Long-term changes in phyto- and zooplankton communities of Lake Hawassa, Ethiopia

Abstract

We studied phyto- and zooplankton communities of Lake Hawassa (or Awassa)  between November 2003 and August 2004, and compared findings with historical data since the 1980s to assess potential inter-decadal changes. The lake is  located in the Ethiopian Rift Valley in the vicinity of the growing city Hawassa which receives adverse effluents from textile and ceramics industry and municipal sewage apparently with little treatment. In 2003/04, phytoplankton abundance comprised 54% Chlorophyta, 26% cyanoprokaryotes, 18% diatoms and 2% others, a  proportion similar to previous records. However, the mean phytoplankton biomass in terms of chlorophyll a (19 μg L^-1) was lower than reported from previous studies but similar to a report in 2010. In contrary, areal rate of gross photosynthesis had increased in the last two decades with higher values recorded in 2003-04 (0.35 to 2.21 g O₂ m^-2 h^-1). Even though zooplankton community composition remained the same, the abundance and dominance of taxa had changed in the last decades. In 2003/04, the mean abundance of cyclopoid-copepods was 58 000 ± 9200 (SE) Indl m^-3 whereas cladocera abundance (2 600 ± 640 SE Indl m^-3 was very low in the lake. Rotifers outnumbered other zooplankton with a maximum value of 264 000 Indl m^-3, which was about five times greater than previous reports. Adult cyclopoid to nauplii ratio of 0.27 in 2003-04 study was indicative that cannibalism had diminished, probably due to increased rotifers as prey, which contradicted a previous hypothesis. Despite its closed nature, the plankton of Lake Hawassa changed erratically during a decade, probably due to intense biological interactions in the system.

Key words/phrases: Decadal change, Interactions, Rift valley, Phytoplankton, Zooplankton.