Ocean Deoxygenation Conference | Kiel 2018

Oxygen minimum zones (OMZs) are regions of the ocean where the depletion of oxygen leads to favorable conditions for microbial nitrogen (N) loss processes. Although OMZs make up less than 1% of the global ocean (oxygen $< 20~\mu M$), they host 20-40% of global oceanic N-loss through the processes of denitrification and anammox. Recent studies indicate that marine snow aggregates play a vital role in OMZ N-loss by transporting organic matter from surface layers to the ocean interior. Aggregates are also hotspots of microbial activity, and it is speculated that a large proportion of N cycling occurs within and around aggregates. However, due to their fragile nature and difficulties in sampling, studies on single aggregates are limited and N-loss determinations are mostly based on bulk water incubations, which likely exclude sinking aggregates. To investigate the role of single aggregates in OMZ N cycling, we collected $>200$ aggregates with sizes larger than 0.3 mm from the OMZ offshore Peru using a marine snow catcher. The aggregates, together with bulk water samples, were incubated and amended with stable nitrogen isotopes ($^{15}N$) to determine anammox and denitrification rates.

Based on our bulk water incubations, the areal anammox rates ranged between $1.7$ and $10~mmol \cdot N_2 \cdot m^{-2} \cdot day^{-1}$, with highest rates observed at coastal stations. Denitrification occurred more sporadically than anammox, ranging between $1.0$ and $1.9~mmol\cdot N_2\cdot m^{-2}\cdot day^{-1}$. In contrast, $N_2$ production by denitrification was detected in the majority of the single aggregate incubations with rates in the range of pico- to nano-mole N per aggregate per day. Denitrification associated with these large ($>0.3 mm$) marine snow aggregates contributed between $2.5 \%$ and $50 \%$ to total N-loss from the investigated OMZ waters. Anammox rates were mostly insignificant for the large aggregates but bulk anammox rates strongly correlated with the abundance of smaller particles ($128-256~\mu m$). Our results indicate that marine snow aggregates play a major role in N loss from the Peruvian OMZ.