The impact of climate change on benthic-pelagic coupling and the biogeochemical cycling 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 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 N2-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 N2 fluxes. Inorganic enrichment of the overlying water caused no change in net flux of N2. 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 N2 flux was developed. ^
Biology, Ecology|Biology, Oceanography|Biogeochemistry
Robinson Walter Fulweiler,
"The impact of climate change on benthic-pelagic coupling and the biogeochemical cycling of Narragansett Bay, R.I."
Dissertations and Master's Theses (Campus Access).