Estimating the Influence of a Tributary on Primary Productivity in Delaware Bay from Continuous Data: Biogeochemical and Ecological Responses to Inputs from the Murderkill

Abstract:

Since April 2012, the University of Delaware has operated a real-time water quality monitoring station at Bowers, DE, to determine the effect of nutrient loads from the Murderkill Estuary to the Delaware Bay. The Land Ocean Biogeochemical Observatory (LOBO; Hach/Sea-Bird Electronics) is deployed under a public dock, with support from the Kent County Board of Public Works and Delaware’s Department of Natural Resources and Environmental Control, and in collaboration with the USGS that operates a gauging station at the same site (01484085). Every hour the LOBO measures salinity, temperature, pressure, dissolved oxygen, turbidity, chlorophyll, CDOM, nitrate (NO₃^-) and orthophosphate (PO₄^3-). Ebb tide measurements reflect conditions in the Estuary and flood tide measurements reflect conditions in Delaware Bay. Real-time data are available to the public at kentcounty.loboviz.com.

Using hourly data from the LOBO, we are able to assess the nutrient and particle exchange between the Murderkill and the Delaware Bay, as well as biogeochemical and ecological responses to this exchange. Primary production in the turbid Estuary is low and nutrients from the Murderkill are efficiently delivered to Delaware Bay, where O₂ production response to nitrate influx is almost instantaneous. In the summer, during the day, nutrients originating in the Estuary (ebb tide NO₃^- > 50-60 μM) stimulate phytoplankton growth in Delaware Bay, indicated by chlorophyll increases and O₂ supersaturation during flood tide. This suggests that the Murderkill and other similar tributaries are important nutrient sources to the Delaware Bay. We use a phase-sensitive analysis to extract data characterizing the Delaware Bay, and then use dissolved oxygen to calculate rates of primary production and respiration. Newly produced Delaware Bay phytoplankton (often > 40 µg Chla L^-1) may be an important labile carbon source supporting respiration in the Estuary and its surrounding salt marshes.