Spatial heterogeneity of zooplankton community in an eutrophicated tropical estuary

Abstract

Studies on the zooplankton community are essential for diagnosing the health of aquatic ecosystems, as these systems respond quickly to environmental changes. Using the multimetric TRIX index for the assessment of trophic status, we assessed the trophic state and its association with zooplankton composition, distribution, and environmental variables in a tropical estuary on the Brazilian equatorial margin. The results showed that significant seasonal and sectoral environmental differences contributed to biological heterogeneity, with the second spatial sector (SII) exhibiting the greatest impact, leading to decreased alpha diversity compared to that of the first spatial sector (SI). Salinity, pH, SiO₂³, and NO₂⁻ exhibited significant seasonal and sectoral variations (p < 0.05). The community consisted of 65 taxa, with copepods (81.5%), mainly from the Oithonidae and Paracalanidae families, dominating species number. Overall, the community exhibited medium diversity, low richness, and heterogeneity. Beta diversity, calculated using PERMDISP, reflected environmental heterogeneity with significant seasonal differences and biological variability between rainy and dry periods. Indicator species analysis identified 15 taxa, including copepods such as Euterpina acutifrons and Clytemnestra scutellata. Of these, nine taxa (60%) were indicators for the SI, and six (40%) were indicators for the SII. This study underscores the importance of identifying environmental filters and indicator species to understand estuarine dynamics and assess ecosystem trophic states.