Environmental controls of anammox and denitrification in southern New England estuarine and shelf sediments

We measured denitrification and anammox potential rates in homogenized sediments over two annual cycles from two estuarine sites (Providence River estuary and Narragansett Bay) and two continental shelf sites (Block Island and Rhode Island sounds). Denitrification varied both spatially and seasonally, but anammox only varied spatially. Denitrification ranged from 2.5 to 112 nmol N h(-1) mL(-1) sediment, and anammox from 0 to 8.7 nmol N h(-1) mL(-1) sediment, with the contribution of anammox to N-2 fluxes (ra) ranging from undetectable to 4% in estuarine sites and 8-42% on the shelf. Oxygen microprofiles were analyzed in intact cores and concentrations of nitrate, nitrite, and ammonium were measured in the sediment slice corresponding to the depth layer in which potential rates were measured. Denitrification rates correlated positively with diffusive O-2 consumption and temperature and potential anammox rates correlated positively with pore-water NO3- concentrations. Anammox was positively correlated with denitrification in shelf sediments where anammox was regularly detected, but not in estuarine sediments. ra was negatively correlated with diffusive O-2 consumption and positively correlated with pore-water nitrate. Both organic matter and nitrate were important factors regulating the contribution of anammox to N-2 production. As a whole, our results demonstrate that incorporating processes that control these two factors into models of N cycling, rather than focusing on organic matter availability alone, may improve predictions of the relative importance of anammox to N-2 production in marine sediments.