Anthropogenically Driven Changes in the Carbon to Phosphorus Ratio of Marine Dissolved Organic Matter

Abstract

Marine dissolved organic matter (DOM) cycles play a pivotal role in sustaining marine ecosystems and regulating the ocean's carbon sequestration from the atmosphere. However, the response of DOM cycles, including dissolved organic carbon (DOC) and dissolved organic phosphorus (DOP), to future climate change remains highly uncertain. Using the Community Earth System Model version 2 large ensemble simulations, we find that the C:P ratios in DOM are projected to increase by up to two-fold in oligotrophic gyres by 2100. Increased upper ocean stratification reduces surface phosphate availability, thereby elevating phytoplankton C:P ratios and enhancing phytoplankton utilization of DOP, both acting to deprive DOM of P. Moreover, ocean stratification has a direct effect on exporting less DOC to the subsurface while accumulating more DOC at the sea surface. As a result of the strong sensitivity to ocean surface warming, the anthropogenically driven trends in upper ocean DOM concentration and its C:P ratios are estimated to emerge earlier from the simulated natural variability than upper ocean phosphate concentrations and net primary production-two key biogeochemical variables that are frequently monitored. This study suggests that changes in the C:P ratios of DOM could serve as a sensitive fingerprint of anthropogenic ocean warming, potentially exerting broad impacts on marine microbes. Our estimated 4% reduction in the globally integrated DOC export below 100 m is comparable to a 2% reduction in particulate organic carbon (POC) export by 2100, implying that global warming is likely to weaken the biological carbon pump through both DOC and POC. Organic matter is produced by phytoplankton photosynthesis in the sunlit surface ocean. Particulate organic matter sinks to the deep ocean and decomposes back into inorganic forms over relatively short timescales. In contrast, dissolved organic matter (DOM), much smaller than particulate organic matter and formed through biogeochemical reprocessing, can be transported by ocean circulations and stored in the intermediate and deep ocean over timescales ranging from decades to millennia. Using an Earth system model, we show that DOM cycles respond sensitively to upper ocean warming through changes in biological production and uptake as well as changes in ocean stratification. Notably, the intensification of upper ocean stratification has a multiplicative effect on the carbon-to-phosphorus ratio of surface DOM due to its overall opposing effects on dissolved organic carbon (increasing) and dissolved organic phosphorus (decreasing). We also find that the future subsurface ocean may store less DOM, implying that more respired carbon could escape to the atmosphere through the shallow decomposition of organic carbon. The C:P ratios in dissolved organic matter are projected to increase by up to two-fold in low latitudes and decrease in high latitudes as of 2100 Anthropogenically forced trends in C:P ratios of marine organic matter emerge earlier than those of marine net primary production Ocean stratification increases the accumulation of surface dissolved organic carbon while exporting less to the subsurface