Algal Blooms

Aquaculture significantly contributes to the increase in harmful algal blooms (HABs) by releasing nutrient-rich waste, including nitrogen and phosphorus, into the surrounding waters. These excess nutrients–from uneaten feed, fish feces, and effluent discharge–accelerate eutrophication, stimulating algae overgrowth. This nutrient loading in open-water and land-based aquaculture systems promotes dense algal blooms that block sunlight, reduce oxygen levels, and degrade water quality.

As these HABs die and decompose, they consume dissolved oxygen, creating dead zones where marine life struggles to survive. This oxygen depletion forces fish and invertebrates into shallower water and disrupts local food webs. HABs can also produce potent toxins harmful to both marine organisms and humans. Shellfish and other filter feeders accumulate these toxins, poisoning humans. Additionally, some blooms damage fish gills directly or release mucus that suffocates marine animals.

Aquaculture itself is also a victim of HABs. Harmful blooms cause mass mortalities in farmed fish and shellfish through oxygen depletion or exposure to toxins. These events can lead to large-scale harvest delays and production losses, resulting in high economic losses. The growing frequency and severity of HABs are further exacerbated by climate change and warming waters, increasing the risk for aquaculture operations and global coastal ecosystems. Aquaculture drives algal blooms through nutrient pollution, and these blooms in turn harm aquaculture and the environment – creating a destructive feedback loop that threatens marine biodiversity, water quality, and food safety.

Photo credit Woods Hole Oceanographic Institute's Harmful Algal Blooms Report.