Trait-mediated effects of Macoma balthica (L.) on nitrogen and carbon biogeochemistry in estuarine ecosystems

Coastal ecosystems rely on microbially mediated biogeochemical processes for ecosystem functioning, particularly carbon (C) and nitrogen (N) cycling across the water-sediment interface, supported by bivalves through bioturbation activity. Here, we conducted an 8-week coastal microcosm experiment using Macoma balthica sampled from the Rhode River (Chesapeake Bay) to assess how bivalve presence and individual traits (burial position and shell length) influence the N cycle (NH₃ and NO_X) and C cycle (CDOM, measured as a_CDOM 250 m^-1) in the water column and interstitial water. Our results show that bivalve presence significantly affected N cycling: NH₃ initially rose in the water column before declining, followed by exponential increases in NO_x in both water and interstitial water. Burial position influenced NH₃ and NO_x concentrations throughout the experiment, and larger individuals were associated with higher NO_x levels. Additionally, a_CDOM 250 m^-1 increased over time regardless of bivalve presence, likely due to daily feeding with artificial algae, suggesting that future studies should include additional C parameters such as dissolved and particulate forms of organic and inorganic C. Overall, our findings show that M. balthica's influence on N cycling is shaped by individual traits and temporal dynamics, creating functional heterogeneity with implications for Rhode River nutrient fluxes and ecosystem functioning. While microbiomes might contribute to the nitrification processes observed, future research should clarify the pathways of bivalve-microbiome interactions in M. balthica.