Dominant macrobenthic populations experience sustained impacts from annual disposal of fine sediments on sandy beaches

Despite increasing use of dredged materials as beach fill to protect coastal property and public beaches from storm damage, knowledge of how this practice affects sandy beach ecosystems remains poor. We coupled field monitoring of 2 successive beach disposal events with mesocosm experiments to assess mechanisms of ecological effects of fine sediment disposal. Macrobenthic sampling on Topsail Island, North Carolina, revealed that disposal of dredge spoils transformed beach grain sizes from medium to fine sand. Water sampling documented substantially elevated surf-zone turbidity during and occasionally after sediment deposition. When disposal occurred before spring invertebrate recruitment to the beach, it negatively influenced recruitment of the mole crab Emerita talpoida and the amphipod Parahaustorius longimerus. When disposal followed recruitment, it affected abundances of the bean clam Donax variabilis and 3 species of haustoriid amphipods negatively, and abundance of the spionid polychaete Scolelepis squamata positively. Effects lasted for the full warm season, and suppressions of invertebrate abundances were repeated across successive annual disposal events. In mesocosms, turbidity matching that induced in the field slowed growth of clams and modified habitat choices by predatory fishes. Hence, annual disposal of fine-grained dredge spoils on these sandy beaches maintained depressed abundances of 5 of 6 macroinvertebrate prey of shorebird and surf fish, without sustaining elevated volumes of beach sediments as long as a year. Implementation of disposal projects before the beginning of the seasonal recruitment of benthos resulted in fewer negative impacts on abundance than disposal projects conducted after the recruitment season, probably reflecting a more universal risk from burial and suffocation.