This thesis examines soft-sediment benthic community structure and factors affecting that structure. The body of work consists of five separate studies which can be divided into two distinct, yet related, parts: (1) examination of the interconnections of different size-groups of benthic organisms (bacteria, meiofauna and macrofauna) with the sediment chemical environment and chemical fluxes (2) patterns of, and factors affecting, organism community structure measured as benthic biomass size-spectra (BBSS).

The first two studies (I, II) examine the impact of organic input from a salmon aquaculture farm on benthic fluxes, sediment sulphide storage, abundance of capitellid worms and meiofauna BBSS. Near the farm, where organic input was greatest, sediment fluxes of oxygen and ammonium, and sulphide concentrations were greatest relative to areas more distant from the farm. This was accompanied by a decreased meiofauna diversity, biomass and average body size, as well as capitellid abundance nearer the fish farm. Some meiofauna were, however, more resistant to organic enrichment than the capitellids and meiofauna community specific-respiration (R/B) was higher near the farm that at more distant locations. These characteristics were incorporated into a simulation model of a soft-sediment benthic community in a third study (III). The model also incorporated positive feedbacks between chemical and biological components. The model simulated reasonable seasonal dynamics and response to increasing organic carbon input rates for benthic organisms and carbon mineralisation rates and pathways. The model was a means of summarising results of the first two studies into a structure which incorporated positive feedbacks and a rough organism size-distribution, and produced a reasonable conception of benthic community dynamics.

The final two studies examined patterns of BBSS in the Baltic Sea. In the first of this last group of studies (IV), patterns of Baltic BBSS were examined when grouped according to geographic location and when grouped by properties of the sediment environment. This study showed that Baltic BBSS did not follow the bimodal pattern suggested by theory but instead increased in biomass with body size. This pattern was less marked in the northern Baltic where large bivalves are not present. Baltic BBSS did not show any coherent relationship with sediment properties, which also does not conform to theory. To confirm these patterns and to elucidate the influence of sediment grain size on BBSS patterns, a final study (V) was conducted. This confirmed the previous BBSS pattern of increasing biomass with body size, and found little evidence of consistent change in the shape of the spectrum with changes in sediment grain-size.