14/11/2014

Eutrophication

Following on from my post on tumors in sea turtles (see below), I wanted to investigate eutrophication further.

During my research, I came across a paper written by a group of high school students in Alaska describing eutrophication on marine ecosystems and its effect on their villages. It's an impressive piece of work (albeit it with some minor errors), especially considering the students had never heard of eutrophication prior to starting the project (Matthias et al. 2011)!

Eutrophication is a complex issue involving many factors. Below are some of the key facts courtesy of the Alaskan High Schoolers:

Eutrophication is the enrichment of a body of water due to the influx of nutrients, particularly those that are nitrogen and phosphorous-based. A major contributor of this input is the anthropogenic (over)use of fertilisers, which infiltrate rivers and streams and eventually discharge into the ocean (Matthias et al. 2011). According to the 2011 United Nations Environment Programme Year Book (UNEP 2011) "the global use of fertilisers that contain phosphorous, nitrogen and potassium increased by 600 per cent between 1950 and 2000". UNEP estimate that about 22 million tonnes of phosphorous enter the marine environment from land every year, mainly due to inefficient farming methods and "failure to recycle wastewater".

Nitrates and phosphates (nitrogen and phosphorous-based compounds) are required in algal and plant growth. When excess nutrients enter coastal waters, nutrient concentrations in these waters increase leading to a surge in algal growth, a phenomenon known as algal blooms [Figure 1]. When the organisms die they sink to the bottom of the ocean and are decomposed by bacteria, a process that requires oxygen. Consequently, these areas are depleted of oxygen becoming hypoxic (oxygen-poor) and in some cases anoxic (oxygen-free). This often results in the death of fish and other aquatic organisms (Matthias et al. 2011).


Figure 1. Satellite image of an algal bloom in the Baltic Sea (National Geographic)
 
Areas in which the deep water oxygen concentration is so low it cannot sustain sea life are called dead zones [Figure 2].
Figure 2 shows the location and size of dead zones around the world. It is perhaps unsurprising that the majority of these zones are found along the coastlines of developed countries, where population is high and the use of fertilisers is abundant (NASA 2008).

Figure 2. Location and size of marine dead zones (NASA 2008)


Of course, areas that are naturally low in oxygen are not uncommon in oceans and in such zones, marine life has adapted to these conditions. However, increases in both global population and global demand for meat and dairy suggests the use of fertilisers is likely to escalate thus exacerbating the problem (NASA 2008).

Moreover, problems linked to eutrophication are not only confined to marine species but affect communities and economies that rely on fishing and tourism (Matthias et al. 2011). I don't believe anthropogenic eutrophication can be stopped, but it can be slowed with adequate management strategies aimed at reducing the amount of nutrients entering both freshwater and marine ecosystems.

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