Eutrophication

Eutrophication is the enrichment of water with nitrogen and phosphorus — from uneaten feed, fish excretion, and fertilizer — that fuels excessive algal and phytoplankton growth, whose decomposition then consumes dissolved oxygen and destabilizes the whole culture system. In ponds the progression is familiar: water turns green and turbid, Secchi depth drops below 20–30 cm, a dominant algal bloom forms, and when the bloom crashes — often after a cloudy spell or nutrient shift — decomposition drives a hypoxia event that can kill stock overnight. Symptoms farmers watch for are elevated chlorophyll-a (above roughly 30 µg/L signals bloom pressure), high pH above 9 during afternoon photosynthesis, and unstable morning DO. Chronic eutrophication also produces off-flavor compounds (geosmin, MIB) that taint harvest, and in cage farming the same process drives organic deposition beneath cages, smothering benthic life. Management combines feeding discipline to minimize nutrient input, water exchange, aeration to buffer oxygen, algal management, and in discharge-regulated regions, effluent treatment before water leaves the farm. For the operator, eutrophication is the central water-quality failure mode of intensive farming, connecting feed waste, pond photosynthesis, hypoxia, and off-flavor risk into one causal chain.