Modelling the seasonal cycle of dissolved organic carbon at station E1 in the English Channel

Seasonal cycles of DOC and DON at Station E-I in the English Channel were investigated using an ecosystem model. Important features of the model are: (i) both carbon and nitrogen are incorporated by means of a set of varying C/N ratios; (ii) separate labile and semi-labile dissolved organic pools are distinguished; and (iii) there is a detailed treatment of processes associated with DOM production. The model yields a seasonality in DOM C/N ratios. Results suggest that the observed DOC increase of 34.4 mmol m(-3) is almost entirely due to a buildup of a semi-labile pool (turnover similar to 1% per day), with phytoplankton excretion acting as a major contributary flux (with lesser additions via zooplankton messy feeding, detrital breakdown, bacterial lysis and phytoplankton mortality). The model indicates that the food supply from DOM via bacteria may provide up to 50% of the needs of zooplankton at certain times of year, notably during the overwintering period. (C) 1998 Academic Press Limited.