The spatiotemporal development of two shellfish populations and their associated filtration capacity on a living shoreline near Milford, Delaware, USA

Bivalve shellfish are common in coastal ecosystems where their aggregate structure attenuates wave energy and provides habitat, while delivering water quality benefits through their feeding activity. These factors make them appropriate candidates for inclusion in living shorelines to facilitate positive ecological outcomes. In 2014, a 61 m shellfish-based living shoreline was constructed along a salt marsh at the DuPont Nature Center in Milford, Delaware with the goal to maximize shellfish populations for water quality benefits. Monitoring was conducted to assess oyster and ribbed mussel population development and evaluate their relative contribution to cumulative filtration at three positions— on structures near mean low water (low), along the marsh edge (high), and on the untreated mudflat between (mid). Oyster and ribbed mussel counts and size demographics were converted to population and biomass densities to calculate filtration capacity in each position through 2020. Cumulatively, shellfish on the Mispillion living shoreline filtered 6763 kg of seston, but population and biomass development varied spatiotemporally between species. Between 2018 and 2020, oyster population and biomass densities declined at the low, but increased at the high positons, while ribbed mussel densities increased at both. Despite differences among species and position, the annual summed filtration of the low and high positions continually increased. These results indicate that a multi-species approach, across a variety of appropriate spatial niches, can help maintain or enhance overall filtration capacity through either complementary species contributions at a single positon, or spatial compensation by a single species across positons. •Oysters and ribbed mussels can develop sympatric populations on living shorelines if provided suitable, stable substrate•Appropriate substrate accross their range can provide for spatial and temporal resilience.•A multi-habitat approach to living shoreline design can maximize spatial refuge for multiple species.•A multi-species approach to living shoreline design can balance ecological impacts from species-specific declines.