Impacts of nutrient loading and fish grazing on the phytoplankton community and cyanotoxin production in a shallow tropical lake: Results from mesocosm experiments

Abstract

Given the increasing eutrophication of water bodies in Africa due to increasing anthropogenic pressures, data are needed to better understand the responses of phytoplankton communities to these changes in tropical lakes. These ecosystems are used by local human populations for multiple purposes, including fish and drinking water production, potentially exposing these populations to health threats if, for example, an increase in toxic cyanobacterial blooms is associated with increasing eutrophication. To test the short‐term response of the phytoplankton community to the addition of nutrients (phosphorus and nitrogen, alone or in combination) and Nile tilapia, we developed an in situ mesocosm experiment in a freshwater lagoon located near Abidjan (Ivory Coast). We found that phytoplankton growth (estimated by chlorophyll‐a quantification) was highly stimulated when both nitrogen and phosphorus were added, while there was no clear evidence for such colimitation by these two nutrients when considering their concentrations in the lagoon. Phytoplankton growth was accompanied by significant changes in the diversity and composition of this community and did not lead to an increase in the proportions of cyanobacteria. However, the addition of fish to some mesocosms resulted in a drastic decrease in phytoplankton biomass and a dominance of chlorophytes in this community. Finally, these experiments showed that the addition of nitrogen, alone or combined with phosphorus, stimulated microcystin production by cyanobacteria. In addition, no evidence of microcystin accumulation in the fish was found. Taken together, these data allow us to discuss strategies for controlling cyanobacterial blooms in this tropical ecosystem.

Assessment of cyanotoxins in water and fish in an African freshwater lagoon (Lagoon Aghien, Ivory Coast) and the application of WHO guidelines

“…During this monitoring of Lagoon Aghien, the N:P ratio values ranged between 7 and 14(Ahoutou et al 2020), which corresponds to the low N:P ratio and low microcystin concentrations in the paper ofMowe et al (2015). Moreover, during mesocosm experiments performed in Lagoon Aghien(Ahoutou et al 2022), we found that the addition of nitrogen, alone or combined with phosphorus in the mesocosms lled with the water of the lagoon, stimulated MC production by cyanobacteria. Taken together, these results are interesting to consider in light of the data provided byChianu et al (2012) showing that nitrogen fertilizer consumption is low in African farming systems compared to northern countries.…”

“…The same protocols as those described in Ahoutou et al (2022) were used for all toxin analyses. Brie y, for intracellular cyanotoxins (in the phytoplanktonic biomass), the extraction was performed from biomass on polycarbonate lters (Whatman® Nuclepore) stored at −80 °C.…”

Changes in Phytoplankton Community Composition and Phytoplankton Cell Size in Response to Nitrogen Availability Depend on Temperature

“…A shift towards dominance by green algae of the phytoplankton community with increasing N has been reported in mesocosm studies simulating shallow lakes [ 63 ] and natural field samples in shallow lakes at high N loading [ 17 , 64 , 65 , 66 ]. Despite the well-recognized harm of cyanobacteria blooms to the aquatic ecosystem, the blooming of other phytoplankton groups may also have serious implications for the ecosystem’s biodiversity and functioning [ 67 , 68 ].…”

Fish gut-associated bacterial communities in a tropical lagoon (Aghien lagoon, Ivory Coast)

“…To do so, reductionist approaches under controlled conditions are necessary, alongside holistic approaches using more environmental samples. These may for example take advantage of the existence of aquaculture settings in the Aghien lagoon, or may involve field-based mesocosm experiments ( Ahoutou et al, 2022 ).…”