Environmental and sediment conditions, infaunal benthic communities and biodiversity in the Celtic Sea

While it is recognised that the nature of the sediment is a major driver of benthic macro-infaunal community structure, it is also true that diverse environmental factors determine the distribution and composition of sediments. Among those factors are depth, tidal stress and seasonal stratification of the water column. In the Celtic Sea an area of seabed approximately 20 km wide and 125 km long was selected in which variation in water depth, stratification, primary production and current velocity were minimised, but which contained sediments ranging from fine muds to coarse gravelly sands. 55 stations were sampled across the area using a box-corer. At each station a comprehensive suite of sediment and biogeochemical measurements were made. Macrofauna were identified and weighed. Of the stations sampled, four had been chosen as focal sites for a study of relationships between benthic biogeochemistry and sediment type. Relationships between variation in environmental and sediment variables and macrofaunal community structure were analysed using a range of non-parametric multivariate techniques. Environmental variables were discriminated into situational variables that broadly encapsulate potential drivers of spatial heterogeneity in the benthos such as depth and fishing effort, and in-situ variables that were measured at each site concurrently with the sampling of the macrobenthos, such as sediment properties and biogeochemical measurements. Among the former, analyses tended to identify the importance of average shear stress and depth in explaining observed variation in benthic community structure, even though the area had been chosen to minimise variation in those factors. Analyses using in-situ measurements of sedimentary conditions at each site identified very fine sand content (correlated with average shear stress) as the most important explanatory variable. Most of the measured biogeochemical variables varied with sediment structure, particularly reflecting differences between finer-grained sediments with higher organic content (generally from deeper areas) and coarser sediments with lower organic content. While clear spatial heterogeneity in sediments and associated biogeochemical variables could be demonstrated, spatial variation in benthic abundance and biomass was less clear. Benthic community structure varied significantly with sediment type, but did not vary closely with the in-situ environmental variables measured at the same sites. This may indicate that the samples collected were too small to accurately characterise the benthic assemblage at each site, or that most species inhabiting the area inhabit a range of sediment types, or that processes which are not reflected in sediment or biogeochemical measurements are also important determinants of benthic community structure.