General background

Freshwater streams and lakes are habitats for complex ecosystems, of which plankton is an important component. Even more extensive are the oceans, which cover about 70% of the Earth’s surface. Marine ecosystems including their plankton have very great ecological and economic significance. Although our knowledge of biodiversity patterns in marine phyto- and zooplankton (compared to terrestrial systems) is still very limited (Irigoien et al., 2004), much work, some of it theoretical, some experimental, has led to important insights.

The study of plankton has played a crucial historical role in our understanding of ecological processes. The famous “paradox of the plankton” formulated by Hutchinson (1961) drew attention to the fact that many more species coexist in the supposedly homogeneous habitat than permitted under the competitive exclusion principle of Gause. Hutchinson suggested that nonequilibrium conditions might lead to the greater than expected diversity, a suggestion shown to be correct by many subsequent studies. Hutchinson himself thought that seasons and weather-induced fluctuations were responsible. But, in addition, as reviewed by Scheffer et al. (2003), homogeneity due to mixing hardly exists, and even in the open ocean meso-scale vortices and fronts result in spatial heterogeneity. Moreover, modeling of plankton communities has shown that even in homogeneous and constant environments plankton may never reach equilibrium, because multi-species interactions may lead to oscillations and chaos. This is supported by laboratory experiments, which have shown highly irregular and unpredictable long-term fluctuations at the species level (Figure 4.1), although total algal biomass and other indicators at higher aggregation levels may show regular patterns.