Benthic-pelagic coupling as a function of changing organic inputs in coastal ecosystems

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 climateinduced 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 (R=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 the Providence River Estuary and the mid-Bay. This may be due to the export of organic material down-bay before an excess amount can fall to the benthos. Block Island Sound (BIS) and Rhode Island Sound (RIS) are adjacent, phytoplankton-based ecosystems on the inner continental shelf off Southern New England with contrasting hydrographic regimes. The water column of Block Island Sound is more well mixed as a result of year-round energetic tidal mixing, while Rhode Island Sound typically becomes stratified during the summer. I aimed to examine the effect of hydrography on benthic-pelagic coupling in transitional shelf areas. To address these goals, I measured parameters in both the water column and the benthos. I compared annual cycles of surface chlorophyll a and C-measured primary production between these two ecosystems using samples collected over 22 months (chlorophyll) and approximately monthly for 12 months (production). In the benthos, I made the first measurements of biogeochemical fluxes of oxygen and inorganic nutrients at the sediment-water interface in Block Island and Rhode Island Sounds. I hypothesized that seasonal water column stratification in Rhode Island Sound would result in increased summer nutrient limitation, lower primary production, and reduced biogeochemical exchanges at the sediment-water interface compared to the relatively more well-mixed Block Island Sound, and that stratification would ultimately weaken the link between the benthos and the water column. In situ measurements of surface chlorophyll a in Block Island Sound and Rhode Island Sound revealed no significant differences in chlorophyll concentrations between the regions. However, the regional validation of satellite chlorophyll a data enabled a comparison over a larger extent of space and time that clearly showed higher overall concentrations in Block Island Sound than Rhode Island Sound. Empirical models of primary production indicated that annual primary production was also higher in relatively well-mixed Block Island Sound (230-329 g C m y) than in seasonally stratified Rhode Island Sound (162-256 g C m y). Despite the higher rate of euphotic zone primary production in Block Island Sound, benthic metabolism (measured as sediment oxygen demand) was not significantly different between the two areas (BIS=953.8 μmol m h; RIS=912.2 μmol m h). This lack of differences was likely due to differences in water column hydrography between the sounds, where the energetic water column mixing in Block Island Sound resuspended organic matter back to the water column to be decomposed before reaching the benthos. Additionally, the seasonal presence of a strong pycnocline in Rhode Island Sound likely prevented mixing of regenerated DIN and DIP to surface waters for use by phytoplankton. Apparent differences in benthic macrofaunal communities between Block Island Sound and Rhode Island Sound translated to differences in dissolved inorganic nutrient fluxes between the two areas, despite the similarities in benthic metabolism. Excretion and irrigation activities by the dense amphipod communities in Block Island Sound caused higher effluxes of DIN (NH4=36.9 μmol m h; NOX=23.5 μmol m h) and DIP (7.2 μmol m h) compared to fluxes in Rhode Island Sound (NH4=22.8 μmol m h; NOX=11.1 μmol m h; DIP=3.2 μmol m h). These findings indicate that the hydrographic regime of the water column may exert a strong influence on benthic-pelagic coupling dynamics on the Southern New England shelf and in other inner continental shelf