Hydrogen sulphide dynamics, benthic community responses, and post-production recovery beneath cage aquaculture systems at Anyanga Beach, Lake Victoria, Kenya

The rapid expansion of cage aquaculture in Lake Victoria has raised concerns regarding organic waste accumulation and degradation of benthic habitats beneath cage farms. Deposition of uneaten feed and fish feces can create anaerobic sediment conditions that promote hydrogen sulphide (H₂S) production, yet information on sulphide accumulation beneath inland cage aquaculture systems in Africa remains limited. This study assessed the temporal and spatial dynamics of sediment H₂S accumulation beneath an operating Nile tilapia cage farm at Anyanga Beach, Lake Victoria, and evaluated its relationship with sediment quality indicators, benthic macroinvertebrate communities, and post-production recovery. Monthly sampling was conducted over a six-month production cycle at stations located directly beneath cages (0 m), 50 m, 150 m, and 500 m from the cage farm (reference site). Monitoring continued for four months following production cessation. Sediment samples were analyzed for H₂S concentration, organic matter, total organic carbon (TOC), redox potential, and macrozoobenthic community composition, while dissolved oxygen and temperature were measured in situ. Sediment H₂S concentrations exhibited a strong spatial gradient, averaging 25.6 ± 0.1 mg kg⁻¹ beneath cages compared with 7.3 ± 0.5 mg kg⁻¹ at 150 m and 1.8 ± 0.2 mg kg⁻¹ at the reference site. Organic matter and TOC increased threefold and fivefold, respectively, while redox potential declined from +120 mV to −180 mV beneath cages. H₂S concentrations were positively correlated with organic matter (r = 0.82; p < 0.001) and TOC (r = 0.89; p < 0.001), but negatively correlated with redox potential (r = −0.78; p < 0.0001) and dissolved oxygen (r = −0.72; p < 0.0001). Macrozoobenthic abundance, richness, and sensitive taxa declined markedly beneath cages, with richness decreasing by 86% and pollution-tolerant taxa becoming dominant. Four months after production ceased, sediment H₂S concentrations remained 2.5-fold higher than at the reference site, while macrozoobenthic richness recovered to only 45% of pre-production levels. These findings demonstrate that cage farming promotes localized sulphide accumulation, sediment organic enrichment, and loss of benthic biodiversity, while recovery of benthic conditions remains incomplete several months after production ceases, highlighting the need for routine benthic monitoring and evidence-based fallowing strategies.