Water-quality monitoring project sheds new light on pollution problems

August 28, 2021

A new research effort by the Delaware Center for the Inland Bays is offering deeper insight into the true state of water quality in the bays’ most polluted areas.

Continuous monitoring devices called “sondes” are part of a new network of sensors that collect water-quality data every 30 minutes, 24 hours a day, at five locations in the Inland Bays watershed. The establishment of this network is part of the center’s recently updated environmental monitoring plan for Delaware’s Inland Bays. The plan aims to track the status and trends of key environmental indicators to gauge the health of the bays, their tributaries, and the fish and shellfish that depend on them.

Expanding water-quality monitoring efforts is a key plan recommendation that will help scientists learn more about the true state of water quality and environmental conditions in the Inland Bays. That knowledge will help inform management decisions while also educating the public, and providing scientific support for the resources and policies needed to restore the bays.

“Long-term monitoring is key to not only understanding the health of our bays, but also to evaluating the effectiveness of the many investments that have been directed at improving water quality and habitats,” said Dr. Marianne Walch, the center’s science and restoration coordinator. “The environmental monitoring plan is intended to guide future research, and increase the integration and efficiency of monitoring efforts across organizations involved in collecting data.”

The center’s plan also outlines the need for studies of bay grasses which are good indicators of water quality, identifying emerging contaminants, tracking sea-level rise impacts, oyster growth, shoreline and marsh changes, and much more.

The center is already gaining insight from continuously collected water-quality data. During several recent fish kills reported in Rehoboth and Indian River bays, the center’s monitoring devices recorded extremely low oxygen levels in multiple areas of the bays – indicating that the unhealthy conditions and resulting death of marine life, including over a million menhaden in June alone, are neither isolated nor uncommon this time of year.

“Despite the millions of dollars that have been put into management actions, water quality in many parts of our bays is not improving,” said Walch, especially considering data collected from portions of the Indian River. “Data collected monthly or bimonthly by the state of Delaware does not reveal the full picture of what is actually happening in polluted areas. That points out the importance of the center’s work to collect and share high-frequency data.”

Intense algae blooms, which are driven by nutrient pollution from agricultural and urban runoff, are common in the bays this time of year. They also decrease levels of dissolved oxygen, which fish and shellfish need to survive.

The center’s sondes serve as continuous eyes on the bays, collecting data on dissolved oxygen levels, salinity, water clarity, pH and temperature from sites in the Indian River, Pepper Creek and Guinea Creek.

Data shows that between June 1 and Oct. 1, 2020, oxygen levels in the upper Indian River failed to meet the state’s water-quality standard for dissolved oxygen on 75 percent of mornings. On five occasions, dissolved oxygen levels remained below the healthy limit for eight hours or longer. Similar conditions are being observed in summer 2021.

The center has partnered with the University of Delaware on this project, with plans to install additional stations in 2021 and 2022.

This new research effort is featured in the Spring/Summer 2021 edition of the Inland Bays Journal, a free, seasonal publication that highlights the center’s projects and accomplishments. The journal is mailed to anyone who signs up at

Nutrient pollution from nitrogen and phosphorus remains the greatest threat to water quality in the bays. The center is currently working on the five-year update to its State of the Inland Bays report, which will provide the latest information on how the bays are faring. 

The Delaware Center for the Inland Bays is a nonprofit organization established in 1994, and is one of 28 National Estuary Programs. With its many partners, the center works to preserve, protect and restore Delaware’s Inland Bays and their watershed. To learn more, go to


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