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

Benthic communities in the Bay of Fundy are characterized by the distribution of biomass among logarithmic size classes of organisms ranging from 0.25 μm to 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 carbon over a seasonal cycle and, spatially, with grain surface area and carbon content of sediments.