Benthic response to a pelagic front

With the aim of studying the influence of pelagic front primary production on the benthic system underneath, biomarkers of benthic organic matter constituents and macrofaunal abundance and biomass were measured on stations in a grid extending through the area of the Skagerrak-Kattegat pelagic plume front. A time persistent pattern of chl a and phaeopigments in the surface sediment was observed, with chl a/chl a + phaeopigments ratios exceeding 0.5, suggesting high input of phytoplankton to the bottom near the front. Of the sediment variables chl a, phaeopigments, particulate organic nitrogen (PON), particulate organic carbon (POC) and biogenic silica (BSi), the pigments showed the highest correlation with benthic biomass and abundance. Chl a and phaeopigments together explained nearly half of the variation in benthic biomass and the non-polychaete fraction of abundance. C/N ratio showed the expected negative relationship with biomass but was not statistically significant. PON, BSi and POC were poor indicators of faunal variables. Results suggest that sediment chlorophyll and its breakdown products may be useful biomarkers of labile organic matter. The organic matter (OM) gradient significantly influenced faunal structure. Polychaete and echinoderm AFDW were positively correlated with chl a and with phaeopigments. A major part of the positive faunal response was due to the burrowing ophiuroid Amphiura filiformis and its commensal Mysella bidentata. While the host was best correlated with phaeopigments, the commensal correlated equally well with both chl a and phaeopigments. Faunal changes in composition suggested increased importance of subsurface feeding deep in the sediment in response to increased OM loading. Surface deposit-feeders did not respond to the high levels of labile OM in the middle of the area. Results indicate strong pelagic-benthic coupling near the front and in the area with a mixed water column and are consistent with the hypotheses that pelagic-benthic energy coupling is stronger in mixed areas compared to those which are stratified and that increased OM loading may increase subsurface dwelling and OM: processing through benthic burrowing biomass.