Phytoplankton in two tropical mangroves of São Tomé Island (Gulf of Guinea): A contribution towards sustainable management strategies

Phytoplankton are important primary producers in tropical coastal lagoons, with a critical role in sustaining food webs. However, most studies conducted in tropical systems with high anthropogenic influence have focused only on biomass. Water samples were collected, for phytoplankton (pigments and microscopy) and other environmental parameters, from two mangrove lagoons in São Tomé Island. The highest nutrient concentrations were observed in the mangrove of Praia das Conchas, indicating stronger nutrient inputs in this catchment area. Bottom oxygen deficiency was observed in the mangrove of Malanza, which is likely to be associated with the existence of a bridge that constrains the water exchange with the sea. Pigment analyses showed that overall chlorophyll a concentrations were low, indicating low phytoplankton biomass. The highest values were found at Praia das Conchas. It is suggested that chlorophyll a can be used as an indicator of anthropogenic pressure in this tropical system. Microscopy reveales the presence of Dinophysis sp. and Ostreopsis sp. cells, which can be harmful even at low densities. Water quality could be improved by enhancing oxygen condition in Malanza and by monitoring the occurrence of algal blooms, with potential benefits for public health and tourism.