Coastal Marine Eutrophication

Feedback mechanisms


Nutrient limitation (regional, well-established). Nutrient availability limits the growth rate of phytoplankton populations for the populations present in the water body (Goldman et al. 1979) and for the potential rate of primary production (e.g. Boynton et al. 2009), which limits the possible shifts in species compositions, and, finally, the net ecosystem production is limited (Howarth 1988). The understanding of whether nitrogen or phosphorus is more significant and the effect of nitrogen and phosphorus limitation in marine ecosystems is limited due to the complexity of biogeochemical cycling and nutrient inputs.


Hypoxia or anoxia (nutrient recycling) (Local, well-established). A feedback loop occurs between increased hypoxia, enhanced regeneration of phosphate and increased primary production (Mort et al. 2010; Mort et al. 2007). Hypoxia and anoxia affect nutrient transformation processes (nitrification, denitrification) and the capacity of the sediments to bind phosphorus. In the absence of oxygen, decomposing sediments release significant quantities of phosphorus into the water. The ability of the ecosystem to lose nitrogen through denitrification is limited to regions with low oxygen concentrations. The increased phosphorus and nitrogen concentrations accelerate the rate of eutrophication. This feedback creates a persistent internal loading of phosphate even if external nutrient loads are reduced.

Water clarity feedback (local, well-established). The increased turbidity and abundant algae mean less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which once again means more algae and turbidity (Kemp et al. 2005; Bonsdorff, E. M. Blomqvist, et al. 1997).

Cyanobacteria (Local, well-established). Some eutrophic ecosystems are trapped in a feedback mechanism encouraging algal blooms although the inputs of nitrogen and phosphorus have been reduced (Vahtera et al. 2007). Anoxia facilitates the release of phosphorus from the sea floor sediments, fueling the growth and blooms of cyanobacteria. Some cyanobacteria species can fixate nitrogen gas. In addition, especially during their bloom in the late summer, cyanobacteria may release nitrogen compounds, which can partly be used by other organisms.


The main external direct drivers that contribute to the shift include:

Anthropogenic nutrient loads (local, well-established). Cultural eutrophication increases the nutrient concentrations significantly, contributing to or triggering the eutrophication in the marine system.

The main external indirect drivers that contribute to the shift include:

Change in land use (local/regional, well-established). The nutrient loads from land to sea have increased due to, for instance, land clearing for agriculture or urban areas.

Fishing (regional, well-established). Removal of top predators may cause food web reorganization, changing the resilience of the system to eutrophication.

Climate change (global, speculated). Climate change is predicted to intensify eutrophication due to increased water temperature strengthening vertical stratification, which may lead to more anoxic deepwaters, and due to higher rainfalls increasing the river discharge.


Summary of Drivers

# Driver (Name) Type (Direct, Indirect, Internal, Shock) Scale (local, regional, global) Uncertainty (speculative, proposed, well-established)
1 Anthropogenic nutrient loads Direct Local Well-established
2 Change in land use Indirect Local/regional Well-established
3 Fishing Indirect Regional Well-established
4 Climate change Indirect Global Speculated

Key thresholds

Shift from oligotrophic to eutrophic regime:

Intense algal growth. The threshold of which decomposition of the abundant algal biomass leads to oxygen deficiency and hydrogen sulphide production.

Anoxic deepwater conditions. The threshold of which the living conditions become intolerable for fish and benthic fauna.

Nutrient loads. The threshold of water nutrients at which algal blooms occur.

Leverage points

Nutrient inputs (Regional, well-established). Recent research has shown that the management of nutrient loadings is the key to maintaining the preferred water quality (Smith et al. 2006; Smith 2003), and that nitrogen and phosphorus are the nutrients which critically determine the growth of primary producers. Conley et al. (2009) highlight the importance of recognizing the varying role of different nutrients in different marine systems and even under different seasons, making it important to have understanding on the multiple drivers of the marine regime shifts and consequently a reduction strategy for both nitrogen and phosphorus.

Hypoxia/anoxia (Local, speculative). Hypoxia and anoxia make deepwater habitats unsuitable for fish and benthic fauna. Pilot studies aimed at artificially oxygenating deep-water basins to combat oxygen deficiency are carried out for instance in the Baltic Sea.

Summary of Ecosystem Service impacts on different User Groups

References (if available)
Provisioning Services
Food Crops
Feed, Fuel and Fibre Crops
Fisheries +/-
Wild Food & Products
Regulating Services
Air Quality Regulation
Climate Regulation
Water Purification -
Soil Erosion Regulation
Pest & Disease Regulation
Protection against Natural Hazards
Cultural Services
Recreation -
Aesthetic Values -
Cognitive & Educational
Spiritual & Inspirational