MSI Technical Seminars

Monday, May 7, 2007, 12:30 p.m.

Robinson W. "Wally" Fulweiler, Ph.D.
Graduate School of Oceanography
University of Rhode Island

"Climate Change Impacts on Benthic-Pelagic Coupling and the Biogeochemistry of Narragansett Bay, R.I."

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 has been 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. 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 mmol N₂-N m^-2 h^-1) were measured at four sites. This is particularly unusual since nitrogen fixation in marine sediments has traditionally been considered inconsequential. In an effort to understand why this dramatic shift in N₂ fluxes occurred, a series of experiments were conducted using sediment cores and large marine mesocosms. The experiments were designed to study the effects of enriching the overlying water with nitrate, ammonium, or phosphate on the net N₂ fluxes across the sediment water-interface. In addition to inorganic nutrient enrichment of the water we also carried out separate experiments in which organic matter was added to the sediment surface. While inorganic enrichment of the overlying water caused no change in the net sediment N₂ flux, organic matter enrichment ultimately switched the sediments from net N-fixation to net denitrification.