The effects of land use disturbance vary with trophic position in littoral cichlid fish communities from Lake Tanganyika

Abstract

Impacts of anthropogenic disturbance are especially severe in freshwater ecosystems. In particular, land use disturbance can lead to increased levels of pollution, including elevated nutrient and sediment loads whose negative impacts range from the community to the individual level. However, few studies have investigated if these impacts are uniform across species represented by multiple trophic levels. To address this knowledge gap, we focused on Lake Tanganyika cichlid fishes, which comprise hundreds of species representing a wide range of feeding strategies. Cichlids are at their most diverse within the near‐shore environment; however, land use disturbance of this environment has led to decreasing diversity, particularly in herbivores. We therefore tested if there is a uniform effect of pollution across species and trophic groups within the hyper‐diverse rocky shore cichlid fish community. We selected three sites with differing levels of human impact along the Tanzanian coastline and 10 cichlid species, comprising varying taxonomic and trophic groups, common to these sites. Nitrogen and carbon stable isotope values for 528 samples were generated and analysed using generalised linear mixed models. We also estimated stomach contents including sediment proportions. Our study highlights that multiple sources of pollution are having differing effects across species within a diverse fish community. We found that nitrogen stable isotope values were significantly higher at the most disturbed (urbanised) site for benthic feeding species, whereas there was no difference in these isotopes between sites for the water column feeding trophic group. Stomach contents revealed that the elevated δ15N values were unlikely to have been caused by differences in diet between sites. However, at the most disturbed site, higher proportions of sediment were present in most herbivores, irrespective of foraging behaviour. It is likely that anthropogenic nitrogen loading is the cause of higher nitrogen stable isotope values since there was no evidence of species shifting trophic levels between sites. Results support our previous study showing herbivore species to be most affected by human disturbance and make the link to pollution much more explicit. As lower diversity of consumers can negatively affect ecosystem processes such as stability, alleviating environmental impact through sewage treatment and afforestation programmes should continue to be a global priority for the conservation of aquatic ecosystems, as well as human health.

Environmental impacts of tilapia fish cage aquaculture on water physico-chemical parameters of Lake Kivu, Democratic Republic of the Congo

“…Furthermore, cases of algal bloom, ammonium buildup, increased level of suspended solids, and decreased water clarity (increased turbidity) and pH have been reported (Pitta et al, 2006;Degefu et al, 2011;Verdegem, 2013;Karikari et al, 2016;. The water quality status of various African lakes has been sufficiently studied, with most studies showing that these waters are increasingly organically polluted over the years (Helmer and Hespanhol, 1997;Odada et al, 2003;Branchu et al, 2005;UNEP, 2010;Ouma et al, 2016;Britton et al, 2019). Among other causes of this pollution, floating cages have been mentioned mainly in Lakes Volta (Clottey et al, 2016;Osei et al, 2019), Victoria (Nyakeya et al, 2022) and Kariba (Mhlanga et al, 2014).…”

Physico‐chemical characterization of littoral water of Lake Kivu (Southern basin, Central Africa) and use of water quality index to assess their anthropogenic disturbances

“…In this context, Ouma et al (2016) using physico-chemical parameters showed that the littoral waters in Lake Victoria were polluted, rendering them unfit for human consumption, who use them for their various domestic needs. Donohue and Irvine (2003), using the same approach, showed that in Lake Tanganyika, the physico-chemical pollution of the littoral waters due to contamination from its watershed has caused a decline in the biodiversity of this area, biodiversity that served as a source of food for pelagic fish that come here to reproduce (Britton et al, 2019). So, a water quality survey program is necessary for monitoring the water bodies' safety and freshness (Pesce & Wunderlin, 2000).…”