Eutrophication of freshwater and coastal marine ecosystems is a global problem caused by human activities that have significantly altered nutrient fluxes from landscapes to water bodies. This review discusses how eutrophication affects algal biomass and species composition in both freshwater and coastal marine systems. Recent research shows that eutrophication leads to predictable increases in algal biomass in lakes, reservoirs, streams, rivers, wetlands, and coastal marine ecosystems. Cyanobacteria dominance in phytoplankton is also increasing globally. The response of algal biomass to nitrogen and phosphorus availability is consistent across different ecosystems. Nutrient loading control efforts have positive effects on coastal marine ecosystems. However, understanding of eutrophication effects on estuaries and coastal marine ecosystems is limited. Despite this, marine phytoplankton biomass responds sensitively to changes in nitrogen and phosphorus inputs. Efforts to manage nutrient inputs to the seas can improve coastal water quality. Future research should focus on developing models that link ecosystem responses to nutrient loading in both freshwater and marine systems. The review highlights the need for continued research and management to address the impacts of eutrophication on aquatic ecosystems.Eutrophication of freshwater and coastal marine ecosystems is a global problem caused by human activities that have significantly altered nutrient fluxes from landscapes to water bodies. This review discusses how eutrophication affects algal biomass and species composition in both freshwater and coastal marine systems. Recent research shows that eutrophication leads to predictable increases in algal biomass in lakes, reservoirs, streams, rivers, wetlands, and coastal marine ecosystems. Cyanobacteria dominance in phytoplankton is also increasing globally. The response of algal biomass to nitrogen and phosphorus availability is consistent across different ecosystems. Nutrient loading control efforts have positive effects on coastal marine ecosystems. However, understanding of eutrophication effects on estuaries and coastal marine ecosystems is limited. Despite this, marine phytoplankton biomass responds sensitively to changes in nitrogen and phosphorus inputs. Efforts to manage nutrient inputs to the seas can improve coastal water quality. Future research should focus on developing models that link ecosystem responses to nutrient loading in both freshwater and marine systems. The review highlights the need for continued research and management to address the impacts of eutrophication on aquatic ecosystems.