Varying influence of phytoplankton biodiversity and stoichiometric plasticity on bulk particulate stoichiometry across ocean basins

Michael W Lomas,Kristina X Terpis,Debra A Lomas,Celine Mouginot,Adam C Martiny,Steven E Baer,Mark A Altabet

doi:10.1038/s43247-021-00212-9

Abstract

Concentrations and elemental ratios of suspended particulate organic matter influence many biogeochemical processes in the ocean, including patterns of phytoplankton nutrient limitation and links between carbon, nitrogen and phosphorus cycles. Here we present direct measurements of cellular nutrient content and stoichiometric ratios for discrete phytoplankton populations spanning broad environmental conditions across several ocean basins. Median cellular carbon-to-phosphorus and nitrogen-to-phosphorus ratios were positively correlated with vertical nitrate-to-phosphate flux for all phytoplankton groups and were consistently higher for cyanobacteria than eukaryotes. Light and temperature were inconsistent predictors of stoichiometric ratios. Across nutrient-rich and phosphorus-stressed biomes in the North Atlantic, but not in the nitrogen-stressed tropical North Pacific, we find that a combination of taxonomic composition and environmental acclimation best predict bulk particulate organic matter composition. Our findings demonstrate the central role of plankton biodiversity and plasticity in controlling linkages between ocean nutrient and carbon cycles in some regions.

Highlights

Concentrations and elemental ratios of suspended particulate organic matter influence many biogeochemical processes in the ocean, including patterns of phytoplankton nutrient limitation and links between carbon, nitrogen and phosphorus cycles
Spatial variability in the average taxonomic composition of natural phytoplankton populations has been hypothesized as an important driver of observed patterns in bulk particulate stoichiometry[5,6]
In the North Atlantic have studies directly measured stoichiometric ratios in phytoplankton taxa physically separated from natural populations

Summary

Introduction

Concentrations and elemental ratios of suspended particulate organic matter influence many biogeochemical processes in the ocean, including patterns of phytoplankton nutrient limitation and links between carbon, nitrogen and phosphorus cycles. Galbraith and Martiny[12] observed a robust relationship between ambient phosphate concentrations and bulk particulate N:P ratios, which was mechanistically linked to plasticity in the ability of certain phytoplankton to modulate phosphorus content. The results of these two studies are not mutually exclusive, owing to the covariance of elevated nutrient concentrations and phytoplankton communities dominated by diatoms[13]. What is the variability in element contents and stoichiometric ratios for specific phytoplankton populations across ocean regions? Does environmental forcing modulate the observed stoichiometric ratios within a taxonomic group? Do shifts in biodiversity contribute to variation in bulk C:N:P of marine communities?

Methods

Results

Conclusion