Impacts OF EUTROPHICATION

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Fig 1. Red tide in Xiamen, in China’s Fujian Province, April 21, 2007. Red tides are nutrient-fueled blooms of phytoplankton that discolor water with their pigments. Several species are known to have toxic effects on marine life and pose a risk to human health through the consumption of exposed fish. Image Credit: Gu Liuzhang | Asianewsphotos

Excess nutrients in coastal waters can cause excessive growth of phytoplankton, microalgae (i.e. epiphytes and microphytes), and macroalgae (i.e. seaweed).
In turn, the increase in phytoplankton and algae can lead to more severe secondary impacts such as:

• Loss of subaquatic vegetation as excessive phytoplankton, microalgae and macroalgae 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.
• A shift in phytoplankton species composition, creating favorable conditions for the development of nuisance, toxic, or otherwise harmful algal blooms.
• Low dissolved oxygen and formation of hypoxic or “dead” zones (oxygen-depleted waters), which in turn can lead to ecosystem collapse.

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.

Fig 2. A series of phytoplankton blooms. A cyanobacterial (blue-green algae) in the Baltic Sea (upper left). Red tide bloom (dinoflagellate) in the Sea of Japan (upper right). Cyanobacterial bloom in the St John’s River Estuary, Florida (lower left). Cyanobacteria-chlorophyte bloom in New Zealand (lower right)

Today nearly half of the world’s population lives within 60 kilometers of the coast, with many communities relying directly on coastal ecosystems for their livelihoods. This means that a significant portion of the world’s population is vulnerable to the effects of eutrophication in their local coastal ecosystems.
Two of the most acute and commonly recognized symptoms of eutrophication are harmful algal blooms and hypoxia.

Harmful Algal Blooms

Harmful algal blooms (Figures 1 and 2) can cause fish kills, human illness through shellfish poisoning, and death of marine mammals and shore birds. Harmful algal blooms are often referred to as “red tides” or “brown tides” because of the appearance of the water when these blooms occur. One red tide event, which occurred near Hong Kong in 1998, wiped out 90 percent of the entire stock of Hong Kong’s fish farms and resulted in an estimated economic loss of $40 million USD.

Fig 3. A menhaden (Brevoortia sp.) fish kill in August 2003 was caused by severe hypoxic conditions in Greenwich Bay, part of Narragansett Bay, Rhode Island, USA. Image Credit: Chris Deacutis | IAN

Hypoxia

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 lead to stratification, limiting oxygen replenishment from surface waters and creating conditions that can lead to the formation of a hypoxic or “dead” zone. The formatioin of dead zones can lead to fish kills (Figure 3) and benthic mortality. Because benthic organisms are bottom dwelling and cannot easily flee low-oxygen zones, they are often the most severely impacted.