Principles of terrestrial ecosystem ecology.pdf

240 10. Aquatic Carbon and Nutrient Cycling
ability. As headwater streams merge to form
broader streams, the greater light availability
supports more in-stream production, and the
input of terrestrial detritus contributes proportionately
less to stream energetics. This
coincides with a change in the invertebrate
community from one dominated by shredders
to one dominated by collectors and grazers
(Fig. 10.3). These middle reaches of rivers are
typically less steep than headwaters and begin
to accumulate sediments from upstream
erosion. These sediments support rooted vascular
plants and a benthic detrital community of
collectors. The largest downstream reaches of
rivers typically have a sediment bed and are
dominated by collectors that live in the sediments.
These large rivers may have submerged
or emergent vascular plants, depending on the
stability of the flow regime. There is a gradual
increase in fish diversity from headwater
streams to large rivers, whereas the diversity of
benthic invertebrates is generally greatest in
middle reaches of rivers (Poff et al. 2001).
There is massive variation among streams
and rivers in their structure and functioning,
just as in terrestrial and marine ecosystems.The
river continuum concept provides a framework
for predicting patterns of variation within a
region but does not capture the large variation
due to substrate and climate. Nutrient-poor
regions of the tropics and boreal peatlands, for
example, have large inputs of dissolved organic
carbon (DOC) leading to black-water rivers.
White-water rivers result from inputs of silt
and other mineral particulates from glacial and
agricultural erosion. Clear-water rivers lack
these dissolved and suspended materials. Island
streams are much shorter than the large rivers
that drain the interiors of continents. Lakes and
impoundments on rivers create abrupt shifts in
habitats, food resources, and biota, punctuating
the gradual changes of the river continuum
(Ward and Stanford 1983).
Carbon and Nutrient Cycling
Stream productivity is governed by its interface
with terrestrial ecosystems (Hynes 1975).
Terrestrial ecosystems influence stream productivity
directly through the input of detritus
that fuels the detritus-based food chain and
indirectly by determining the light environment
that supports in-stream production. In forest
headwater streams, the dominant energy input
is terrestrial detritus that enters as coarse
particulate organic matter (CPOM) (Fig. 10.10).
This includes leaves, wood, and other material
larger than 1mm diameter. In forests, there is
relatively little algal production in headwater
streams because of low light availability.
Algal production becomes proportionately more
important in ecosystems with low canopy cover,
such as in grasslands, tundras, and deserts. Fine
particulate organic matter (FPOM) comes primarily
from within the stream through the processing
of CPOM by shredders, the abrasion of
periphyton from rocks, and other processes.
About a third of the leaf material consumed
by shredders, for example, is released into the
stream as FPOM (Giller and Malmqvist
1998). The third major organic carbon input to
streams comes as dissolved organic carbon
from terrestrial groundwater. DOC is the
largest pool of organic carbon in most streams.
In tropical black-water rivers and boreal peatlands,
this carbon source to streams is particularly
large and/or persistent. DOC inputs to
streams can be an important energy source if
the compounds are readily assimilated and
metabolized by microbes. Tannins and other
recalcitrant substances, however, are processed
slowly in streams. Headwater streams are dominated
by a detritus-based food chain, including
fungi, shredders, and their predators. Heterotrophic
respiration therefore considerably
exceeds photosynthesis. Downstream, where
rivers are wide enough to allow substantial
light input, photosynthesis may be similar to
or exceed heterotrophic respiration (Vannote
et al. 1980). In these middle sections of rivers,
heterotrophic respiration is supported by a
mixture of FPOM imported from upstream,
terrestrial inputs of CPOM (litter), DOC, and
algal production. In large rivers with large sediment
loads, water clarity may limit algal production,
and detrital processing again dominates.
The frequency and magnitude of nutrient
limitation to algal production in streams and
rivers are more variable than in lakes (Newbold
1992). Many streams, particularly headwater