BENTHIC-PELAGIC COUPLING AS A FUNCTION OF CHANGING ORGANIC INPUTS IN COASTAL ECOSYSTEMS

Abstract

Benthic-pelagic coupling links the sediments and the water column in shallow coastal marine ecosystems. Measurements of benthic-pelagic coupling have been made for decades in shallow, estuarine ecosystems, but relatively few measurements have been made in transitional inner continental shelf areas. Accordingly, the Providence River Estuary and mid-Narragansett Bay have many measurements while ecosystems on the Southern New England inner continental shelf have few. However, even in estuarine areas, the reactions and response times of benthic-pelagic coupling to recent anthropogenic and climate-induced changes are poorly constrained. Recently, a climate-induced oligotrophication has weakened the relationship between the benthos and the water column in Narragansett Bay. When benthic metabolism and nutrient flux measurements were first measured in the 1970s, benthic nutrient regeneration supplied 50 to over 200 percent of the required nitrogen and phosphorus for phytoplankton production. By the mid-2000s, a considerable reduction in benthic fluxes was observed. These decreases were driven by climate induced ecosystem changes (e.g. altered winter-spring diatom bloom timing, warming water, increased cloudiness, etc.). Similar changes did not occur in the Providence River Estuary, an area in the upper Bay heavily fertilized by effluent from wastewater treatment facilities. I measured benthic fluxes of oxygen, and dissolved inorganic nutrients across two annual cycles in these areas to (1) determine if mid-Bay sediments have responded to the recent return of the traditional winter-spring diatom bloom, and (2) compare benthic-pelagic coupling in two stations at different locations on a north-south gradient of anthropogenic impacts. I hypothesized that the response time of mid-Bay benthic-pelagic coupling would be fast and regulated on relatively short time scales, and that differences in water column biology (i.e. primary production, phytoplankton biomass) and nutrients between the Providence River Estuary and the mid-Bay would be mirrored in the benthic fluxes. In the mid-Bay, I measured substantial increases in regeneration of ammonium (176%) and phosphate (266%) regeneration compared to rates measured in 2005-2006, and a significant relationship between surface water phytoplankton biomass and sediment oxygen demand (R2=0.23, p=0.02). Even though these changes occurred concurrently with the recent return of the winter-spring phytoplankton bloom, the lack of difference in sediment oxygen demand over time indicated either a lack of rapid response or loss of organic matter through water column consumption. Despite strong gradients in some of the drivers of benthic mineralization such as organic matter (phytoplankton biomass) and primary production, I found no significant differences in average benthic nutrient fluxes between