The physics of the warming of Lake Tanganyika by climate change

Abstract

Climate warming over the 20th century has increased the density stratification and stability of Lake Tanganyika, a deep rift valley lake. Here we examine the physical processes involved in and affected by the warming of the lake, and we discuss effects on lake productivity. The rate of net heat absorption by Lake Tanganyika has been 0.4 W m 22 since 1913, twice the rate in the global ocean, indicating stronger climate forcing in the East African region. Lakes warm through increased incoming long-wave radiation. While lakes in general will increase heat outputs in a warming climate, heat outputs will increase more slowly in deeper lakes than in shallower lakes. Temperatures have increased by 0.2uC at 1000 m in depth, in part because of reduced cool marginal inflows, while water surface temperatures have increased by about 1.3uC. This differential heating over depth has increased the density gradient through the water column, reducing the potential for vertical mixing and thereby limiting nutrient fluxes to the phototrophic zone. An increase in transparency, indicating a reduction in productivity as a result of the reduced vertical mixing, occurred both in Lake Tanganyika and in Lake Malawi, a similar deep tropical lake in which warming has also been documented.

Climate warming reduces fish production and benthic habitat in Lake Tanganyika, one of the most biodiverse freshwater ecosystems

“…In each case, we quantified geochemical proxies for temperature and algal production as well as the abundance of fossils from pelagic fishes and benthic invertebrates (ostracodes and molluscs). Benthic animals are of special concern because stronger stratification reduces oxygenated habitat in Lake Tanganyika (16,17). In modern sediments, benthic invertebrates are generally absent from sediments deposited under anoxic conditions, although some ostracodes tolerate low oxygen (as low as 1 mg·L ) (17)(18)(19)(20).…”

Lakes and reservoirs as sentinels, integrators, and regulators of climate change

“…The first example deals with the multiple effects of predicted increases in thermal stability predicted by climate models and observed under climate warming scenarios in both inland waters and oceans Stramma et al 2008). Data from systems ranging from small Arctic to large, deep tropical lakes demonstrate that warming increases thermal stability and decreases mixing (MacIntyre et al 2009;Verburg and Hecky 2009). Increased stability leads to oxygen depletion and the creation of hypoxic or anoxic dead zones that have many far-reaching consequences.…”

“…Several papers examine climatic control of the physical structure (MacIntyre et al 2009;Mueller et al 2009;Verburg and Hecky 2009) and carbon chemistry (Catalan et al 2009;Finlay et al 2009;Weyhenmeyer and Karlsson 2009) of lake ecosystems. Climatic forcing of biological responses in lake ecosystems (Castañ eda et al 2009;Drö scher et al 2009;Graham and Vinebrooke 2009;Wagner and Adrian 2009a;Winder et al Fig.…”

Large lakes as sources and sinks of anthropogenic heat: Capacities and limits

“…Observed increasing water temperature is strongly correlated to the signal of increasing air temperatures [Livingstone, 2003;Straile et al, 2003]. As a result, periods of summer stagnation of the water column may become longer and the strength of the stratification and heat content may increase [Verburg and Hecky, 2009]. Both climate change and heat use affect the sensitive aquatic ecosystems as water temperature is a key control for most physicobiological and biogeochemical processes [Davidson and Bradshaw, 1967;Fry, 1967].…”