IMPACT OF PHOTOCHEMISTRY ON CARBON CYCLING IN THE SEA

PROJECT SUMMARY (NSF grant OCE0096426):

Physical and biological processes are historically considered to be the most important factors affecting the carbon cycle in the ocean. Specific processes include air-sea CO₂ exchange, surface mixing, venting of deep waters, carbon fixation, respiration, calcium carbonate formation and sedimentation. Recent studies now suggest that light-initiated (photochemical) processes also strongly impact carbon cycling at the sea surface, particularly with respect to the photodegradation of dissolved organic carbon (DOC). In our previous grant, we calculated that from 0.2-1.5% of DOC is directly remineralized daily yielding a photochemical half-life for DOC in surface seawater of 46-347 days. This half-life estimate is significantly reduced when photochemically enhanced microbial utilization of substrates is also taken into account. Combining our measured abiotic CO and CO₂ photoproduction with measured enhanced microbial carbon uptake (production + respiration) yields a photochemical enhancement of carbon cycling of about 3-5 µM-C/day in coastal surface waters. Given a DOC concentration of about 80-100 µM-C, our results suggest that photochemical degradation has a major impact on carbon cycling in coastal waters. On a global scale, we estimate that the photochemical carbon flux is about 5 % of the average for total marine primary production, and is more than an order-of-magnitude larger than global estimates for carbon burial to marine sediments. Even though our preliminary results indicate that this photochemical carbon flux is large, the quantitative importance of photochemistry in the oceanic carbon cycle is not well established. A comprehensive global field study is needed to parameterize spatio-temporal variations in CO₂ photoproduction because global photochemical conversion rates of DOM into CO and CO₂ will be important for quantifying DOM turnover rates in the oceans and for constraining air-sea CO₂ fluxes. Therefore, we propose to expand upon our initial work to gain a better understanding of the factors that control the photoremineralization of DOM in the oceans, and to increase our database on photoremineralization rates (both direct and indirect) in contrasting oceanic environments. Specifically, we will determine: (1) the photochemical production of CO and CO₂ from marine DOM, (2) the photochemical loss of DOC, and (3) the microbial uptake and remineralization of photooxidized DOM. From this study we will be able to estimate the impact of photochemistry on carbon fluxes and organic carbon mass balances in the water column, including primary and bacterial productivities, and microbial respiration. Another important goal of our proposed renewal is to develop algorithms, based on DOM absorbance and surface irradiance, to predict photochemical rates in surface waters. Our long-term goal is to use these algorithms in combination with remotely sensed DOM fluorescence to predict CO and CO₂ photochemical production and DOC photodegradation rates over large areas of the ocean. Our previous successes in studying photochemical processes in a variety of marine environments gives us a strong foundation for assessing the importance of these processes in the oceanic carbon cycle.