Patterns of phytoplankton structure in response to environmental gradients in a macrotidal estuary of the Equatorial Margin (Atlantic coast, Brazil)

The structure of phytoplankton community was studied in a tropical macrotidal estuary (Paciencia river estuary), which is socially and ecologically important because is surrounded by one of the most extensive mangrove forests of the world and plays a key role in the local economy, through artisanal fishing and bivalve extraction activities in natural environments. To understand the regulation mechanisms driving the phytoplankton dynamics and the key indicator species for monitoring ecological change, the study considered seasonal and spatial analysis of environmental and biological variables in thirteen sampling points during the three seasonal periods in 2017. The ecological patterns of phytoplankton were explored by Cluster Analysis and key species were selected by the indicator value method (IndVal). From 171 identified taxa, Bacillariophyta was the most representative division (128 taxa). The phytoplankton structure was well-defined by a salinity gradient governed by rainfall and river discharge. Thus, three scenarios related to seasonality and salinity gradient were identified: tidal river zone, mixing zone and coastal zone. During the rainy and transitional seasons, it was observed a spatial gradient of salinity, characterized by the gradual increase in phytoplankton abundance and biomass from the freshwater zone towards the coastal zone, following an opposite nutrient gradient. The highest salinity and light availability (low turbidity and total particulate matter) in the dry season favored a significant growth of phytoplankton (cell abundance and biomass). Thus, the key species selected by IndVal method was Discostella stelligera, Coscinodiscus oculus-iridis and Trieres sinensis in the rainy period, Nitzschia reversa, Thalassiosira sp. and Bellerochea horologicalis in the transitional period, and Protoperidinium sp., Thalassiosira subtilis, Cyclotella striata and Thalassiosira rotula in the dry period. Hence, this work contributes to the understanding of the macrotidal estuaries functioning, reinforcing the importance of phytoplankton monitoring for the identification of ecological changes.