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1. Initation of different phytoplankton groups by inorganic nutrients, nutrient enrichment bioassays using the addition of iron (Fe) and the combined addition of nitrogen and phosphorus (NP) were carried out in the north and the south of Lake Tanganyika during the rainy and dry seasons in 2003 and 2004. 2. Nutrient additions resulted in an increase in phytoplankton growth rate relative to control treatments in all experiments. HPLC pigment data and epifluorescence microscopy counts indicated differential stimulation of the dominant phytoplankton groups. Iron additions mainly stimulated prokaryotic picophytoplankton, while enrichments with nitrogen and phosphorus stimulated green algae and in some cases diatoms. Extended incubation (3 days) indicated co-limitation of Fe and NP, in particular for picocyanobacteria

BR Staff Publications [a] (paper in international, peer reviewed journal) --- lake Tanganyika --- nutrient limitation --- iron --- phytoplankton --- picocyanobacteria

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Northern Ethiopia highlands are a densely populated region that suffers from strong ancient and ongoing deforestation. The region is dependent on rain-fed agriculture and has a history of recurrent droughts. To alleviate the dependency on rain-fed agriculture, microreservoirs have been constructed during the course of the last decades to harvest water during the rainy season to be used for irrigating fields during the dry season. The reservoirs serve many purposes and have strong added value, but many suffer from excessive algal blooms, including potentially toxic cyanobacteria blooms. As there was no previous information on the ecology of these semi-arid highland tropical reservoirs, we engaged in a study designed to document the baseline data of the limnological and ecological characteristics of the reservoirs, to assess the variables that determine zooplankton community structure, to document temporal variation in abiotic and biotic variables, to assess whether the trophic structure is bottom-up or top-down controlled, and document the effect of fish (mainly the small riverine genus Garra) on biotic and abiotic variables in the reservoirs. We started with a field survey of a set of 32 reservoirs that are representative for the permanent reservoirs in the region. This field survey was intended to describe the ecological conditions in the reservoirs and obtain an overview on the variation in reservoir ecology. This survey was carried out in both the wet and dry season to capture seasonal variation, and included regional, morphometric, abiotic and biotic variables, the latter ranging from bacteria to fish. Seasonal variation was also assessed in more detail by monitoring eight selected reservoirs on a monthly basis during a whole year. In an effort to better understand the trophic structure of the reservoirs and identify mechanisms that potentially lead to cyanobacteria blooms, we also carried out a field enclosure experiment testing for the impact of fish on abiotic conditions in the water column as well as the dynamics of zooplankton species composition and densities, and phytoplankton and cyanobacteria biomass. In general, the studied reservoirs were characterized by high nutrient concentrations and high turbidity. Most of the reservoirs harbour a population of the riverine fish Garra . Among the zooplankton, the water flea Daphnia was observed in a majority of the reservoirs, and most reservoirs suffered from cyanobacteria blooms (mainly Microcystis ). The most important axis of variation in abiotic variables was represented by a gradient in nutrients and altitude that was positively associated with suspended matter and oxygen concentration, and negatively with water transparency. Phytoplankton and fish biomass tended to be positively related with the concentration of total phosphorus, whereas cladoceran biomass was not associated with nutrient concentrations. The former suggests a bottom-up control, whereas the latter may reflect a moderate top-down impact. The biomass of cyanobacteria was negatively related to the biomass of cladocerans ( Daphnia ), which likely reflects a top-down effect. The occurrence and abundance of macrophytes tended to be positively related to water transparency and was negatively associated to phosphorus concentration and the amount of livestock frequenting the reservoirs. However, macrophytes were not limited to clear-water reservoirs. We identified a total of fifteen cladoceran species in the highland reservoirs of Tigray. Within the zooplankton community, the genus Daphnia was observed in all reservoirs and was the most abundant cladoceran in our study. Using presence-absence data, no association between cladoceran community and geographic distance was observed. Depth, altitude and fish biomass showed a significant association with cladoceran community composition during the wet season, whereas variation in cladoceran community structure was associated with phytoplankton biomass in the dry season. We observed pronounced temporal variation in both abiotic and biotic variables in the reservoirs. This involved both seasonal variation (wet / dry season) and year-to-year variation. In the survey study, correlations between the wet and dry season were weak for most variables, which suggests that individual reservoirs behaved rather differently in their response to seasonal changes. In our monitoring study, temporal patterns for limnological variables, such as temperature, pH, conductivity, suspended matter, total phosphorus and total nitrogen were pronounced and seemed to a large degree associated with patterns of rainfall, thus also showing some consistency among reservoirs. In contrast, water transparency, phytoplankton chlorophyll a and biomass of cyanobacteria were variable among reservoirs and did not show a consistent temporal pattern. Among the zooplankton, only the genera Diaphanosoma and Ceriodaphnia tended to show a significant temporal pattern, with higher abundances in the dry than in the wet season. Due to the pronounced year-to-year variation for biological variables, it is difficult to make strong predictions on when to expect algal blooms or high densities of zooplankton. Yet, one recurrent finding in both our survey and monitoring study was a tendency for a negative association between the relative abundance of Daphnia and Diaphanosoma in the cladoceran community, and a negative association between the biomass of Daphnia carinata and the relative abundance of cyanobacteria in the phytoplankton community. We conducted a controlled field enclosure experiment in two reservoirs in both the wet and dry season testing for the impact of fish (Garra) on abiotic and biotic