Search

The elemental composition of particulate organic matter in the surface ocean significantly affects the efficiency of the ocean's store of carbon. Though the elemental composition of primary producers is an important factor, recent observations from the western North Atlantic Ocean revealed that carbon-to-nitrogen ratios (C:N) of phytoplankton were significantly higher than the relatively homeostatic ratio of the total particulate pool (particulate organic carbon:particulate organic nitrogen; POC:PON). Here we use an idealized ecosystem model to show how interactions between primary and secondary producers maintain the mean composition of surface particulates and the difference between primary producers and bulk material. Idealized physiological models of phytoplankton and microzooplankton, constrained by laboratory data, reveal contrasting autotrophic and heterotrophic responses to nitrogen limitation: under nitrogen limitation, phytoplankton accumulate carbon in carbohydrates and lipids while microzooplankton deplete internal C reserves to fuel respiration. Global ecosystem simulations yield hypothetical global distributions of phytoplankton and microzooplankton C:N ratio predicting elevated phytoplankton C:N ratios in the high-light, low-nutrient regions of the ocean despite a lower, homeostatic POC:PON ratio due to respiration of excess carbon in systems subject to top-down control. The model qualitatively captures and provides a simple interpretation for, a global compilation of surface ocean POC:PON data.

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.

The ratio of carbon (C) to phosphorus (P) in marine phytoplankton is thought to be constant throughout the worlds'oceans. Known as the Redfield ratio, this relationship describes the links between carbon and phosphorus cycling and marine ecosystems. However, variations in the stoichiometry of phytoplankton have recently been identified, in particular strong latitudinal variability. Here we assess the impact of this variability in the C:P ratio of biomass on the C:P ratio of organic matter that is exported to the deep ocean using a biogeochemical inverse-model based on a data-constrained ocean circulation model and a global database of dissolved inorganic carbon and phosphate measurements. We identify global patterns of variability in the C:P ratios of exported organic matter, with higher values in the nutrient-depleted subtropical gyres, where organic matter export is relatively low, and lower ratios in nutrient-rich upwelling zones and high-latitude regions, where organic matter export is high. This suggests that total carbon export is relatively constant throughout the oceans, in agreement with recent estimates of carbon fluxes. We conclude that the latitudinal patterns of C:P in exported organic matter are consistent with the large-scale stoichiometric variations in phytoplankton C:P. We suggest that a future expansion of nutrient-depleted waters could result in a shift to more efficient C export that compensates for the expected decline in productivity.

Abstract. Marine microbial communities mediate many biogeochemical transformations in the ocean. Consequently, processes such as primary production and carbon (C) export are linked to nutrient regeneration and are influenced by the resource demand and elemental composition of marine microbial biomass. Laboratory studies have demonstrated that differential partitioning of element resources to various cellular components can directly influence overall cellular elemental ratios, especially with respect to growth machinery (i.e., ribosomal RNA) and phosphorus (P) allocation. To investigate whether allocation to RNA is related to biomass P content and overall C : P biomass composition in the open ocean, we characterized patterns of P allocation and C : P elemental ratios along an environmental gradient of P-supply in the North Atlantic subtropical gyre (NASG) from 35.67° N 64.17° W to 22.67° N 65.52° W. Because the NASG is characterized as a P-stressed ecosystem, we hypothesized that biochemical allocation would reflect sensitivity to bioavailable P, such that greater P supply would result in increased allocation toward P-rich RNA for growth. We expected these changes in allocation to also result in lower C : P ratios with increased P supply. In contrast to our predictions however, bulk C : P ratios were decoupled from allocation to nucleic acids and did not vary systematically across a P supply gradient of 2.2–14.7 μmol m–2 d–1. Overall, we found that C : P ratios ranged from 188–306 along the transect, and RNA represented only 6–12% of total particulate P, whereas DNA represented 11–19%. However, we did find that allocation to RNA was positively correlated with SRP supply rate, suggesting a consistent physiological response in biochemical allocation to resource supply within the whole community. These results suggest that community composition or non-nucleic acid P pools may influence ecosystem scale variation in C : P stoichiometry more than nucleic acid allocation or prevailing environmental conditions in diverse marine microbial communities.

Nearly 75 years ago, Alfred C. Redfield observed a similarity between the elemental composition of marine plankton in the surface ocean and dissolved nutrients in the ocean interior. This stoichiometry, referred to as the Redfield ratio, continues to be a central tenet in ocean biogeochemistry, and is used to infer a variety of ecosystem processes, such as phytoplankton productivity and rates of nitrogen fixation and loss2-4. Model, field and laboratory studies have shown that different mechanisms can explain both constant and variable ratios of carbon to nitrogen and phosphorus among ocean plankton communities. The range of C/N/P ratios in the ocean, and their predictability, are the subject of much active research. Here we assess global patterns in the elemental composition of phytoplankton and particulate organic matter in the upper ocean, using published and unpublished observations of particulate phosphorus, nitrogen and carbon from a broad latitudinal range, supplemented with elemental data for surface plankton populations. We show that the elemental ratios of marine organic matter exhibit large spatial variations, with a global average that differs substantially from the canonical Redfield ratio. However, elemental ratios exhibit a clear latitudinal trend. Specifically, we observed a ratio of 195:28:1 in the warm nutrient-depleted low-latitude gyres, 137:18:1 in warm, nutrient-rich upwelling zones, and 78:13:1 in cold, nutrient-rich high-latitude regions. We suggest that the coupling between oceanic carbon, nitrogen and phosphorus cycles may vary systematically by ecosystem. © 2013 Macmillan Publishers Limited. All rights reserved.