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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
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.
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).
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).
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.
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.
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.
Respiration
"Life is energy", all living organisms burn organic matter in a slow, controlled way.
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.
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".
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.
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.
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).
Two different mechanisms have to occur simultaneously or within a short space of time. Firstly, P bound to particles or aggregates in the sediment must be mobilized by being transferred to the pool of dissolved P (primarily phosphate) in the pore water. Secondly, processes which transport the dissolved phosphorus to the lake water must function. Important mobilization processes are desorption, dissolution, ligand exchange mechanisms, and enzymatic hydrolysis. These processes are affected by a number of environmental factors, of which redox potential, pH and temperature are the most important. Essential transport mechanisms are diffusion, wind-induced turbulence, bioturbation, and gas convection.
Redox-controlled dissolution and diffusion are considered as the dominant mechanisms for P release from stagnant hypolimnetic bottom areas. All the mobilization and transport processes can theoretically contribute to the overall P release from sediments in shallow lakes. At high temperatures microanaerobic zones are formed very rapidly, and redox-controlled liberation of phosphate can occur to well-aerated water. Wind-induced turbulences often have a dominating role among the transport processes.
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).
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).
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.
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.
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.
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.
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).
Decreases of APA in the presence of ultraviolet radiation could increase P-stress in low nutrient aquatic environments.
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.
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.
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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