Climate change has altered the phenology or seasonal sequencing of events in many ecosystems, including Narragansett Bay, RI (USA). The timing and magnitude of the winter-spring bloom has changed dramatically over the last three decades resulting in a 60% decline in water column chlorophyll a concentrations in mid-Narragansett Bay. This large decline is linked to a long-term warming trend. We hypothesized that the decline in chlorophyll led to a decrease in the quantity and/or quality of organic matter deposition to the benthos and fundamental changes in sediment biogeochemical cycling and benthic-pelagic coupling.
We observed significant decreases in sediment respiration and dissolved inorganic nitrogen fluxes in mid and upper Narragansett Bay compared to rates measured in the 1970s/80s. In the more eutrophic Providence River estuary, the decline in oxygen is less clear, nitrogen fluxes appear unchanged, and dissolved inorganic phosphate fluxes have declined significantly. Summer sediment nutrient regeneration supplied <13% of the nitrogen and <5% of the phosphate needed by phytoplankton, a 75-98% decrease compared to previously measured regeneration rates.
Net sediment denitrification in mid-Narragansett Bay also decreased significantly. In the summer of 2006, high rates of net sediment nitrogen fixation (-5 to -650 μmol N 2 -N m -2 h -1 ) were measured at four sites. This is remarkable since N-fixation in marine sediments is traditionally inconsequential. During the summer, sediment N-fixation added an additional 90 million moles of N to Narragansett Bay. In contrast, net sediment denitrification in the summer of 2005 removed only 26 million moles of N. Thus, the sediments of Narragansett Bay switched from being a sink of nitrogen to a source.
Experiments studied the effects of various inorganic nutrient and organic matter amendments on net sediment N 2 fluxes. Inorganic enrichment of the overlying water caused no change in net flux of N 2 . Organic matter enrichment first increased N-fixation, but ultimately switched the sediments from being a net source to a net sink of nitrogen. A clear threshold was observed where N-fixation took place when organic matter deposition was <0.3 g C m -2 d -1 . A preliminary conceptual model for the possible mechanisms regulating estuarine sediment N 2 flux was developed.
