Light-dependent grazing can drive formation and deepening of deep chlorophyll maxima

Holly V Moeller,Matthew D Johnson,Edward M Sweeney,Charlotte Laufkötter

doi:10.1038/s41467-019-09591-2

Abstract

Deep Chlorophyll Maxima (DCMs) are subsurface peaks in chlorophyll-a concentration that may coincide with peaks in phytoplankton abundance and primary productivity. Work on the mechanisms underlying DCM formation has historically focused on phytoplankton physiology (e.g., photoacclimation) and behavior (e.g., taxis). While these mechanisms can drive DCM formation, they do not account for top-down controls such as predation by grazers. Here, we propose a new mechanism for DCM formation: Light-dependent grazing by microzooplankton reduces phytoplankton biomass near the surface but allows accumulation at depth. Using mathematical models informed by grazing studies, we demonstrate that light-dependent grazing is sufficient to drive DCM formation. Further, when acting in concert with other mechanisms, light-dependent grazing deepens the DCM, improving the fit of a global model with observational data. Our findings thus reveal another mechanism by which microzooplankton may regulate primary production, and impact our understanding of biogeochemical cycling at and above the DCM.

Highlights

Deep Chlorophyll Maxima (DCMs) are subsurface peaks in chlorophyll-a concentration that may coincide with peaks in phytoplankton abundance and primary productivity
Deep chlorophyll maxima (DCMs), subsurface peaks in the concentration of the photosynthetic pigment chlorophylla, are widespread phenomena observed in aquatic ecosystems, from freshwater lakes to coastal oceans to oligotrophic gyres[1,2,3]
As nutrients become depleted from surface waters, phytoplankton accumulate at progressively deeper depths until light becomes co-limiting[18,19]. This co-limitation mechanism is supported by DCMs that coincide with the nutricline[20] and that deepen as light increases or nutrients decrease[21,22]

Summary

Introduction

Deep Chlorophyll Maxima (DCMs) are subsurface peaks in chlorophyll-a concentration that may coincide with peaks in phytoplankton abundance and primary productivity. We propose a new mechanism for DCM formation: Light-dependent grazing by microzooplankton reduces phytoplankton biomass near the surface but allows accumulation at depth. Grazers can regulate phytoplankton abundance and composition[29,30], viruses may drive termination of blooms[31,32], and in several Earth System Models grazers can even cause a decrease in future primary production[33] Such mechanisms have generally been disregarded in DCM formation, perhaps because we expect an individual grazer’s functional traits (e.g., per-capita ingestion rate) to be independent of depth. We challenge this conventional view by proposing an alternate mechanism for DCM formation: light-dependent grazing of phytoplankton by microzooplankton can drive the formation, and increase the depth, of DCMs due to elevated rates of removal of phytoplankton in shallower waters

Methods

Results

Conclusion