Eutrophication events cause an increase in greenhouse gas release from intertidal flats

Intertidal flats, as part of coastal wetlands, play a crucial role in the global carbon cycle as they bury substantial quantities of carbon belowground (126 Tg C y^-1). Identifying the extent and rate of carbon turnover and thus greenhouse gas release from these ecosystems is essential to better understand their role in the global carbon cycle. Located at the interface of marine and terrestrial environments, intertidal flats may act as mixing zones between riverine nutrient input and incoming tides that carry nutrients and particles.

Eutrophication, i.e., the stimulation of phytoplankton primary production, is particularly concerning in coastal waters. Phytoplankton exude carbon; during its decomposition, further nutrients are released. The effect of such an input on intertidal flat carbon cycles is unknown. Here, we investigated greenhouse gas fluxes from an intertidal flat during a simulated algal bloom. We chose the Wadden Sea coast, northern Germany as a representative field site.

Preliminary characterization data suggested that CO₂ fluxes at our field site are not limited by the presence of electron acceptors, e.g., sulfate, belowground but by the concentration and composition of organic carbon. Hence, we hypothesize that with the addition of organic carbon inputs during an algal bloom, higher CO₂ fluxes are emitted. We performed tidally influenced microcosm experiments in which the sediment was inundated twice a day during high tide with algae enhanced artificial seawater. In addition, we also added organic carbon only (lactate/acetate) to determine if the co-occurring nutrients during the algal bloom, e.g., nitrogen play a role.

We observed that the algae treatment emitted twice as much CO₂ compared to the control over the course of the experiment. In the recovery phase (after the algae bloom) the CO₂ fluxes were still elevated compared to the control. The carbon only treatment showed CO₂ fluxes 1.6 times higher than the control. Thus, not only carbon but also other nutrients that are part of the algal bloom control the CO₂ fluxes from this ecosystem; for example, ammonium as a nitrogen source may enhance the release of CO₂. Aqueous and solid Fe data show an increase in Fe(II) in the algae and carbon only treatment, indicating Fe(III) reduction. Both treatments also show increased acid volatile sulfide concentrations, suggesting sulfate reduction. The results show that CO₂ release from the Wadden Sea increases significantly during an algal bloom and remains elevated during recovery. Therefore, further control on nutrient inputs to the Wadden Sea is necessary to prevent eutrophication.