Ocean acidification could influence nitrogen cycling in coastal soft sediments, which are moderated by bioturbating macrofauna. The functioning of coastal ecosystems has a strong connection with nitrogen fluxes that occur at the sediment–seawater interface; the disturbance of the sediment matrix via bioturbation can significantly alter these fluxes. To investigate how decreasing seawater pH affects the fluxes of 02, NH4+, NO2– and NO3–, I incubated sediment core samples of intact coastal subtidal silt in four seawater recirculating systems and injected CO2 to adjust their pH to 8.0, 7.8, 7.6 and 7.4. I also incorporated bioturbation via a Bivalve treatment by adding 10 Theora lubrica (introduced infaunal bivalve) to a sediment core. Furthermore, the experiment was done in full darkness to eliminate photosynthesis, and salinity and temperature were controlled variables.

Initial measurements at in situ pCO2 indicated, that the Bivalve treatment significantly increased NH4+ and NO3– effluxes, and O2 influxes, but had no effect on NO2- fluxes. After a 20-day incubation, the final measurements revealed, that seawater acidification significantly increased NH4+ and NO2– effluxes, but had no effect on the fluxes of NO3- and O2. Furthermore, I detected no significant effects on nitrogen fluxes by the interaction between the pH and Bivalve treatments; however, the interaction significantly decreased O2 influxes.

I hypothesise that the addition of T. lubrica stimulated ammonification and nitrification at in situ pCO2 during the initial measurements. I also suspect that seawater acidification decreased coupled nitrification-denitrification during the final measurements. Furthermore, I suggest that T. lubrica caused both direct and indirect effects on the sediment matrix, leading to the significant decrease in O2 influxes during lower seawater pH within the Bivalve treatment cores.

Overall, my study was conclusive because I was able to prove that T. lubrica had no influence on coastal soft sediment nitrogen cycling during seawater acidification. Furthermore, I demonstrated that seawater acidification significantly affected sediment nitrogen cycling, which means ocean acidification could have a profound impact on coastal ecosystem functioning in the future.