In the tropics, thermal stratification (during warm conditions) may contribute to a shallowing of the mixed layer above the nutricline and a reduction in the transfer of nutrients to the surface lit-layer, ultimately limiting phytoplankton growth. Using remotely sensed observations and modelled datasets, we study such linkages in the northern Red Sea (NRS)-a typical tropical marine ecosystem. We assess the interannual variability (1998-2015) of both phytoplankton biomass and phenological indices (timing of bloom initiation, duration and termination) in relation to regional warming. We demonstrate that warmer conditions in the NRS are associated with substantially weaker winter phytoplankton blooms, which initiate later, terminate earlier and are shorter in their overall duration (∼ 4 weeks). These alterations are directly linked with the strength of atmospheric forcing (air-sea heat fluxes) and vertical stratification (mixed layer depth [MLD]). The interannual variability of sea surface temperature (SST) is found to be a good indicator of phytoplankton abundance, but appears to be less important for predicting bloom timing. These findings suggest that future climate warming scenarios may have a two-fold impact on phytoplankton growth in tropical marine ecosystems: 1) a reduction in phytoplankton abundance and 2) alterations in the timing of seasonal phytoplankton blooms.
Bibliographical noteFunding Information:
The authors are grateful to the ESA Ocean Colour CCI team for providing and processing the Chl-a dataset. We also acknowledge the NOAA National Oceanographic Data Centre (NODC), the University of Miami Rosenstiel School of Marine and Atmospheric Science (RSMAS) and the NASA Physical Oceanography Distributed Active Archive Centre (PODAAC) for making the GHRSST AVHRR_OI dataset available. The authors also thank Dr Hari Dasari for providing modelled outputs of heat fluxes, and Dr Marie-Fanny Racault for her valuable input regarding the implementation of the phenology threshold criterion algorithm. This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. 3268.
© 2018 The Author(s).
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