An investigation into the variability of the nutrient and carbon biogeochemistry in the North Atlantic

Abstract

The oceans as a whole and the North Atlantic in particular are key regulators of our planet’s climate and elemental cycles. The high inherent variability in the circulation of the North Atlantic contributes to significant inter annual variability in the biogeochemical properties that can only be adequately assessed through sustained observations. This study used multiple observation platforms including ships-of-opportunity and hydrographic research cruises to quantify this variability, investigate its potential causes and evaluate the consequences on a range of scales. Between 2002 and 2017, large decoupled variability was observed in the surface concentrations of the major inorganic macronutrients(nitrate+nitrite, phosphate and silicate). In the North Atlantic Subtropical Gyre(NASTG), nitrate and phosphate displayed long term increases of 0.019 μmol L-1 yr-1and 0.003 μmol L-1 yr-1 respectively, while silicate decreased throughout the study area (−0.022 to -0.109 μmol L-1 yr-1). Previous knowledge on the nitrogen limitation regime of the NASTG was experimentally confirmed. The long-term observations allowed the identification of anomalously high surface nitrate concentrations (up to 1 μmol L-1 higher)in a region in the Central North Atlantic that more than doubled new production. Throughout the study period, the North Atlantic remained an overall carbon sink. Critically, the mid-latitude region became a stronger sink since the seawater carbon dioxide partial pressure (pCO2) did not follow the increasing trend in atmospheric pCO2. Over 70% of the changes in pCO2 could be explained by variations in temperature, alkalinity and surface dissolved inorganic carbon. Biological processes were important in maintaining this sink, especially since carbon overconsumption at a C:N ratio of 9.6 was observed in the spring bloom. Full-depth nutrient concentrations across a repeat transect at 24° Nin the NASTG also featured variability. Particularly important was the 2010 reversal of the usual net southwards nitrogen transport. This was due to a strong northward Florida Straits dissolved organic nitrogen transport. Events such as this one have an impact on local nutrient stoichiometry and implicitly phytoplankton production, but also on regional elemental budgets.

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