NOT SO SHELLFISH AFTER ALL: HOW NATIVE OYSTERS (OSTREA LURIDA) MAY AID EELGRASS (ZOSTERA MARINA) RESTORATION BY NITROGEN FILTRATION

Eelgrass (Zostera marina) is a foundation species in coastal waters that provides vital ecosystem services, from habitat provision to trophic support. However, populations have declined globally at alarming rates within the last century. A recent restoration project within Upper Newport Bay (UNB), California, USA implemented a multi-habitat restoration approach using Z. marina and Ostrea lurida, which may render additional ecosystem services and simultaneously promote more successful eelgrass restoration. Oysters may increase nutrient availability for eelgrass in sediment porewater by mediating nitrogen transfer via filter-feeding and subsequent deposition of nitrogenous waste. In summer 2021, I collected eelgrass shoots, porewater samples, and shoot and oyster densities from three sites within UNB, each restored with eelgrass alone and eelgrass adjacent to oysters. I measured leaf and rhizome growth rates, organic tissue nitrogen, above and below-ground dry weight, and porewater NH₄⁺ and NO₃^- concentrations. Since sedentary species may have limited spatial range of impact, spatially explicit co-variogram models, novel to this field of study, related response metrics to distance from and density of associated oysters in restored beds. I found positive associations among oyster density and eelgrass below-ground dry weight and rhizome growth rates that increased in strength inversely to distance between samples. Porewater ammonium and tissue nitrogen also increased with presence, density, and proximity to oyster beds. For above-ground metrics, including dry weight, growth rates, and shoot density, the relationship between oysters and eelgrass was site-dependent, varying from negative to non-significant to positive. The enhanced eelgrass below-ground growth attributable to interaction with oysters suggests that these species can be restored together in a spatial design similar to that used for this study. To capture the full complexity of oyster-eelgrass interactions, future studies should incorporate spatially explicit models and consider a large suite of both above-ground and below-ground eelgrass metrics.