The influence of nutrient enrichment and fish addition on microbial communities in water and fish guts during mesocosm experiments conducted in a tropical freshwater lagoon (Lagoon Aghien, Cote dIvoire)
Freshwater ecosystems in the intertropical zone have received considerably less scientific attention than those in temperate regions, with the notable exception of a few large and iconic lakes, such as Lake Victoria. Consequently, the diversity, functioning, and pollution sensitivity of microbial communities in these systems remain poorly understood. To investigate the short-term responses of microbial communities to nutrient enrichment and fish addition, we conducted in situ mesocosm experiments at two stations in a freshwater lagoon near Abidjan (Cote dIvoire). Phosphorus enrichment, either alone or in combination with nitrogen, led to a consistent increase in bacterial abundance across mesocosms, suggesting a potential phosphorus limitation of bacterial growth by these nutrients and consequently a potential competition for this resource between bacteria and microphytoplankton. In contrast, picophytoplankton abundances showed no clear or consistent response to nutrient enrichment, suggesting that other ecological processes may play a more important role in regulating these communities. High-throughput Illumina sequencing revealed contrasting patterns in the relative abundance of cyanobacterial and chloroplast-associated reads, depending on both nutrient enrichment and fish addition. These results, consistent with those obtained by cell counting in a previous study, suggest a potential top-down control of cyanobacteria by fish in Lagoon Aghien. In addition, nutrient (phosphorus and nitrogen combined) enrichment and fish addition significantly affected the amplicon sequence variant (ASV) composition of bacterial communities. Overall, these findings provide new insights into the drivers shaping microbial community structure in tropical freshwater ecosystems and are discussed in the context of previously published data on phytoplankton dynamics from the same experimental system.