Environmental Gradients and Benthic Community Responses to Cage Aquaculture in Lake Victoria: A Spatial and Seasonal Assessment at Mulukoba fish farms

Cage aquaculture can influence aquatic ecosystems through nutrient enrichment, organic matter accumulation, and changes in water column structure, with consequences for both pelagic and benthic habitats. This study assessed spatial (offshore), vertical, and seasonal variation in water quality and benthic macroinvertebrate communities along aquaculture-influenced gradients in Mulukoba, Lake Victoria. Sampling was conducted during wet (May 2026) and dry (June, 2026) seasons along two parallel offshore transects (A and B) extending from near-cage stations (A1–A3, B1–B3) to an offshore control site. Physicochemical water column profiles were collected together with surface and bottom nutrient samples, while benthic macroinvertebrates were used to assess community structure. Distinct offshore gradients were observed in key water quality parameters, including chlorophyll fluorescence, turbidity, dissolved oxygen, conductivity, and nutrient concentrations, with the strength and direction of these gradients varying between wet and dry seasons. Vertical profiles revealed pronounced stratification in temperature, dissolved oxygen, and related variables, with station-specific differences indicating spatial heterogeneity in water column structure. Nutrient concentrations (nitrogen and phosphorus fractions) also varied between surface and bottom waters, highlighting vertical differentiation in biogeochemical conditions. Benthic macroinvertebrate communities exhibited clear spatial structuring along the offshore gradient. Near-cage stations showed higher abundances of pollution-tolerant taxa such as Chironomidae and Tubificidae, while offshore stations supported more balanced assemblages. Diversity indices reflected these patterns, with lower species richness and Shannon diversity observed at impacted stations compared to the offshore control, indicating reduced community complexity near aquaculture sites. Multivariate analyses confirmed significant differences in community composition among stations, consistent with environmental gradients. Overall, the findings demonstrate that cage aquaculture is associated with measurable offshore, vertical, and seasonal changes in water quality and benthic community structure. The associated reductions in diversity indices near cages further suggest localized ecological stress. These results highlight the importance of integrating diversity-based metrics with physicochemical monitoring to assess aquaculture impacts in Lake Victoria and support evidence-based management of cage farming systems (299 words).