Principles of terrestrial ecosystem ecology.pdf

238 10. Aquatic Carbon and Nutrient Cycling
erotrophs. This situation is particularly severe
in winter, when low temperature limits oxygen
production from photosynthesis. In ice-covered
lakes, ice and snow reduce light inputs that
drive photosynthesis (providing oxygen) and
prevent the surface mixing of oxygen into the
lake. Lakes in which the entire water column
becomes anaerobic during winter do not
support fish. Even during summer, the accumulation
of algal detritus at times of low surface
mixing can deplete oxygen from the water
column, leading to high fish mortality.
Carbon and Nutrient Cycling
Carbon and nutrient cycling processes in lakes
are similar to those described for the ocean.
Phytoplankton account for most primary production
in large lakes. As in the ocean, most
phytoplankton are grazed by zooplankton, so
phytoplankton biomass is relatively low. Lakes
without fish have large-bodied zooplankton
like Daphnia that are efficient feeders on
plankton and suspended organic matter. When
fish are present, however, they reduce populations
of large-bodied zooplankton and probably
reduce the efficiency of energy transfer up
the food chain (Brooks and Dodson 1965). As
in the ocean, bacterial production is substantial
in lakes, and the bacteria are an important component
of the pelagic food web. Terrestrial dissolved
and particulate organic matter is an
important substrate for bacterial production in
some lakes. Some of this terrestrial organic
matter may be more recalcitrant than the bacterial
substrates in the oceans and may be consumed
more slowly by bacteria or accumulate
in sediments. Decomposition in the sediments
tends to be more important in lakes than in
the ocean because more detritus reaches the
bottom before it decomposes, so there is often
a well-developed benthic food web similar to
that in coastal sediments. Some of the nutrients
released by decomposition return to the water
column and are mixed to the surface by wave
action and lake turnover.
The species composition of lakes strongly
influences their physical properties and biogeochemistry.
Inadvertent experiments in which
fishermen or management agencies have intro-
duced fish or benthic organisms to lakes have
provided a wealth of evidence that species
traits strongly affect the functioning of aquatic
ecosystems (Spencer et al. 1991). In many
lakes, the abundance of a top predator alters
the abundance of their prey and indirectly the
abundance of phytoplankton (see Chapter 11).
Changes in benthic fauna can have equally
large impacts. Introduction of the zebra mussel
to the United States, for example, has displaced
native mussels from many rivers and streams.
The zebra mussel is a more effective filter
feeder than their native counterparts, filtering
from 10 to 100% of the water column per day
(Strayer et al. 1999). The resulting decrease in
density of phytoplankton and other edible
particles reduced zooplankton abundance and
shifted energy flow from the water column to
the sediments.
Streams and Rivers
The structure of stream and river ecosystems
depends on stream width and flow rate. The
physical environment and therefore the biotic
structure of stream ecosystems are dramatically
different from those of lakes or the open ocean.
Water is constantly moving downstream across
the riverbed, bringing in new material from
upstream and sweeping away anything that is
not attached to the substrate or able to swim
vigorously. Phytoplankton are therefore unimportant
in streams, except in slow-moving or
polluted rivers.The major primary producers of
rapidly moving streams are periphyton, algae
that attach to stable surfaces such as rocks and
vascular plants. The slippery surfaces of rocks
in a riverbed consist of periphyton and associated
bacteria in a polysaccharide matrix.
Submerged or emergent vascular plants and
benthic mats become relatively more important
in slow-moving stretches of the river. Within a
given stretch of river, alternating pools and
riffles differ in flow rate and ecosystem structure.
Seasonal changes in discharge often
radically alter the flow regime and therefore
structure of these ecosystems. Desert streams,
for example, have flash floods after intense
rains but may have no surface flow during dry