Principles of terrestrial ecosystem ecology.pdf

236 10. Aquatic Carbon and Nutrient Cycling
The biological pump that transports carbon
to depth carries with it the nutrients contained
in dead organic matter. Decomposition continues
as particles sink, so much of the decomposition
occurs in the water column rather than in
the sediments, particularly in the deep oceans.
The rapid (about weekly) turnover of carbon
and nutrients in phytoplankton in the euphotic
zone (Falkowski et al. 1999) makes these nutrients
vulnerable to loss from the ecosystem and
contributes to the relatively open nutrient
cycles of pelagic ecosystems. The longer-lived
and larger primary producers on land can store
and internally recycle nutrients for years. This
reduces the proportion of nutrients that are
annually cycled and contributes to the tightness
of terrestrial nutrient cycles.
Benthic decomposition is more important
on continental shelves than in the deep ocean
because the coastal pelagic system is more productive,
generating more detritus. In addition,
the dead organic matter has less time to decompose
before it reaches the sediments. Here
oxygen consumption by decomposers depletes
the oxygen enough that decomposition
becomes oxygen limited, and organic matter
accumulates or becomes a carbon source for
methanogens and denitrifiers.
Lakes
Lakes consist of a range of ecosystem types,
from pelagic systems to wetlands dominated by
vascular plants (Wetzel 2001). The centers of
deep lakes are structurally similar to marine
ecosystems with discrete pelagic and benthic
systems; phytoplankton are the major primary
producers, and zooplankton are the major herbivores.
The littoral zone of lakes generally
experiences less disturbance from wave action
and currents than in marine systems.This allows
mats of algae to grow directly on lake sediments,
even where light is only 0.1% of that
present at the surface. The littoral zone of lakes
often has rooted vascular plants, whose leaves
extend above the water surface and shade the
water, reducing the light available to phytoplankton
and benthic algae. Floating aquatic
plants like water lilies cause a similar reduction
in light availability in the water column. Many
lakeshore ecosystems and salt marshes, their
marine equivalent, are structurally and functionally
similar to terrestrial wetland ecosystems.
There is therefore a continuum between
the structural and functional properties of
aquatic and terrestrial ecosystems.
The origin of lakes strongly affects their
structure and functioning (Lodge 2001). Glacial
lakes are abundant in young landscapes at high
latitudes and altitudes. They are frequently
interconnected with other lakes by short stream
segments and have a low degree of endemism.
Rivers create lakes in several ways, including
isolation of former river channels (oxbow
lakes) and periodically inundated swamps and
floodplains such as the Pantanal of Bolivia and
Brazil. These lakes are generally shallow and
occasionally reconnect with adjacent rivers
during floods. Tectonic lakes form along faults.
They are often large, deep, and isolated from
one another, providing an environment for substantial
diversification, such as in the rift lakes
in eastern Africa and Lake Baikal in Siberia.
These large tectonically derived lakes harbor
most of the endemic freshwater organisms.
Over 80% of the open water animals of Lake
Baikal, for example, are endemic (Burgis and
Morris 1987). Other lakes form in volcanic
craters, by damming of rivers, and other
processes.
Controls over NPP
Photosynthesis in fresh-water ecosystems is
seldom carbon limited, just as in the ocean.
Groundwater entering fresh-water ecosystems
is supersaturated with CO2 derived from root
and microbial respiration in soil (Kling et al.
1991). Most streams, rivers, and oligotrophic
lakes are net sources of CO2 to the atmosphere
because the rates of water and CO2 input from
groundwater generally exceed the capacity of
primary producers to use the CO2 (Cole et al.
1994, Hope et al. 1994). Eutrophic lakes with
their high algal biomass have a greater demand
for CO2 to support photosynthesis than do oligotrophic
systems, but their organic accumulation
and high decomposition rate in sediments
provide a large CO2 input to the water column.