Estimates of particulate organic carbon export and loss rates in the warmest sea of the global ocean from a bio-optical profiling float

The mesopelagic zone of the Red Sea, the region between 150 and 1000 m, represents an extreme environment due to low food concentrations, high temperatures and low oxygen waters. This unique environment provides conditions which are likely to occur in other marine ecosystems in the future and thus offers many opportunities as a “natural laboratory” to study the ocean carbon cycle in response to climate change. Using autonomous optical backscattering observations made by a BGC profiling floats, we estimated for the first time the seasonal particulate organic carbon (POC) export and loss rates into the mesopelagic layer. We observed that most the POC was degraded in the upper part of the mesopelagic layer (about 72%), while less than 10% of POC reached the bottom of the mesopelagic zone. We also observed that POC is mainly rapidly (few days) degraded in a first stage and slowly (weeks to months) in a second stage in the upper layer of the mesopelagic zone by heterotrophic processes. These differences may be dependent on the palatability of the organic matter (labile and nutritive vs recalcitrant and nonnutritive particles). We also found that the sum of the fast and slow POC losses lead to a neutral budget for the small slowly-sinking particles in the mesopelagic zone and that these particles could be a potential supplementary carbon source capable of balancing carbon budgets in the mesopelagic zone. Then, a significant correlation between the so-called Martin’s exponent b values and the median upper 500 m water temperature with greater b values in warm waters. Such variability in b showed that an open ocean composite uniform b value is not appropriate to estimate or predict the feedback of ocean carbon remineralization on atmospheric carbon in the future. We thus surmise that variations in b should be considered in model frameworks for predictions. For future studies, we suggest to focus on shallow observations to study the processes involved in the POC turnover associated to the size and the nature of the particles due to rapid remineralization within the first 500 m of the water column in such environment.