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A Brief Treatise on Eutrophication
Soil & Water Conservation Society of Metro Halifax (SWCSMH)
Updated: August 30,
2020
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.
"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.
- Trophic classification is most commonly performed using parameters which reflect pelagic phytoplanktonic autotrophy (total phosphorus [TP], Chlorophylla [Cha],
Secchi [SD]). In lakes dominated by paralimnetic or littoral organic
sources, the TSI will be low because autochthonous (pelagic,
phytoplanktonic) production is low, e.g. dystrophic lakes.
- On the other hand, respiration-based trophic indices include littoral production as autotrophy. The anoxic factor, AF (Nurnberg 1984) relates
areal oxygen deficit to lake surface area (facilitating comparison to
areal autochthonous productivity). Hutchinson (1957) presented a trophic
classification scheme based on areal oxygen deficit rate. Kortmann et al., (1988) used the scheme by Hutchinson
(1957), converting areal dissolved inorganic carbon (DIC) increment
rate to oxygen deficit equivalents. Use of these respiration-based
approaches almost always indicates a higher degree of biological
activity (trophy) than indices based on planktonic productivity
parameters (TP, Cha, SD).
- Oligo-Eutro classification scheme: 1, 2, 3
- Oligotrophic lakes are poorly supplied with plant nutrients and
support little plant growth. As a result, biological productivity is
generally low, the waters are clear, and the deepest layers are well
supplied with oxygen throughout the year.
- Mesotrophic lakes are intermediate in characteristics. They
are moderately well supplied with plant nutrients and support moderate
plant growth.
- Eutrophic lakes are richly supplied with plant nutrients and
support heavy plant growths. As a result, biological productivity is
generally high, the waters are turbid because of dense growths of
phytoplankton, or contain an abundance of rooted aquatic plants;
deepest waters exhibit reduced concentrations of dissolved oxygen
during periods of restricted circulation.
- The boundary categories of the above are ultraoligotrophy and hypereutrophy.
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 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 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).
- Phosphorus mobilization and transport:
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.
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 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).
- Caution: The relationship between
phosphorus and algal chlorophyll (i.e. algal biomass) is a log-log plot
of the data, and there is considerable scatter to the linear data,
indicating effects from other factors in the pelagic environment, such
as light, nitrogen, or zooplankton grazing in limiting biomass. This
indicates that there can be wide variation in the expected chlorophyll
from any given
phosphorus concentration.
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.
- Measurement of DIC increment yields a more comprehensive
estimate of total hypolimnetic respiration than oxygen consumption rate.
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.
- Preventing spatial separation between
the trophogenic boundary and epilimnetic-metalimnetic interface (mixing
depth) is critical to managing trophic quality (especially buoyancy
controlled Cyanobacteria), and can be achieved by reducing
autochthonous production by reducing nutrient influx and/or by in-lake methods.
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).
- Twenty years of climatic, hydrologic,
and ecological records for the Experimental Lakes Area of northwestern
Ontario show that air and lake temperatures have increased by 2°C and
the length of the ice-free season has increased by 3 weeks (this trend
primarily reflects earlier ice-out dates in the spring). Higher than
normal evaporation and lower than average precipitation have decreased
rates of water renewal in lakes. Concentrations of most chemicals have
increased in both lakes and streams because of decreased water renewal
and forest fires in the catchments. In Lake 239, populations and
diversity of phytoplankton also increased, but primary production
showed no consistent trend. Increased wind velocities, increased
transparency, and increased exposure to wind of lakes in burned
catchments caused thermoclines to deepen. As a result, summer habitats
for cold stenothermic (tolerant of a narrow range in temperatures)
organisms like lake trout and opposum shrimp decreased (Schindler et al., 1990).
- A number of cold
stenothermic glacial relicts are unable to survive in lakes that are
too shallow to have cold, well-oxygenated hypolimnions. Climatic
warming would certainly shift northward the southern boundary for the
occurrence of these species in stratified lakes. It would also
extirpate them from small lakes where deepening of the thermocline
would destroy cold, oxygen-rich hypolimnions. Even though the
extirpated fauna might be replaced by warm-water assemblages, it is by
no means certain that fisheries of comparable value or ecosystems of
comparable diversity would be reestablished quickly.
Algal communities of small lakes and ponds in alpine and arctic environments may be sensitive indicators of climatic variation (Vinebrooke and Leavitt, 1999).
- For instance, warmer climates can cause
stained, unproductive lakes to become more chemically concentrated and
translucent, thereby altering resource availability and exposure to
damaging ultraviolet (UV) radiation.
- Recently, climatic warming
has increased conductivity and altered diatom assemblages in European
alpine lakes because of enhanced catchment weathering rates.
- Similarly, fossil algal
pigment stratigraphies in Canadian Rocky Mountain lakes suggest that
periods of drought affect phytobenthos by reducing dissolved organic
carbon (DOC) concentrations and increasing exposure to high ambient UV
irradiance.
- Further, experimental
evidence has suggested that phytobenthos, rather than phytoplankton,
are responsive to changes in allochthonous DOC, inorganic nutrients,
and UV exposure in alpine littoral habitats.
- Epilithon (rock-attached algae) appears
better suited than either phytoplankton or epipelon (sediment-dwelling
algae) as a bioindicator of climatically induced variations in the
abiotic environments of shallow mountain lakes and ponds.
- Abundance of the epilithon was
negatively correlated to lake elevation, and positively correlated to
conductivity and DOC content. Redundancy analysis (RDA) showed that
elevation, conductivity, and DOC were also significant predictors of
epilithon community composition.
- Epilithic diatoms (diatoxanthin,
diadinoxanthin, fucoxanthin) declined disproportionately with
increasing water transparency and decreasing chemical concentrations.
- In contrast, epipelon abundance and community composition were not well-explained.
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.
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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