Chlorophyll trends are negative for lakes but positive for estuarine–coastal waters

Nutrient pollution degrades water quality by increasing phytoplankton biomass and occurrences of harmful algal blooms while decreasing oxygen concentrations. Policies have been implemented to reduce nutrient inputs to lakes and estuarine–coastal waters. Success of these policies is shaped by processes that regulate conversion of nutrients into phytoplankton biomass. These processes are changing, and our study is an assessment of how phytoplankton biomass has changed over the period 2000–2019, using chlorophyll a trends measured in 191 lakes and 159 estuarine-coastal sites. Our results show that phytoplankton biomass has decreased in most lakes but increased in most coastal sites. Nutrient pollution is a challenging environmental problem, and our results show that it might be particularly challenging for ecosystems situated in densely populated coastlines.