Estuaries along the U.S. coast are showing similar patterns of rising water temperatures, increased algal growth, and reduced oxygen levels, all of which can harm water quality and fish populations. A new study led by Kaitlin Reinl, assistant professor of biological sciences at Rensselaer Polytechnic Institute (RPI), highlights these trends and suggests they could significantly influence how water management and environmental policies are developed. The research, published in the journal Communications Earth & Environment, involved a team from 14 reserves in the National Estuarine Research Reserve System (NERRS), NOAA, state water agencies, and academic institutions, and was partly funded by NOAA. The study analyzed over 250 million data points collected from 129 locations in 29 of the 30 NERRS estuaries between 2002 and 2022. These estuaries are spread across a variety of climate zones, giving the study a broad geographic scope. The researchers focused on understanding the factors behind the widespread increase in eutrophication—when water becomes overly enriched with nutrients—and hypoxia, or low oxygen levels, which can be harmful to aquatic life. To achieve this, they used data from the NERRS System-wide Monitoring Program (SWMP), which includes automated sensors that take measurements every 15 minutes and monthly chemical analyses by reserve staff. The research found that changes in nutrient levels are strong indicators of algal growth, measured by chlorophyll levels, and that decreasing oxygen levels are linked to warmer water and increased algal biomass. These nutrient changes are a primary cause of increased algal growth across the U.S., which can block sunlight from reaching underwater plants essential to fish and other aquatic life. Additionally, rising temperatures and algal growth contribute to lower oxygen levels, which can be dangerous for fish and other marine species. The researchers referenced previous studies that estimate the annual economic cost of coastal eutrophication at $100 million in the U.S. and $1 billion in European Union countries. These costs come from public health issues, loss of recreational activities, and damage to commercial fisheries. The study aims to help guide future water management by identifying specific actions that can protect and restore these vital ecosystems.