Asterionellopsis tropicalis as a potential bioindicator of human impacts on the semi-arid Southwestern Atlantic coast

Resumo

Abstract Diatoms are sensitive bioindicators to environmental changes and can be useful in monitoring disturbances caused by engineering works in the coastal areas. This study examines the occurrence, density, and relative abundance of Asterionellopsis tropicalis in the semi-arid coast of the Southwestern Atlantic. Moreover, we analyse the impact of coastal constructions on its population dynamics and potential use as a bioindicator in environmental monitoring. A total of 130 phytoplankton samples from estuaries, surf zones of sand beaches, and the shelf waters were analyzed. The quantification of A. tropicalis was performed using the Utermöhl method with optical microscopy. A. tropicalis showed a wide distribution, being found in all studied habitats. Diatom accumulations, a high-biomass phenomenon, formed by this species were recorded at Aquiraz coast (Ceará state), a second site 30 km from the first previous known location, suggesting it is not a local anomaly, but may be more widespread in equatorial SW Atlantic waters. The average densities of A. tropicalis were lower in estuaries (1.2 × 103 cell/L ± 2.6 × 103), and higher in shelf waters the shelf waters (3.1 × 103 cell/L ± 6 × 103). In the estuarine environment, A. tropicalis occurred exclusively in the intermediate and lower estuarine regions, at salinity equal to or greater than 33. The highest densities (reaching 29.7 × 106 cell/L) were observed during diatom accumulation events in the surf zone (mean: 8.6 × 106 cel/L ± 9.9 × 106). The port dredging and the artificial beach nourishment caused a significant and temporary increase in water turbidity, which was concomitantly accompanied by an increase in the diatom density. These results indicate that this species is adapted to high turbulence, where there is a large amount of suspended material and a reduction in Secchi disk depth. Similar environmental conditions were observed in dissipative surf zones, where A. tropicalis can form accumulations and tropical coasts under human interventions that resuspended sediments. Therefore, this species has the potential to be used to monitor and evaluate human impacts caused by coastal engineering works, due to its rapid response to increased suspended material.