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 North Sea were visited to study the relationship between phytopigment input to the sediment and the subsequent response by the benthic community. Duplicate sediment cores were taken, sliced and analysed for chlorophyll a, and sediment oxygen demand was measured. The downcore distribution of chlorophyll a was modelled with both a random diffusive model and a non-local mixing model. In most cases, the random diffusive model gave the best fit. In some cases, however, when a subsurface maximum of chlorophyll was found, the non-local mixing model gave a better description of the depth-distribution of chlorophyll. From these models, the bioturbation rates in and the fluxes of chlorophyll a to the sediments were calculated. Fluxes were also calculated according to the inventory of chlorophyll in the sediments. Results from these 2 methods correlated well. Fluxes were highest in spring, lower in summer and lowest in autumn and winter. At one station a good correlation between the chlorophyll flux to the sediment and the bioturbation rate was found. Another station demonstrated a positive trend between these 2 para meters, and the third station showed no relation at all. Sediment oxygen demand at the 3 stations exhibited a clear seasonal cycle, with high values in mid or late summer. When both chlorophyll fluxes and sediment oxygen demand were converted into carbon units, the annual averages of carbon supply (chlorophyll) and carbon mineralisation (oxygen demand) were reasonably balanced. It appeared that in spring, carbon build-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.