I got this picture from cichlid-forum.com I would assume that the official reference would be NASA or Hubbel but I’m not sure. It is a shot from space of Lake Tanganyika on a pretty clear day in that region.
this is a copy of a response from Unprecedented Warming in Lake Tananyika thread over at Mr. Watt’s site.
Pat Moffitt says:
May 22, 2010 at 8:11 am

The claim by Tierney and others is that in this relatively nutrient poor lake- primary productivity is a function of upwelling of the nutrient stores from the lake’s deeper waters. The lake has a relatively low thermal (density) gradient meaning it takes a good deal of wind to disrupt the lakes stability and bring the relatively more nutrient rich waters to the surface.

Tierney assumes that winds are higher in periods of aridity (cooler periods) and thus lake productivity. Her cores assume that productivity is linked to biogenic silica (BSI) production– basically diatoms. Tierney shows a correlation between BSI and the TEX86 lake surface temperatures as support for the link between increasing temperature and declining productivity. Tierney assumes that the increasing water temperatures have led to greater stability of the water column making it both more difficult to turn over the water column and cycle the nutrients as well as reducing the intensity of the required winds. She draws on support from Verburg of increasing lake clarity as further evidence of declining production. She then links this decline in primary production to a possible decline in fish biomass.

There are a number of concerns with Tierney’s basic assumptions and best summarized in Victor Langenberg’s 2008 thesis (Wageningen University) On the Limnology of Lake Tanganyika:
-The highest productivity on the lake is found on the end with the lowest winds- contradicting a major premise of Tierney’s that productivity and lake stability are linked
– there is no measured evidence by Secchi disc that the lake has been getting clearer(less productive)
-There is no evidence of a climate and fish biomass link
-There is evidence of severe overfishing
-The simple association of wind speed and temperature lake stability and lake upwelling-is not so simple (A wind model in the hydrodynamics of the Lake showed that temperature is of only secondary importance-LT Regional Fisheries Management Programme)
-Found that allochtonous (outside the lake) sources of nutrients were more important than assumed by Tierney
-Found no evidence for a decline in lake productivity and that the lake’s current production is within the expected range for this type of lake
-Phytoplankton chlorophylla has not materially changed from the 1970s to 1990s

Tierney draws heavily on the BSI (biogenic silica index) as evidence for the TEX86 lake surface temperatures (LSTs) being related to productivity. She assumes that diatoms are a major component and a representative proxy for productivity from the work of Verbum. Langenburg contradicts Verbum’s finding that diatoms are an indicator for Lake Tanganyika productivity finding picocyanobacteria may be the Lake’s dominant form of phytoplankton. A 2009 paper in Journal of Plankton Research by Stenuite et al supports this position . A paper by Hecky and Kling 1987. Phytoplankton ecology of the great lakes in the rift valleys of Central Africa. Arch. Hydrobiol., Beih. Ergebn. Limnol., 25, 197–228 found that in Africa rift lakes upwelling is associated with diatom production and stratification stability with the production of cyanobacteria.
A presentation by Hecky and Verburg http://www.espp.msu.edu/climatechange/…/Physical%20and%20Ecological%20Responses%20of%20the%20Great%20 showed the switch from cyanobacteria in the wet season (warm) to diatoms in the dry season (cool) on an annual basis. The cyanobacteria do not appear as biogenic silica in the cores and as such will not be measured as productivity.

Tierney’s correlation of BSI with LST may be nothing more than diatoms being relatively more plentiful in periods of lake upwelling (aridity and high T) and cyanobacteria during periods of low upwelling (wet, low T and stable stratification). Tierney’s BSI as a result may say nothing about the overall changes in productivity of the lake. (The BSI simply reflecting the Lake’s primary productivity mode switching between cyano bacteria and diatoms.) Without a reliable proxy for total productivity the assumed correlations to temperature and fishery catch becomes less grounded.

You have commented on the heat from thermal vents- and direct you to comments made by Coulter (1968?) before the Banza vents were discovered. http://www.aslo.org/lo/toc/vol_13/issue_2/0385.pdf He proposed the possibility of heat flow at the bottom of the lake to explain the apparent lack of an expected salinity gradient and that given the depth of the lake and hydraulic residence time only small inputs of heat would be required to produce deep lake convection currents. Deep water generated convection currents would make the nutrient dynamics far more complicated than that proposed by Tierney’s simple wind/temperature mixing.

Tierney has acknowledged a “potentially” large role for overfishing in her Nature paper- however the media interviews have tended to diminish the relative threat of overfishing. There have been over two decades work trying to get the multiple interests and nations involved in the Lake fishery to agree to an enforceable/workable fishery harvest plan. Pointing a misguided finger of blame at global warming may very well undo these vital efforts. If so- the threat of global warming may have a far greater impact on the Lake and the food supply to its residents than any warming- real or imagined.


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