Cry me a river - Part One

So far we've focused on the implications of climate change on marine fisheries. In today's post we will look at the problems faced by freshwater environments, mainly rivers and lakes, and their ecological assemblage.

It's important to recognise that the effects of climate change on freshwater environments are not exactly the same as those acting on marine fisheries. As discussed in previous posts, changes in chemical dynamics are responsible for damage to coral reefs and fish stocks. Rivers and lakes on the other hand are susceptible to changes in the hydrological cycle. This is a very broad topic in itself, and there is a great blog you can visit to find out more if you're interested.

First of all, increased inland temperatures can have a direct effect on lakes and rivers. Obviously, it can intensify aridity and therefore lead to a higher dry season mortality of fish and other important species in ecosystem. Additionally, many species have a particular acceptable range of temperature (known as their temperature niche) and any change in temperature can drive fish species out of their habitats. Worse still, it can lead to the succession of more competitive invasive species, parasites and pathogens which threaten 'native' species.

Furthermore, increased temperatures can lead to changes in the mixing of lake water. Mixing regimes are highly important due to the internal circulation of nutrients. Lake Tanganyika, occupying territory in the Democratic Republic of Congo, Tanzania, Burundi and Zambia, has experienced a slowdown in the mixing of water strata. This has meant that nutrients are no longer resuspended as much, leading to a decline in plankton species assemblages and a 30% decrease in the yield of planktivorous fish.

Figure 1 - Lake Tanganyika

The complexity of the hydrological cycle means that it is quite difficult to predict with certainty how it will respond in the face of future temperature increases. A policy briefing by the WorldFish Center suggests that continued climate change is likely to lead to increased seasonal and annual variability in precipitation patterns. In turn, this is likely to unpredictable flood and drought extremes that can potentially have severe impacts on inland fisheries. These changes mean that Bangladesh, a country that relies on fisheries for 80% of its protein intake, is likely to see an increase of 23-39% of areas prone to flooding. Flooding is a particular issue as the high wet season discharge and low dry season flows can lead to a disruption of the spawning season of various species.

An extensive analysis of historical data of salmon ecology in the Thames, the Severn, the Wye, the Lune and the Dee suggests that the increase in frequency of summer droughts and winter floods due to climate change are likely to lead to lower survival rates and a diminished abundance of the species.

The importance of the climate on hydrological processes is therefore quite obvious.