Abstract
In marine oxygen (O2) minimum zones (OMZs), the transfer of particulate organic carbon (POC) to depth via the biological carbon pump might be enhanced as a result of slower remineralisation under lower dissolved O2 concentrations (DO). In parallel, nitrogen (N) loss to the atmosphere through microbial processes, such as denitrification and anammox, is directly linked to particulate nitrogen (PN) export. However it is unclear (1) whether DO is the only factor that potentially enhances POC transfer in OMZs, and (2) if particle fluxes are sufficient to support observed N loss rates. We performed a degradation experiment on sinking particles collected from the Baltic Sea, where anoxic zones are observed. Sinking material was harvested using surface-tethered sediment traps and subsequently incubated in darkness at different DO levels, including severe suboxia (<0.5 mg l−1 DO). Our results show that DO plays a role in regulating POC and PN degradation rates. POC(PN) degradation was reduced by approximately 100% from the high to low DO to the lowest DO. The amount of NH4+ produced from the pool of remineralising organic N matched estimations of NH4+ anammox requirements during our experiment. This anammox was likely fueled by DON degradation rather than PON degradation.
Highlights
Dissolved oxygen concentration (DO) profiles were taken from the CTD optode and calibrated against DO concentration measurements performed using a semi-automated whole bottle Winkler titration unit[57]
In the bottles corresponding to 110 m samples, the DO concentration in the bottles at the start of the experiment was measured using Pyroscience sensor TROXR430 designed for low DO concentration
A brine solution was prepared by dissolving 50 g l−1 of NaCl with filtered surface seawater. This was subsequently filtered through a 0.2 μm cartridge to remove any particulate material. 0.5 l of this brine solution was slowly pumped into each Particle Interceptor Traps (PITs) with a peristaltic pump beneath the 1.5 l (3/4) of filtered seawater
Summary
Biogeochemical conditions. surface satellite-derived Chl-a concentrations in the Gotland Deep peaked in mid-June (8–10 mg m−3, Julian day 160) during sampling (Fig. 1A). Given the similarity of the DIs (both in trap material and incubation bottles) at 40 and 60 m, but the large difference in POC loss rate between 40 and 60 m, we believe that DI has a moderate control on POC degradation rate at high DO concentrations. It is unclear why NO3− concentration decreased with time (Fig. 7) for the 180 m treatments This has been observed in other degradation studies[11] and is potentially due to the balance between the actual remineralisation of ON and the reduction of NO3− through pathways 3, 4 and 5 (see text above). (3) DON ammonification can be a source of NH4+ for anammox bacteria
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