1. 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.
2. 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.
3. 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.
4. 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.