Relationships between freshwater mussel abundance and physical and nutritional conditions in the tidal Delaware River
Authors:   , Roberts Spencer Alan , Kreeger Danielle , Kilham Susan Soltau 
Published: July 2021

Freshwater mussels (Order Unionoida) are one of the most imperiled taxonomic groups in North America. Recent attention on population restoration and augmentation in tidal areas of the Delaware Estuary with historic populations has proven challenging. Benthic conditions in urbanized areas may not provide readily suitable habitat to support larger mussel populations. Lower benthic shear stress and increased sediment stability may promote mussel retention and recruitment; however, few studies have examined how such physical factors govern habitat suitability in tidal freshwater systems. Various metrics associated with mussel population robustness and benthic physical conditions were correlated to explore feature of the benthic environment that might govern habitat suitability in the tidal Delaware River. Comparisons among several study sites suggest that sediments with lower percentages of silt, clay, and organic matter along with higher gravel content correlated with higher mussel density and biomass. The high mussel density site, at the Betsy Ross Bridge, demonstrated an average gravel content of 41%, sand 54.4%, and fine particles 4.6%. Particle transport rates over a 3 week period in May-June 2018 were higher at Betsy Ross, which could explain the site's higher percent gravel and lower percent fine sediments. Alternatively, the low mussel density Frankford Arsenal site demonstrated low particle transport and a high percentage fines and lower percent gravel. The low-density Cooper River site, however, demonstrated high particle transport and a high fine particle content and low gravel content. An experiment was designed at the low-density Cooper site to manipulate habitat conditions in order to stabilize sediments, reduce benthic shear stress, and potentially enhance habitat conditions for mussels. In fall 2017, experimental structures comprised of salvaged logs and cobble were constructed along a shallow subtidal shoreline of the Delaware River where the preexisting mussel abundance was greatly reduced compared to a nearby reference site. The structures were oriented to stabilize sediments and reduce benthic shear stress while not interfering with seston delivery. Passive Integrated Transponder (PIT) tagged Utterbackiana implicata and Elliptio complanata were subsequently deployed into each structure and untreated controls. Changes in Plaster-of-Paris "Clod" sphere dissolution rates, sediment grain size, and organic content were monitored for a full year starting in Fall 2017. Dissolution rates of clods were significantly lower in experimental structures and mussel retention averaged 71.6% in structures compared to 67.4% in control plots after 1 year. This suggests that the structures may have served as refugia from high flow that typically mobilize the substrate. If confirmed with extended monitoring, these results should support design of living shoreline projects aimed at boosting populations of native unionids. Bivalve shellfish food conditions in the freshwater tidal Delaware River were monitored from April 2017 to April 2018. Previous studies in the tidal Delaware River characterized general seston and chemical conditions; however, the availability and quality of seston food for bivalve shellfish was less studied. The quantity of food was measured as total suspended solids (TSS) and particulate organic matter (POM) and quality was measured as the percentage organic content, protein content, and carbohydrate content. Seston quantity was found to vary by season, site, and tide, whereas the seston quality was found to vary seasonally and tidally. Compared to previous studies in Delaware Bay and other bivalve food studies, the availability in the freshwater tidal Delaware River did not appear to be limiting bivalve shellfish during this study. As demonstrated in this study, sites like Cooper do have the capacity to support the nutritional needs of more mussels. However, sediment stability and benthic shear stress should be addressed when attempting to augment this population. Future studies to further resolve the relationships and mechanisms between freshwater mussels and the physical habitat constraints in tidal areas will help inform restoration planning and design.