Freshwater Lake and Pond Water Quality

RIDEM is responsible for assessing the water quality conditions in Rhode Island's lakes and ponds. Fortunately, due in large part to the long-running volunteer monitoring program coordinated by URI-Watershed Watch and RIDEM Office of Water Resources Aquatic Plant Surveys, there is a sizable amount of data for many of the freshwater lakes and ponds RIDEM is tracking.

The following information from RIDEM's 2022 Integrated Water Quality Monitoring and Assessment Report summarizes the assessment results for lakes and ponds.

Water Pollution

Water pollution problems in Rhode Island freshwater lakes and ponds include mercury in fish tissue, excessive nutrients, and to a lesser extent metals and pathogens. To restore water quality conditions, RIDEM develops water quality restoration plans, known as TMDLs, which identify necessary pollution reduction actions. 

As of April 2022, the most prevalent cause of water pollution for lakes and ponds documented in the Integrated Report was mercury in fish tissue. Mercury is a potent neurotoxin that poses risks to human health. Fifty-two lakes are identified as having high mercury in fish tissue. A regional mercury TMDL that applies to 19 Rhode Island lakes has been completed. The majority of mercury in the environment is released into the air, and it reaches waterbodies through atmospheric deposition. 

Collection of fish tissue data continues to be a major data gap in Rhode Island, but RIDEM has collaborations with state and federal partners to continue a limited fish tissue sampling program to document mercury in fish tissue impairments in publicly accessible lakes and ponds. The Rhode Island Department of Health has developed guidance on eating fish safely.

The second most common pollution in Rhode Island's lakes and ponds is elevated levels of the nutrient phosphorus and associated high chlorophyll and/or low dissolved oxygen. More than half of assessed lake acres had high nutrient pollution. When lakes or ponds receive too many nutrients, the response typically is excess algal or plant growth that decays and may deplete dissolved oxygen levels. Low dissolved oxygen impacts fish and other aquatic life and may lead to fish kills. Excess algal or plant growth can alter the lake or pond's fish and aquatic life and lead to low dissolved oxygen.

Aquatic Invasive Species

The largest non-pollutant cause of impairment in lakes and ponds is due to the presence of aquatic invasive species (AIS), primarily plants. As of April 2022, RIDEM reported 73 lakes or ponds had aquatic invasive species in sufficient quantities to impair one or more beneficial uses of the waterbody. RIDEM field survey work has documented AIS to be widespread in Rhode Island freshwaters.

In many of Rhode Island's impacted lakes and ponds, aquatic invasive plants are well established and require a long-term plant management strategy. RIDEM recommends addressing aquatic invasive plant management as part of a comprehensive lake management plan. See the side navigation bars on the left of this page for more information.

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