Meteorological Forcing of the Biogeochemistry of the Murderkill River and Estuary (Delaware, USA) Determined by High Frequency Continuous Monitoring

Abstract:

Comparison of high-frequency meteorological, physical (tide, salinity, and flow), and biogeochemical monitoring data (dissolved oxygen, turbidity, chlorophyll, organic matter, and nitrate) in the Murderkill River and Estuary and Delaware Bay during normal and extreme climatic events illustrates how the primary productivity (PP) response of the freshwater riverine system is dominated by seasonal and local (PAR, temperature, streamflow) factors, while that of the Estuary is dependent on season and the balance between regional (winds and tides) and local forcing factors (runoff).

Wind events have little impact on river discharge and PP, but have dramatic impacts on estuarine hydrodynamics, turbidity, exchange with Delaware Bay, and PP, depending on wind direction, duration, and intensity and tides during the event. Most rainfall events have little impact on the biogeochemistry of the estuary but are a dominant factor in determining PP in the river. Rainfall and wind together have larger impacts on the estuary than either alone, particularly on turbidity, nitrate and chlorophyll concentrations. Increased turbidity within the estuary leads to decreased estuarine chlorophyll and oxygen concentrations and PP, leading to increased export of nitrogen to nitrogen-limited Delaware Bay where it supports a zone of persistently high PP.

Nitrate concentrations and flux in the river peak in the non-growing season when runoff is largest and retention times, PAR, and PP are smallest. Because nitrate is delivered by groundwater discharge, rainfall dilutes nitrate concentrations in the river. In contrast, nitrate concentrations and flux are minimum in the growing season when runoff is smallest and retention times, PAR, and PP are greatest. Nitrate concentrations increase during rainfall and low PAR events during the growing season because of lower rates of PP, and increase in nitrate input from ground and surface-water runoff.

Simultaneous monitoring of meteorological, physical, and biogeochemical parameters provides invaluable insights into the response of PP to estuarine loads, biogeochemistry, and physical forcing. These results can be integrated with process models to make predictions concerning the evolution of coastal ecosystems in light of climate change.