Generating ecological hypotheses from biomass spectra using causal analysis: a benthic example

Abstract

Benthic communities in the Bay of Fundy [Canada] are characterized by the distribution of biomass among logarithmic size classes of organisms ranging from 0.25 .mu.m-64 mm equivalent spherical diameter. This distribution, a biomass spectrum, exhibits features that are conservative over a wide range of environmental conditions and over a seasonal cycle, having 3 distinct size groups of heterotrophic organisms: grain surface dwellers (bacteria), interstitial fauna (meiofauna) and macroscopic surface dwellers (macrofauna). Causal analysis was used to construct plausible, hypothetical models of interactions among macrofauna, meiofauna, micro-algae, bacteria and environmental variables. In the models, macrofaunal biomass was largely a function of exogenous predation and sediment disturbance; meiofauna were most abundant in fluid, fine-grained sediments where algal biomass was high; and there was evidence of size-dependent competition for food between macrofauna and meiofauna. Benthic micro-algae at an intertidal station appeared to be controlled by macrofaunal cropping and nutrient conditions in the sediment. Bacteria showed a very strong positive relationship with macrofauna and sediment C over a seasonal cycle and, spatially, with grain surface area and C content of sediments.