Cry me a river - Part Two

Last week we saw how climate can impact the hydrological cycle, and how this can in turn have disruptive implications for freshwater fisheries. Today we will see how climate change can directly affect ecosystems by influencing nutrient dynamics.

Eutrophication is the process by which a lake or fluvial ecosystem is overwhelmed by an unprecedented influx of nutrients. This can lead to the formation of dense algal blooms on the surface of the water which prevent photosynthesis from occurring in the deeper strata of the water column. In turn, there fewer bottom-dwelling plants can grown, leading to a shortage of food supply for smaller fish, and the effect cascades through the food web right the way to the top predator, often leading to disastrous fish kills.

 Additionally, dense algal blooms can deplete the dissolved oxygen in the water. This can lead to an environment that is unsuitable for fish species, both large and small, that previously populated the ecosystem.

The main cause of eutrophication is the massive influx of nutrients due to human activity. The widespread use of fertilisers and the discharge of industrial and domestic waste in watercourses can lead to increased inputs of nitrates and phosphates. However, scientists are beginning to examine how, if at all, climate change will impact eutrophication processes in terrestrial freshwater environments.

Figure 1 - Potomac River, USA, with dense green cyanobacterial bloom typical of eutrophication

A paper by Ficke et al. (2007) suggests a possible mechanism by which increased global temperatures could augment the process of eutrophication. Warmer temperatures have the potential to increase algal growth and bacterial metabolism. Enhanced growth and metabolic rates provide an opportunity for algal and bacterial populations to explode, leading to scenes like the one above.

Heino et al. (2009) draws attention to the fact that temperature increase will intensify precipitation events (discussed in the last post), which leads to a more intense process of nutrient leeching from fertilised soil and a greater input of nitrates and phosphates into lakes and rivers.

If these predictions are true the implications for freshwater fish stocks are quite severe, with a limited source of food for many established fish species and the increased likelihood of fish kills due to oxygen depletion.

However, the mechanisms are not yet fully understood, and there is much discussion as to whether climate change can actually ameliorate the disruptive process. Research by Schindler (1997) found that climate change coincided with declines in phosphorus levels in certain Canadian lakes. Ficke et al. (2007) suggest that the increased stratification of the water column in response to warmer temperatures could lead to a sequestration of nutrients in the hypolimnion, where they are inaccessible to algae and bacteria that may try to grow on the surface of the water.

Now while this might seem inconclusive, present knowledge suggests that we really cannot give any definitive answers as to how climate change will impact eutrophication processes. The articles looked at in this post suggest that increased temperatures work alongside other factors to determine the outcome. So for the best results we'd best examine lakes and rivers on a case by case basis. Hopefully further research will shed more light on the topic, but the present lack of consensus is testament to the complexity of the matter at hand.