Long-term Trends of Phyto- and Zooplankton Communities of Lake Hawassa, Ethiopia
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Long-term monitoring of plankton composition and productivity can impart information useful for lake management purposes, though such study is sporadically documented in tropical lakes. The phyto- and zooplankton communities of Lake Hawassa (previously also known as Lake Awasa or Awassa), Ethiopia, was studied between November 2003 and August 2004, and it is compared with historical data since the 1980-ies to assess potential interdecadal changes. Lake Hawassa, located in the Ethiopian rift-valley and neighborhood of the growing city of Hawassa, receives effluents from textile and ceramics industry and municipal sewage 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 (as surrogate Chlorophyll a, 19 µg L-1) was lower than reported from previous studies (e.g. 43 µg L-1). However, areal rate of gross photosynthesis had increased in the last two decades with
higher values recorded at present (0.35 to 2.21 g O2 m-2 h-1). In a similar way, even though zooplankton community composition remained the same, the abundance and dominance of taxa have changed in the last decades with lower abundance of cladocera (2600 ± 640 SE Indl m-3) and high abundance of the small-sized rotifers (264 000 Indl m-3, more than fivefold decadal increase) and cyclopoid nauplii. The adult cyclopoid to nauplii ratio of 0.27 in the present study indicated that cannibalism previously suggested has diminished at present, probably due to high number of rotifers as prey. Plankton data of 2003/4 compared with those of two decades earlier indicated that Lake Hawassa has become eutrophic with higher gross phytoplankton production rate, reduced phytoplankton biomass due to overtake by nanoplanktonic forms and Nitrogen dominance by NH4. Zooplankton composition has shifted to small-sized groups with Rotifer and Nauplii dominance and Cladocera have become reduced. Most plankton parameters indicate that Lake Hawassa is trending towards eutrophication
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