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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 5 | Copyright
Biogeosciences, 15, 1395-1414, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Mar 2018

Research article | 07 Mar 2018

Delineation of marine ecosystem zones in the northern Arabian Sea during winter

Saleem Shalin1,a, Annette Samuelsen2, Anton Korosov3, Nandini Menon1, Björn C. Backeberg4,5,3, and Lasse H. Pettersson3 Saleem Shalin et al.
  • 1Nansen Environmental Research Centre (India), Kochi, India
  • 2Nansen Environmental and Remote Sensing Center and Bjerknes Centre for Climate Research, Bergen, Norway
  • 3Nansen Environmental and Remote Sensing Center, Bergen, Norway
  • 4Coastal Systems Research Group, Natural Resources and the Environment, Council for Scientific and Industrial Research, Stellenbosch, South Africa
  • 5Nansen-Tutu Centre for Marine Environmental Research, Department of Oceanography, University of Cape Town, Cape Town, South Africa
  • apresent address: Central Marine Fisheries Research Institute, Kochi, India

Abstract. The spatial and temporal variability of marine autotrophic abundance, expressed as chlorophyll concentration, is monitored from space and used to delineate the surface signature of marine ecosystem zones with distinct optical characteristics. An objective zoning method is presented and applied to satellite-derived Chlorophyll a (Chl a) data from the northern Arabian Sea (50–75°E and 15–30°N) during the winter months (November–March). Principal component analysis (PCA) and cluster analysis (CA) were used to statistically delineate the Chl a into zones with similar surface distribution patterns and temporal variability. The PCA identifies principal components of variability and the CA splits these into zones based on similar characteristics. Based on the temporal variability of the Chl a pattern within the study area, the statistical clustering revealed six distinct ecological zones. The obtained zones are related to the Longhurst provinces to evaluate how these compared to established ecological provinces. The Chl a variability within each zone was then compared with the variability of oceanic and atmospheric properties viz. mixed-layer depth (MLD), wind speed, sea-surface temperature (SST), photosynthetically active radiation (PAR), nitrate and dust optical thickness (DOT) as an indication of atmospheric input of iron to the ocean. The analysis showed that in all zones, peak values of Chl a coincided with low SST and deep MLD. The rate of decrease in SST and the deepening of MLD are observed to trigger the algae bloom events in the first four zones. Lagged cross-correlation analysis shows that peak Chl a follows peak MLD and SST minima. The MLD time lag is shorter than the SST lag by 8 days, indicating that the cool surface conditions might have enhanced mixing, leading to increased primary production in the study area.

An analysis of monthly climatological nitrate values showed increased concentrations associated with the deepening of the mixed layer. The input of iron seems to be important in both the open-ocean and coastal areas of the northern and north-western parts of the northern Arabian Sea, where the seasonal variability of the Chl a pattern closely follows the variability of iron deposition.

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This work objectively classified the northern Arabian Sea into six ecological zones based on surface Chl a distribution patterns during winter. Distinct Chl a characteristics within each delineated zone show that this classification method is a good way of separating regions with different phytoplankton dynamics during winter. The study provides improved understanding of how environmental factors control the spatio-temporal variability of the marine Chl a concentration in the area during winter.
This work objectively classified the northern Arabian Sea into six ecological zones based on...