Description :

Predictive climate models based on observations from the present and instrumental period must be extended to historical periods from proxies (sediments, ice cores, pollen, tree rings). This is an important objective of several international programs (IGBP/PAGE, IDEAL). East African lakes are particularly interesting in this prospect because they are highly sensitive to climate variability (Johnson and Odada, 1996).
The density stratification and stability of tropical lakes fluctuate widely at high water temperature (Lewis, 1996). For this reason, lake hydrodynamics, which are heavily dependent of weather conditions, govern nutrient distribution in the water column, thereby affecting phytoplankton and food web productivity. During El Niño, air T° is significantly greater in the intertropical area (Bradley et al, 1987) as observed at Lake Tanganyika (Plisnier, 1998). This influences the lake stability, nutrient distribution, and planktonic communities. Diatoms are valuable indicators of such environmental and climate changes over hundreds/thousands of years (Gasse et al., 1995) while information over the instrumental period may also be obtained from molluscs shells geochemistry.

In the anoxic waters of a deep lake as Lake Tanganyika, laminated sediments are well preserved which constitute proxies of world interest. It is however essential to correctly interpret the source of those variations. This has to be based on a series of high frequency measurement of many environmental variables ideally integrated into a descriptive and interpretative model. We propose to carry out a study of the present ecological conditions in Lake Tanganyika as influenced by climate, and to develop a model (ECO-HYDRO) to be used as a tool for interpretation of the proxy records observed in the lake.

The ECO-HYDRO model will be developed in three steps:

(1) Present conditions: (2001-03): Implementation of a 3D hydrological model based on the CLIO model (Coupled Large-scale Ice Ocean) of UCL using data on weather, hydrodynamics, and integration into the ECO-HYDRO 3D of data and relationships on nutrient availability, planktonic production, plankton community structure and geochemistry.
(2) Instrumental period: (last 0-100 years): ECO-HYDRO Model tuning based on molluscs geochemistry, past limnological and weather data.
(3) Historical period: (last 1500 years): Application of ECO-HYDRO Model based mainly on relative abundance and productivity of diatoms in sediments.

We will particularly focus on the high resolution necessary to study interannual modes of variability such as ENSO (3 to 7 years). Our ECO-HYDRO model will be used with forcing conditions issued from general and regional circulation models collaborating closely with atmospheric modellers (Prof. Semazzi and his team). Local lake and weather conditions will be related to broad patterns of atmospheric and oceanographic conditions (Semazzi et al, 1996).

The results will be used to:

(1) analyse the time series of climate conditions during the last 1500 years (cycles, trend, comparison of natural vs industrialised period...),
(2) compare the series obtained with paleo-series recorded from various sites, particularly in Europe to investigate historical global teleconnections
(3) provide climate modellers with high resolution paleo-series necessary to validate high resolution forecasting models aiming at forecasting interannual variability, including ENSO. Annually resolved tropical paleoclimatic reconstructions of the upper Holocene are of direct relevance to understanding the longer period modulation of the ENSO and monsoon phenomena (Duplessy and Overpeck, 1996).


Methodology

- Set-up of a new program for data acquisition, consisting of meteorological measurements (continuous) and limnological sampling (biweekly) at two stations of Lake Tanganyika during 3 years (2002 - 2004);

- Development of an hydrodynamic model for predicting upwelling and internal waves from wind pattern and velocity and other relevant variables available from existing data sets of previous studies; validation of the model will be carried out with the new data acquired in the study;

- Use of trace elements indicators (Sr, B, Ba, Mn, Rare Earth Elements, Cd, Zn, Pb, Cu) in order to quantify the water movements;

- Test a bivalve mollusc (Pleiodon spekii) as a recorder for environmental change using microdrill and laser ablation with ICP-MS to perform reliable weekly to daily trace element distributions in chemically marked shells. After validation, we hope to use these shells to study past changes during the instrumental period (last 100 years) and tune the model;

- Use a new isotopic indicator (based on silicon isotopes) to track changes of diatom production in the surface waters by calibration against primary production measurements and other productivity proxies (e.g. Ba excess);

- Use of limnological indicators, in particular phytoplankton composition, which responds in the short term (days, weeks) to fluctuations of various factors (light, temperature, mixing regime, nutrient inputs to surface layers);

- Continuation of analysis of sediment cores addressing environmental change in Lake Tanganyika during the past 1500 years using diatoms, sedimentology and other proxies;

- Development of a descriptive lake model integrating climate and weather data, hydrodynamics, nutrient availability, primary production, plankton community structure and geochemical indicators;

- Development of a predictive model to relate climate change and ecosystem function.


