Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 468

COMPARISON OF FRESHWATER ECOSYSTEMS

All the generalizations presented in previous chapters can be used to
classify ecosystems. The difficult issue is how to weight the importance of
the various factors. Keep in mind that classification is mainly a tool for sci-
entists to deal with a very complex world. Primary abiotic differences are
associated with hydrologic throughput, the availability of light, the amount
of allochthonous input, and the extent of benthic habitat (depth). If we view
each of these factors as gradients, where most combinations are possible, in
addition to variance associated with each of the factors over spatial and
temporal scales, we can describe many essential characteristics of aquatic
habitats (Fig. 22.14). Classification of abiotic parameters can provide in-
formation about constraints on evolutionary and physiological processes.
For example, a forested temperate headwater stream can be characterized
as being highly variable (hydrologically and with seasons), receiving low
light, and having a high degree of terrestrial influence (including allochtho-
nous carbon input). In contrast, deep groundwaters are highly stable, re-
ceive no light, receive low amounts of
allochthonous input and even less au-
tochthonous input, and have very low
hydrological throughput. The river
continuum concept is an excellent ex-
ample of a conceptual model that uti-
lizes classifications of gradients and
spatial linkages to make specific pre-
dictions about ecosystem function and
community structure.
Of course, problems arise when
few abiotic gradients are used to clas-
sify ecosystems. Very shallow lakes
may have very little emergent vegeta-
tion or macrophyte production but
high cyanobacterial biomass. Wind,
unstable benthic substrata, high graz-
ing, and high nutrients all lead to these
blooms. The complexity of natural
systems precludes our ability to com-
pletely classify and make predictions
about their processes. Perhaps the
most interesting ecosystems are those
that are the most difficult to classify.
Abiotic gradients lead to biotic
differences in the food webs of vari-
ous freshwater habitats (Fig. 22.15).
Simple planktonic food webs charac-
terize the pelagic zone of lakes and
simple food webs based on consump-
tion of biofilms characterize ground-
waters. Shallow benthic habitats with
heterogeneous substrata (wetlands,

(Pteronura brasiliensis). Numerous waterbirds
utilize the wetland, as does a diverse assem-
blage of parrots. The Pantanal also has more
than 400 species of fish and attracts large num-
bers of sport fisherman to the mostly unregu-
lated fishery.
The greatest threats from development are
channelization and wetland modification that
could lower water levels. A major project to in-
crease the navigability of the Paraguay River
is being considered. The dredging and chan-
nelization project will cost approximately $1
billion, and economic assessments have not
carefully considered ecological impacts and
the altered hydrology with associated flooding
down river (Gottgens et al., 1998). With only a
0.25-m decrease in water level, the inundated
area of the wetland will be decreased by more
than half in the upper regions of the wetland
and 5790 km 2 overall (Hamilton, 1999). The proj-
ect will negatively impact the livelihoods of
thousands of indigenous people that inhabit
the region. Political pressure has caused a de-
crease in the scale of the plans, but a com-
prehensive development plan is lacking, and
dredging and channelization plans continue. If
past cases of exploitation of wetlands are any
indication (e.g., the Everglades), the Pantanal
ecosystem will suffer greatly as humans de-
velop the area (Gottgens et al., 1998).