The Delaware Estuary is approximately 220 km long with five estuarine regions: upper tidal river, urban river, upper bay (turbidity maximum), mid-bay, and lower bay. The flushing rate is about 80 days. The upper tidal river receives significant nutrient inputs from agriculture and waste treatment plants in the greater Trenton area. The urban river portion of the estuary receives large nutrient inputs from the greater Philadelphia area. As the bay widens downstream, nutrient concentrations are diluted. The lower bay has relatively low nutrient concentrations and clearer waters than the upper regions. There is no appreciable input of nutrients in the saline portion of the estuary. Major wastewater treatment plant upgrades in the 1970s and 1980s along with a phosphate detergent ban caused significant decreases in nutrient loading to the estuary.
The estuary stratifies in the spring, coinciding with the only algal blooms consistently observed in the estuary, diatom blooms occurring primarily in the mid-estuary. There is minimal zooplankton grazing and cells sink to the bottom but because of the cold temperatures in the bottom waters there is little microbial respiration and DO demand. The water column is well-mixed at other times of the year. DO depletion sometimes occur in the urban river region but generally hypoxia is not a problem in the estuary.
The estuary is relatively turbid with the upper bay, or turbidity maximum region, being the most turbid. The high turbidity levels are primarily due to the resuspension of bottom sediments by tidal currents. Total suspended solids account for 88 percent of light attenuation.
Nutrient concentrations are high in the Delaware estuary with large seasonal variations in concentration. The urban river has higher average chlorophyll a than the bay in the summer. At different times and places, the estuary shows limitations of primary production by light, nutrients, grazing, and possible contaminant inhibition. Because the Delaware Estuary is so turbid, there are no submerged aquatic vegetation and no major macroalgal populations.
Water quality data are collected at stations located throughout the Delaware Estuary and used to examine spatial and temporal trends and relationships among parameters. Despite very high nutrient loading, the Delaware Estuary has no hypoxia problem or harmful algal blooms. High turbidity, high flushing, and a well-mixed water column in the summer months are factors (National Estuarine Experts Workgroup 2010).
Reference:
National Estuarine Experts Workgroup. 2010. Nutrients in Estuaries: A Summary Report of the National Estuarine Experts Workgroup 2005–2007. U.S. Environmental Protection Agency. Accessed October 2016. https://www.epa.gov/sites/production/files/documents/nutrients-in-estuaries-november-2010.pdf.