Narrowing the gap: Phytoplankton functional diversity in two disturbed tropical estuaries

The functional diversity of phytoplankton communities was studied in two tropical estuaries subjected to anthropogenic pressure, despite one being environmentally protected. We applied functional diversity indices, analysed the dominant morphology traits and explored morphology-based functional groups. The less disturbed estuary showed slightly higher diversity and functional redundancy, during a wet season (on average 1.3 higher). Functional groups including nuisance species were more abundant in the heavily disturbed estuary. During a dry season, results for both systems were similar. Overall, abundance and functional diversity were higher in the upstream areas, associated with the river runoff containing freshwater species and essential nutrients for algae development. In both systems, diatoms were dominant (> 75% of the community), however, the group included small and larger individuals, and freshwater to cosmopolitan species. This dominance and the change in traits were associated with sufficient nutrient supply, low light conditions, ressuspension and salinity changes, also associated with river runoff. Overall, the different functional diversity approaches enabled us to improve our understanding of the changes occurring in both systems.