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Nile Perch were obtained from Winam Gulf of Lake Victoria in 1998 and 2022. Thirty eight and 43 perch samples ranging in total length from 1.2 - 158 cm and 7.7 to 139.5 cm were processed in 1998 and 2002, respectively. A smaller number of samples of nile tilapia and unionids were co-collected across years. Samples were analyzed for δ13C and δ15N stable isotopes and total Hg. Carbon isotopes were depleted in mussel, tilapia and perch in 2022 compared to 1998 but δ13C was independent of body size for both fish species. Nitrogen isotopes were not dependent on size for tilapia (1998) but strongly size dependent for perch (1998 and 2022). For size matched tilapia, δ15N decreased in 2022 with a similar, but non-significant temporal decrease observed for perch. Total Hg showed a significant decrease in tilapia in 2022 compared to 1998. For perch, the bioaccumulation slopes of Hg vs total length and Hg vs δ15N were significantly different between years with higher bioaccumulation rates observed in 2022. These shifts were most evident in large perch > 80 cm and highest trophic level fish. Total length was a stronger predictor of Hg (R² of 0.45 and 0.86 for 1998 and 2002 ) compared to δ15N (R² of 0.07 in 1998 and 0.52 in 2002). Risk analysis on the legal 50 to 85 cm commercial slot size indicates a shift in maximum recommended monthly meals from >32 meal/month in 1998 to 16 meal/month for perch over 70 cm in length for women of childbearing age and children. [ABSTRACT FROM AUTHOR]

Cyanobacteria harmful algal blooms (cHABs) are increasing in frequency and extent globally and their growth is known to be driven by eutrophication. Lake Victoria has year-round blooms of the cosmopolitan cyanobacteria Microcystis aeruginosa, which can produce microcystins that are dangerous at µg/L concentrations and endanger fisheries by bloom decomposition inducing hypolimnion anoxia. Lake Victoria is shared by Kenya, Tanzania, and Uganda and supports the livelihood of more than 73 million people. Using bottle incubations, we assessed the effects of nutrient type (nitrogen (N), phosphorus (P), micronutrients) and form (urea, nitrate, ammonium; phosphate, G6P) on phytoplankton growth and microcystin production across four sites in the Nyanza Gulf of Lake Victoria during June-July 2022. Results suggest primary N limitation followed by serial P and micronutrient colimitation across the gulf. Nitrate addition induced 2-3 times more growth than other N forms at one site, while urea, ammonium and nitrate had a similar positive effect on growth at the remaining three sites. Micronutrient and P effects were spatially variable, but increased growth under micronutrient enrichment suggest that organic nutrient recycling may be limited by micronutrients in parts of the gulf. Though blooms of Microcystis aeruginosa were common, microcystin was only detected (>2 ug/L) at one site (Homa Bay). Our findings will inform best management practices for nutrient management in Lake Victoria's watershed to reduce cHABs in this highly relied upon natural resource. [ABSTRACT FROM AUTHOR]