Relationship between structural and functional aspects of microbial and macrofaunal communities in different areas of the North Sea

Spatial similarities of structural and functional (trophic) aspects of microbial and macrofaunal community patterns were studied in relation to sediment parameters in the German Bight, the Oyster Ground, the Dogger Bank, eastern North Sea, the Skagerrak, and northern Kattegat in May and September 1999 and 2000. Spatial patterns of microbial and macrofaunal communities were congruent. Differences in spatial distribution of communities were caused by utilisation of the different food sources available in the different areas. In the German Bight, the communities were correlated significantly with chlorophyll a (chl a) concentrations in the sediments, an indicator of fresh nitrogen-rich organic matter. In contrast, communities in the Skagerrak and Kattegat were correlated with total organic carbon (TOC) and mud concentrations, indicating the presence of more refractory organic matter for the microbial and macrofaunal organisms. For the communities in the Oyster Ground, the eastern North Sea and the Dogger Bank, no such significant correlations were found. The community in the Oyster Ground was characterised by the ophiurid Amphiura filiformis foraging on relatively refractory macrofloculate organic matter from the benthic boundary layer. Preferred substrates of the microbial communities in this area were macrofaunal excretion products. The Skagerrak and Kattegat stations were also characterised by the A. filiformis community, indicating lower food supply than in the German Bight, Although microbial community patterns were similar to those in the German Bight, lower chl a concentrations also suggested the presence of relatively refractory organic matter. Reflected by the dominance of sand-licking species, macrofaunal communities at the Dogger Bank and in parts of the eastern North Sea seem to rely on benthic primary production. Here, microbial communities were adapted to hydrodynamically induced stress, sediment disturbance and limited food supply. Our results revealed that in areas with high food supply such as the German Bight, no competition for food between microorganisms and macrofauna occurs. But in areas with less or limited food supply from the water column, macrofauna provides the microorganisms with organic matter or microorganisms utilise macrofaunal excretion products and are physiologically adapted to the exposed environment.