Decadal Shifts in Zooplankton Size Structure and Biomass in a Warming Tropical Lake

Climate warming can alter lake zooplankton through changes in thermal structure, oxygen availability, nutrient cycling, and phytoplankton dynamics, but decadal evidence from African lakes remains scarce. We compared zooplankton size structure and biomass in Lake Bosumtwi, Ghana, between 2005–2006 and 2018–2020, using harmonised historical data and recent observations of zooplankton, temperature, dissolved oxygen, nutrients, chlorophyll a, and water transparency. Water temperatures increased across all depth strata, with the largest increase in the epilimnion (+0.30 °C), while dissolved oxygen declined by approximately 27%. Epilimnetic total nitrogen, total phosphorus, and chlorophyll a increased, and water transparency decreased. Chlorophyll a rose by 142% between periods, while mean total zooplankton biomass increased 2.33-fold, mainly because of higher copepod biomass. Mean body length decreased in nauplii but increased in copepodites and adult copepods, indicating contrasting stage-specific shifts in size structure. The median zooplankton biomass-to-chlorophyll a ratio increased from 59.46 to 79.24 (Mann–Whitney U = 210, p = 0.032). Biomass differed significantly among years for copepods, rotifers, and total zooplankton, but not for cladocerans. These findings show that warmer and less oxygenated conditions with higher chlorophyll a concentrations were accompanied by a substantial increase in copepod-dominated biomass and stage-specific changes in zooplankton size structure.