Objective 2:
In order to elucidate the importance of photochemistry in the production and loss of trace gases we will undertake a number of incubation experiments using natural light +/- UVA and UVB exclusion filters linking CDOM photobleaching with the photochemical transformation of NH₄, O₂, iodocarbons, oVOCs and DMS [these latter pending additonal SOLAS funding to Nightingale and Uher]. A recent open ocean diel study, with iodocarbon surface seawater concentrations measured every 2 hours, showed a 50 and 200% increase for iodomethane and chloroiodomethane, respectively.

These patterns are significant in terms of net daily fluxes and warrant further investigation. Photochemical transformations will be separated from microbial transformation in these experiments by ultrafiltration and monitoring of the filtrate using flow cytometry. We will assess the cumulative effect of photo- and microbial oxidation of DOM by determining turnover rates of labile organic molecules and their consumption by dominant microbial groups using a tracer dilution technique and flow cytometric sorting. We will also compare rates of sugar and amino acid turnover in light /dark and UV irradiated / screened bottles. CDOM also plays an important role in seawater spectral quality and hence the efficiency of nitrogen and carbon uptake during photosynthesis. The absorption coefficient of CDOM will be determined at discrete depths in the photic zone by both spectrophotometry and fluorometry following published methods. These will be coupled with vertical profiles of the apparent optical properties of the water column using both free fall PAR and UV optical profilers and the inherent optical properties of the water column using ac-9, ECO-VSF and Hobilabs Bb6. We will assess the magnitude of photoinhibition of photosynthesis and CDOM absorption using short term 14C irradiance and benchtop FRRF experiments.