Nutrient enrichment of marine waters promotes the growth of algae, either as attached multicellular forms (e.g. sea lettuce) or as suspended microscopic phytoplankton, because algae can grow faster than larger vascular plants. Small increases in algal abundance or biomass have subtle ecological responses that can increase production in food webs sustaining fish and shellfish, even producing higher fish yields. However, over-stimulation of algal growth leads to a complex suite of interconnected biological and chemical responses that can severely degrade water quality and threaten human health and sustainability of living resources in the coastal zone.
As algal biomass builds during blooms it forms aggregates that sink and fuel bacterial growth in bottom waters and sediments. Bacterial metabolism consumes oxygen. If the rates of aeration of water by mixing are slower than bacterial metabolism, then bottom waters become hypoxic (low in oxygen) or anoxic (devoid of oxygen), creating conditions stressful or even lethal for marine invertebrates and fish. Seasonal occurrences of dead zones devoid of oxygen and animal life have expanded in the Gulf of Mexico (where the dead zone has approached the size of New Jersey), the Baltic Sea, and Sea of Marmara as a consequence of eutrophication from nutrients delivered by large rivers.