5.7.3.3. Biogeochemical Cycling and Pollution
Understanding of interactions of climate change with other human-caused pressures
on lakes and streams is still in its infancy. No simulation models have been
developed to assess the combined effects of these pressures on these systems.
5.7.3.3.1. Eutrophication
Figure 5-8: Diagram of complex interactions between climate change,
watershed and lake processes, and water clarity of a eutrophic lake [modified
from Lathrop (1998) and information in Lathrop et al. (1999)]. The left
panel diagrams a warmer, dryer climate with less wind; the right panel diagrams
a warmer, windier, and wetter climate with more extreme rain events. In
both cases, the altered climate would be expected to change the water quality
of the lake, but the complexity of relations leads to uncertain future water
clarities. A "+" means an increase and a "-" means a
decrease in the condition or process; a "?" means conflicting
expectations. Greater blooms of phytoplankton lead to lower water clarity,
and reduced blooms lead to greater clarity. Blooms depend on external and
internal loading of phosphorus (P). Dryer climates lead to less external
loading, whereas wetter climates or more episodic rains lead to more external
loading. Warmer climates lead to warmer surface waters and increased vertical
stability in the water column, thus less mixing and internal loading. Warmer
waters also allow potential invasion by warmer loving, zooplaktivorous fish
that can reduce zooplankton species that in turn reduce algal populations.
Windier climates result in increased vertical mixing, thus greater internal
loading. |
Nutrient cycling would be altered by climate change in
ways that could exacerbate existing water quality problems such as eutrophication
(see Figure 5-8). Eutrophication of lakes results when
nutrient inputs from catchments and recycling from bottom sediments are large.
The result is excessive production of algae; blue-green algae reduce water quality
for recreation and drinking. Deep coldwater habitats become anoxic, owing to
greater rates of decomposition of sinking organic mater (Horne and Goldman,
1994).
Interaction between climate change and eutrophication is complex, and projections
are somewhat contradictory (see Figure 5-8) because climate-influenced
processes have interacting and often opposing effects (Magnuson et al., 1997;
Schindler, 1997). Consider, for example, phosphate release from anoxic sediments.
In a warmer climate, the longer period of summer stratification would increase
the likelihood that anoxia develops below the thermocline (Stefan and Fang,
1993); this would increase the solubility of phosphates in sediment and increase
nutrient recycling. At the same time, warmer climates would reduce the duration
of ice cover in lakes, which would reduce winter anoxia and decrease sediment
phosphate release in winter. This is further complicated by water column stability.
In Lake Mendota, Wisconsin (Lathrop et al., 1999), it is not surprising that
one-third of observed year-to-year variation in summer water clarity is associated
with variability in runoff (see Figure 5-8); more nutrient
input leads to higher populations of phytoplankton, which reduce water clarity.
Runoff would be influenced by differences in precipitation and the frequency
of extreme rainfall events during autumn, winter, and spring. Precipitation
trends differ around the world, and there is evidence for increased frequency
of extreme rainfall events that may occur in different seasons of the year (see
Chapter 4). Wetter climates or climates with more extreme
rainfall events would increase export of nutrients and sediment to lakes and
streams; dryer climates or those with more even rainfall would reduce export
to lakes and streams. Extreme rainfall events would export more if they occurred
at seasons when the earth was bare in agricultural watersheds.
In Lake Mendota, the other two-thirds of the variation in water clarity also
is related to climate (see Figure 5-8). One-third is through
climatic influences on vertical mixing, where warmer summers lead to greater
water column stability, less recycling from deep water, and greater water clarity.
The remaining one-third is related to the abundance of herbivorous zooplankton
that eat phytoplankton. A warming climate would allow invasion of new species
of fish that forage on the herbivorous zooplankton; in Lake Mendota, it probably
would be the gizzard shad that lives in reservoirs south of Wisconsin (see Dettmers
and Stein, 1996).
Several other complexities would lead to different results, depending on the
change in precipitation patterns. For example, in a dryer climate, increases
in water residence time (Schindler et al., 1996a) would increase the importance
of nutrient recycling within lakes, and storage of nutrients in the sediments
of lakes would be reduced (Hauer et al., 1997). This is complicated by changes
in light penetration that occur if dissolved organic carbon (DOC) inputs are
reduced (Magnuson et al., 1997; Schindler, 1997). DOC can reduce light penetration,
so a reduction in DOC input would increase light availability and could lead
to increases in primary production in deeper water. This influence would depend
on whether climate becomes dryer or wetter.
Several empirical and simulation studies support the idea of increased eutrophication
with climate change. Results of these studies contradict the expectations of
reductions in nutrient loading from catchments in drier climates and greater
stability of the water column in warmer climates.
Empirical relations (Regier et al., 1990; Lin and Regier, 1995) suggest that
annual primary production by phytoplankton, zooplankton biomass, and sustained
yields of fisheries all increase with temperature. Simulations for Lake Erie
(Blumberg and Di Toro, 1990) and smaller lakes (Stefan and Fang, 1993) indicate
that climate change leads to more eutrophic conditions with respect to loss
of oxygen beneath the thermocline in summer. Ogutu-Ohwayo et al. (1997) suggest
that recent changes in the regional climate of Lake Victoria in Africa may have
reduced physical mixing and contributed to increases in deepwater anoxia and
thus nutrient recycling contributing to eutrophication.
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