Seasonal phytoplankton blooms in the Gulf of Aden revealed by remote Sensing

Abstract The 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.

[1]  T. Weisse,et al.  Significance of Picocyanobacteria in the Red Sea and the Gulf of Aden , 1992 .

[2]  W. Gladstone,et al.  Sustainable use of renewable resources and conservation in the Red Sea and Gulf of Aden: issues, needs and strategic actions , 1999 .

[3]  D. Smeed.  Seasonal variation of the flow in the strait of Bab al Mandab , 1997 .

[4]  E. Boss,et al.  Regional ocean-colour chlorophyll algorithms for the Red Sea , 2015 .

[5]  E. Möbius,et al.  Charge states of energetic (≈0.5 MeV/n) ions in corotating interaction regions at 1 AU and implications on source populations , 2002 .

[6]  T. Özgökmen,et al.  How does the Red Sea outflow water interact with Gulf of Aden Eddies , 2011 .

[7]  Peter Regner,et al.  The Ocean Colour Climate Change Initiative: III. A round-robin comparison on in-water bio-optical algorithms , 2015 .

[8]  T. Prasad,et al.  Spring evolution of Arabian Sea High in the Indian Ocean , 2001 .

[9]  Ibrahim Hoteit,et al.  Remote Sensing the Phytoplankton Seasonal Succession of the Red Sea , 2013, PloS one.

[10]  Annick Bricaud,et al.  Natural variability of phytoplanktonic absorption in oceanic waters: Influence of the size structure of algal populations , 2004 .

[11]  S. Saitoh,et al.  Bio-optical characteristics of the western Arctic Ocean: implications for ocean color algorithms , 2007 .

[12]  T. Yamagata,et al.  The Red Sea outflow regulated by the Indian monsoon , 2006 .

[13]  Robert J. W. Brewin,et al.  Influence of light in the mixed-layer on the parameters of a three-component model of phytoplankton size class , 2015 .

[14]  M. Veldhuis,et al.  Abundance and productivity of bacterioplankton in relation to seasonal upwelling in the northwest Indian Ocean , 1997 .

[15]  Jean-Marc Nicolas,et al.  Seasonal and interannual variability of ocean color and composition of phytoplankton communities in the North Atlantic, equatorial Pacific and South Pacific , 2004 .

[16]  A. Robinson,et al.  Eddy-induced nutrient supply and new production in the Sargasso Sea , 1997 .

[17]  P. Oke,et al.  An avenue of eddies: Quantifying the biophysical properties of mesoscale eddies in the Tasman Sea , 2012 .

[18]  W. Gregg,et al.  Global and regional evaluation of the SeaWiFS chlorophyll data set , 2004 .

[19]  P. Vinayachandran,et al.  Westward movement of eddies into the Gulf of Aden from the Arabian Sea , 2007 .

[20]  S. Shenoi,et al.  Water masses in the Gulf of Aden , 2007 .

[21]  J. D. Wilson,et al.  The use of cumulative monthly mean temperature anomalies in the analysis of local interannual climate variability , 1989 .

[22]  A. Bower,et al.  Mesoscale eddies in the Gulf of Aden and their impact on the spreading of Red Sea Outflow Water , 2012 .

[23]  D. Fratantoni,et al.  Somali Current rings in the eastern Gulf of Aden , 2006 .

[24]  C. Saydam,et al.  Transport and distribution of nutrients and chlorophyll-a by mesoscale eddies in the northeastern Mediterranean , 1990 .

[25]  P. J. Werdell,et al.  A multi-sensor approach for the on-orbit validation of ocean color satellite data products , 2006 .

[26]  Ralf Goericke,et al.  Top‐down control of phytoplankton biomass and community structure in the monsoonal Arabian Sea , 2002 .

[27]  K. Banse.  Grazing, Temporal Changes of Phytoplankton Concentrations, and the Microbial Loop in the Open Sea , 1992 .

[28]  I. Hoteit,et al.  Comparison of chlorophyll in the Red Sea derived from MODIS-Aqua and in vivo fluorescence , 2013 .

[29]  M. Veldhuis,et al.  Seasonal fluctuations in plankton biomass and productivity in the ecosystems of the Somali Current, Gulf of Aden, and southern Red Sea , 1995 .

[30]  James Willard Nybakken,et al.  Marine Biology: An Ecological Approach , 1982 .

[31]  William E. Johns,et al.  Arabian Marginal Seas and Gulfs , 1999 .

