This laboratory study of microcosms showed that adding both nitrogen and phosphorus caused the phytoplankton biomass to increase and also stimulated bacteria to grow and reproduce. Laboratory studies have shown that lethal and sub-lethal effects in several common frog species are detected at nitrate concentrations above 2.5 milligrams per litre. Sub-lethal effects include altered growth and development, potentially resulting in late maturation and emergence from the water before development is complete. This in turn may limit the ability of amphibians to move in the land environment, making them more vulnerable to predators and dessication. The spring use of nitrogen fertilizer and the reproduction period of amphibians correspond, which creates a vulnerable period.
Excessive nutrients can also promote the growth of potentially toxic organisms.
In inland settings, toxic algae called cyanobacteria grow in warm, stratified surface waters that are enriched with phosphorus and low in nitrogen. Toxic cyanobacteria produce neurotoxins and hepatotoxins that have been fatal to fish and livestock when ingested. In coastal waters, nutrient enrichment can promote the growth of certain algae that produce toxins, which can accumulate in shellfish. The shellfish are only marginally affected, but the toxins can be acutely toxic to humans. A 1987 case of food poisoning from eating contaminated mussels was linked to a toxin produced by marine algae that were stimulated by nutrients, particularly nitrate, from a river in an agricultural district of Prince Edward Island.