Phytoplankton community and physico-chemical seasonality and changes in Lake Hora-Kilole, Ethiopia, a tropical crater lake

Abstract

Phytoplankton community structure and physico-chemical variables, sampled monthly at an offshore station in Lake Hora-Kilole from August 2007 to May 2008, were compared with previously reported data. In 1989 the Mojo River was temporarily diverted to flow into the lake. The lake subsequently changed markedly, first from being a small polymictic system to being a large monomictic system with a maximum depth of 29 m in 1990/1991, and then by 2007/2008 to again being a small continuously polymictic system less than 8 m deep. Secchi depth averaged 0.35 m and varied from 0.15 to 0.78 m, indicating a marked decline in water transparency from the levels recorded in 1990/1991. Nitrate levels, rarely detectable in the 1960s, subsequently became high, whereas phosphorus levels have declined since the 1960s. The phytoplankton community was dominated by cyanobacteria and dinoflagellates, in contrast to the dominance by cyanobacteria before 1989, and by dinoflagellates and chlorophytes shortly thereafter. Total phytoplankton Chl a averaged 83.5 mg m−3 and ranged from ∼36 to 148 mg m−3, peaking in October 2007 and February 2008. The impacts of river diversion and shoreline modification on the lake were reflected in changes in the physico-chemical conditions, including low light penetration and high nitrate, and changed phytoplankton community structure. The development of algal blooms in future is anticipated.