Climate change / Lake Tanganyika, Africa

[Les Schaffer <schaffer@xxxxxxxxxxxxx>]

1.) Nature 424, 731 - 732 (14 August 2003) 
 
Global change: The heat on Lake Tanganyika 

DIRK VERSCHUREN 


Warming of surface waters and declining fish catches in Lake
Tanganyika have been linked to global climate change. The impact of
global warming on natural ecosystems may be starting to affect local
economies.


The effects of global climate change on ecosystems and the
geographical distribution of species are already clearly visible1, 2,
but concrete examples of their impact on the livelihood of sizeable
human populations are still scarce. Now two groups ? Verburg et
al. writing in Science3 and O'Reilly et al. on page 766 [snippet
below] of this issue4 ? have combined documentary and field data on
long-term ecosystem dynamics in Lake Tanganyika, East Africa, to show
how increased heat accumulation by this deep tropical lake, linked to
climate warming, has led to a marked reduction in fish yields.

Lake Tanganyika (Fig. 1, overleaf) is one of the world's great
freshwater ecosystems. It is the second deepest lake and second
largest by volume (after Lake Baikal), the second largest tropical
lake by surface area (after Lake Victoria), and holds the second
greatest biological diversity (after Lake Baikal). This great species
richness is primarily accounted for by endemic fishes, snails and
crustaceans that inhabit nearshore environments. Its high ecosystem
productivity, however, with a carbon transfer from algae to fish that
is comparable to that of the most efficient marine fisheries5, comes
mostly from the offshore, open-water food web, which is relatively
poor in species.

[snip]

The seven great lakes of East Africa are all dissected by
international boundaries. Lake Tanganyika is shared by the countries
of Burundi, Tanzania, Zambia and the Democratic Republic of Congo, and
so systems for sustainable management and conservation are subject to
the difficulty of reaching multilateral agreements in a politically
unstable region. Creating such systems is also hampered by our
fragmentary understanding of the lakes' functioning. At present it is
probably enough to provide a qualitative explanation for the distinct
responses of lakes Tanganyika and Victoria to the effects of human
activities. But to quantify the exact causes, evaluate the
consequences and find preventative measures, much more data are
needed, as are realistic mass-balance models based on a better
knowledge of the inputs, outputs and biogeochemical cycling of
essential nutrients in the water column.

In the meantime, we must hope that managers do not mistakenly conclude
from these studies that fish yields in Lake Tanganyika can be restored
by stimulating the fertilization brought by deforestation and
agriculture, or that the precarious condition of Lake Victoria may be
cured over time by the expected effects of global warming on
water-column stability and nutrient cycling.

 


 

2.) Nature 424, 766 - 768 (14 August 2003)
 
Climate change decreases aquatic ecosystem productivity of Lake
Tanganyika, Africa


Although the effects of climate warming on the chemical and physical
properties of lakes have been documented1, biotic and ecosystem-scale
responses to climate change have been only estimated or predicted by
manipulations and models1. Here we present evidence that climate
warming is diminishing productivity in Lake Tanganyika, East
Africa. This lake has historically supported a highly productive
pelagic fishery that currently provides 25 - 40% of the animal protein
supply for the populations of the surrounding countries2. In parallel
with regional warming patterns since the beginning of the twentieth
century, a rise in surface-water temperature has increased the
stability of the water column. A regional decrease in wind velocity
has contributed to reduced mixing, decreasing deep-water nutrient
upwelling and entrainment into surface waters. Carbon isotope records
in sediment cores suggest that primary productivity may have decreased
by about 20%, implying a roughly 30% decrease in fish yields. Our
study provides evidence that the impact of regional effects of global
climate change on aquatic ecosystem functions and services can be
larger than that of local anthropogenic activity or overfishing.

[snip] 

The combined historical and paleolimnological data provide evidence
that climate change has contributed to diminished productivity in Lake
Tanganyika over the past 80 years. Within the next 80 years, air
temperature increases of 1.3 - 1.7 °C are predicted for the Great
Lakes region of East Africa[9], which may further increase thermal
stability and reduce productivity in these large lakes, provided that
wind velocities remain low. The human implications of such subtle, but
progressive, environmental changes are potentially dire in this
densely populated region of the world, where large lakes are essential
natural resources for regional economies.