Influence of freshwater and terrigenous material on nutrients, bacteria and phytoplankton in a high island lagoon: Mayotte, Comoro Archipelago, Indian Ocean

Abstract

Freshwater and terrigenous suspended material inputs were studied in Longoni Bay, an area of the lagoon of Mayotte (Comoro, Indian Ocean) that is unaffected by the influence of the most populated area of the island. The influence of terrigenous material on the lagoon waters was limited to the coastal zone and resulted in the increase of inorganic nitrogen and silica. The response of the bacteria was an increase of their cell volume; that of the phytoplankton was an increase of cyanobacteria numbers and chlorophyll a related to ammonia. Ammonification was apparently enhanced in freshwater whereas nitrification was enhanced in seawater. The high nitrate and silicate content of the reef zone were explained neither by river or rainfall inputs nor by enrichments by ocean waters. The hypothesis of the participation of marine groundwater discharges to this enrichment was examined and found to be supported by the range of concentrations in the reef zone and because enrichment did not concern ammonia and nitrite but only nitrate and silicate and occurred during the wet period when the aquifer is fed by abundant rainfall. The response of bacteria to the characteristics of the reef zone was an increase in biomass and a lower level of heterotrophic activity related to nitrate concentration. The changes observed during the wet period confirmed that the nitrogen increase was linked to the terrigenous material, and possibly to discharge of groundwater, and yielded a lower number of bacteria but increased biomass, and despite a drop in large phytoplankton numbers, a strong increase in chlorophyll a related in part to small cyanobacteria. This enrichment is high enough to place the Mayotte Lagoon among the richest high island lagoons, with regard to nitrogen and chlorophyll a.