Diel variations in density and diversity of micro-phytoplankton community in and around a barachois-based oyster culture farm

Knowledge of diel variations in phytoplankton communities is important to set up and plan a successful and sustainable bivalve aquaculture industry. This study investigated the diel (24-hour cycle) variations in micro-phytoplankton community during two high and two low tides over two alternate days in summer at a barachois-based oyster farm in Mauritius, a micro-tidal tropical island. Micro-phytoplankton density and diversity, sea surface chlorophyll a concentration, and physico-chemical parameters, such as sea surface temperature, dissolved oxygen, pH and salinity, were assessed each day during high and low tides, at an interval of approximately six hours at 11 stations. Despite the small tidal range, significant diel variations in micro-phytoplankton density were noted throughout the tidal cycle. Highest densities of micro-phytoplankton and diversity of diatoms, dinoflagellates and cyanobacteria were recorded at stations three and six (S3 and S6). The range of total micro-phytoplankton at S3 and S6 varied above 2.0 x 10⁵ cells L-¹ and for the other stations, it varied mainly between 1 to 2 x 10⁵ cells L-¹ throughout all the tides, except on the second sampling day, where at S1 and S7, the density was slightly above 2 x 10⁵ cells L-¹. The diatom Coscinodiscus (14 %), the dinoflagellates Peridinium (18 %) and the cyanobacteria Anabaena (24 %) were dominant. S3 and S6 within the barachois having a low flushing rate were characterized by the highest availability of food stock, inferring that these stations may sustain an optimal growth of bivalves. These findings may be useful in directing the barachois-based bivalve culture site towards a more effective and sustainable management by locating the most appropriate culture areas in the system.