A simple optical index shows spatial and temporal heterogeneity in phytoplankton community composition during the 2008 North Atlantic Bloom Experiment

I. Cetinić,M. J. Perry,N. Poulton,C. M. Lee,N. Briggs,M. E. Sieracki,E. D'Asaro

doi:10.5194/bg-12-2179-2015

Abstract

Abstract. The ratio of two in situ optical measurements – chlorophyll fluorescence (Chl F) and optical particulate backscattering (bbp) – varied with changes in phytoplankton community composition during the North Atlantic Bloom Experiment in the Iceland Basin in 2008. Using ship-based measurements of Chl F, bbp, chlorophyll a (Chl), high-performance liquid chromatography (HPLC) pigments, phytoplankton composition and carbon biomass, we found that oscillations in the ratio varied with changes in plankton community composition; hence we refer to Chl F/bbp as an "optical community index". The index varied by more than a factor of 2, with low values associated with pico- and nanophytoplankton and high values associated with diatom-dominated phytoplankton communities. Observed changes in the optical index were driven by taxa-specific chlorophyll-to-autotrophic carbon ratios and by physiological changes in Chl F associated with the silica limitation. A Lagrangian mixed-layer float and four Seagliders, operating continuously for 2 months, made similar measurements of the optical community index and followed the evolution and later demise of the diatom spring bloom. Temporal changes in optical community index and, by implication, the transition in community composition from diatom to post-diatom bloom communities were not simultaneous over the spatial domain surveyed by the ship, float and gliders. The ratio of simple optical properties measured from autonomous platforms, when carefully validated, provides a unique tool for studying phytoplankton patchiness on extended temporal scales and ecologically relevant spatial scales and should offer new insights into the processes regulating patchiness.

Highlights

Autonomous observations of phytoplankton are becoming increasingly ubiquitous, including in situ optical sensing from Argo-type and Lagrangian floats, gliders and moorings, as well as remote sensing from space
Phytoplankton biomass is assessed through several different optical proxies, including in situ chlorophyll a fluorescence (Chl F ; Lorenzen, 1966), the phytoplankton absorption coefficient (aphy(λ)) or particulate absorption coefficient in waters dominated by phytoplankton (Bricaud et al, 1995; Roesler and Barnard, 2014), and chlorophyll derived from in situ or remotely sensed ocean reflectance at visible wavelengths (O’Reilly et al, 1998)
Simple optical measurements made from autonomous platforms allow us to follow the variability in phytoplankton biomass (Chl F ) and particulate organic carbon (POC) concentration on highly resolved spatial and temporal scales

Summary

Introduction

Autonomous observations of phytoplankton are becoming increasingly ubiquitous, including in situ optical sensing from Argo-type and Lagrangian floats, gliders and moorings, as well as remote sensing from space. High-frequency optical measurements are ideal for detecting temporal change and spatial patchiness and for improving our understanding of the role of meso- and submesoscale physics in the distribution of phytoplankton in the ocean (Denman and Platt, 1976; Yoder et al, 1987; Munk, 2000). Autonomous optical observations have enabled advances in understanding the timing of and mechanisms responsible for initiating blooms (Perry et al, 2008; Boss and Behrenfeld, 2010; Ryan et al, 2011; Mahadevan et al, 2012; Matrai et al, 2013). Cetinicet al.: A simple optical index shows spatial and temporal heterogeneity

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Results

Conclusion