Oligotrophication trend of Lake Ziway, Ethiopia
Some ecological changes have been noted in Lake Ziway since the 1980s, such as lowering of the lake level, introduction and dominance of the catfish Clarias gariepinus in the fishery (53%) and establishment of cladocerans such as Daphnia barbata and Ceriodaphnia cornuta. This prompted us to study the phytoplankton biomass to see whether these changes were cascaded through the trophic food chain of Lake Ziway. The dynamics of some chemical and biological parameters were studied in two littoral and one offshore sites in the lake from November 2003 to August 2004. Nutrients showed temporal as well as spatial variations. Nitrate increased in the offshore whereas soluble reactive phosphate (SRP) was high in littoral sites (20–380 µg/L), indicating anthropogenic impacts. Mean phytoplankton biomass was high at the offshore (43.85 mg Chl a m-3) and almost similar at the littoral sites (Mean values 33.73 and 33.68 mg Chl a m-3), but much lower than values reported earlier. Hourly rate of integral photosynthesis (∑a) ranged from 57.4–726 mg O2 m-2 h-1at the offshore and 95–300 mg O2 m-2 h-1 at littoral sites, respectively. Biomass-specific rate of photosynthetic production at light saturation, (photosynthetic capacity, Φmax) ranged from 5.06–28.8 mg O2 (mg Chl a)-1 h-1, slightly higher than values reported in the 1980s (9.6–22.5), due to depressed algal biomass. Although nutrients have increased, phytoplankton biomass (as Chl a) has decreased over the last two decades, possibly due to heavy grazing by zooplankton and introduced fish. If this continues for some time, Lake Ziway will head towards oligotrophication, instead of eutrophication, as speculated by previous workers.