«Plankton Communities and Ecology of Tropical Lakes Hayq and Awasa, Ethiopia angestrebter akademischer Grad Doktor der aturwissenschaften (Dr. rer. ...»
Plankton Communities and Ecology of Tropical Lakes Hayq and Awasa,
angestrebter akademischer Grad
Doktor der aturwissenschaften (Dr. rer. nat.)
Verfasser: Tadesse Fetahi
Dissertationsgebiet (It. Studienblatt): Őkologie
Betreuer: Univ.-Prof. Dr. Michael Schagerl
Dr. Seyoum Mengistou, Dr. Demeke Kifle
Vienna, June 2010
Addis Ababa University
Science Faculty, Biology Department
P. O. Box 1176 Addis Ababa, Ethiopia
Email: email@example.com Professor Michael Schagerl Vienna University, Limnology Department Althanstraße 14, A-1090 Vienna, Austria Email: firstname.lastname@example.org Dr. Seyoum Megistou Addis Ababa University Science Faculty, Biology Department P. O. Box 1176 Addis Ababa, Ethiopia Email: email@example.com Dr. Demeke Kifle Addis Ababa University Science Faculty, Biology Department P. O. Box 1176 Addis Ababa, Ethiopia 2 Contents Dedication 4 Acknowledgments 5 General introduction 6 Objectives and research questions 9 Thesis outline 10 Paper I: Phytoplankton community, biomass and primary production of 12 the Ethiopian highland Lake Hayq (Submitted to Hydrobiologia) Paper II: Zooplankton Community Structure and Ecology of a Tropical-highland 32 Lake, Lake Hayq, Ethiopia (Submitted to Journal of Plankton Ecology) Paper III: Food web structure and trophic interactions of the tropical highland 51 lake Hayq, Ethiopia (Submitted to Ecological Modelling) Paper IV: Decadal changes in phyto- and zooplankton communities of 71 Lake Awasa, Ethiopia (Submitted to International Review of Hydrobiology) Summary and Curriculum Vitae 87 3 Dedication EMAHOY ABEBECH ASFAW, mom, represents all mothers in particular Ethiopian mothers who devoted themselves to the best of their children. Her belief in education reached me here: when I complained the difficulty of learning during childhood, she always kindly responded “you are not requested to carry tons of grain; you are requested to read – just read”. That is how I learn ‘reading is a secret of knowledge’. Thanks mom and long live to you!
Several people and institutions have contributed a lot to the completion of this PhD program, and I earnestly thank all of them very much for their understanding and timely support, including those inadvertently oversight.
Special thanks are due to my supervisors Dr. Seyoum Mengistou and Professor Michael Schagerl for their consistent guidance, encouragements, proofreading and patience. My beginning in Austria was memorable, as Professor Michael (Michi) had organized courses in various places—out of them the town Mondsee remained vivid: Thank you very much Michi. I also thank Dr. Demeke Kifle, cosupervisor, who supported me financially, materially and also for his earnest encouragements.
Mr. Gerold Winkler, IPGL manager, supported me in soliciting a supervisor, kindly and constantly follow-up until the scholarship granted: I thank you so much for your right-hand support. I am indebted to Dr. Abebe Getahun for his financial support and also for his willingness to supervise MSc students Workyie and Esayas, as their outputs were an input to this study. I also thank Workyie Worie, Esayas Alemayehu and Beti Assefa for their indispensable contribution through their MSc thesis. I deeply appreciate the support I received from Ms Menber Simegn, laboratory assistance, during limnological analysis in Addis Ababa University. I also thank Aba Haile-Giorgis, Mesfin Shiferaw, and Mohammed who were courageous and trusted field assistance. I thank St. Estifanos Monastery and the monks for offering me a store, kind support and prayers.
I greately acknowledged various invaluable assistances and encouragements from my friends and colleagues: Asrat Fetahi, Getachew Beneberu, Gashaw Beyen, Gumataw Kifle, Wassie Anteneh, Abebe Amha, Paul Yilla (Dr.), Nabea Wendo (Dr.), Yosef Teodros, Adamneh Dagne, Eden Ezra, Sintayehu, drivers Gashaw and Shimeles, Ato Melke, Fasil Degefu, Aschalew, Goraw, Martin Mosel, Helen, Getnet (Finance), Miruts Hagoss, Yohannes (Jony), William (Dr.), Tizazu, Mehrdad Zamanpoore. I also thank Gashaw for introducing me FiSAT II software. I am indebted to Mr.
