MEPS 380:19-32 (2009)  -  DOI: https://doi.org/10.3354/meps07921

ABSTRACT: Organic matter input and processing was studied in 2 contrasting sediments (Stn 115_FINE and Stn 330_COARSE) in the southern North Sea. The sediments are subjected to similar hydrodynamic conditions, but Stn 115_FINE underlies a high turbidity zone, making it a fine, low-permeability sediment. Monthly data on chlorophyll a (chl a), δ¹³C and δ¹⁵N of particulate organic matter in the water column and sediment showed that the algal spring bloom deposition created a strong vertical gradient of sedimentary chl a at Stn 115_FINE. Macrobenthic biomass (78 ± 60 g C m^–2, mean ± SD) was dominated by suspension feeders, suggesting biological mediation of the organic matter input. In contrast, the offshore Stn 330_COARSE is a coarse, high-permeability sediment in which chl a penetrated centimeters deep due to physically mediated input. The macrobenthic community, low in biomass (3.8 ± 2.4 g C m^–2), was dominated by mobile polychaetes and epibenthic amphipods, which is characteristic of physically disturbed sediments. Overall, sediment characteristics played an important but indirect role in the organic matter input and processing. At Stn 115_FINE, a large macrobenthic community developed that mediated the input of organic matter to the sediment through herbivore and predatory pathways. At Stn 330_COARSE, in contrast, organic matter input seemed to be dominated by physical processes. Overall, the fraction of algal carbon degraded in the sediment was higher at Stn 115_FINE than at Stn 330_COARSE, indicating that the physical input at Stn 330_COARSE was less efficient than the biological input at Stn 115_FINE.

KEY WORDS: Food web · Carbon isotope · Nitrogen isotope · Coastal sediment · North sea · Permeable sediment · Benthic · Pelagic coupling