distribution and abundance of benthic invertebrates can convey a picture of ecosystem interactions in a stream and constrain the hypotheses about ecosystem processes. For example, many species of mayflies graze on algae growing on rocks in the streambed (periphyton), whereas many species of stoneflies shred and ingest leaves. The presence of both mayflies and stoneflies in a stream indicates significant quantities of organic material, either produced in the stream by periphyton or entering the stream from trees adjacent to it in the riparian zone.

For algal species, the ecological relevance of species data may be useful, but less refined than for benthic invertebrates. Hutchinson (1961) asked why so many algal species are present in a 1-liter sample of lakewater, a seemingly uniform environment. He referred to this question as the "paradox of the plankton." Whether or not this is a true paradox can be debated. For algal species, some general habitat requirements and ecological functions are known at the division level (i.e., diatoms require silica and heterocystous cyanobacteria can fix atmospheric nitrogen). However, more detailed habitat information is inferred from distributional patterns rather than by direct study of individual algal species. The significance of interannual shifts in algal species, whether within a lake or stream ecosystem, or the presence of particular rare species, may be more difficult to interpret.

Question: Is understanding of important microbial processes in aquatic ecosystems (the microbial loop, heterotrophic degradation of natural organic material or organic contaminants) constrained by lack of species-level information?

For bacteria, the species concept may seem hardly relevant to ecological studies. In the 1970s and 1980s, microbial ecology classified bacteria functionally, rather than by detailed, recognizable, morphological traits. Their habitat requirements were described broadly (e.g., aerobic, anaerobic, facultative, obligate). However, more current laboratory studies of isolates of aquatic bacteria show that genetic characteristics may be significant in showing relatedness among bacterial strains, but that functional characteristics are not unique.

For example, some species of sulfate-reducing bacteria may be capable of reducing ferric iron under certain conditions. Current cutting-edge research is examining genetic characteristics in the context of the individual and populations; the context of communities or ecosystems is probably the next frontier. This is an important complement to another cutting-edge research area with much practical significance: the development of engineered bacterial species to remediate certain compounds in situ. This research area has much promise and is a certain future direction of applied limnology.

Question: How does biodiversity in aquatic ecosystems develop and change? How much time is needed?

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