) is not normally considered to be important in photodecay. Hence there is growing evidence of a connection between UV radiation, bacterial consumption, and the regulation of DOC levels in aquatic systems.

Absorption of light by humic substances (humic and fulvic acids), referred to as `color', is often used as an index of DOC by limnologists. Though color is a poor surrogate for total DOC during short-term experiments. However, mean color is a good surrogate for mean DOC only when time periods exceed one year.



Impact of Reservoir Creation (ELA)


Impact of Reservoir Creation on Greenhouse Gas Fluxes from Forested Uplands:

Reservoirs created for hydroelectric power have recently been identified as sources of greenhouse gases (GHG), including methane (CH4) and carbon dioxide (CO2 ), to the atmosphere. Following flooding plants die and stop taking up atmospheric CO2 via photosynthesis. In addition, bacteria mineralize carbon stored in plants and soils to CO2 and CH4, which then flux to the atmosphere. The long-term impact of reservoir creation on GHG emissions should be related to the amount of organic carbon stored in ecosystems prior to flooding. In the northern Boreal landscape, where many Canadian reservoirs are developed, carbon stores range from large in peatlands to small in pockets of ridge-top forests.


Effects of Reservoir Creation on Mercury Methylation Rates:

The Upland Flooding Experiment has been designed to test the hypothesis that ethylmercury (MeHg) and greenhouse gas production in reservoirs is related to the amount of carbon stored in the reservoir. The three sites have been chosen to represent three different types of upland forests; namely, a moist forest (Site 1: jack pine stands, with Sphagnum and Ledum), a dry forest (Site 2: thick jack pine stands with some birch and alder), and a very dry forest (Site 3: jack pine stands with exposed bedrock outcrops). Each of these sites has different amounts of organic matter stored in the vegetation and soils. The specific research objective is to determine, using a whole ecosystem mass balance approach, if Hg methylation rates increase, and therefore lead to increased MeHg concentrations in fish.


Flooded Upland Dynamics Experiment (FLUDEX):

The purpose of the upland flooding experiment is to study the greenhouse gas and mercury impacts of flooding forested upland areas. Three forested uplands, a moist forest and two dry forested areas located in the watershed of Roddy Lake, were experimentally flooded, beginning in June 1999, to create experimental hydroelectric reservoirs. Greenhouse gas fluxes before and during flooding were measured at all three sites. Carbon dioxide, methane and nitrous oxide were monitored. Fluxes will be compared to the previously flooded boreal wetland (ELARP) and to existing hydroelectric reservoirs to determine the potential greenhouse gas contribution of global, freshwater reservoirs. The production of methyl mercury from flooded soils and the bioaccumulation of methyl mercury through the food chain were measured in the experimental reservoirs. Mitigation strategies with direct planning application will be developed.



Prologue

As has been succinctly described by Vallentyne (1974), a common result of misuse of the drainage basin and excessive nutrient loading of fresh waters is an accelerated eutrophication; our lakes are literally turning into "algal bowls". It has to be emphasized that the metabolism of all aquatic systems, and indeed of a major portion of the biosphere is dominated by detrital metabolism. Accelerated eutrophication leads to accelerated pelagial and littoral primary productivity with progressive intensification of detrital metabolism, effectively relagating lakes to "detrital bowls" in an operational sense. Metabolically mediated changes in the environment leading to strata of prolonged anoxia and attendant reductions in catabolism of detrital organic matter result in decreased efficiencies of utilization and degradation of organic matter.

A conscientious individual must view these changes in his natural environment with concern. As the exploitative pressures of demophoric growth increase, man's concern must involve more than simply his aesthetic values and those of future generations of humans. The very survival of man centers on the wise utilization of finite freshwater resources; to think otherwise is naive and myopic.


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