Assessing Fish Community Structure and Diversity Across Environmental Gradients in a Tropical Bay

Abstract

Understanding the relationship between species distribution and the environment across spatial gradients is crucial for biodiversity evaluation. We surveyed fish populations in a tropical bay, covering a spatial gradient from the outer zone with favorable marine conditions to the inner zone affected by human activities. The tested hypothesis is that natural environmental gradients (e.g., temperature, salinity, turbidity) and anthropogenic influences (e.g., pollution, organic enrichment) affect fish communities and that environmental filters reduce taxonomic and functional diversity in more impacted zones. Species richness increased, while fish abundance and biomass decreased from the inner to the outer zone. Only functional divergence changed spatially, being higher in the outer zone. This suggests changes in species but the maintenance of most functions. Fourteen functional groups were identified based on locomotion and food acquisition traits. Higher taxonomic diversity and functional divergence in the outer zone likely stem from greater resource utilization differentiation and more favorable environmental conditions near the sea, fostering species with distinct functional attributes. Conversely, the inner zone, burdened by high organic and pollutant loads, favors species highly tolerant of harsh environmental conditions, such as marine catfishes. Fish communities exhibited spatial changes due to environmental gradients and anthropogenic influences. Environmental filters altered taxonomic indices, while functional indices remained stable, except for functional divergence, which was lower in the most impacted area, partially confirming our hypothesis. These findings advance our understanding of environmental influences on species distribution along spatial gradients in coastal systems, proving to be a promising and increasingly utilized tool for ecological assessment.