The influence of ocean acidification on nitrogen regeneration and nitrous oxide production in the North-West European shelf sea

D. R. Clark, I. J. Brown, A. P. Rees, P. J. Somerfield, P. I. Miller
2014 Biogeosciences Discussions  
The assimilation and regeneration of dissolved inorganic nitrogen, and the concentration of N 2 O, was investigated at stations located in the NW European shelf sea during June/July 2011. These observational measurements within the photic zone demonstrated the simultaneous regeneration and assimilation of NH + 4 , NO − 2 and 5 NO − 3 . NH + 4 was assimilated at 1.82-49.12 nmol N L −1 h −1 and regenerated at 3.46-14.60 nmol N L −1 h −1 ; NO − 2 was assimilated at 0-2.08 nmol N L −1 h −1 and
more » ... L −1 h −1 and regenerated at 0.01-1.85 nmol N L −1 h −1 ; NO − 3 was assimilated at 0.67-18.75 nmol N L −1 h −1 and regenerated at 0.05-28.97 nmol N L −1 h −1 . Observations implied that these processes were closely coupled at the regional scale and nitrogen recycling played an important 10 role in sustaining phytoplankton growth during the summer. The [N 2 O], measured in water column profiles, was 10.13 ± 1.11 nmol L −1 and did not strongly diverge from atmospheric equilibrium indicating that sampled marine regions where neither a strong source nor sink of N 2 O to the atmosphere. Multivariate analysis of data describing water column biogeochemistry and its links to N-cycling activity failed to explain the 15 observed variance in rates of N-regeneration and N-assimilation, possibly due to the limited number of process rate observations. In the surface waters of 5 further stations, Ocean Acidification (OA) bioassay experiments were conducted to investigate the response of NH + 4 oxidising and regenerating organisms to simulated OA conditions, including the implications for [N 2 O]. Multivariate analysis was undertaken which 20 considered the complete bioassay dataset of measured variables describing changes in N-regeneration rate, [N 2 O] and the biogeochemical composition of seawater. While anticipating biogeochemical differences between locations, we aimed to test the hypothesis that the underlying mechanism through which pelagic N-regeneration responded to simulated OA conditions was independent of location and that a mech-25 anistic understanding of how NH + 4 oxidation, NH + 4 regeneration and N 2 O production responded to OA could be developed. Results indicated that N-regeneration process responses to OA treatments were location specific; no mechanistic understanding of 3114 BGD how N-regeneration processes respond to OA in the surface ocean of the NW European shelf sea could be developed. 25 studies, present a complex view of the response of marine biota to projected OA conditions (Reibesell et al., 2008; Flynn et al., 2012) . At the organismal level there will 3115
doi:10.5194/bgd-11-3113-2014 fatcat:zpj7kgwy4fdfzlfmri4qe6z7qi