Human agriculture and industry reshape nutrient balance in coastal waters, fueling harmful algae

A comprehensive analysis by scientists led by John Harrison of Washington State University’s College of Environmental Science shows that terrestrial activities, especially intensive farming and industrial effluents, are dramatically changing the nutrient makeup of near-shore waters. By merging global estimates of riverine, atmospheric and oceanic nutrient inputs, the team examined 66 major and 152 minor coastal zones. While natural ocean currents still deliver the bulk of nutrients, many regions now receive more human-origin nitrogen than marine sources, and phosphorus levels are rising due to waste streams.

Simultaneously, silicon concentrations are falling because dams trap it upstream, disadvantaging diatoms and favoring harmful, non-silicon-requiring algae. Using the ICEP (Coastal Eutrophication Potential Index), the study identified 128 smaller coastal areas with rising scores and 35 sites where the index has crossed a threshold that signals imminent algal blooms. Modeling suggests that cutting nitrogen and phosphorus inputs by half could remove many coasts from danger zones, though some areas would still need additional measures because of strong natural oceanic nutrient supply.