IDENTIFYING ECOSYSTEM PROPERTIES - A CASE-STUDY USING PLANKTON BIOMASS SIZE DISTRIBUTIONS

Concepts regarding the hierarchical organization of ecosystems postulate the existence of specific properties that emerge at the ecosystem level. As an example, empirically based biomass size distributions of a pelagic ecosystem (Lake Constance) were investigated. The plankton biomass was approximately uniformly distributed over the entire size-range from bacteria (10(-14) g C/cell) to crustaceans (10(-4) g C/individual) on a seasonal average. The continuum of organisms with respect to body size may indicate the existence of a continuum of functional guilds in the planktonic food web, owing to the close relationship between body size and physiological and ecological features of planktonic organisms. It is hypothesized that the internal organization of the pelagic food web (mediated by interspecific competition and successful invasion into "empty" size-ranges) promotes an approximately regular distribution of biomass over the range of functional guilds. This process may be an underlying mechanism for the observed regular size distribution of biomass in pelagic ecosystems. Regarding second-order effects, systematic deviations from the overall rule of a uniform biomass size distribution were observed. The plankton community of Lake Constance was divided into two autotrophic and five heterotrophic groups to analyze potential causes for biomass accumulation in certain size ranges. The allocation of organisms into groups was based on the vulnerability of the algae to grazing, and on the feeding modes (raptorial feeders vs. filter feeders) and prey sizes of the heterotrophs. The overlapping size spectra of the different groups were calculated for different time periods during the season. They revealed that specific properties of component organisms influencing the flow of matter from small to larger sized organisms may cause deviations from a regular size distribution of biomass. For example, algae with a low vulnerability to grazing, and daphniids which feed on a broad range of relatively small prey organisms, both formed modes in the size spectrum. The dominance of size-ranges (almost-equal-to groups of organisms) with respect to biomass changed seasonally. These findings contradict a former conjecture that biomass accumulations in size spectra reflect the action of ecological second-order scaling factors.