Eutrophication

Primary reference(s)

Cloern, J., 2001. Our evolving conceptual model of the coastal eutrophication problem. Marine Ecology Progress Series, 210, 223–253.

Nixon, S. W., 1995. Coastal marine eutrophication: A definition, social causes, and future concerns. Ophelia, 41(1), 199–219.

NOAA, 2007. National Estuarine Eutrophication Assessment: Update. National Centers for Coastal Ocean Science, National Oceanic and Atmospheric Administration (NOAA). Accessed 21 January 2025.

Pannard, A., Souchu, P., Chauvin, C., Delabuis, M., Gascuel‐Odoux, C., Jeppesen, E., Le Moal, M., Ménesguen, A., Pinay, G., Rabalais, N. N., and Souchon, Y., 2024. Why are there so many definitions of eutrophication? Ecological Monographs, p.e1616.

UNEP, 2015. Mediterranean Action Plan. Report of the Online Groups on Eutrophication, Contaminants and Marine Litter. United Nations Environment Programme (UNEP). Accessed 21 January 2025.

Annotations

Key relevant UN convention / multilateral treaty

The UNECE Convention on Long-range Transboundary Air Pollution adopted the Protocol to Abate Acidification, Eutrophication and Ground-level Ozone in Gothenburg, Sweden, on 30 November 1999. The Protocol sets national emission ceilings from 2010 to 2020 for four pollutants: sulphur (SO₂), nitrogen oxides (NOₓ), volatile organic compounds (VOCs), and ammonia (NH3).

Sustainable Development Goal (SDG) Target 6.3 (water quality) and SDG Target 14.1 (marine pollution) directly address nutrient pollution from land-based sources, aligning eutrophication with the broader global sustainability agenda. (https://sdgs.un.org/2030agenda). Although these are not binding, they provide useful guidance.

Drivers

While there are some causes of natural eutrophication (Knight, 2021), the primary drivers stem from human activity. Untreated or inadequately treated sewage remains a major source of nitrogen and phosphorus, often originating from municipal wastewater or agricultural runoff containing human waste (Akinnawo, 2023). Biomass processing industries such as food processing, animal feed production, and pulp and paper mills also contribute organic matter and nutrient-rich wastewater when not properly managed (Tusseau-Vuillemin, 2001).

Intensive agricultural practices, particularly overuse of fertilisers and manure, as well as soil erosion due to cultivation techniques, contribute to nutrient runoff into water bodies (Akinnawo, 2023). Aquaculture can also contribute to eutrophication through nutrient-rich feed and fish waste (Talbot, 1994). Atmospheric deposition of nitrogen oxides (NOₓ) from fossil fuel combustion may increase nutrient enrichment in water bodies via precipitation (Paerl, 1995).

Land-use changes-including urbanisation, deforestation, drainage of wetlands, removal of riparian vegetation, and damming of rivers-disrupt natural nutrient cycling and filtration, increasing nutrient loads in aquatic systems (Rodríguez-Gallego et al., 2017).

Environmental stressors such as rising sea surface temperatures and increased salinity can intensify stratification, limiting vertical mixing and promoting algal growth (Coffey et al., 2018). Flooding can flush fertilisers and sewage into water bodies, increasing nutrient loads (Nazari-Sharabian et al., 2018). Forest fires also contribute to eutrophication by releasing nitrogen oxides into the atmosphere, which are later deposited over water bodies (Paerl, 1995).

Impacts

Eutrophication causes cascading effects on aquatic ecosystems. Nutrient enrichment favours fast-growing phytoplankton, leading to shifts in phytoplankton community structure (Chrislock et al., 2013). Reduced light penetration and algal overgrowth result in the loss of seagrass habitats essential for marine biodiversity (Pick, 1991). Oxygen depletion from decaying biomass leads to hypoxic or anoxic conditions, causing fish kills and harming benthic organisms (Akinnawo, 2023). 

These changes disrupt food webs, reduce biodiversity, and compromise the overall health and resilience of aquatic ecosystems (Chrislock et al., 2013). 

Eutrophication acts as a trigger point for a series of secondary hazards, further exacerbating the problem. As organic matter from algal blooms decomposes under anoxic conditions, it releases toxic gases such as hydrogen sulfide, which can affect aquatic life and pose a serious threat to human health (Dunnette et al., 1985). Additionally, eutrophication creates ideal conditions for the accumulation of persistent organic pollutants (POPs) and heavy metals. These pollutants, originating from various sources including industrial waste, bind to organic particles in the water and become incorporated into the food web. As they move up the food chain through a process known as biomagnification, their concentrations increase, posing long-term ecological and human health risks (Larsson et al., 2000). 

The consequences of eutrophication extend beyond ecological damage. Certain algal blooms can produce toxins such as microcystins and saxitoxins that pose a public health risk to humans (Gody et al., 2019; Brooks et al., 2016). These toxins can also sicken or kill animals within the aquatic ecosystem and threaten human health through contaminated drinking water or seafood consumption (Brooks et al., 2016). Additionally, fish kills and decreased fishery productivity can lead to economic losses for commercial and recreational fisheries (Kerr et al., 1992). Finally, eutrophication can render water bodies unsuitable for recreational activities such as swimming and boating due to the unpleasant aesthetics associated with algal blooms (Priskin, 2008).

Multi-hazard context

The figure below summarises common interactions between eutrophication and other hazards. This information should be used with caution and not be solely relied upon in disaster risk management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify, or be otherwise related to each other. Specific examples of the multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.

Multi-hazard diagram

Risk Management

Risk management combines source control (e.g. wastewater treatment), in-system measures (e.g. biomanipulation), and policy (e.g. effluent standards). Context-specific approaches are essential (UNESCO, 2015).

Monitoring

The section and the table below offer an overview of monitoring for eutrophication. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.