Differences in Rate and Direction of Shifts between Phytoplankton Size Structure and Sea Surface Temperature

Hisatomo Waga,Shintaro Takao,Toru Hirawake,Shigeto Nishino,Sei-Ichi Saitoh,Takashi Kikuchi,Koji Suzuki,Amane Fujiwara

doi:10.3390/rs9030222

Abstract

Species distributions are changing with various rates and directions in response to recent global warming. The velocity of sea surface temperature (SST) has been used to predict species migration and persistence as an expectation of how species track their thermal niches; however, several studies have found that evidence for species shifts has deviated from the velocity of SST. This study investigated whether estimation of the velocity of shifts in phytoplankton size structure using remote sensing data could contribute to better prediction of species shifts. A chlorophyll-a (Chla) size distribution (CSD) model was developed by quantifying the relationships between the size structure of the phytoplankton community and the spectral features of the phytoplankton absorption coefficient (aph(λ)), based on the principal component analysis approach. Model validation demonstrated that the exponent of CSD (hereafter, CSD slope), which can describe the synoptic size structure of a phytoplankton community, was derived successfully with a relative root mean square error of 18.5%. The median velocity of CSD slope across the ocean was 485.2 km·decade−1, broadly similar to Chla (531.5 km·decade−1). These values were twice the velocity of SST, and the directions of shifts in CSD slope and Chla were quite different from that of SST. Because Chla is generally covariant with the size structure of a phytoplankton community, we believe that spatiotemporal changes in Chla can explain the variations of phytoplankton size structure. Obvious differences in both rate and direction of shifts were found between the phytoplankton size structure and SST, implying that shifts of phytoplankton size structure could be a powerful tool for assessing the distributional shifts of marine species. Our results will contribute to generate global and regional maps of expected species shifts in response to environmental forcing.

Highlights

Global warming has brought dramatic changes to the marine environment [1]
It is possible to derive the Chla size distribution (CSD) slope from any combination of Chlasize ; we succeeded in developing a CSD model using the entire dataset
This study proposed a CSD model to derive the synoptic size structure of the phytoplankton community using satellite remote sensing data

Summary

Introduction

Global warming has brought dramatic changes to the marine environment [1]. Physical and chemical variations have strong effects on the behavior of marine species [2], and responses of species to changing environmental conditions are manifest at the population level as shifts in distribution [3].Empirical and theoretical studies have suggested that while species tend to migrate toward higher latitudes [4,5], up to 60% of species have not responded as expected, and that shifts in species distributions have shown various rates and directions [6].Improved understanding of how species shift their distributions is crucial for effective conservation under conditions of climate change [7]. Empirical and theoretical studies have suggested that while species tend to migrate toward higher latitudes [4,5], up to 60% of species have not responded as expected, and that shifts in species distributions have shown various rates and directions [6]. Climate velocity is derived as the ratio of the long-term temperature trend to the two-dimensional spatial gradient in temperature, and it enables predictions regarding species migration and persistence as an expectation of how species track the location of their thermal niches [8]. Some recent studies have exposed a complex mosaic of velocity of SST, suggesting non-uniform shifts in the global oceans with a wide range of rates and directions that deviate from simple poleward migration [9]. The velocity of SST is expected to describe observed species shifts [10,11]; unexpected shifts that are inconsistent with predictions generated by the velocity of SST have been reported for some species and in certain regions [12,13]

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