Biogeochemical Stoichiometry of the Sargasso Sea

Recent studies have highlighted that macronutrient (carbon (C), nitrogen (N), and phosphorus (P)) stoichiometry in the ocean is not static but rather has been hypothesized to vary with changes in phytoplankton community composition, which co-varies with changes in nutrient concentrations and supply. In the open ocean at the Bermuda Atlantic Time-series Study (BATS) site, a 10-year time-series (2004 to present) shows a range in particle stoichiometric ratios nearly as large as that seen globally. On average, particle C:P and N:P ratios in the upper euphotic zone were twice the Redfield Ratio, 210:1 and 36:1 respectively. While no multi-year trend was apparent, substantial (4-fold) short term, shorter than seasonal, variability was observed perhaps associated with mesoscale physical features. The dissolved organic matter pools exhibited stoichiometric ratios much greater than in the corresponding ratios in particulate pools. This was particularly evident for the DOC:DOP ratio which was nearly an order of magnitude greater than the particulate C:P ratio, due to low DOP concentrations resulting from rapid biological uptake. The time-series of shallow (150m) particulate matter export shows a significant increase in P export fluxes that has decreased the N:P ratio of exported particulate matter from ~80:1 to ~40:1. During this period of increased P export, total phytoplankton carbon represents, on average, a greater fraction of total suspended carbon, driven primarily by relative increases in small eukaryotic phytoplankton. These changes are also co-occurring with increases in abundance of a wide range of zooplankton grazers. This analysis shows that macronutrient stoichiometry in the Sargasso Sea is not only static in time, but also is not the same ratio for different macronutrient pools. Furthermore, temporal changes in the stoichiometry of these different pools suggest changes in the ecosystem that may impact the strength of the biological carbon pump.