Resource competition in a discrete environment: Why are plankton distributions paradoxical?

For nearly ail natural waters, planktonic organisms will be distributed discretely in the fluid mechanical sense. This means that the dynamics of planktonic ecosystems occur among discrete particles, not continuous scalar fields. This idea was first suggested by Hurlburt (1990) as an explanation for the paradox of the plankton. However, the discrete nature of organism distributions has many important implications for the interpretation and modeling of planktonic ecosystems. In particular, mass conservation relationship approaches to the modeling of planktonic populations (i.e., resource competititon and its result, competitive exclusion) will not be valid for all natural conditions. A microscale model of competition among individual phytoplankton cells was used to investigate the role of discreteness on phytoplankton competitition for a single, limiting nutrient substrate. This scaling analysis demonstrates that rates of competitition should increase with cellular abundance and phytoplankton size. For a typical eutrophic planktonic ecosystem (relatively large cell abundances and cell sizes), resource competitition among individual phytoplankters appears to be likely. However, for oligotrophic conditions (low cell abundances and small cells), rates of competitive displacement should be greatly reduced. The microscale competition model does not predict that the final outcome of competition will differ from resource competition theory when evaluated over thousands of division cycles. However the time required for this outcome to occur may be so long that other processes, such as episodic nutrient inputs, imposed diel cycles, and specialization of the grazer assemblage, are likely to have a dominant role in determining the species composition of an oligotrophic phytoplankton community. This seeming violation of the principle of competitive exclusion occurs because nutrient competition in oligotrophic environments is governed by interactions among discrete individuals rather than entire populations. Discreteness in plankton distributions also creates an ecological subgrid scale (SGS) problem that must be solved as part of most mathematical descriptions of plankton population dynamics. Approaches towards solution of the ecological SGS problem are suggested; however, a great deal of theoretical and experimental work remains.