U.S. Estuaries Getting Warmer, “Greener,” and Lower in Oxygen

Algal blooms occur across U.S. estuaries, including Ohio’s Old Woman Creek (left) and lakes like Lake Erie (right). Photos courtesy Old Woman Creek Reserve and NOAA.

A study made possible by the National Estuarine Research Reserve System (NERRS) has found that many U.S. estuaries are getting warmer, greener, and lower in oxygen. Drawing on decades of data collected in 29 estuaries, researchers found that water temperature and levels of chlorophyll-a (a proxy for algae abundance) are significantly increasing at roughly two-thirds of the sites, while dissolved oxygen is significantly declining at about half. Published in Nature’s Communications Earth & Environment, the findings could have profound implications for water management and policy.

“We’ve had consistent problems with hypoxia [low oxygen] and eutrophication [over-accumulation of nutrients] that leads to excessive algae growth in our coastal waters,” says Kaitlin Reinl, the study’s lead author and an assistant professor of biological sciences at Rensselaer Polytechnic Institute. (Reinl began the work while serving as research coordinator at Wisconsin’s Lake Superior Reserve.) “We wanted to know what the big, primary drivers of these kinds of changes are on a large scale.”

To address that question, the team leveraged decades of data from the NERRS System-Wide Monitoring Program (SWMP), a long-running, standardized monitoring effort spanning Reserves across the coastal United States. Since its inception in the 1990s, SWMP has generated the most comprehensive data set on estuary water quality in the country. To identify factors contributing to the rise of eutrophication and hypoxia on a wide scale over time, the team analyzed over 250 million data points collected between 2002 and 2022 from 129 locations in 29 estuaries spanning multiple climate zones.

The NERRS System-wide Monitoring Program’s capacity for tracking short-term changes and long-term trends across a national system supports local action and national science and policy. SWMP data is available to the public at the Centralized Data Management Office at the University of South Carolina. Photos courtesy (top left clockwise) the South Slough Reserve, University of South Florida, Ohio’s Old Woman Creek Reserve, and Texas’ MIssion-Aransas Reserve.

“We can now say that altered nutrient concentrations are important predictors of increasing algal biomass and that declining oxygen is associated with warming waters and algal biomass,” says Reinl.

Increasing concentrations of nutrients leads to more algal biomass that prevents sunlight from reaching the plants below the surface that are needed by fish and other organisms. At the same time, higher water temperature and chlorophyll growth contribute to the decline of oxygen concentrations that can harm fish and other aquatic life.

Since many recreational and cultural industries rely on healthy estuaries, this knowledge may have profound effects on the economy. The researchers cited studies suggesting that coastal eutrophication costs the U.S. $100 million a year and European Union countries $1 billion annually “due to public health concerns, lost recreational opportunities and degraded commercial fisheries.”

Despite the widespread negative trends, the study found only a handful of sites with significant increases in the frequency of low oxygen events, suggesting that while the overall trajectory is concerning, most estuaries have not yet tipped into more severe, sustained low-oxygen conditions. Some of the steepest warming and oxygen declines occurred in the Northeast U.S., including in New Hampshire’s Great Bay. Notable warming has also occurred at more southerly sites like California’s Tijuana River and Georgia’s Sapelo Island.

“No single factor explains what’s happening in all estuaries, as nutrients, temperature, precipitation, turbidity, and local geography all interact differently from site to site,” says Denise Sanger, research coordinator of South Carolina’s ACE Basin Reserve. “That complexity requires access to a standardized, long-term monitoring dataset like SWMP; it makes it possible for us to discern the difference between short-term swings and decades-long change.”

This kind of large-scale synthesis is only possible because the SWMP framework ensures that Reserves collect data in a standardized way year after year. The program runs 300 stations nationwide, collecting more than 48 million publicly available readings on water quality and weather conditions every year.

“This study illustrates why the SWMP dataset is an irreplaceable national resource,” says Rebecca Roth, executive director of the National Estuarine Research Reserve Association. “It’s a deep record of how our estuary waters are changing and it belongs to everyone. It’s for scientists doing studies like these, for educators, for students who will be tomorrow’s scientists, and for communities and states who use this data to manage fisheries, support aquaculture, protect public health, and increase resilience.” 

Funding for this effort was provided by 2024 and 2025 awards from the National Oceanic and Atmospheric Administration (NOAA). The study engaged staff from 14 Reserves, NOAA, state water management agencies, and institutions of higher learning, including the University of South Carolina, Mississippi State University, Virginia Institute of Marine Science, University of North Florida, Rutgers University, and University of Wisconsin-Madison.