Vertical migration by bulk phytoplankton sustains biodiversity and nutrient input to the surface ocean

Kai Wirtz,S Lan Smith

doi:10.1038/s41598-020-57890-2

Abstract

Phytoplankton subsumes the great variety of unicellular photoautotrophs that perform roughly half of Earth’s primary production. They achieve this despite their challenging oceanic habitat, with opposing vertical gradients of nutrients (which often limit their growth near the surface) and light (which becomes limiting with increasing depth). Most phytoplankton species are commonly assumed to be incapable of moving actively between the zones of light and nutrient availability, which are separated vertically by from 30–120 m. Here we propose that a considerable fraction of phytoplankton vertically traverse these gradients over time scales from hours to weeks, employing variations of a common migration strategy to acquire multiple resources. We present a mechanistic Lagrangian model resolving phytoplankton growth linked to optimal migration behaviour and demonstrate unprecedented agreement of its calculated vertical CHL-a distributions with 773 profiles observed at five prominent marine time-series stations. Our simulations reveal that vertically cycling phytoplankton can pump up enough nutrient to sustain as much as half of oceanic Net Primary Production (NPP). Active locomotion is therefore a plausible mechanism enabling relatively high NPP in the oligotrophic surface ocean. Our simulations also predict similar fitness for a variety of very different migration strategies, which helps to explain the puzzling diversity of phytoplankton observed in the ocean.

Highlights

Phytoplankton subsumes the great variety of unicellular photoautotrophs that perform roughly half of Earth’s primary production
We suggest that slow vertical migration on time spans of a week or so could allow cells to enhance their growth by accessing light near the surface and nutrients near the nutricline
The vertical cycling described has the important impact of displacing autotrophs from the chemocline, which favours the formation of subsurface chlorophyll maxima (SCM)

Summary

Introduction

Phytoplankton subsumes the great variety of unicellular photoautotrophs that perform roughly half of Earth’s primary production They achieve this despite their challenging oceanic habitat, with opposing vertical gradients of nutrients (which often limit their growth near the surface) and light (which becomes limiting with increasing depth). The two zones where either nutrients or light become replete are separated by about 30–120 m, a distance that challenges the locomotion ability of most phytoplankton species, which are classically assumed to be passive drifters This picture of drifters has been corrected through evidence of active and fast migration through the water column at 20–100 md−1 for a few phytoplankton groups, such as diatom mats[1,2] and cyanobacteria[3] in the open ocean, and for dinoflagellates in coastal waters[4]. If operated by bulk phytoplankton rather than just a few species, this active biological nutrient pump would constitute a powerful mechanism to sustain the marine cycles of carbon, nitrogen, and other elements, despite the unfavorable conditions typical of the surface ocean

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Discussion

Conclusion