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During a Lagrangian study in the Mauritanian upwelling (April 15 to May 27 2009), we investigated the biogeochemical cycling of carbon monoxide (CO) through a series of experiments, field observations and a simple 1-D model. We carried out ten photochemical experiments in a solar simulator and a further dark experiment in order to determine the magnitude of the photochemical source and microbial sink for CO. Parallel irradiation experiments using a long-pass filter (390 nm cutoff) showed that CO photoproduction was dominated by UV-light. Assuming that the apparent quantum yield (AQY) of CO photoproduction followed an exponential decrease with increasing wavelength, we applied a non-linear fit to derive the AQY for each of 10 experiments. The average AQY at 350 nm from our irradiation experiments was 1.4 × 10− 5 mol CO produced per mol photons absorbed. One of our photochemical experiments showed a distinct dissolved organic matter absorption ‘shoulder’ in the UV-B, consistent with the presence of mycosporine-like amino acids. Though this feature was rapidly photobleached during irradiation, CO AQY did not differ from other experiments. During the microbial CO oxidation experiment we found first-order loss of CO with a rate constant of 0.17/h. We further used our experimental data to parameterise a 1-D steady state model which included photochemical and dark production of CO, microbial oxidation, mixing and sea to air loss. Our sea–air flux estimates were in the range of 4.6–9.6 μmol CO per square metre per day. The estimated annual flux of CO to the atmosphere from the Mauritanian upwelling was 0.04 Tg CO yr− 1.

Dissolved methane and nitrous oxide in seawater were measured along a 6700 km transect of the North-West Passage between the North Atlantic Ocean and Beaufort Sea in the Arctic Ocean. Over- and under-saturation with respect to atmospheric equilibrium were observed for both gases. Methane and nitrous oxide were in the range of 58–528% and 82–181% saturation, respectively. Under-saturation was attributed to melt-water with low methane and nitrous oxide, while over-saturation was found under multi-year sea-ice. Elevated methane was also found in the vicinity of the marginal ice zones and the Mackenzie River plume. Our data support both water column and sedimentary sources of methane and nitrous oxide. We found first-order methane oxidation in surface seawater with a rate constant of 3.8 × 10−3 h−1. Based on these results and a conceptual model, we suggest that future sea-ice retreat may decrease the residence times of methane and nitrous oxide in the surface Arctic Ocean and thus enhance the sea–air flux of these climatically active gases.

We carried out 12 irradiation experiments under artificial light using samples collected from two UK upland temperate rivers, the Tyne and the Tay, and from the Tyne estuary and adjacent coastal North Sea waters. During these experiments, we followed concentration changes of ammonium (NH 4 + ) and spectral changes of chromophoric dissolved organic matter (CDOM). In all experiments CDOM absorbance followed pseudo-first-order loss kinetics and photochemical NH 4 + production occurred consistently. However, the patterns of NH 4 + concentration changes with time were complex. Experiments in which irradiated waters were sampled at approximately 10 min intervals revealed an initial lag phase of ∼ 30 to 120 min during which NH 4 + concentrations did not change significantly. This lag phase was followed by a net photoproduction phase, and finally an apparent NH 4 + consumption phase. Such behaviour has not previously been reported and appeared to be independent of sample origin, initial NH 4 + concentration and initial CDOM absorption coefficient at 350 nm ( a 350 ). The observed behaviour could be described by a 3-stage irreversible reaction model: A → B → NH 4 + → D with first-order rate constants ka , kb and kc for the 3 consecutive reaction steps. Both ka and kc were significantly correlated ( p a 350 and salinity over the CDOM spectral slope ( S 290–350 ) through multiple linear regression analysis, suggesting that dissolved organic matter (DOM) properties associated with CDOM absorbance characteristics play an important role in this photochemical process. NH 4 + photoproduction rates, calculated over the lag- and production-phases, were significantly correlated with the initial a 350 for the River Tyne (including tributaries) samples, but not when samples from the estuary, coastal North Sea and River Tay were included. We also tested for possible correlations between CDOM photobleaching and spectral properties, but found no significant correlations between NH 4 + photoproduction rates and the initial spectral slope of CDOM or the CDOM photooxidation rate constants at 285, 310, 350 and 390 nm.