On the Limnology of Lake Tanganyika
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On the limnology of Lake Tanganyika Promotor: Prof dr M. Scheffer Hoogleraar Aquatische Ecologie en Waterkwaliteitsbeheer Wageningen Universiteit, Nederland Co-promotoren: Prof dr J. Sarvala Head of Department Biology, section of Ecology Universiteit van Turku, Finland Dr R.M.M. Roijackers Universitair Docent, leerstoelgroep Aquatische Ecologie en Waterkwaliteitsbeheer, Wageningen Universiteit, Nederland Prof dr A.A. Koelmans Persoonlijk hoogleraar bij de leerstoelgroep Aquatische Ecologie en Waterkwaliteitsbeheer, Wageningen Universiteit, Nederland Samenstelling promotiecommissie: Prof dr J-P Descy, Université de Namur, Belgium Prof dr J.A.J. Verreth, Wageningen Universiteit Dr J. Vijverberg, Nederlands instituut voor Ecologie, Nieuwersluis Dr P.C. Goudswaard, IMARES. Yerseke Dit onderzoek is uitgevoerd binnen de onderzoekschool WIMEK-SENSE On the limnology of Lake Tanganyika Victor Theodorus Langenberg Proefschrift Ter verkrijging van de graad van doctor Op gezag van de rector magnificus Van Wageningen Universiteit prof dr M.J. Kropff in het openbaar te verdedigen op vrijdag 14 maart 2008 des namiddags te vier uur in de aula V. T. Langenberg On the limnology of Lake Tanganyika Thesis Wageningen University, The Netherlands– with summary in Dutch and French ISBN 978-90-8504-784-1 Voor mijn ouders en mijn gezin Cover photos of R/V Tanganyika Explorer by V. T. Langenberg. Front: Light play through future door opening of wet laboratory including Greek chalk drawings of building scheme. Back: Trawling gear at the stern. Contents Chapter 1 General introduction 11 Chapter 2 Limnological annual cycle inferred from physical-chemical fluctuations at three stations of Lake Tanganyika. 21 Chapter 3 Comparison of thermal stratification, light attenuation and chlorophyll-a dynamics between the ends of Lake Tanganyika. 39 Chapter 4 Phytoplankton in Lake Tanganyika; vertical and horizontal distribution of in vivo fluorescence. 53 Chapter 5. Effect of wind induced water movements on nutrients, chlorophyll-a and primary production in Lake Tanganyika. 73 Chapter 6 Wind-induced changes in physico-chemical and plankton community structures. 87 Chapter 7 External nutrient sources for Lake Tanganyika. 103 Chapter 8 Trophic structure of Lake Tanganyika: carbon flows in the pelagic food web. 119 Chapter 9 Climate change versus productivity in Lake Tanganyika: cases fish and primary production. 151 Chapter 10 The problems confronting survival of biodiversity in ancient lakes 163 Summary 171 Résumé 179 Samenvatting 187 Acknowledgements 193 Curriculum vitae 205 List of publications 209 Chapter 1 General Introduction Chapter 1 General Introduction 11 Chapter 1 General Introduction 12 Chapter 1 General Introduction Research for the Management of the Fisheries on Lake Tanganyika (LTR) The large lakes of the East African Rift Valley are amongst the oldest on earth. Lake Tanganyika is the largest and oldest of the Rift Valley lakes of East Africa and the second largest and deepest freshwater body in the world (Horne and Goldman, 1994). Lake Tanganyika is shared by four nations, i.e. Burundi, Tanzania, Democratic Republic of Congo and Zambia. It is an ancient lake with a history of geographical isolation of some 20 million years. Flora and fauna therefore have unique characteristics not found elsewhere (Coulter, 1991). The lake is situated in a region characterised by low latitude, high temperatures and favourable light conditions year round and from ancient times, the lake’s fish productivity has been a major source for the nutritional, economic and social wellbeing for the indigenous human populations This unique ecosystem with its characteristic fluctuations in fish catches and continuous changes in species composition of the catch, defies standard fisheries planning, management and development practice (Hanek et al., 1997). Development programmes and studies undertaken under the Food and Agriculture Organization of the United Nations (FAO) auspices in the 1960s and 1970s in each of the four countries had concluded that simultaneous lake-wide research was needed to understand the observed phenomena, and more specifically how the lake demonstrates that high fish yields are not incompatible with apparently high water transparency, low nutrient concentrations, low algal biomass and other favoured water quality characteristics commonly found in oligotrophic waters. To this end, the four riparian countries requested FAO's Committee for Inland Fisheries of Africa (CIFA) in 1977 to create a Subcommittee for the Development and Management of the Fisheries of Lake Tanganyika. This body was specifically requested to formulate and subsequently find funding for a regional fisheries research project on the