2025 Baltimore Harbor Water Quality Update
With the 2025 water recreation season upon us, it’s a good time to review recent trends in water quality throughout Baltimore Harbor. For recreational activities like swimming and kayaking, Waterfront Partnership monitors bacteria levels in the Inner Harbor daily (M-F) and posts the results online by the next business day. To measure how healthy the Harbor is for wildlife, Blue Water Baltimore monitors ecosystem health throughout the Harbor as part of a larger regional monitoring program.
Recreational health and ecosystem health are different
Recreational health and ecosystem health measure very different things. Most animals are perfectly happy living alongside bacteria that could make people sick. Similarly, ecosystem indicators like water clarity, dissolved oxygen, and nutrients can negatively impact aquatic life but pose no direct risk to people. Waterfront Partnership issues the Healthy Harbor Report Card, which assesses both recreational and ecosystem health annually.
Recreational Health 🏊
The Baltimore Harbor is conditionally safe for swimming
In 2024, the location of Harbor Splash in Fells Point met the Maryland standard for water recreation 83% of the time. Of the 17% of samples that resulted in water contact advisories, the vast majority were collected during or just after rain events. When it had not rained, sites generally passed.
Sampling sites throughout the Inner Harbor tend to meet the swimming standard so long as it has not rained within 48 hours. Per MDE and EPA recreational water quality criteria, we use a pass/fail threshold of 104 MPN (Most Probable Number) enterocci /100mL. See results for all monitoring sites here.
Ecosystem Health 🐟
Baltimore Harbor receives "C" for ecosystem health in 2024
The reporting region for this analysis is a subset of sites monitored by Blue Water Baltimore. It includes the Inner Harbor, Middle Branch, and Mainstem Patapsco out to Fort McHenry. Water samples collected from this region show the Harbor once again received a “C” for ecosystem health. The Harbor also received a “C” in 2022 and 2023.
A “C” is a moderate score. This means Baltimore Harbor supports a diverse community of wildlife, but lacks resilience, meaning the ecosystem is vulnerable to intermittent changes in water quality caused by algal blooms, turnover events, and pollution. For example, in 2021, ecosystem health declined 10 points due to issues at the Patapsco Wastewater Treatment Plant, but scores rebounded in subsequent years.
Ecosystem health has significantly improved since 2010
In 2010, a State of the Harbor Report produced by the University of Maryland Center for Environmental Science gave Baltimore Harbor a “D” for ecosystem health. Between 2010 and 2013, ecosystem health scores improved by 22 points, increasing overall scores by a full letter grade - from “D” to “C”. Since that time, the overall ecosystem health grade has fluctuated only slightly between “C” and “C-”.
Dissolved Oxygen gets Harbor's first ever "A”
For the first time in the history of the monitoring data, the Baltimore Harbor region received an “A” for an ecosystem health indicator. Specifically, the Harbor received a score of 87/100 for dissolved oxygen, the highest score since monitoring began in 2010. Dissolved Oxygen has previously received an "A-," but this is the first "A".
River Otter spotted at the floating wetlands at the National Aquarium and marinas around the Baltimore Harbor
Dissolved oxygen is important for all aquatic animals. Just like animals that live on land, fish, crabs, oysters, and even zooplankton need sufficient oxygen to survive. An A means the Harbor is meeting the desired level of oxygen for a healthy habitat. This has likely contributed to an increase in the abundance of diversity of wildlife found in the Harbor in recent years, including rockfish, American eels, pipefish, and even river otters.
Chlorophyll a has been the worst-performing indicator since 2020
In 2024, Baltimore Harbor received a “D” for Chlorophyll a, the same score it received in 2022 and 2023. Chlorophyll a tells us if there is too much algae in the water. Algal blooms may be toxic to fish and block sunlight from reaching underwater plants. In 2024, Chlorophyll scores were the best in the Inner Harbor and the worst southeast of the City, where the Tidal Patapsco mixes with the Chesapeake Bay.
More information
The overall ecosystem health of the harbor includes additional indicators. To see grades and graphs going back to 2010, click here.
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