The changing conditions

Over the 20^th century the environmental change had a massive impact on the Lake Tanganyika. Today I would like to focus on the elevation of the density stratification and stability. The warming influences the physical conditions of the lake and therefore the productivity of the lake (Verburg& Hecky, 2009).

But first, it can be said that climate warming effects can have sizeable repercussions for tropical lakes like the Tanganyika lake. In my last blog entry, I have mentioned the economic dependence in which the neighbouring countries find themselves. That’s why the knowing about the consequences of climate change is in this region so important. So the local people, the states and other involves parties can get prepared for the changing and challenging situation ahead.

To come back to physical conditions, which change during the climate change, it is essential to explain the key point just as “warming surface waters lead to increasing differences in water density that will reduce vertical circulation in these deep, already-meromictic tropical lakes, resulting in reduced rates of nutrient renewal and reduced vertical penetration of oxygen” (Verburg & Hecky, p. 2481, 2009). The evidence that climate change is the perpetrator of the mentioned is that observation show that in all oceans the primary cause for lower oxygen content is reduced vertical circulation by global warming (Verburg & Hecky, 2009). But what exactly is circulation important for? The vertical circulation and the upwelling driven by southeast trade winds provide the prime nutrient source to sustain primary production by internal nutrient loading (Verburg & Hecky, 2009). This event is season dependent. Over the 20^th century there has been an increase of the water temperatures below 100 m in depth hence the nutrient balance is affected by this event. 

(Verburg & Hecky, 2009)

In the graph you can see the rising temperatures throughout the years 1913-2000. The depth temperatures as well as the temperatures of the surface have increased. To be more precise from 1913 to 2000 the lower metalimnion of the Tanganyika Lake warmed by 0.9 °C, the hypolimnion warmed by 0.2°C and phytoplankton biomass between 1975 and 2000 decreased by 70% (Tierney et al,2010).

Science speaks of other quantities that influence the shift to more negative carbon isotopes. This includes, for example, the combustion of fossil fuels. Because of that there is a decrease in atmospheric isotope ratio d13C.This phenomenon is also known as the Suess effect, an “increase in terrestrial organic matter, or a change in phytoplankton composition, can be discounted, and the shift occurs independently of the rates of sediment mass accumulation” (O’Reilly, p. 767, 2003). There is evidence that this shift (more negative carbon isotopes ratios) has begun at the same time as the climatic changes (O'Reilly, 2003). The changed state of water quality has an influence on life in the lake. Phytoplankton is an important basis of many organisms and when the mass of phytoplankton decreases, the number of living organisms whose existence is thus assured also changes. A higher competition for food emerges. For this reason, fishing (among other things) suffers from these changes. Especially the sardine fishery, which provides a major source of animal protein for people in the region as well as employment for approximately 1 million people (Tierney et al, 2010). As I have written before, the fishery depends on the nutrients found in the lake. In this case (sardine fishery) “depends on high algal productivity in the open-water food web and an efficient carbon transfer from algae to fish” (Tierney et al., p. 422, 2010). This shows that water is (always) connected to food and the slightest changes can cause massive consequences.

Next week I would like to see if there are similar observations in comparable lakes in Africa and how changes and impacts behave there.