Status of phytoplankton biomass and physico-chemical parameters of water during fish cage culture fallowing: a case study of Southeast Arm of Lake Malawi

The prospect of using fallowing as an ecosystem management measure to restore ecosystem functions in disturbed aquatic environments under cage culture was evaluated in this study. The cage aquaculture site in the Southeast arm of Lake Malawi was studied following the abandonment of cage aquaculture activities in 2022 after 18 years of operation. A Hydrolab CTD MS5 series probe was cast at each station to a maximum depth to collect electronic data and provide in situ depth profiles of temperature, dissolved oxygen, oxygen saturation, pH and salinity. Chlorophyl-extraction was done using a mixture of acetone and methanol and analyzed on a turner series 10 fluorometer. The study found that physical-chemical parameters such as temperature, pH, ammonia, turbidity and dissolved oxygen were not significantly different (p > 0.05) across the cage and non-cage sites and between seasons. However, ammonia levels recorded in this study were lower than the values reported during the cage culture operations period, an indication that some recovery processes are happening at the site. Chlorophyl a level was found to be insignificantly higher in cage and non-cage sites (p > 0.0.5) and significantly different between season (p < 0.05). High levels of Chl-a observed in this study are independent of cage aquaculture but might be linked to external inputs associated with tributaries as they collectively drain into the lake. Chl-a was found to be significantly and directly correlated to temperature, followed by total suspended solids and pH. Overall, the site is undergoing recovery with respect to the abandonment, but the effect is masked by external inputs of nutrients into the Lake. Although the study confirmed the oligotrophic status of the lake, the increasing levels of Chl-a are alarming and calls for inclusion of watershed management in agriculture and environmental management programming to sustainably safeguard the health of the Lake.