Mengistu who provided me Bird data of Lake Hayq. I thank Dr. Tenalem for allowing me to use his laboratory and encouragements. I thank Katharina Engel, OeAD, for fruitfull discussion and support.
Aquatic Team (Phycology Group): Melanie Zwirn, Charlotte Wöber (Lota), Alfred Burian, Martin Gruber, Katharina Bürger (Kathi), Nadja Straubinger, Elisabeth Haberleitner, Kaggwa Teddy (Mary), Julia Wukovits, Roland, Bernard, Johanna Zeilinger, Caroline Linhart: you are an excellent team with excellent and prompt cooperation: thank you so much.
My deepest appreciation goes to my family: Abebech Asfaw (Emahoy, mom), Asrat, Kelem, Fetle, Eney, Adey, Dereje, Desalegn, Woyneshet and Dave for their encouragements, moral support and prayers.
I thank Biology Department of AAU and Limnology Department of Vienna University, and the staffs for their encouragements, support and friendship. I thank librarians of Vienna University, Life Science Faculty for their kind support. I also thank Amehara Regional Fishery Research Institute, Bahir Dar Ethiopia and the staff for their support.
This PhD program was sponsored by Austrian Agency for International Cooperation in Education and Research (OeAD-GmbH): ‘Vielen Danke’. I also thank Addis Ababa University for granting me leave of absence and financial assistance for field work. Ministry of Science and Technology also supported me for fishery data collection for which I am grateful.
Above all I thank the almighty God and His Mother Mary.
5 General Introduction and Thesis Outline Ethiopia, situated in Eastern Africa, has become a land locked country since 1993 but is still endowed with various inland water resources covering an estimated surface area of 18587 km2 (Ethiopian Environmental Protection Authority, 2010). Because of their location and opportunity for comparative study, the rift-valley and Bishoftu crater lakes have been relatively better studied (Tudorancea and Taylor, 2002). Phytoplankton community, biomass and primary production has been studied primarily in these lakes (Kebede and Belay 1994; Belay and Wood 1984; Kifle and Belay; Talling et al. 1973).
The investigations of physico-chemical environmental variables, zooplankton and fishery were also concentrated on the same lakes (Kebede et al., 1994; Talling and Talling, 1965; Mengistou and Fernando, 1991; Mengistou et al., 1991; Dadebo 2000). These studies were able to compare within themselves to understand similarities and differences along ecosystem gradients, and represent Ethiopian inland water-bodies when compared with other tropical and/or temperate lakes (Talling and Lemoalle, 1998). However, highland lakes have been marginally investigated predominantly due to their remote location and associated logistic constraints. They are located at relatively high altitude (1800 m), have intense solar radiation around noon, relatively mean lower water temperature (for tropical lakes), some are comparatively deep lakes, have high primary production, and high zooplankton numerical abundance and biomass. The study of these lakes will increase local and global understanding and comparison of aquatic ecosystems. Furthermore, it is important for designing inland water management and sustainable utilization of the resources. Ironically, these highland areas are densely populated in Ethiopia (i.e. about 70% of the population lives in highland area); consequently demographic pressure and anthropogenic effect on natural resources could be high. The highland of northern Ethiopia, where the present study Lake Hayq is located, exhibits severe land degradation, erosion and is periodically prone to drought and famine caused by deforestation (Darbyshire et al., 2003).
Lake Hayq provides a habitat to different fish species, water birds and aquatic organisms. It also plays an economical role via tourism and fishery, and most importantly it provides drinking water to the local inhabitants. The lake has been described as oligotrophic by different visitors. Zanon (1941), for example, described the lake as ‘limpida e verdastra’, which means ‘crystal clear water’. Some 30 years later, Baxter and Golobitsch (1970) also described the lake as ‘an unusual clear-water lake’ with a Secchi depth of 9 m, very low algal biomass ( 1 µg L-1 Chlorophyll a) and oxic conditions down to 40 m depth. Indeed, this lake was a clear-water lake when compared with other Ethiopian rift-valley lakes (Tudorancea and Taylor, 2002). In the meantime, the lake was stocked with Tilapia fish (Oreochromis niloticus) in the late 1970-ies to fill an empty pelagic niche of planktivorous fish and to provide cheap protein to the local people (Kebede et al., 1992). This lake was without piscivororous fish species and the stocked Tilapia fish dominated the ecosystem. In the 1990s an annual harvest of 200 ton Tilapia was recorded (Kebede et al., 1992) and a biomass of 298 ton was calculated in the present study. Approximately 10 years later after the introduction of Tilapia, the lake had undergone eutrophication and many Tilapia fish died (Kebede et al., 1992).
