Algal resource availability shapes trophic convergence of deposit feeders in a low-turbidity dredged estuary

Abstract

Dredging alters sediment composition and hydrodynamics, disrupting organic matter distribution and deposit-feeder communities, yet its long-term effects on benthic food webs remain underexplored. We examined how dredging-induced sediment changes influence trophic niches of deposit feeders by analyzing abundance, resource use, and trophic structure of three deposit-feeding species and one predator in Gwangyang Bay, Korea, using stable isotope analysis. Sediment characteristics differed markedly between regions: the northern inner bay was dominated by fine, organic-rich sediments, whereas the dredged main channel contained coarser, organic-poor sediments associated with short water residence times. Despite these contrasts, sediment conditions alone did not explain deposit-feeder distributions; instead, recolonization capacity following disturbance appeared more important. Isotopic niche metrics (delta 13C range, delta 15N range, and corrected standard ellipse area) revealed substantial trophic niche overlap among deposit feeders (Capitella capitata, Magelona japonica, and Theora fragilis), with primary reliance on phytoplankton and microphytobenthos and minor contributions from riverine and marsh-derived organic matter. Although species-specific traits influenced feeding strategies, high phytoplankton availability likely promoted trophic convergence rather than niche partitioning. Pronounced niche overlap occurred in both dredged and non-dredged areas, indicating that resource availability, rather than sediment conditions, primarily structures benthic trophic interactions. Predatory polychaete, Scoletoma longifolia, occupied a higher trophic position but exhibited similar isotopic overlap across dredged and non-dredged areas, supporting trophic homogenization mediated by shared prey resources. Overall, our results demonstrate that in low-turbidity estuaries such as Gwangyang Bay, autochthonous microalgal availability-rather than dredging status-primarily governs benthic trophic dynamics, underscoring resource-driven controls on trophic structure following dredging.