Water Quality: Eutrophication and Hypoxia

Photo: red algal bloom at Leigh, near Cape Rodney, NZ. Photo by Miriam Godfrey. Used by permission of NIWA Science

Within the past 50 years eutrophication–the over-enrichment of water by nutrients such as nitrogen and phosphorus–has emerged as one of the leading causes of water quality impairment. The rise in eutrophic and hypoxic events has been attributed to the rapid increase in intensive agricultural practices, industrial activities, and population growth which together have increased nitrogen and phosphorus flows in the environment. The Millennium Ecosystem Assessment (MA) found that human activities have resulted in the near doubling of nitrogen and tripling of phosphorus flows to the environment when compared to natural values.

Once nutrients reach coastal systems, they can trigger a number of responses within the ecosystem. Two of the most acute and commonly recognized symptoms of eutrophication are harmful algal blooms and hypoxia.

Harmful algal blooms can cause fish kills, human illness through shellfish poisoning, and death of marine mammals and shore birds. They are often referred to as “red tides” or “brown tides” because of the appearance of the water when these blooms occur.

Hypoxia, considered to be the most severe symptom of eutrophication, has escalated dramatically over the past 50 years, increasing from about 10 documented cases in 1960 to at least 169 in 2007. Hypoxia occurs when algae and other organisms die, sink to the bottom, and are decomposed by bacteria, using the available dissolved oxygen. Salinity and temperature differences between surface and subsurface waters leads to stratification, limiting oxygen replenishment from surface waters and creating conditions that can lead to the formation of a hypoxic or “dead” zone – so called because the oxygen deprivation causes the death or evacuation of sea creatures. Two of the most well-known hypoxic areas are the Gulf of Mexico and the Black Sea. In 2002, dead zone in the Gulf of Mexico reached 22,000 km2, roughly the size of Massachusetts.

Other harmful impacts of eutrophication include:

• Loss of subaquatic vegetation as excessive phytoplankton and algae growth reduce light penetration.
• Change in species composition and biomass of the benthic (bottom-dwelling) aquatic community, eventually leading to reduced species diversity and the dominance of gelatinous organisms such as jellyfish.
• Coral reef damage as increased nutrient levels favor algae growth over coral larvae. Coral growth is inhibited because the algae outcompetes coral larvae for available surfaces to grow.

The scientific community is increasing its knowledge of how eutrophication affects coastal ecosystems, yet the long-term implications of increased nutrient fluxes in our coastal waters are currently not entirely known or understood. We do know that eutrophication diminishes the ability of coastal ecosystems to provide valuable ecosystem services such as tourism, recreation, the provision of fish and shellfish for local communities, sportfishing, and commercial fisheries. In addition, eutrophication can lead to reductions in local and regional biodiversity.

Strategy

In order to combat these problems, WRI is disseminating its research findings and recommendations through a series of Policy Notes in order to educate decision makers about the impacts of eutrophication and hypoxia, and enable them to take appropriate concrete steps towards stemming their growth. Specifically, these Notes aim to:

1. Complete an assessment of the state of knowledge about the extent of eutrophication and hypoxia worldwide, identifying data gaps and providing recommendations for closing them.
2. Identify the underlying drivers and causes of eutrophication.
3. Provide a policy framework which outlines strategies for halting or reversing the impacts of eutrophication.