Experimental studies of the effect of organic deposition on the metabolism of a coastal marine bottom community

Abstract

Experimental laboratory microcosm studies using intact sediment cores of a benthic community from mid-Narragansett Bay, Rhode Island (USA) were conducted to determine the influence of the rate of supply of organic matter on benthic metabolism and nutrient remineralization. Enrichment experiments of 3 to 4 mo duration were carried out at lSC using seston filtered from bay water. Replicate benthic microcosms were either 'starved', given regular (every 3 d) organic inputs, or exposed to large 'pulses' of organic matter equivalent to 3 to 5 mo of metabolic loss. With particulate starvation, sediment oxygen uptake and inorganic nitrogen release rates decreased only slowly, approximately halving in 80 to 120 d. 0, uptake, C O , release, and NH,+ release all increased immediately in response to a pulse input, subsequently declining exponentially towards the original levels within 1 to 2 mo. Rates increased gradually under regular organic additions and declined quickly when the inputs were discontinued. The magnitude of the increase in dissolved fluxes was a function of both the quantity deposited per unit time and the initial remineralization rate of the added organic matter. Our results offer direct evidence that organic input from the water column can exert a major influence on the magnitude (and variability) of sediment-water exchange rates. Within 2 mo, about 24 to 30 % of the organic nitrogen and l l to 20 % of the organic carbon experimentally deposited to the sediment surface had returned to the overlying water as dissolved inorganic decomposition products. We calculate that, for the in situ mid-Narragansett Bay sediment community, over 80 % of the annual C and N deposition may be remineralized and returned to the overlying water. We suggest that, although having demonstrated a pronounced effect of organic inputs upon benthic metabolism, we may be underestimating the effect since the initial remineralization rates of field deposition may be even more rapid than in our experiments.