In situ response of bay productivity to nutrient loading from a small tributary: The Delaware Bay-Murderkill Estuary tidally-coupled biogeochemical reactor
Graphical abstract
Introduction
Phytoplankton production in estuarine and coastal systems provides a food supply to heterotrophs and regulates cycling of inorganic nutrients, such as nitrogen and phosphorus (Cloern, 1996). Primary production can be used to assess ecosystem health, nutrient limitation (Elser et al., 2007), and the extent of eutrophication (Smith, 2007, Collins et al., 2013); it can be regulated by nutrient supply, light availability, and circulation (Cloern, 1996, Elser et al., 2007). Phytoplankton blooms can be episodic and short-lived in response to changes in physical forcing, reflecting tides (0.5–1 day), winds (3–5 days), and storms (Cloern, 1996), but blooms may also persist over much longer intervals. In Delaware Bay, spring freshwater inflow can generate stratification that supports persistent blooms (Pennock, 1985, Sharp et al., 1986, Yoshiyama and Sharp, 2006). Coastal and estuarine production of organic carbon during blooms can equal or exceed allochthonous inputs, and therefore can play an important role in coastal and global carbon budgets (Cai, 2011, Bauer et al.,2013).
Tidal creeks and tributary estuaries that branch off from a larger estuary are a common feature in coastal regions and can be affected by both local tributary inflows, winds and tides, as well as by the processes and behavior that take place in the main estuary and its larger watershed (Uncles and Stephens, 2010). Primary production and phytoplankton dynamics at the confluence of tributaries with an estuary are influenced by freshwater and nutrient influxes, from both the tributary and the main-stem watersheds, often with a lag between the two sources (Gallegos et al., 1992). The coupling between tributaries and the main estuary becomes even more complex due to the diel and tidal modes that can differently affect sea level (Dzwonkowski et al., 2014), turbidity (Uncles and Stephens, 2010), organic matter transport (Lake et al., 2013) and extent and occurrence of hypoxia (Tyler et al., 2009). In these complex systems, continuous high-frequency monitoring of multiple parameters that both regulate and are regulated by phytoplankton growth can be used to determine primary production, respiration and net ecosystem metabolism rates; all of these processes can help characterize the state of coastal and estuarine ecosystems (Yoshiyama and Sharp, 2006, Tyler et al., 2009, Collins et al., 2013). Understanding the coupling between the tributaries and the main stem estuary can also provide insight on carbon and nutrient cycling in coastal tidal regions.
In this study, we use continuous automated measurements of DO and other biogeochemical parameters, collected at the confluence of the Murderkill Estuary and the Delaware Bay at Bowers, Delaware, to assess the role of nutrient inputs from a small, nutrient-rich tributary (the Murderkill) on primary production and nutrient cycling at the shallow Delaware Bay margins. Specifically, we (1) determine the role of nutrient loads from the Murderkill Estuary in supporting primary production in the adjacent Delaware Bay, and (2) estimate respiration, gross primary production, and net ecosystem metabolism (NEM) in the Bay. Our methods of analysis target a coupled tidally-dominated biogeochemical reactor (Murderkill Estuary – Delaware Bay) that supports a high level of planktonic primary production in the Bay during the summer.
Section snippets
Study site
One of the largest estuarine ecosystems in the United States, Delaware Bay drains a 36,570 km2 watershed that encompasses parts of Delaware, New Jersey, Pennsylvania, and New York State. The Bay and Delaware Tidal River extend 215 km from Trenton, New Jersey, to the Atlantic Ocean between Cape Henlopen, Delaware and Cape May, New Jersey (Fig. 1). Sections of the tidal river and upper Bay from Trenton, New Jersey, to Wilmington, Delaware, receive nutrients and other contaminants from urban areas
Data verification, description and frequency analysis
Regressions between discrete samples and corresponding LOBO values were used to evaluate LOBO performance and to correct LOBO measurements for interferences when necessary (Fig. S4). We applied corrections to the chlorophyll and nitrate LOBO data, to be consistent with discrete measurements, and calculated DOC from CDOM and TSS from turbidity. The corrected LOBO datasets are shown in Fig. 2, Fig. 3. On May 10 and May 30, when we took discrete samples both at the surface and at the LOBO depth,
Discussion
The LOBO accurately determined a number of biogeochemical parameters. Exceptions were nitrate and chlorophyll; the turbid water column, high DOC (Pellerin et al., 2013), or initial instrument calibration, probably affected the quality of our SUNA NO3 measurements. Fortunately, the observed interference appeared to be consistent and therefore the LOBO measurements were corrected using direct comparisons with discrete samples. These observations suggest that it is essential that in situ optical
Conclusion
Continuous automated biogeochemical observations at Bowers, Delaware, suggest that the Murderkill Estuary and the adjacent Delaware Bay are an example of an efficient, tidally-coupled biogeochemical reactor during the summer. Nutrient loads from point and nonpoint sources in the watershed are not consumed in the Estuary due to high turbidity, but have a significant and immediate effect on primary production in the adjacent Delaware Bay. In the Bay, transport velocities probably decrease,
Acknowledgments
We would like to thank the Kent County, Delaware, Board of Public Works, (Hans Medlarz, Director) for its financial support of this research and technical assistance in the field. We would also like to thank the Satlantic/WET Labs/Sea-Bird technical assistance teams for their support in maintaining the LOBO instrument, Peter Raymond (Yale School of Forestry and Environmental Studies) for the use of his lab to process the DOC samples, Deb Jaisi (College of Agriculture and Natural Resources,
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Now at Environmental Program, Colorado College, Colorado Springs, CO 80903, USA.