Biological traits of benthic macroinvertebrates from a large area of the North Sea soft sediments were used to explore habitat occupancy within seascapes of contrasting hydrodynamics. The area, the Dutch sector of the North Sea, is mainly composed of 2 habitats: shallow dynamic bottoms of heterogeneous geomorphologies and deep homogeneous muddy bottoms. Higher within-habitat heterogeneity was hypothesized to more specifically select benthic life strategies according to environmental filtering, i.e. through the action of abiotic forces. Functional community patterns were explored through the RLQ method, which relates habitat and trait variables, at different spatial scales of specific seascape heterogeneity, and functional diversity indices were used to shed light on community assembly mechanisms. Locally, 3 associations between habitat characteristics and biological traits were shown to correspond with predictions of life history theories, whereas only 2 emerged when considering all types of seascapes. This spatial scaledependence was explained by abiotic alternations masked over the larger scale at which all the existing strategies could not be properly disentangled. The relative composition in strategies obeyed specific assembly rules as identified by functional diversity indices. Seascape geomorphology was locally discriminant of functional patterns, and could account for biodiversification, much beyond basic taxonomic counts. Whereas habitats of higher physical stability hosted the taxonomically richest communities, stress or disturbance frequency increased functional variations within communities due to different strategist habitat occupancies. This study proposes a generic mechanism of benthic community structuring in soft sediment shelves.

We investigated seasonal changes in the production of extracellular polymeric substances (EPS) and short-chain organic acids (SCOA) exuded by benthic diatoms, and the use of these exudates as a carbon source by heterotrophic bacteria. An in situ ¹³C pulse-chase method was used to follow the fate of EPS for 5 consecutive days. These experiments were done at 2 mo intervals for 1 yr. The EPS were recovered from the sediment as 2 operationally defined fractions (i.e. water-extractable and EDTA-extractable EPS). Seasonal differences in EPS production correlated to light intensity and temperature. From February until June the biomass and production of diatoms and bacteria were closely coupled. It was concluded that SCOA were the most important substrates for the bacteria. Sulfate-reducing bacteria (SRB) in particular benefited from SCOA released by diatoms. From August on, the coupling of biomass and production of diatoms and bacteria weakened and was almost lost in December. During the period from August to December, EPS produced by diatoms promoted the growth of other bacterial taxa rather than SRB, and the production of SCOA was low. Thus, it appears that the seasonal variation in exudates produced by diatoms plays an important role in shaping community composition and maintaining the diversity of the associated bacteria.