The publication Delineation of marine ecosystem zones in the northern Arabian Sea during winter, resently published in Biogeosciences, is a result of collaboration between the Nansen Centers in India, Norway and South Africa. It was inititated under the EU FP7 project Indo-Mareclim: Indo-European research facilities for studies on marine ecosystem and climate in India, coordinated by the Nansen Center in India. The northern Arabian Sea is a dynamic ocean area, influenced by the seasonal monsoon cycles, where upwelling, downwelling, convective overturning, mesoscale eddies, fronts and planetary waves commonly occur. This dynamic have a significant influence on the biological production in the region.

- Classification of the ocean into ecological zones is a useful tool to understand the interactions between physical and biochemical marine processes, says one of the authors, Annette Samuelsen, at NERSC. She tells that the spatial and temporal variability of phytoplankton abundance, expressed as chlorophyll concentration, is monitored from space and here used to delineate the surface signature of marine ecosystem zones with distinct bloom characteristics.

In this study Chlorophyll a (Chl a) satellite remote sensing data from the winter seasons (November to March) were used to delineate the marine ecological zones to study phytoplankton variability and its drivers in the northern Arabian Sea. Though we know that significant primary production occurs in summer in the Arabian Sea, it so cloudy that there are insufficient remote sensing observations to perform the analysis. During winter season, high-quality satellite data are available and high values of Chl a (>0.5 mgm-3) prevailed in the study area. Principal component analysis (PCA) and cluster analysis (CA) were used to statistically delineate the region into zones with similar surface distribution patterns and temporal variability of Chl a.

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 such as mixed-layer depth (MLD), wind speed, sea-surface temperature (SST), photosynthetically active radiation (PAR), nitrate and dust optical thickness (DOT). DOT is an indication of atmospheric input of iron to the ocean.

Samuelsen tells that the analysis showed that in all zones, peak values of Chl a are associated with low SST and deep MLD and lagged cross-correlation analysis shows that peak Chl a usually follows peak MLD and SST minima. Cool SST occurs before the MLD deepening, 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 nutrient 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.

Citation: Shalin, S., Samuelsen, A., Korosov, A., Menon, N., Backeberg, B. C., and Pettersson, L. H.: Delineation of marine ecosystem zones in the northern Arabian Sea during winter, Biogeosciences, 15, 1395-1414, https://doi.org/10.5194/bg-15-1395-2018, 2018.