Eutrophication

As discussed in the previous post, the inefficient use of fertilisers in agriculture has lead to an excess of nutrients such as Nitrogen and Phosphorous in the soil, which is then leached or released as a gas to the atmosphere.  I have discussed the impacts of nitrous oxide, this post will analyse the impact of those nutrients leached.  

Eutrophication occurs where nutrients, namely phosphorous and nitrogen, found in agricultural fertilisers are washed (leached) by rainfall into a freshwater or coastal systems.  Here I am going to focus on freshwater systems.  Leaching causes increased productivity  lakes, causing large algal blooms to grow.  These dominate the lake preventing other plants from getting the nutrients and light they need, reducing the biodiversity of the lake and the water quality.  When these large algal blooms die they sink to the bottom where they decompose, causing deoxygenation of the water, affecting fish and other organism.  It becomes difficult for these organisms to live due to the deoxygenation of the lake and they eventually die.  Consequently lakes end up a green colour and are species limited, dominated by algae, these lakes referred to as being in a turbid state (see picture below).  These lakes are very common in agricultural areas.  

Below is a simple but effective animation of the process of eutrophication. 

There have been several strategies to prevent the situation getting worse, and to restore these freshwater systems.  As I mentioned in my previous post, better, more efficient use of fertilisers in agriculture is key as this reduces losses.  It is important to attempt to stop the problem at its source.  Over recent years buffer areas have been introduced.  Haycock and Burt (1993) showed this to be a successful measure at reducing the level of nutrients reaching freshwater systems.  They undertook a study on sections of the River Leach, making a grid of bore holes on the buffer area, from which they could take water samples to measure nutrient concentrations.  They found that there were sharp losses in nitrates with increasing distance into the buffer. This strategy has been successfully adopted.  Re-meandering rivers and streams is also a possible approach, as this means the water flows more slowly allowing more time for deposition before it reaches lakes.  These ideas intend to reduce the level of nutrients entering the lakes. 

However. restoring turbid lakes is much more difficult.  Even when the excess of nutrients being added to the lake has been stopped the lake still remains eutrophic due to nutrients embedded internally in the lake.  Therefore it is possible to dredge lakes of all there sediment, however this is a very expensive strategy and causes loss of the lake habitat.  Another possible restorative measure is biomanipulation.  This is where the lake ecosystem is manipulated, for example by removing fish, to see if the original ecosystem can be restored.  Sondergaard et al (2007) evaluated data from more than 70 restoration projects conducted in shallow, eutrophic lakes in Denmark and the Netherlands.  They found the most common biomanipulation measure to be removal of zooplanktivorous fish.  They showed some success, over half the lakes had decreased levels of phosphorous, nitrogen and chlorophyll a.  This shows potential for biomanipulation to work, however it is a very complex process.  The article states on the long term only a few lakes recovered, most returned to a turbid state after ten years or so.  Consequently, biomanipulation could work but it needs to be maintained and repeated over long time frames otherwise the fish recolonise and the turbid state returns. 

For more information on eutriphication, this article by Smith et al (1999) explains it well, explaining both coastal and freshwater eutriphication giving examples of restoration success stories.  

Reference: 
Haycock, N.E. & Burt, T.P. (1993) The sensitivity of rivers to nitrate leaching: The effectiveness of near-stream land as a nutrient reduction zone. In: D.S.G. Thomas & R.J. Allison. Landscape Sensitivity. John Wiley & Sons, 261-272.