| Lake Victoria serves as the basic food and economic
resource for millions of people. The last 30 years have
produced remarkable and alarming changes in the
ecosystem. Phytoplankton productivity has increased 2 to
3-fold and the biomass has increased 10-fold. Water
clarity, oxygen content, and silica concentrations have
been dramatically reduced. The tremendous diversity of
endemic fishes (300 species) has been all but lost, due
in part to the introduction of exotic species such as the
predatory Nile perch. There are three competing alternatives to explain these extensive changes: (1) the introduction of Nile perch has altered the food web so that biomass now accumulates lower in the food chain, (2) nutrient inputs from the basin and atmosphere have caused eutrophication, and (3) recent changes in climate have altered water column structure and caused mixing patterns to favor development of blue-green algae and the loss of oxygen in bottom waters. An a priori elimination of potential alternatives is not possible, mainly due to our incomplete understanding of ecosystem function. Theory predicts a weak trophic cascade in a tropical lake, a forecast clearly at odds with the observations in Lake Victoria. The phosphorus-driven eutrophication models fail to explain the change in productivity of the lake. Is it because the outcome of biogeochemical control is different at high and uninterrupted temperatures, or is there an unrecognized feedback through trophic interactions or variations in climate? In this research we will explore these apparent contradictions and test these basic but unproven ideas. A recent joint research project of the Ugandan Fisheries Institute and the University of Michigan found that that the effects of eutrophication were widespread across the lake. The spatial extent of oxygen depletion in bottom waters was large, and the spatial variability was dependent on physical mixing processes that appear to be driven mostly by seasonal rather than short-term changes in climate. Our cross-lake transects indicated substantial upwelling on the northern shore of the lake in late May. This upwelling appeared to be caused by a unimodal, horizontal internal seiche. Using CTD transect data we found that water circulation between inshore and offshore areas is partially driven by horizontal differences in temperature structure, with water from cooler regions flowing at depth into other parts of the lake. |
These circulation patterns are probably responsible
in part for the transport and distribution of nutrients
within the lake. Sediment microcosm experiments showed
that nitrogen flux from the sediments to the water column
is more pronounced than the phosphorus flux, which
suggests that the phosphorus loading to the lake may be
primarily external rather than from the sediments. In
these microcosms respiration rates consumed about 1.2 g O2 m-2
d-1, which is similar to the rates of primary
productivity in the water column. Rates of O2 consumption
increased when SO42- was added as an electron acceptor.
This increase indicates that anaerobic microbial
degradation is controlled at least in part by low rates
of sulfate reduction caused by the low concentrations of SO42- found in the lake. Measurements of DIC and dissolved CO2 indicate that epilimnetic waters are consistently undersaturated with CO2 with respect to the atmosphere. This undersaturation occurs at both inshore and offshore stations, and our cross-lake transects show that the phenomenon is widespread across the lake. Such low CO2 levels in the epilimnion suggest some limitation of algal production. In addition, stable isotope studies of 13C of DIC and POC indicate that there is greater primary production occurring inshore than offshore, and that the drawdown of CO2 could be a factor in limiting lake primary production in the water column. Sediment cores from several sites around the lake have also been collected to look at the changes in primary production over time, and as expected show striking changes over the last 30 years. Various species of haplochromines and Tilapia either exist or are returning to the open water. A bioenergetics model of Nile perch predation and exploitation effects produced three main conclusions: (1) the advent of large-mesh gill net fisheries reduced total predation by Nile perch to approximately one half of that during highest predator densities, (2) the development of small-mesh gill net and beach seine fisheries for juvenile Nile perch had a similar effect, and (3) when combined, these two fisheries reduced total predation to approximately 25% of the previous highest levels. Reduction in predatory impact corresponds with the recently observed resurgence of diversity where fisheries are locally intense. |
Overall Conclusions: Next Section
A Joint Project of :
The Government of Uganda
(Fisheries Research Institute, NARO)
The University of Michigan
(U.S. National Science Foundation)
Project Directors:
Dr. Fred Bugenyi, Fisheries
Research Institute
P.O. Box 343, Jinja,
Uganda
ph: 256-43-22071 FAX:
256-43-21727 Email:
Dr._Fred_Bugenyi@mukla.gn.apc.org
Dr. George Kling, Department of
Biology, University of Michigan
University of Michigan,
Ann Arbor, MI 48109-1048 USA
ph: 734-647-0894 , -0898 FAX:
734-647-0884 Email: gwk@umich.edu
Taken from a Progress Report to NSF
NSF Program: Ecosystems NSF Award Number:
DEB-9318085
PI Name: George Kling Period Covered by this
Report: Jan 95 - Jan 96
PI Organization: University of Michigan Date:
8 January 1996
Co-PIs: A.Giblin, B. Hecky, J. Kitchell, S.
MacIntyre, P. Reinthal
Research Plans for 1996:
In the third year of the grant we will complete our field and laboratory investigations and simulation modeling, and continue the synthesis of our results into the conceptual framework of our major questions about biological, physical, and chemical controls on the ecosystem. We will pursue our interesting findings about the spatial variability of processes across the lake by making additional cross-lake transects and by continuing our research to understand the linkages between nutrient transport and our observed water circulation patterns. We are planning on enhancing our understanding of rates of gross primary production by using a new technique that measures the stable isotopes of oxygen. We will collaborate with Paul Quay (Univ. Washington), Ray Hesslein (Freshwater Institute), and Steve Hamilton (Michigan State) to estimate primary productivity using this method. Sediment cores will be analyzed to further understand the rates of limnological change in this ecosystem.
We will continue to work with our colleagues at FIRI in developing models that are used to evaluate trophic interactions in Lake Victoria. Among the next challenges are (1) further analyses of biodiversity effects due to selective predation by Nile perch and the food web responses to intensified exploitation of Nile perch, (2) evaluation of zooplanktivory, and (3) estimation of the change in herbivory due to reduction and recovery of haplochromine stocks. The results of our initial IBA modeling analyses will be available to guide development of a general size-based modeling approach to understand trophic interactions within the lake.
Publications:
Reinthal, P. N. and G. W. Kling. 1994. Exotic species, trophic interactions and ecosystem dynamics: a case study of Lake Victoria. pp. 295-313 In, D. Stouder, K. Fresh, and R. Feller. Theory and Application in Fish Feeding Ecology. Univ. of South Carolina Press, Columbia.
MacIntyre, S. and J. M. Melack. 1995. Vertical and horizontal transport in lakes: linking littoral, benthic, and pelagic habitats. J. N. Am. Benthol. Soc. 14(4): 599-615.
Kitchell, J. F., D. E. Schindler, R. Ogutu-Ohwayo, and P. N. Reinthal. In Press. The Nile perch in Lake Victoria: Interactions between predation and fisheries. Ecological Applications.
Schindler, D. E., J. F. Kitchell and R. Ogutu-Ohwayo. In Press. Ecological Consequences of Alternative Gill Net Fisheries for Nile Perch in Lake Victoria. Ecological Applications.
Page Author: Dr Rick T Leah, Univ of L'pool - Contents last reviewed15/12/2005