C : N : P stoichiometry at the Bermuda Atlantic Time-series Study station in the North Atlantic Ocean

A Singh,U Riebesell,A C Martiny,M W Lomas,S E Baer

doi:10.5194/bg-12-6389-2015

Abstract

Abstract. Nitrogen (N) and phosphorus (P) availability, in addition to other macro- and micronutrients, determine the strength of the ocean's carbon (C) uptake, and variation in the N : P ratio of inorganic nutrient pools is key to phytoplankton growth. A similarity between C : N : P ratios in the plankton biomass and deep-water nutrients was observed by Alfred C. Redfield around 80 years ago and suggested that biological processes in the surface ocean controlled deep-ocean chemistry. Recent studies have emphasized the role of inorganic N : P ratios in governing biogeochemical processes, particularly the C : N : P ratio in suspended particulate organic matter (POM), with somewhat less attention given to exported POM and dissolved organic matter (DOM). Herein, we extend the discussion on ecosystem C : N : P stoichiometry but also examine temporal variation in stoichiometric relationships. We have analyzed elemental stoichiometry in the suspended POM and total (POM + DOM) organic-matter (TOM) pools in the upper 100 m and in the exported POM and subeuphotic zone (100–500 m) inorganic nutrient pools from the monthly data collected at the Bermuda Atlantic Time-series Study (BATS) site located in the western part of the North Atlantic Ocean. C : N and N : P ratios in TOM were at least twice those in the POM, while C : P ratios were up to 5 times higher in TOM compared to those in the POM. Observed C : N ratios in suspended POM were approximately equal to the canonical Redfield ratio (C : N : P = 106 : 16 : 1), while N : P and C : P ratios in the same pool were more than twice the Redfield ratio. Average N : P ratios in the subsurface inorganic nutrient pool were ~ 26 : 1, squarely between the suspended POM ratio and the Redfield ratio. We have further linked variation in elemental stoichiometry to that of phytoplankton cell abundance observed at the BATS site. Findings from this study suggest that elemental ratios vary with depth in the euphotic zone, mainly due to different growth rates of cyanobacterial cells. We have also examined the role of the Arctic Oscillation on temporal patterns in C : N : P stoichiometry. This study strengthens our understanding of the variability in elemental stoichiometry in different organic-matter pools and should improve biogeochemical models by constraining the range of non-Redfield stoichiometry and the net relative flow of elements between pools.

Highlights

Nitrogen (N) and phosphorus (P) are critical elements that control primary production in large portions of the surface ocean
Suspended euphotic zone particulate organic N (PON) : particulate organic P (POP) ratios were generally lower than total organic N (TON) : total organic P (TOP) ratios (Fig. 2, Table 1)
particulate organic C (POC) : POP ratios were much lower than total organic C (TOC) : TOP, varying from 45 to 532

Summary

Introduction

Nitrogen (N) and phosphorus (P) are critical elements that control primary production in large portions of the surface ocean. In the context of this proxy, subsurface nutrient N : P ratios > 16 : 1 suggest net nitrogen gain, while ratios < 16 : 1 suggest net nitrogen loss (e.g., Gruber and Deutsch, 2014) This relatively simple point of view has been shown to yield N2 fixation rates that are overestimated by up to 4 times when compared to directly measured rates (Mills and Arrigo, 2010). In part, this overestimation is due to the production and sedimentation of nonN2 fixer biomass that can occur at ratios much greater than the Redfield ratio, in the subtropical and tropical oceans (Singh et al, 2013; Martiny et al, 2013; Teng et al, 2014). With a better understanding of N cycle processes, the validity of the Redfield model for nutrient uptake has been questioned (SañudoWilhelmy et al, 2004; Mills and Arrigo, 2010; Zamora et al, 2010)

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