Dissolved Oxygen and Nutrients
All About Dissolved Oxygen (DO)
DRBC staff collects a sample from the tidal Delaware River to monitor dissolved oxygen levels. Photo by DRBC.
DRBC staff collects a sample from the tidal
Delaware River to monitor dissolved oxygen
Photo by DRBC.

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. 

Oxygen enters water

•  by direct absorption from the atmosphere; and

•  as a 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

DO in the Non-Tidal Delaware River

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

DO in the Tidal Delaware River

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. 

Check out this animated graphic showing summertime (July and August) DO improvement from the late 1960's through 2021.

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

Current DRBC Studies

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, there are, at times, DO sags mid-summer in areas near Philadelphia's Ben Franklin Bridge and Chester, Pa. 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 mortality can occur.

The DRBC is currently examining whether current DO criteria need revision to be better protective of fish reproduction.

The DRBC is also studying:

  • How lower DO levels are connected to elevated nutrient levels (called "eutrophication") in the Delaware Estuary

  • How DO varies at different depths (i.e., stratification) in the Delaware Estuary

  • How climate change impacts of increasing temperature and sea level rise will affect estuary DO levels
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.

High Nutrients - Low DO

While nutrients are good at certain levels, high concentrations in aquatic ecosystems - known as eutrophication - overstimulates 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.

In 2024, the DRBC is collaborating with the N.J. Department of Environmental Protection (NJDEP) to collect nutrient samples from 4 N.J. tidal tributaries: Blacks and Crosswicks creeks (sampled as one system), Pennsauken Creek, Rancocas Creek and Raccoon Creek. This monitoring effort is targeting tributaries with point-source discharges (NPDES permit holders) and known water quality impairment to get a better understanding of numerical nutrient concentrations in these waterways. The nutrient grab samples will be collected every other week. Sondes will also be deployed to collect DO, temp, pH, SC, turbidity and phycocyanin data. This project could go through 2028.

The DRBC also works with its Water Quality Advisory Committee (WQAC), which includes state and federal agencies and the regulated and environmental communities, to focus 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

  • Development and use of the Delaware Estuary Eutrophication Model

Learn More

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.

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