Biogeochemical and seasonal nutrient dynamics of restored estuarine oyster reefs in Mobile Bay, Alabama.

Nutrient overloading in aquatic environments can degrade water quality and cause eutrophication, disrupting ecosystem function and local economies, including Mobile Bay, Alabama. Oyster reef restoration is a promising strategy for nutrient sequestration and estuarine recovery. Yet, few studies have examined how oysters affect the biogeochemical dynamics of nutrients such as nitrogen (N), phosphorus (P), and carbon (C), limiting our ability to develop restoration best management practices (BMPs). This study investigated the effects of eastern oyster (Crassostrea virginica) size class (juvenile and adult) and set style (single and floret) on nutrient removal through a one-year field experiment across 32 experimental plots in Mobile Bay, Alabama. Seasonal water, sediment, and oyster tissue samples were collected quarterly, which were analyzed for total N, P, and C to assess nutrient dynamics, biogeochemical processes, and oyster bioassimilation. Sediment nutrient concentrations showed strong seasonal variability for P and C, which peaked in April (0.019 ± 0.001% and 1.66 ± 0.18% respectively), whereas sediment N varied less markedly and reached its highest concentration in July (0.09 ± 0.011%). Oyster tissue nutrient content followed similar patterns, with the highest total P and C (0.82 ± 0.04% and 49.15 ± 0.39%, respectively) occurring in April, while the highest total N (7.95 ± 0.23%) in July, indicating coupling between oyster bioassimilation and sediment processes. Furthermore, nutrient removal varied among oyster treatments, with adult single oysters showing the lowest nutrient uptake, whereas floret configurations, particularly those incorporating juvenile oysters, supported greater nutrient retention. These findings demonstrate that nutrient removal benefits of oyster restoration depend not only on oyster presence, but also on life stage, reef configuration, and seasonal biogeochemical conditions. For restoration BMPs, deploying juvenile oysters in clustered floret configurations and accounting for seasonal nutrient dynamics may enhance nutrient capture and retention compared with restoration strategies dominated by individually set adult oysters.