Behaviour of Dissolved Organic Matter in Pore Waters of Near-Shore Marine Sediments

Pore water samples, collected from short sediment cores in the Gulf of Trieste over a yearly cycle, were analysed for dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP), as well as ammonia and phosphate. The concentrations of DOC, DON and DOP in pore waters were up to 10-fold higher than in the overlying water. The concentrations of DOC increased with sediment depth, while DON and DOP showed no trend. Pore water DOM concentrations were highest in late summer. This is thought to be caused by a higher microbial activity and restricted upward fluxes towards the overlying water because of a strong pycnocline just above the bottom. A mean DOC:DON:DOP ratio (atomic) of 35:9:1 was computed. Low DOC:DON ratios, ranging between 2–4, were taken to indicate the predominance of proteinaceous matter. The benthic fluxes of DOC, DON and DOP, measured in parallel with O₂ consumption in situ with benthic chambers, showed in general a net influx, except for DOC in the period of high pore water DOC content and benthic O₂ consumption when strong DOC fluxes out of the sediment prevailed. The bulk of DOC was found to be refractory or decomposed in anoxic conditions. In contrast to DOC behaviour, DON and DOP are more reactive involving aerobic and anaerobic pathways of decomposition. The DOC benthic fluxes were strongly temperate dependent, and the high apparent activation energy computed indicated a significant proportion of a high molecular weight fraction of DOC, while DON and DOP fluxes were not directly temperature dependent, probably because of a random fluctuating supply of degradable sedimentary organic matter and oxidants. Constructing the budgets of C, N and P, including the DOC, DON and DOP benthic fluxes, it was found that the total N and P inputs are finally all utilized in benthic primary production while the balance of C remains obscure.