Evaluating tropical phytoplankton phenology metrics using contemporary tools

Hervé Claustre,Dionysios E Raitsos,Marie-Fanny Racault,Malika Kheireddine,John A Gittings,Ibrahim Hoteit

doi:10.1038/s41598-018-37370-4

Abstract

The timing of phytoplankton growth (phenology) in tropical oceans is a crucial factor influencing the survival rates of higher trophic levels, food web structure and the functioning of coral reef ecosystems. Phytoplankton phenology is thus categorised as an ‘ecosystem indicator’, which can be utilised to assess ecosystem health in response to environmental and climatic perturbations. Ocean-colour remote sensing is currently the only technique providing global, long-term, synoptic estimates of phenology. However, due to limited available in situ datasets, studies dedicated to the validation of satellite-derived phenology metrics are sparse. The recent development of autonomous oceanographic observation platforms provides an opportunity to bridge this gap. Here, we use satellite-derived surface chlorophyll-a (Chl-a) observations, in conjunction with a Biogeochemical-Argo dataset, to assess the capability of remote sensing to estimate phytoplankton phenology metrics in the northern Red Sea – a typical tropical marine ecosystem. We find that phenology metrics derived from both contemporary platforms match with a high degree of precision (within the same 5-day period). The remotely-sensed surface signatures reflect the overall water column dynamics and successfully capture Chl-a variability related to convective mixing. Our findings offer important insights into the capability of remote sensing for monitoring food availability in tropical marine ecosystems, and support the use of satellite-derived phenology as an ecosystem indicator for marine management strategies in regions with limited data availability.

Highlights

In tropical oceans, phytoplankton constitute a direct food source for coral reef fauna and pelagic larvae[1,2,3,4], whose survival contributes to healthy, diverse marine ecosystems
We synergistically utilise satellite-derived Chl-a observations with data from an autonomous Biogeochemical-Argo float (BGC-Argo float) to evaluate (1) the capability of remote-sensing data to estimate phytoplankton phenology metrics in a typical tropical marine ecosystem – the northern Red Sea; and (2) extend the phenological analysis to the part of the upper water column that is not seen by satellites
Ocean-colour remote sensing is currently the only platform from which synoptic estimates of phytoplankton phenology – an important ecosystem indicator – can be acquired. Prior to this analysis, research dedicated to the substantiation of satellite-derived phenology metrics was limited

Summary

Introduction

Phytoplankton constitute a direct food source for coral reef fauna and pelagic larvae[1,2,3,4], whose survival contributes to healthy, diverse marine ecosystems. This translates to economic support, services and well-being for maritime nations via fisheries and tourism[5]. We synergistically utilise satellite-derived Chl-a observations with data from an autonomous Biogeochemical-Argo float (BGC-Argo float) to evaluate (1) the capability of remote-sensing data to estimate phytoplankton phenology metrics in a typical tropical marine ecosystem – the northern Red Sea; and (2) extend the phenological analysis to the part of the upper water column that is not seen by satellites. We corroborate surface signatures detected by satellites by investigating the physical mechanisms that control vertical phytoplankton dynamics

Methods

Results

Conclusion