BENTHIC METABOLISM AND NUTRIENT CYCLING IN BOSTON HARBOR, MASSACHUSETTS

To gain insight into the importance of the benthos in carbon and nutri ent budgets of Boston Harbor and surrounding bays, we measured sedimen t-water exchanges of oxygen, total carbon dioxide (DIC), nitrogen (amm onium, nitrite + nitrite, urea, N2O), silicate, and phosphorus at seve ral stations in different sedimentary environments just prior to and s ubsequent to cessation of sewage sludge disposal in the harbor. The ra tio of the average annual DIC release to O-2 uptake at three primary s tations ranged from 0.84 to 1.99. Annual average DIC:DIN flux ratios w ere consistently greater than predicted from the Redfield ratio, sugge sting substantial losses of mineralized N. The pattern was less clear for P: some stations showed evidence that the sediments were a sink fo r P while others appeared to be a net source to the water column over the study period. In general, temporal and spatial patterns of respira tion, nutrient fluxes, and flux ratios were not consistently related t o measures of sediments oxidation-reduction status such as Eh or disso lved sulfide. Sediments from Boston Harbor metabolize a relatively hig h percentage (46%) of the organic matter inputs from phytoplankton pro duction and allochthonous inputs when compared to most estuarine syste ms. Nutrient regeneration from the benthos is equivalent to 40% of the N, 29% of the P, and more than 60% of the Si demand of the phytoplank ton. However, the role of the benthos in supporting primary production at the present time may be minor as nutrient inputs from sewage and o ther sources exceed benthic fluxes of N and P by 10-fold and Si by 4-f old. Our estimates of denitrification from DIC:DIN fluxes suggests tha t about 45% of the N mineralized in the sediments is denitrified, whic h accounts for about 17% of the N inputs from land.