Resumen de Modelling of isoprene distribution and cycling in the ocean
Isoprene (C5H8) is the most widely emitted biogenic volatile organic carbon (BVOC). It is produced in the biosphere both on land and in the ocean, and in the atmosphere it acts as a precursor of secondary organic aerosols. Isoprene has an eminently biological origin in phytoplankton; but its agents, production and recycling mechanisms, including photochemistry, are very poorly known. There still are large discrepancies in the estimations of global oceanic emission of isoprene (0.1 - 11.6 Tg C yr-1). Despite lower marine emissions than terrestrial ones, they play a key role in cloud formation and brightness in remote regions of the oceans. Due to the unfeasibility of getting synoptic measurements of isoprene emissions over the global ocean, they need to be calculated with numerical models that use variables that can be measured using remote sensing data from satellites or generated through ecosystem models. To achieve the capacity to predict the distribution and emission of isoprene in the surface ocean in time and space, in this thesis computational tools for the statistical treatment of data and for the diagnosis/prognosis were used. Thus, different approaches to predict and study isoprene in surface waters, including statistical modeling, biogeochemical-ecological modeling, and remote sensing retrieval were tested.
Regarding the SO, isoprene concentration levels are driven by phytoplankton abundance over environmental or physical descriptors. Simple statistical models based on chlorophyll-a were developed showing different slopes and intercepts above and below a sea surface temperature threshold of 3.4ºC. The strong relationship between isoprene and photoprotective pigments brought new evidence to the potential role of marine isoprene as a photoprotective response in phytoplankton. Isoprene concentration levels were retrieved for SO waters using remote sensing algorithm based on chlorophyll-a and sea surface temperature products from MODIS-Aqua. The highest values of isoprene concentration and emissions peak in summer season, in coastal areas of Antarctica, in blooming areas close to islands, and around latitudes of 40ºS. The results suggested a total emission value of 0.063 Tg C yr-1, which supports the range of previous bottom-up estimates. We estimated new values of isoprene production and degradation rates from Lagrangian experiments during the PEGASO cruise. These rates together with others previously published in the literature and estimated in laboratory conditions, were implemented on the ROMS-BEC model, a regional ecological model for the SO which includes 3 Phytoplankton Functional Groups (PFT's): diatoms, coccolitophores and a group of small mixed phytoplankton. Diatoms dominated the isoprene production in SO waters and a value of total emission of isoprene of 0.071 Tg C yr-1 was calculated, agreeing with the values from remote sensing retrieval of isoprene concentration and bottom-up estimates.
As to the global ocean, isoprene production rates were implemented in DARWNIN model, which includes 35 PFT's that are grouped in 6 groups: diatoms, coccolitophores, mixotrophic dinoflagellates, prokaryotes, diazotrophs, and pico-eukaryotes. According to the model outputs, diatoms were the most important PFT in terms of isoprene production at the global scale, being specially relevant in surface waters of the SO. Finally, the turnover of isoprene in the surface ocean was studied from incubation experiments performed in different oceanic regions. Production of isoprene normalized to chlorophyll-a levels increased with temperature until 23ºC, and drastically decreased in warmer waters. Biological degradation rate constants were dependent on chlorophyll-a concentration and were generally similar or faster than ventilation rate constants, and much faster than vertical mixing. Overall, the results suggest that isoprene cycling in the surface ocean is faster than previously thought, with turnover times in the range 1-16 days.
