Seasonal phytoplankton blooms in the Gulf of Aden revealed by remote sensing
Raitsos, Dionysios E.
Brewin, Robert J.W.
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AbstractThe Gulf of Aden, situated in the northwest Arabian Sea and linked to the Red Sea, is a relatively unexplored ecosystem. Understanding of large-scale biological dynamics is limited by the lack of adequate datasets. In this study, we analyse 15 years of remotely-sensed chlorophyll-a data (Chl-a, an index of phytoplankton biomass) acquired from the Ocean Colour Climate Change Initiative (OC-CCI) of the European Space Agency (ESA). The improved spatial coverage of OC-CCI data in the Gulf of Aden allows, for the first time, an investigation into the full seasonal succession of phytoplankton biomass. Analysis of indices of phytoplankton phenology (bloom timing) reveals distinct phytoplankton growth periods in different parts of the gulf: a large peak during August (mid-summer) in the western part of the gulf, and a smaller peak during November (mid-autumn) in the lower central gulf and along the southern coastline. The summer bloom develops rapidly at the beginning of July, and its peak is approximately three times higher than that of the autumnal bloom. Remotely-sensed sea-surface temperature (SST), wind-stress curl, vertical nutrient profiles and geostrophic currents inferred from the sea-level anomaly, were analysed to examine the underlying physical mechanisms that control phytoplankton growth. During summer, the prevailing southwesterlies cause upwelling along the northern coastline of the gulf (Yemen), leading to an increase in nutrient availability and enhancing phytoplankton growth along the coastline and in the western part of the gulf. In contrast, in the central region of the gulf, lowest concentrations of Chl-a are observed during summer, due to strong downwelling caused by a mesoscale anticyclonic eddy. During autumn, the prevailing northeasterlies enable upwelling along the southern coastline (Somalia) causing local nutrient enrichment in the euphotic zone, leading to higher levels of phytoplankton biomass along the coastline and in the lower central gulf. The monsoon wind reversal is shown to play a key role in controlling phytoplankton growth in different regions of the Gulf of Aden.
CitationGittings JA, Raitsos DE, Racault M-F, Brewin RJW, Pradhan Y, et al. (2017) Seasonal phytoplankton blooms in the Gulf of Aden revealed by remote sensing. Remote Sensing of Environment 189: 56–66. Available: http://dx.doi.org/10.1016/j.rse.2016.10.043.
SponsorsThe authors are grateful to the ESA Ocean Colour CCI team for the chlorophyll data, and the NASA Goddard Earth Sciences Data and Information Services Centre for processing MODIS SST data products. The authors also thank the Remote Sensing Systems team and their sponsor, the NASA Ocean Vector Winds Science Team, who made wind data available. The initial analysis and visualization of the remotely-sensed variables (Chl-a and SST) in this study took place using the Giovanni online data system, developed and maintained by the NASA GES DISC. The authors thank James Dingle for technical support, and Fengchao Yao for providing the nutrient, temperature and wind data represented in Fig. 5. The authors also acknowledge the National Centre for Earth Observation and a grant from the Living Planet programme of the European Space Agency (CCI-LPF-EOPS-MM-16-0078).
JournalRemote Sensing of Environment