@phdthesis{622498,
  abstract     = {{Ethiopia is the third largest country in Africa with an area of over one million km2. It is one of the most populous countries in Africa (probably second), with more than 80 million inhabitants. The Ethiopian highlands represent about 43% of the country but support about 88% of the population. The highlands account for 95% of the regularly cropped land, more that 70% of the livestock population, and 90% of the economic activities of the country. They are considered to be amongst the most degraded lands in Africa by some authors. Rain fed agriculture is the main stay for most farmers. The frequent rainfall anomalies suggest that there are recurrent periods of drought every 3- 5 years in the northern parts of Ethiopia and every 6-8 years over the whole country. Part of the variability in the seasonal and annual rainfall across time and space is known to be associated with the El Nino-Southern Oscillation (ENSO) phenomenon. The conditions in Tigray are worse. Extreme spatial and temporal variation in rainfall is characteristic for this region. To tackle the problem associated with the rain fall pattern, several small reservoirs have been constructed over the last two decades. Given the population intensity and long history of agriculture in the highlands of Ethiopia, massive erosion linked to land degradation is a prominent problem. This is expected to bring excessive nutrient to the reservoirs. And many of the reservoirs are expected to be characterized by high nutrient loads and phytoplankton blooms, including cyanobacteria blooms. This has indeed observed in a field survey of reservoirs, most of them suffer from heavy blooms of cyanobacteria. In this study we started with a field survey of a set of 32 shallow semi-arid sub-tropical reservoirs in the highlands of Tigray, Ethiopia. This survey was carried out in both the wet and dry season to capture seasonal variations of phytoplankton communities and associated environmental variables. We assessed seasonal variation Summary 240 in more details by monitoring eight selected reservoirs (sub sets of the 32 reservoirs) on a monthly basis during a whole year. We also carried out field enclosure experiments in an effort to better understand the trophic structure of the reservoirs and identify mechanisms that potentially lead to cyanobacterial blooms. First we tested the impact of fish on abiotic conditions in the water column as well as the dynamics of phytoplankton species composition and cyanobacteria biomass. In the second experiment we assessed the potential top-down effects of zooplankton on the phytoplankton communities including the toxic cyanobacteria. The studied reservoirs were characterized by high nutrient concentrations and high turbidity. Most of the reservoirs harbor the riverine fish Garra. Overall, the local phytoplankton richness was low with most reservoirs dominated by a single genus of cyanobacteria (mostly Microcystis), chlorophytes, euglenophytes, cryptophytes or dinophytes. Similarly the bacterial community richness in the studied reservoirs was also low. Lower bacterial taxon richness was encountered in reservoirs with Microcystis blooms than bacterial communities in reservoirs without blooms. High altitude reservoirs were more nutrient-rich and associated with high abundances of green algae, euglenophytes or cyanobacteria other than Microcystis. Microcystis was associated with high pH in the rainy and high conductivity in the dry season. Additional factors correlated with Microcystis biomass were Daphnia biomass and possibly altitude and fish biomass. Environmental factors explained the bacterial community composition differently among season. Percentage contribution of Microcystis to the total phytoplankton biomass and copepod biomass showed significant association with the bacterial community composition in the wet season whereas variation in bacterial community composition was associated with total nitrogen (TN), total phosphorus (TP), oxygen, the number of cattle frequenting the Summary 241 reservoir, and fish biomass in the dry season. Pronounced temporal variation was observed for both biotic and abiotic variables in our study systems. This variation involved both the intra-annual and interannual variations. For the intra-annual variation, the main limnological changes were associated with seasonal differences in rainfall, while also water temperature differed strongly between winter (sub-tropics) and the rest of the year. We observe two minima for phytoplankton biomass: one in winter and a more pronounced one during August. We also observed two main bloom periods for cyanobacteria: one in September-October and a more pronounced one in May-June. Seasonal variation in total phytoplankton and cyanobacterial biomass was, however, not significant. The first field enclosure experiment was the experiment with fish. The results of this experiment showed that 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 second experiment was carried out to study the effect of zooplankton grazing on phytoplankton community structure, including the relative abundance of toxic cyanobacteria. From this experiment top-down regulation by zooplankton was observed for some of the phytoplankton taxa, including Anabaena, Euglenoids, Chlorophytes and Cryptomonads, whereas the impact of the presence of zooplankton on Microcystis and Peridinum biomass was limited. From the same experiment we observed negative correlation between Anabaena and calanoid copepods and Daphnia carinata. We also detected microcystin from all experimental units in the second experiment; and higher concentrations were detected in the treatment with than without zooplankton. Summary 242 We draw some important associations from the field observations and field enclosure experiments for Microcystis in our system. The results indicate an inter-play between bottom-up (possibility of Microcystis affecting the zooplankton composition) and top-down (zooplankton grazing) regulating the Microcystis. Negative association between Microcystis and Daphnia has been observed mainly from the field survey and to some extent the enclosure experiments with fish and fishless treatment demonstrated a top-down regulation. From these results we can conclude that the zooplankton grazing can not fully regulate Microcystis. But, the possibility remains, and is in fact quite realistic, that Daphnia also control Microcystis, mainly at lower biomasses of Microcystis until it reaches a certain biomass and Microcystis “escapes” potential control by zooplankton after which it may poison the major grazer zooplankton due to the high densities. Based on our results of the present study, we put some general suggestions and recommendations for sustainable utilization, maintaining the ecological integrity of the reservoirs and protecting water quality deterioration. The recommendations follow in the following statements. 1) Cyanobacterial monitoring and survey for hazardous effects to assess if the toxins of the organisms are translated into problems of animal or human health should be set-up. 2) Reduction of nutrient loading and sediment input to the reservoirs to curb the eutrophication of the reservoirs. Catchment treatment with reforestation and setting up of exclosures can serve the purpose. 3) Reduction of cattle trampling by restricting cattle access to the reservoirs at selected sites of the reservoir. 4) Reduction of fish (mainly Garra). Here we recommend the use of methods to reduce the riverine fish with a high level care to protect the reservoirs from unpredictable consequences like the introduction of exotic fish.}},
  author       = {{Asmelash Dejene, Tsehaye}},
  language     = {{eng}},
  pages        = {{242}},
  school       = {{Ghent University}},
  title        = {{Phytoplankton community structure and cyanobacterial blooms in reservoirs in the semi-arid highlands of Tigray, Ethiopia}},
  url          = {{http://lib.ugent.be/fulltxt/RUG01/001/332/703/RUG01-001332703_2010_0001_AC.pdf}},
  year         = {{2009}},
}