variables in the water column. The presence of Garra in general increased the amount of suspended matter, nutrient concentrations (total nitrogen and total phosphorus), phytoplankton and to some extent also Microcystis biomass (including the proportion of Microcystis in the phytoplankton community), and reduced water transparency. The positive effect of the presence of Garra on nutrient concentrations and phytoplankton productivity indicate that Garra may have the potential to indirectly affect food web functioning through bottom-up effects, by enhancing nutrient concentrations through sediment resuspension and excretion of nutrients. However, our data on a subset of the experiments also provide some evidence for a potential of Garra to exert top-down control on large bodied daphnids. Both the results of our field survey and the enclosure experiment indicate that bottom-up control dominates the trophic structure in the reservoirs, but that there is also evidence for some top-down effects, both of fish on zooplankton (Daphnia) as well as of Daphnia (D. carinata) on cyanobacteria. Both our field observations and our experimental results are in agreement with the idea that fish, mainly Garra species in the reservoirs studied, exert both a bottom-up as well as a top-down effect on zooplankton, with the bottom-up effect being mediated by an increase in nutrient and phytoplankton levels. Based on the results we obtained in the present study, we present some guidelines for the further management of the reservoirs. These include a reduction of nutrient and sediment input by stimulating vegetation recovery along hill-slopes as well as a reduction of the impact of cattle by limiting their access to the reservoirs. Our data also suggest that it may be important to reduce the impact of fish on the reservoirs, either by removing them by netting, by inoculating the reservoirs with an indigenous predatory fish, or by replacing them by a marketable fish such as Tilapia. The reservoirs in Tigray, northern Ethiopia serve many purposes and have strong added value, but many suffer from excessive algal blooms, including potentially toxic cyanobacteria blooms. As there was no previous information on the ecology of these semi-arid highland tropical reservoirs, we engaged in a study designed to document the baseline data of the limnological and ecological characteristics of the reservoirs, to assess the variables that determine zooplankton community structure, to document temporal variation in abiotic and biotic variables, to assess whether the trophic structure is bottom-up or top-down controlled, and document the effect of fish on biotic and abiotic variables in the reservoirs. We started with a field survey of a set of 32 reservoirs that are representative for the permanent reservoirs in the region. Seasonal variation was also assessed in more detail by monitoring eight selected reservoirs on a monthly basis during a whole year. In an effort to better understand the trophic structure of the reservoirs and identify mechanisms that potentially lead to cyanobacteria blooms, we also carried out a field enclosure experiment testing for the impact of fish on abiotic conditions in the water column as well as the dynamics of zooplankton species composition and densities, and phytoplankton and cyanobacteria biomass. In general, the studied reservoirs were characterized by high nutrient concentrations and high turbidity. Most of the reservoirs harbour a population of the riverine fish Garra . We identified a total of fifteen cladoceran species in the highland reservoirs of Tigray. Within the zooplankton community, the genus Daphnia was observed in all reservoirs and was the most abundant cladoceran in our study, and most reservoirs suffered from cyanobacteria blooms (mainly Microcystis ). Phytoplankton and fish biomass tended to be positively related with the concentration of total phosphorus, whereas cladoceran biomass was not associated with nutrient concentrations. The biomass of cyanobacteria was negatively related to the biomass of cladocerans ( Daphnia ), which likely reflects a top-down effect. We observed pronounced temporal variation in both abiotic and biotic variables in the reservoirs. This involved both seasonal variation (wet / dry season) and year-to-year variation. In our monitoring study, temporal patterns for limnological variables, such as temperature, pH, conductivity, suspended matter, total phosphorus and total nitrogen were pronounced and seemed to a large degree associated with patterns of rainfall, thus also showing some consistency among reservoirs. In contrast, water transparency, phytoplankton chlorophyll a and biomass of cyanobacteria were variable among reservoirs and did not show a consistent temporal pattern. Due to the pronounced year-to-year variation for biological variables, it is difficult to make strong predictions on when to expect algal blooms or high densities of zooplankton. Yet, one recurrent finding in both our survey and monitoring study was a tendency for a negative association between the relative abundance of Daphnia and Diaphanosoma in the cladoceran community, and a negative association between the biomass of Daphnia carinata and the relative abundance of cyanobacteria in the phytoplankton community. The presence of Garra (fish) in general increased the amount of suspended matter, nutrient concentrations (total nitrogen and total phosphorus), phytoplankton and to some extent also Microcystis biomass (including the proportion of Microcystis in the phytoplankton community), and reduced water transparency. Both the results of our field survey and the enclosure experiment indicate that bottom-up control dominates the trophic structure in the reservoirs, but that there is also evidence for some top-down effects, both of fish on zooplankton (Daphnia) as well as of Daphnia (D. carinata) on cyanobacteria. Both our field observations and our experimental results are in agreement with the idea that fish, mainly Garra species in the reservoirs studied, exert both a bottom-up as well as a top-down effect on zooplankton, with the bottom-up effect being mediated by an increase in nutrient and phytoplankton levels. Based on the results we obtained in the present study, we present some guidelines for the further management of the reservoirs. These include a reduction of nutrient and sediment input by stimulating vegetation recovery along hill-slopes as well as a reduction of the impact of cattle by limiting their access to the reservoirs. Our data also suggest that it may be important to reduce the impact of fish on the reservoirs, either by removing them by netting, by inoculating the reservoirs with an indigenous predatory fish, or by replacing them by a marketable fish such as Tilapia.