[32]  Mark G. Meekan,et al.  Extreme climatic events reduce ocean productivity and larval supply in a tropical reef ecosystem , 2011 .

[33]  R. Franke.  Scattered data interpolation: tests of some methods , 1982 .

[34]  T. Platt,et al.  Ecological indicators for the pelagic zone of the ocean from remote sensing , 2008 .

[35]  Robert J. W. Brewin,et al.  Plankton indicators and ocean observing systems: support to the marine ecosystem state assessment , 2014 .

[36]  D. Ediger,et al.  Phytoplankton fluorescence and deep chlorophyll maxima in the northeastern mediterranean , 1994 .

[37]  M. Reboita,et al.  Synoptic and dynamical analysis of subtropical cyclone Anita (2010) and its potential for tropical transition over the South Atlantic Ocean , 2013 .

[38]  W. Johns,et al.  Atmospherically Forced Exchange through the Bab el Mandeb Strait , 2012 .

[39]  R. Arnone,et al.  Deriving inherent optical properties from water color: a multiband quasi-analytical algorithm for optically deep waters. , 2002, Applied optics.

[40]  David M. Fratantoni,et al.  Gulf of Aden eddies and their impact on Red Sea Water , 2002 .

[41]  H. Dierssen.  Perspectives on empirical approaches for ocean color remote sensing of chlorophyll in a changing climate , 2010, Proceedings of the National Academy of Sciences.

[42]  Hiromi Oaku,et al.  Basin scale estimates of sea surface nitrate and new production from remotely sensed sea surface temperature and chlorophyll , 2000 .

[43]  W. Greve,et al.  On the phenology of North Sea ichthyoplankton , 2005 .

[44]  E. Boss,et al.  Underway spectrophotometry along the Atlantic Meridional Transect reveals high performance in satellite chlorophyll retrievals , 2016 .

[45]  C. McClain.  A decade of satellite ocean color observations. , 2009, Annual review of marine science.

[46]  Ricardo M Letelier,et al.  Nitrogen fixation in an anticyclonic eddy in the oligotrophic North Pacific Ocean , 2008, The ISME Journal.

[47]  Janet W. Campbell,et al.  The lognormal distribution as a model for bio‐optical variability in the sea , 1995 .

[48]  Trevor Platt,et al.  Impact of missing data on the estimation of ecological indicators from satellite ocean-colour time-series , 2014 .

[49]  S. Clemens,et al.  Forcing mechanisms of the Indian Ocean monsoon , 1991, Nature.

[50]  W. Johns,et al.  Direct observations of seasonal exchange through the Bab el Mandab Strait , 1997 .

[51]  Xiaoping Zhou,et al.  Marine ecology: Spring algal bloom and larval fish survival , 2003, Nature.

[52]  H. Sverdrup,et al.  On Conditions for the Vernal Blooming of Phytoplankton , 1953 .

[53]  P. J. Werdell,et al.  An improved in-situ bio-optical data set for ocean color algorithm development and satellite data product validation , 2005 .

[54]  Trevor Platt,et al.  Diagnostic Properties of Phytoplankton Time Series from Remote Sensing , 2010 .

[55]  Ibrahim Hoteit,et al.  Phytoplankton phenology indices in coral reef ecosystems: Application to ocean-color observations in the Red Sea , 2015 .

[56]  Ibrahim Hoteit,et al.  Monsoon oscillations regulate fertility of the Red Sea , 2015 .

[57]  Dudley B. Chelton,et al.  A Global Climatology of Surface Wind and Wind Stress Fields from Eight Years of QuikSCAT Scatterometer Data , 2008 .

[58]  Corinne Le Quéré,et al.  Phytoplankton phenology in the global ocean , 2012 .

[59]  François Steinmetz,et al.  Atmospheric correction in presence of sun glint: application to MERIS. , 2011, Optics express.

[60]  M. D. Schwartz.  Phenology: An Integrative Environmental Science , 2003, Tasks for Vegetation Science.

[61]  Ibrahim Hoteit,et al.  Exploring the Red Sea seasonal ecosystem functioning using a three‐dimensional biophysical model , 2014 .

[62]  Mohammed Ali Yahya Al Saafani.  Physical Oceanography of the Gulf of Aden , 2008 .

[63]  F. Colao,et al.  Analysis of simultaneous chlorophyll measurements by lidar fluorosensor, MODIS and SeaWiFS , 2004 .

[64]  J. Dunne,et al.  A comparison of methods to determine phytoplankton bloom initiation , 2013 .