Estuaries have enormous societal importance. However, the functioning of these ecosystems is impaired by eutrophication (i.e. excess nutrient loading), which reduces water quality, causes harmful algal blooms, and increases the incidence of hypoxic events. Salt marshes act as filters of inorganic nutrients passing from the land to estuaries and ultimately to the open ocean, but coastal eutrophication may reduce the efficacy of this filtering. Furthermore, human disturbances in salt marshes are causing geomorphological change, which in turn can have cascading influences on primary producers and consumers. At the interface of saltmarshes and estuaries are tidal creeks, a potentially important link between the terrestrial ecosystem and the open estuary.
Sediment microalgae and bacteria play pivotal roles in coastal carbon dynamics. Benthic microalgae can strongly influence nutrient cycling and carbon dynamics in shallow , well-lit estuaries. For instance, benthic microalgae are estimated to contribute up to 95% of total primary production in some estuarine habitats. However, few methods exist for quantifying the rate of benthic microalgal production. Carbon fixed by benthic microalgae can be rapidly assimilated by sediment bacteria. While this coupling between sediment microalgae and bacteria has been documented in multiple estuarine systems, it is largely unknown whether the importance of algal carbon to bacteria varies with nutrient availability, primary productivity, and allochthonous organic matter inputs.
This project will address how rates of benthic microalgal production respond to eutrophication and geomorphological changes in human-impacted tidal creeks. Excess nutrient loading increases benthic algal biomass and likely stimulates production rates but the magnitude of nutrient and geomorphological effects on rates of production is unknown. Will changes in benthic algal productivity affect algal-bacterial coupling? Furthermore, how is algal-bacterial coupling affected by geomorphological changes, which may be exacerbated by excess nutrient loading but can also occur in pristine marshes?
This project will take advantage of the infrastructure of the TIDE project, a long-term saltmarsh eutrophication experiment at the Plum Island Ecosystem - Long Term Ecological Research site in Northeastern Massachusetts. Specifically, the PIs will measure benthic metabolism and examine algal- bacterial coupling in fertilized and ambient nutrient tidal creeks in the first field season. The following field season, they will compare sediment metabolism and carbon dynamics on slumped tidal creek walls (i.e. areas where low marsh has collapsed into the tidal creek) to that on the bottom of tidal creeks. In both years, gross and net production will be determined using an innovative triple oxygen isotope technique and traditional dissolved oxygen and inorganic carbon flux measurements. Comparisons between these methods will be useful in informing studies of sediment metabolism. Lipid biomarkers will be used to characterize the sources of organic matter to creek sediments, and stable isotope analysis of bacterial specific biomarkers to identify the sources of organic carbon utilized by sediment bacteria. The biomarkers will reveal whether sediment bacteria use organic matter substrates, such as benthic microalgal carbon, selectively or in proportion to availability. Overall, results from the proposed study will provide important information about how sediment carbon dynamics in shallow tidal creeks respond to long term eutrophication. Furthermore, findings will enhance understanding of the role of tidal creeks in coastal biogeochemistry.
Broader Impacts A significant impact from this project will be better knowledge of how eutrophication - a pressing societal problem - affects biological productivity and organic matter cycling and burial. The scientists will work with local conservancy and education groups to bring an understanding of results to the public. Additionally, this project will promote education by including opportunities for two graduate students and two undergraduate students.
