CHLOROPHYLL-A AS A MARKER FOR BIOTURBATION AND CARBON FLUX IN SOUTHERN AND CENTRAL NORTH-SEA SEDIMENTS

In 1993, 3 stations (depth 28 to 49 m) in the southern and central Nor th Sea were visited to study the relationship between phytopigment inp ut to the sediment and the subsequent response by the benthic communit y. Duplicate sediment cores were taken, sliced and analysed for chloro phyll a, and sediment oxygen demand was measured. The downcore distrib ution of chlorophyll a was modelled with both a random diffusive model and a non-local mixing model. In most cases, the random diffusive mod el gave the best fit. In some cases, however, when a subsurface maximu m of chlorophyll was found, the non-local mixing model gave a better d escription of the depth-distribution of chlorophyll. From these models , the bioturbation rates in and the fluxes of chlorophyll a to the sed iments were calculated. Fluxes were also calculated according to the i nventory of chlorophyll in the sediments. Results from these 2 methods correlated well. Fluxes were highest in spring, lower in summer and l owest in autumn and winter. At one station a good correlation between the chlorophyll flux to the sediment and the bioturbation rate was fou nd. Another station demonstrated a positive trend between these 2 para meters, and the third station showed no relation at all. Sediment oxy gen demand at the 3 stations exhibited a clear seasonal cycle, with hi gh values in mid or late summer. When both chlorophyll fluxes and sedi ment oxygen demand were converted into carbon units, the annual averag es of carbon supply (chlorophyll) and carbon mineralisation (oxygen de mand) were reasonably balanced. It appeared that in spring, carbon bui ld-up took place, while in late summer this carbon was degraded. It is argued that chlorophyll a is a good marker for metabolisable organic matter.