Investigating the relationship between different coral health gradients and plankton communities at selected sites around Mauritius Island.

Supervisor: Ranjeet Bhagooli (University of Liège).
Marine microplankton are microscopic organisms at the key base of the marine food chain. Up to 50% of global primary production depends on them. Microphytoplankton are photoautotrophic creatures boosting the ecosystem productivity while microzooplankton are heterotrophic organisms feeding on the primary producers to obtain energy and make the link with Heloise Ave species from higher trophic levels. Their presence is crucial for coral reefs ecosystems since they are the main food sources for fish and reef building polyps. Previous studies have been done in Mauritius on microphytoplankton and microzooplankton, but none has studied both communities at the same time. The aim of this paper was to investigate the relationship between the density and diversity of the microphytoplankton and microzooplankton communities at different study sites with contrasting gradients of reef degradation. Microplankton were collected with a plankton net; 5 μm net size for the microphytoplankton and 100 μm net size for the microzooplankton. Samples were analysed under inverted light microscope and microplankton were identified until their genus level. Statistical software R-Studio was used to test for Pearson’s correlation among the different variable which are microphytoplankton density, microzooplankton density and chlorophyll-a concentration. Two-way ANOVA analysis and Tukey HSD (Post-Hoc) test were carried out to look for any significant difference for the variables among the sites and months. There was no significant difference in the microphytoplankton density among the studied sites except for Albion in April 2019 where the density was considerably higher. An increase in the seawater temperature could have accelerated microphytoplankton growth. Diatoms were the largest group present followed by dinoflagellates and cyanobacteria. However, the percentage abundance of Peridinium sp. and Alexandrium sp. was quite alarming at some sites, as they are considered as toxic. Additional statistical tests were conducted to assess the diversity of both plankton communities. There was no conclusive relationship between the diversity and density in the microphytoplankton community. Any fluctuation in diversity might be explained by either interspecific competition or alterations in physico-chemical parameters such as an increase in temperature or an input of nutrients. Microzooplankton density was highest at Albion in March 2019 and Flic en Flac Non-Degraded in April 2019 and physical dynamics as water current might have altered the density at those sites. Microzooplankton community showed a clear difference in their distribution. Degraded site such as Albion were largely dominated by Copepods in March 2019 and an inverse relationship was between the diversity and the density. On the other hand, Bivalve veliger and Gastropod veliger were the main genera in the microzooplankton community at the other sites.