Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 453

Mooney, 1994). The first problem is how to define ecosystem function.
Function can refer to rates of basic processes, such as photosynthesis, res-
piration, denitrification, or phosphorus retention. It can also refer to more
specific things, such as production of plant biomass for herbivores. The
crux of this issue is if species are functionally redundant with regard to
their role in the ecosystem (i.e., can a species be removed from a commu-
nity without a change in a specific ecosystem process?). Functional redun-
dancy can vary with the ecosystem process that is being considered, the
specific habitat, and the time. Many continental habitats have diverse as-
semblages with a considerable redundancy.
The links between diversity and ecosystem processes driving nutrient
cycling are unclear because little is known about microbial diversity and the
degree of redundancy of specific functional groups (Meyer, 1999). Although
Hutchinson made early arguments about the functional redundancy of phy-
toplankton species with regard to the paradox of the plankton (see Chap-
ter 16), the degree of functional separation of different phytoplankton
species has not been established (Steinberg and Geller, 1994). Finlay et al.
(1997) argue that microbial diversity is never so impoverished in natural
communities that biogeochemical cycling is seriously altered.
Benthic animal diversity may have strong influences on processes of
material exchange between the water column and the benthos (Covich
et al., 1999). Benthic animals can be important in energy flows and nutri-
ent cycling (Fig. 22.4) and different species alter nutrient flux in different
ways (e.g., burrowing, digging tunnels, stirring up sediments, actively
pumping oxygenated water into the sediments, and processing different
types of benthic materials). This raises the following question: How well
does benthic diversity relate to rates of ecosystem processes?
FIGURE 22.4 Benthic macroinvertebrates that burrow into layered sediments and acceler-
ate nutrient cycling and movement of materials into the water column. Burrowing bivalve,
crayfish, tubifid worms, and aquatic larvae mix O 2 into the sediments with their activities.
Surface-dwelling invertebrates increase turnover of microbial communities and increase sedi-
ment suspension (reproduced with permission from Covich et al., 1999).