Principles of terrestrial ecosystem ecology.pdf

This creates a strong vertical gradient in CO2,
with CO2 concentration being drawn down at
the surface, leading to CO2 absorption from the
atmosphere during the day (Carpenter et al.
2001). Some fresh-water vascular plants such
as Isoetes use crassulacean acid metabolism
(CAM) photosynthesis to acquire CO2 at night
and refix it by photosynthesis during the day
(Keeley 1990). Other fresh-water vascular
plants transport CO2 from the roots to the
canopy to supplement CO2 supplied from the
water column.
Vertical mixing is important in lakes, just
as in marine pelagic ecosystems. Lake mixing
occurs not only by wave action, as in the ocean,
but also by lake turnover in most temperate and
high-latitude lakes (Wetzel 2001). In autumn,
the surface waters cool to 4°C, the temperature
at which water is most dense. Once the surface
waters cool to the point that water temperature
is similar from top to bottom, the water column
is readily mixed by wind. This causes surface
waters to sink and brings nutrient-rich bottom
waters to the surface. Turnover also occurs in
spring, when surface waters warm to 4°C,
leading to a spring bloom in production. When
lakes do not turn over, oxygen becomes
depleted at depth, leading to greater prevalence
of anaerobic conditions.Warm-climate lakes do
not experience this seasonal lake turnover if
the surface waters remain much warmer and
less dense than deep water throughout the year.
Nutrients, rather than light, water, or CO2,
are the resources that most consistently limit
the productivity of aquatic ecosystems. Both
N:P ratios in algae and experimental nutrient
additions show that phosphorus limits algal
production in the majority of unpolluted lakes,
whereas nitrogen is the most common limiting
element in coastal marine and salt marsh
ecosystems (Fig. 10.7) (Schindler 1977, Valiela
1995). Why should nitrogen be the limiting
element in temperate terrestrial ecosystems but
phosphorus the limiting element in lakes that
are embedded within this terrestrial matrix? At
least two factors resolve this apparent paradox.
The low mobility of phosphorus compared to
nitrogen in soils retains phosphorus more effectively
than nitrogen in terrestrial systems. In
addition, lakes that receive large phosphorus
Lakes 237
inputs from pollution or other sources generally
support the growth of nitrogen-fixing
phytoplankton, such as cyanobacteria. These
nitrogen fixers have a competitive advantage
over nonfixers when nitrogen is scarce and
phosphorus is available. Lakes therefore add
their own nitrogen, whenever the phosphorus is
sufficient to support nitrogen fixation. Nitrogen
fixation in the surface water of lakes is seldom
limited by light, as it may be in terrestrial and
some stream ecosystems. For these reasons,
lakes are seldom nitrogen limited. Nitrate concentrations
are typically an order of magnitude
higher in lake than in ocean water (Valiela
1995), again indicating the generally greater
availability of nitrogen than of phosphorus in
lakes.
Nutrient inputs to lakes from streams,
groundwater, and atmospheric deposition
strongly influence lake biogeochemistry. Lakes
are generally small aquatic patches in a terrestrial
matrix; they are therefore strongly
influenced by inputs of macronutrients and
base cations from groundwater and streams
(Schindler 1978). The granitic bedrock of the
Canadian Shield, from which soils were
removed by continental glaciers during the
Pleistocene, for example, have low rates of
nutrient input from watersheds to lakes. The
strong nutrient limitation of many of these
lakes makes them vulnerable to change in
response to nutrient inputs from agriculture or
acid rain (Driscoll et al. 2001). Trout and other
top predators in oligotrophic lakes may require
decades to reach a large size, whereas this may
occur in a few months or years in eutrophic
lakes.
Anthropogenic addition of nutrients to lakes
frequently causes eutrophication, a nutrientinduced
increase in lake productivity. Eutrophication
radically alters ecosystem structure and
functioning. Increased algal biomass reduces
water clarity, thereby reducing the depth of the
euphotic zone. This in turn reduces the oxygen
available at depth. The increased productivity
also increases the demand for oxygen to
support the decomposition of the large detrital
inputs. If mixing is insufficient to provide
oxygen at depth, the deeper waters no longer
support fish and other oxygen-requiring het-