Eutrophication, although reversible, is a real and major environmental problem throughout the world (Bronmark and Hansson, 2005). Until the 1980s, limnologists have focused on bottom-up control as the primary determinant of phytoplankton growth and biomass, and phosphorous was a major responsible factor in the process of eutrophication (Schindler 1977; Bronmark and Hansson, 2005). However, in the 1980-ies it became clear that the functioning of ecosystems was not a simple 6 one-way up the food chain but a more complex web of interactions leading to the top-down control hypothesis. Earlier in the 1960-ies, it was known that planktivorous fish affect the zooplankton assemblage and indirectly influence the phytoplankton community (Brooks & Dodson 1965). There has been considerable debate over the relative importance of top-down versus bottom-up control of lake ecosystems. In bottom-up/top-down hypothesis (McQueen et al., 1986), top-down effects are strong at the top of the food web and weaken towards the bottom, because phytoplankton biomass is thought to be more strongly controlled by resources (bottom-up) than by grazing (top-down). However, trophic cascade hypothesis, which is typically strong in aquatic ecosystem, discusses the importance of top-down effect on lower trophic level (Carpenter and Kitchell 1992). The potential biomass and production of organisms at a given trophic level is determined by bottom-up availability of nutrients (for autotrophs) and food (for heterotrophs), whereas the realized biomass at each trophic level is regulated by the trophic level above through top-down control (Carpenter and Kitchell, 1993; Chase, 2000). For instance, in a three trophic-level food chain, predators can reduce herbivore zooplankton indirectly benefiting the primary producer phytoplankton to grow very well. Trophic cascade hypothesis was supported through a whole lake experiments (Carpenter and Kitchell, 1993). Most planktivorous fish are size-selective predators and feed preferentially on large zooplankton (Hall et al., 1976). An increase in planktivorous fish biomass leads to a correspondence change in zooplankton size structure towards smaller-sized species (Spaak and Hoekstra, 1997). Small zooplankters are less efficient in grazing down phytoplankton than large ones (‘Size Efficiency Hypothesis’; Brooks and Dodson, 1965) and the dominance of small zooplankton species often results in an increased phytoplankton biomass (Shapiro and Wright, 1984). In some cases, increased algal growth can be absorbed by the food chain resulting in increased fish catches, but in many surface waters, algal blooms can have considerable detrimental impacts on tourism, drinking water supply, resulting in fishkill and other organisms (due to oxygen depletion). Eutrophication can lead to decreases in species richness and diversity, which may have negative consequences for the food web structure and functioning (Proulx et al. 1996).
In 1989 after a massive fish kill in Lake Hayq, researchers were invited to study the cause behind this adverse phenomenon. Based on their snap-shot survey, Kebede et al. (1992) reported the remarkable changes of Lake Hayq into a stable eutrophic status, with water transparency of only around 1.2 m, Chl a concentrations between 13 and 23 µg L-1 and the absence of oxygen below 15 m.
Furthermore, the authors proposed two hypotheses for the trophic change of the lake: (i) an increased nutrient to volume ratio and (ii) the introduction of Tilapia (Oreochromis niloticus Linnaeus 1758) in the late 1970-ies. The former could not be fully substantiated due to scarcity of chronological data.
However, nutrient concentration might have been elevated as a result of catchment drain off. In the last three centuries, deforestation and soil erosion has been intensified around Lake Hayq possibly due to human impact (Darbyshire et al. 2003), which could be a signal for nutrient increment in the lake.
Moreover, the land uses in the catchments include agriculture on steep land that uses inorganic fertilizer, and livestock grazing.
Additionally, the water level has dropped as St. Estifanos Monastery, which was once on an island, is now connected to the shore via peninsula (Baxter and Golobitsch, 1970; Pankhurst 1967; Ashebir Abebe, personal communication), suggesting nutrient increase to volume ratio. On the other hand, since adult and juvenile Tilapia regularly consumes zooplankton (Getachew 1993; Tadesse 1988), the grazing pressure of introduced Tilapia on zooplankton might have relieved phytoplankton to grow better; consequently higher phytoplankton biomass could develop. For instance, it was shown that the 7 introduction of fish to previously fishless high-mountain lakes is accompanied by a change in the zooplankton community: the calanoid copepod, Hesperodiaptomus, the cladoceran Daphnia, and other large zooplankton were eliminated and replaced by smaller copepods and rotifers (Anderson 1972;
Gliwicz, 1985; Knapp et al., 2001), which are less efficient filter feeders and eutrophication could result.