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Dissolved Oxygen and Nutrient Levels in the Delaware Estuary
Dissolved Oxygen (DO)

Most forms of aquatic life need dissolved oxygen (DO) to respire or “breathe,” and it is important to maintain adequate levels for both migrating and native fish species, juveniles and adults. Oxygen enters water both by direct absorption from the atmosphere and as a by-product of photosynthesis by algae and aquatic plants.

Warmer water generally contains less oxygen than colder water, so the amount of DO naturally varies seasonally and daily as water and air temperatures change. Salinity also affects DO; saltier water carries less oxygen than fresh water. Other things that can decrease the amount of dissolved oxygen in water include wastewater discharges, decaying leaves and algae, some chemical compounds, and nutrients.

Although the worst of the dissolved oxygen problems have been addressed by the commission over the past 50 years, DO conditions in the Delaware Estuary remain a concern even today. Automatic monitors track dissolved oxygen levels at four locations, and although current conditions typically meet the criteria, mid-summer DO is, at times, only 50% or less of full saturation levels in the areas near the Ben Franklin Bridge.

Here's an animated graph created by DRBC's John Yagecic, P.E. showing summertime (July and August) DO improvement from the late 1960's through 2018. DRBC has been monitoring DO in the Delaware Estuary for over 50 years. While there continues to be a DO sag in the most urbanized part of the estuary, DO levels have greatly improved since the 1960s. Please note: In the graphic, the criteria line is 24-hour mean DO, while the measurements are daytime spot measurements near surface.

The commission is currently examining whether current criteria for dissolved oxygen may need revision to be better protective of fish reproduction. And, looking ahead, temperature and salinity in the tidal river may increase due to sea level rise and global climate change. This could potentially lower the river’s oxygen carrying capacity, therefore making other water quality improvements necessary just to maintain the current, yet still highly changeable, levels of dissolved oxygen in the estuary.

View additional dissolved oxygen information.

Connecting DO and Nutrient Levels in the Delaware Estuary

DRBC and its partners in state and federal agencies, the regulated community, and the environmental community have concurrently begun to look at the connections between dissolved oxygen and nutrient levels in the estuary. Nutrients, such as nitrogen and phosphorus, are natural, essential components of aquatic ecosystems, assimilated by living things to promote growth.

While nutrients are good at certain levels, high concentrations can overstimulate the production of plants and algae, which utilize dissolved oxygen as they decompose, therefore reducing oxygen levels in the water. This leads to poor conditions in streams and reduced water quality. According to the U.S. EPA, “high levels of nitrogen and phosphorus in our lakes, rivers, streams, and drinking water sources cause the degradation of these water bodies and harm fish, wildlife, and human health.”

Nitrogen and phosphorus pollution comes from fertilizers, animal waste, septic systems, storm runoff, and sewage treatment plants. This type of pollution is reported to be a problem in more than half of the water bodies in the nation, including the Delaware Estuary which has an area of reduced dissolved oxygen in the urban river corridor. Elevated levels of nitrogen have been identified as a potential cause.

DRBC serves as the lead coordinating agency among the U.S. EPA and the basin states for evaluating nutrient conditions in the shared interstate waters of the Delaware River and the Delaware Estuary. The DRBC also is the lead agency in determining the nutrient criteria or nutrient-related criteria, if appropriate, that are needed to protect aquatic life, public and industrial water supplies, and recreational uses of these shared resources.

DRBC is currently working to identify appropriate levels of nutrients and necessary measures to take, especially in relation to dissolved oxygen. To address both nutrients and dissolved oxygen, a process has been initiated to measure the point sources of nutrients and oxygen-depleting materials and to build a water quality model to integrate this information and forecast future scenarios for the Delaware Estuary. Although the results from such efforts are not yet available, the goal is to select an appropriate path towards a healthy, functioning ecosystem in all parts of the Delaware Estuary.

View additional nutrients information.