The Association of Benthic Infauna with Fine-Grained Organic-Rich Sediments in a Shallow Subtropical Estuary

Fine-grained organic-rich sediments (FGORSs) from anthropogenic impacts are a growing concern for bays and estuaries around the world. This study explores the relationships of infaunal community diversity and species abundances with FGORSs in the Indian River Lagoon and its tributaries. To examine these potential relationships, infauna was collected monthly using a Petite Ponar grab at 16 stations in the central Indian River Lagoon from October 2015 to August 2016. Abundant taxa in these sediments include polychaete worms (e.g., the polychaete Nereis succinea), mollusks (e.g., clam Parastarte triquetra), and arthropods (e.g., the tanaid Leptochelia dubia), with densities as high as 5.3 × 104 m−2 (L. dubia in July 2016). Increasing organic matter (OM) in the sediments was inversely correlated with species richness (R2 = 0.75; p-value < 0.001), densities (R2 = 0.69; p-value < 0.001), and diversity (R2 = 0.37; p-value < 0.001). Other infaunal community and population data showed similar relationships with silt–clay (%), sediment porosity, and dissolved oxygen (mg L−1). Two thresholds of OM and correlated environmental parameters are discussed: an impairment threshold at 2% OM, above which infauna decreases precipitously, and a critical threshold at 10% OM, above which infauna is generally absent.