Dissolved Oxygen and Nutrients
Dissolved Oxygen (DO)

Oxygen is a fundamental requirement for nearly all animal life on our planet.

Aquatic life, such as small protozoans, insects and fish, use the gaseous oxygen dissolved in water - dissolved oxygen, or DO - to respire (breathe).

The levels of DO required for aquatic life varies. Different species of fish have different DO needs, as do their juvenile or adult life stages. Whether they are migratory or resident species also matters. 

DO Basics

Oxygen enters water:

  • Direct absorption from the atmosphere; and

  • By-product of photosynthesis from algae and aquatic plants.

Lower DO levels occur:

  • Warmer water generally contains less oxygen than colder water, meaning DO naturally varies daily as water and air temperatures change and seasonally;

  • Saltier water (higher salinity) carries less oxygen than fresh water;

  • When oxygen-depleting materials are discharged from wastewater treatment plants;

  • From algal blooms, or other decomposition of organic matter; and

  • From the oxidation of ammonia and other nitrogen-based compounds.

Dissolved Oxygen in the Delaware River

In the non-tidal river, north of Trenton, N.J., dissolved oxygen levels - and water quality in general - is better than standard.

DRBC Special Protection Waters Program

DRBC Biomonitoring Program

In the tidal Delaware River, the Delaware Estuary, DO levels were historically problematic. However, over the past 50+years, the DRBC has addressed the worst of the estuary's DO problems. 

Delaware Estuary Dissolved Oxygen: Historical Problems & Recovery

Check out this animated graphic showing summertime (July and August) DO improvement from the late 1960's through 2019. Please note: In the graphic, the criteria line is 24-hour mean DO, while the measurements are daytime spot measurements near surface.

Created by John Yagecic, P.E., DRBC Water Quality Assessment Manager


Despite significant improvement, DO conditions in the Delaware Estuary remain a concern 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 Philadelphia's Ben Franklin Bridge.

This is a concern for aquatic life, for example juvenile fish species native to the estuary (for example, Atlantic sturgeon). At these depressed oxygen levels, studies have shown that significant mortality can occur.

DRBC Studies

The DRBC is currently examining whether current DO criteria need revision to be better protective of fish reproduction. Also being studied is whether lower DO levels are connected to elevated nutrient levels (called "eutrophication") in the Delaware Estuary.

Learn More: DRBC Aquatic Life Designated Use Study

Presentation: Restoring Water Quality in the Main Stem Delaware River: Programs, Policies and Strategies (pdf 3.6 MB: presentation given by DRBC staff at the 9th Annual Delaware River Watershed Forum, Sept. 2021)

In May, July and August 2021, DRBC will be collecting DO profiles along transects at the Ben Franklin Bridge and Chester, Pa. to better understand how DO varies at different depths (i.e., stratification) in the estuary. 

DRBC is also studying how increasing temperature and salinity due to sea level rise and climate change affect DO levels in the estuary. 

DRBC staff collect a water sample to monitor nutrient levels in the Delaware River. Photo by DRBC.
DRBC staff preps a sample for nutrient analysis. Photo by DRBC.
DRBC staff collect water samples to monitor nutrient levels. Photos by DRBC.

What are Nutrients?

A nutrient is any substance assimilated by living things that promotes growth. The term is generally applied to nitrogen and phosphorus, although it can also be applied to trace nutrients like silica and iron.

While nutrients are good at certain levels, high concentrations in aquatic ecosystems can overstimulate the production of plants and algae, which use DO as they decompose. This reduces oxygen levels in the water, leading to reduced water quality, which harms fish, wildlife and human health.

Where do Excess Nutrients Come From?

Excess levels of nitrogen and phosphorus enters waterways 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.

Along Philadelphia's urban river corridor, an area of reduced dissolved oxygen persists at certain times of the year.

Elevated levels of nitrogen, predominantly stemming from the discharge of ammonia from sewage treatment plants, has been identified as a potential cause. 

Addressing Excess Nutrients in the Delaware River 

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 Estuary; and

--Determining the nutrient criteria or nutrient-related criteria needed to protect aquatic life, public and industrial water supplies and recreational uses of these shared resources.

Monitoring Nutrients

Today, the main focus is to identify appropriate levels of nutrients and the necessary measures to take in order to improve DO levels in the Delaware Estuary.

Working with its Water Quality Advisory Committee (WQAC), which includes state and federal agencies and the regulated and environmental communities, the DRBC has focused on the following nutrient-related initiatives:

--Point Source Nutrient Monitoring in the Delaware Estuary

--Non-Point Source Loading to the Delaware River & Delaware Estuary

--Delaware River (Non-Tidal) Bioassessment (see DRBC Biomonitoring Program)

--Monitoring of long-term, or ultimate, biochemical oxygen demand

--Creation of an Expert Panel to advise the WQAC and the DRBC on the development and use of a Delaware Estuary Eutrophication Model

Learn more: DRBC Water Quality Advisory Committee

Learn more: DRBC Aquatic Life Designated Use Study

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.

Related Reports: