Two decades and counting: 24-years of sustained open ocean biogeochemical measurements in the Sargasso Sea

Abstract

The Bermuda Atlantic Time-series Study (BATS) program has sampled the northwestern Sargasso Sea on a biweekly (January to April) to monthly basis since October 1988. The primary objective of the core BATS program continues to be an improved understanding of the time-variable processes and mechanisms that control the biogeochemical cycling of carbon and related elements in the surface ocean. With 24 years of measurements for most chemical, physical and biological variables, we have moved beyond descriptions of seasonal and interannual variability to examination of multi-year trends and potential controls, however there remain substantial gaps in our knowledge of the ecosystem mechanisms related to organic matter production, export and remineralization. While earlier BATS overviews have focused on describing seasonal and year-to-year variability, this overview provides new information on three long-standing biogeochemical questions in Sargasso Sea biogeochemistry. First, why is there a discrepancy between biological (i.e., sediment trap) and geochemical estimates of carbon export production? Winter storms and mesoscale eddies have now been clearly shown to contribute to annual nutrient budgets and carbon export production. Recent information on phytoplankton natural isotopic nitrogen composition, and data from profiling floats suggests that small phytoplankton are important contributors to new production in summer despite the apparent absence of a mechanism to entrain nitrate into the euphotic zone. These findings aid in closing the gap between these two different estimates of carbon export production. Second, what supports the seasonal drawdown of carbon dioxide in the absence of detectable nutrients? The zooplankton timeseries at BATS highlights the importance of zooplankton as a conduit for carbon removal due to grazing and vertical migration. Although increases in cellular elemental stoichiometry to values greater than the canonical Redfield Ratio, and the seasonal (and interannual) accumulation of euphotic zone dissolved organic carbon (DOC) without accumulation of DON in the surface ocean are also important explanations. Lastly, what are the sources of the elevated nitrate to phosphate ratio in the seasonal thermocline (N:P>30 on average)? While generally accepted that nitrogen fixation is the source of the additional nitrogen, new research suggests that export and remineralization of non-diazotroph particulate matter enriched in nitrogen (alternatively viewed as depleted in phosphorus) may also make substantial contributions. In addition, the ratio of particulate nitrogen to phosphorus captured in sediment traps has decreased from 50–75 to <50, possibly due to enhanced nitrogen remineralization. These and other findings from the core BATS observational program contribute to our improved understanding of biogeochemical cycles and ecosystem mechanisms in the subtropical North Atlantic Ocean and how they are changing over time.