lake. The Finnish International Development Agency (FINNIDA) eventually honoured the request for support of the four countries, and a FAO-executed regional research project of 5 years duration entitled "Research for the Management of the Fisheries on Lake Tanganyika" (henceforth named LTR) became operational in 1992 (Hanek et al., 1997). Financing of the Arab Gulf Programme for United Nations Development Organizations (AGFUND) could also be mobilised for part of the equipment component of the project. It was felt that for the management of the fish stocks in the whole of Lake Tanganyika, a proper understanding of the hydrological, limnological and ecological mechanisms present in the lake is essential; such mechanisms were supposed to ultimately set the upper level for fish production. Earlier research on other African lakes had namely indicated the importance of these mechanisms in determining the chemical and biological characteristics of the upper productive waters (see Hecky and Bugenyi, 1992). At the commencement of LTR, such information for Lake Tanganyika was scarce and could be readily characterised by its short-term observations and poor regional coverage (see Coulter, 1991). Consequently, at that time, scientific speculations were not adequate to give sufficient insight into the Lake Tanganyika’s ecosystem functioning and productivity. Therefore, LTR's principal objective was to gather and synthesise data on the main hydrological, limnological and ecological variables, considered crucial to the further understanding of the mechanisms securing the biological basis for fish production in Lake Tanganyika (Hanek et al., 1997). Consequent the considerable monitoring information gathered in the course of LTR-project new insights were obtained. However it became clear that in-depth analyses of the data banks were needed to scrutinise existing theories as well as to formulate new ones. Seen the considerable effort needed to fully analyse LTR’s data banks, it was endorsed to also develop LTR’s data through analyses carried out in different theses (e.g., Mannini, 1998). My studies specifically addressed the limnological dynamics and cycles in Lake Tanganyika using mostly data collected during the LTR-project. Aim of thesis and background research questions This study describes and analyses the intra- and interannual variations of the physical, chemical and biological properties of Lake Tanganyika in order to acquire a better 13 Chapter 1 General Introduction understanding of the relations between constituents that are chemically and biologically active in the productive water layers of Lake Tanganyika. More specifically, based on the knowledge available, five sets of research questions were formulated: 1 What are the spatial and temporal distributions of naturally occurring constituents that are physically, chemically and biologically active in the upper water layers of Lake Tanganyika? The vertical distribution of naturally occurring, chemically and biologically active, constituents in the upper water layers of Lake Tanganyika have been studied and described by several scientists since the early work of Beauchamp in 1939. In general, most scientists agree on the existence of strong vertical gradients of nutrient concentrations like nitrogen, oxygen, silicate and phosphate that result from the lake's meromictic condition (Beauchamp, 1940; Kufferrath, 1952; Degens et al., 1971; Ferro and Coulter, 1974; Ferro, 1975; Well and Chapman, 1976; Coulter, 1991; Hecky and Bugenyi, 1992; Crul, 1993; Edmond et al., 1993 and Plisnier et al., 1996). Most scientific interpretations on the vertical distribution of important constituents and related mechanisms involved are based on short term measurements and many of them do not explain that the upper water layers are occasionally characterised by the occurrence of nutrient concentrations that are atypical to the oligotrophic and meromictic nature of the lake. The spatial and temporal distribution of epilimnetic constituents is still poorly defined, and even a complete annual cycle has not been observed previously on the lake. 2 What is the impact of the external influxes of constituents through terrestrial run-off, river inflow and atmospheric deposition on the chemical and biological characteristics of the upper waters of Lake Tanganyika? Is there a seasonal pattern? In order to solve this question, we try to elaborate on the relative importance of respectively the external influxes of constituents through terrestrial run-off, river inflow and atmospheric deposition to the overall nutrient budget of Lake Tanganyika. In general, loading of plant nutrients (in particular phosphorus and nitrogen) determines the standing crop and primary productivity of phytoplankton (Moss, 1969; Melack et al., 1982; Carpenter and Kitchell, 1984;