Strong effects of occasional drying on subsequent water clarity and cyanobacterial blooms in cool tropical reservoirs

• Published in: Freshwater biology Vol. 59; no. 4; pp. 870 - 884
• Main Authors: Teferi, Mekonen, Declerck, Steven A. J, De Bie, Tom, Lemmens, Pieter, Gebrekidan, Abraha, Asmelash, Tsehaye, Dejenie, Tadesse, Gebrehiwot, Kindeya, Bauer, Hans, Deckers, Jozef A, Snoeks, Jos, De Meester, Luc
• Format: Journal Article
• Language: English
• Published: Oxford Blackwell Science 01-04-2014; Blackwell Publishing Ltd; Blackwell; Wiley Subscription Services, Inc
• Subjects: algal blooms; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; biomass; drying; Ethiopia; fish; fish biomass; fish kills; Fresh water ecosystems; Freshwater; Fundamental and applied biological sciences. Psychology; Microcystis; nutrient content; reservoir ecology; Synecology; turbidity; water clarity; water quality; water shortages; wet season; Ethiopia; East Africa; Sub-Saharan Africa; Africa; Tropical zone; Reservoir; Ecology; Biomass; reservoir ecology; Microcystis; Freshwater environment; fish biomass; Hydrobiology; Cyanobacteria; Bloom; Bacteria; water clarity; Ethiopia; Drying
• ISSN: 0046-5070; 1365-2427
• DOI: 10.1111/fwb.12312

In semi‐arid regions, the construction of small reservoirs is important in alleviating water shortage, although many have poor water quality with high turbidity and dense blooms of algae and cyanobacteria, and there are large differences in the ecology of such reservoirs. We took advantage of two exceptionally dry years in northern Ethiopia to study the effect of a dry period and the associated fish kills on reservoir ecology and water quality. We studied 13 reservoirs, seven of which dried up in 2009. Four of the latter dried up again in 2010. We monitored the ecology of these reservoirs from 2009 to 2011, hypothesising that the pattern of reservoir drying would explain ecological differences among them. Reservoirs that refilled after drying had a significantly lower fish biomass, lower biomass of phytoplankton (expressed as chlorophyll‐a) and cyanobacteria (Microcystis), clearer water, greater macrophyte cover and lower nutrient concentrations than reservoirs that did not dry. Although the differences in water quality were most striking in the wet season after a drying event, there were persistent effects on reservoir ecology. The three categories of reservoirs we distinguished, based on their behaviour in 2009 and 2010, also showed differences in 2004, a year during which none of the reservoirs dried out. While drying evidently results in better water quality, we could not disentangle the effects of drying per se from that of reductions in fish biomass. The total combined effect was highly significant in all 3 years, whereas the separate effects of drying and loss of fish were only significant in 2004. Our results suggest that differences in water quality and ecology among reservoirs depend on their propensity to dry out. Drying might be used as a restoration measure to reduce potentially harmful cyanobacterial blooms in reservoirs.