Interaction between the different partners

During a first period of 3 years, three teams (Namur - FUNDP, Gent (RUG) and Tervuren MRAC-KMMA) are developing a monitoring (in partnerships with Zambian and Tanzanian colleagues) collecting and analysing data in several fields (limnology, plankton and geochemistry mainly). During the first 2 years, a fourth team (UCL) is developing an hydrodynamic model based on previous observations. This information will be updated and tuned at the end of the second year of monitoring with recent data. Ecological information will then be implemented with the hydrodynamic model to constitute a multidisciplinary tool to be used for sediment interpretation and obtaining past climate information. The model will finally be used in the frame of climate change scenarios to forecast possible environmental changes.


Link with international programmes

- The project fits the objectives of IGBP's Core Project PAGES ("Global Paleoclimate and Environmental Variability"). It aims to study the past climate variability (Timestream 1: Holocene with special emphasis on the last 2000 yr.) in a site along the PEP-3 (Pole-Equator-Pole (Afro-European) Transect. - The project is closely collaborating with IDEAL (International Decade for the East African Lakes), a program that is part of PAGES..

- The project meets also the aims of WCR/CLIVAR as it will improve the understanding of decadal to century scale climate variability, especially as relevant to improving predictability, through the use of high resolution paleoclimatic data as well as building a model dealing with the impact of the actual climate variability and changes on a great lake ecosystem.

- The project also corresponds to the themes defined in Key Action 2 "Global change, climate and biodiversity" of the "Energy, Environment & Sustainable Development" Programme of EC DG-XII.

- The CLIMLAKE project works in close collaboration with two African research institutes: TAFIRI (Tanzanian Fisheries Research Institute) in Tanzania and DOF (Departement of Fisheries) in Zambia.


Expected results/outputs

- The project is presented in a web site that is updated every 6 months (http://www.fundp.ac.be/urbo/climlake.html);

- Beside the international peer review publications, the project will present data adapted to diverse types of audiences (scientific, politics, general public...);

- An interdisciplinary database will be made available to the climate modellers. It will hold three types of information: (a) Lake Tanganyika information (limnology), (b) regional data on climate, (c) global data indices;

- The results of the project (paleo-climate series) will be made available to the international research community. This is a participation of Belgium researchers to important international climate research program such as IGBP/PAGE;

- This research is part of the international effort dealing with the global warming issue. The records of global signals and their comparisons, particularly for the recent Holocene is a key point to understand actual climate changes;

- An international symposium will be organised on "The African Rift Great Lakes as recent and past climate impacts recorders";

- Climate-fisheries relationships will help to support the fisheries management and its sustainability at Lake Tanganyika.


Partners

Activities

Laboratoire d'Ecologie des Eaux Douces (FUNDP, Namur)

Limnological sampling (temperature, light, dissolved oxygen, conductivity, pH, nutrient, meta-zooplankton, elemental analysis, pigments,...).

Section de Pétrographie-Minéralogie-Géochimie (MRAC-KMMA, Tervuren)

- Diatom productivity (silicon isotopes) and geochemistry of water, particles and mollusc shells.
- Database (climate, limnology..).

Laboratory of Protistology and Aquatic Ecology (RUG, Gent)

- Study of the present phytoplankton (distribution, dynamics).
- Paleolimnological analysis of fossil diatoms in sediment cores.

Institut d'astronomie et de géophysique G. Lemaître (UCL, Louvain-la-Neuve)

- Building a 3D hydrodynamic model from wind pattern, velocity and other variables.
- Coupling to an ecological model in collaboration with all the teams.