2. Materials and methods
Online databases (http://giovanni.gsfc.nasa.gov,
http://apdrc.soest.hawaii.edu) provided data on sea surface temperature
(SST), surface winds, and chlorophyll-a. Ferret (version 7) was
used for generating the monthly composite maps of these parameters. The
first in-situ data from 2010 – 11was obtained from 15 locations
in the GoM for three seasons [March 2010 (Pre-Monsoon), September 2010
(Southwest Monsoon), and January 2011 (Northeast Monsoon)]. Field
sampling for this was conducted on board a trawler along five
cross-shore transects in the Indian sector of the GoM (GoM1 to GoM5)
(Figure 1). The second in-situ sampling was in September
(Southwest Monsoon) 2018 to verify whether the coastal upwelling process
occurs in the GoM. This sampling was carried out onboard CRV Sagar
Purvi with three sampling locations in each of the five cross-shore
transects (SP1 to SP 5) from the western proximity of the GoM up to 12°N
along the southwest coast of India (Figure 1). A portable Conductivity
Temperature Depth (CTD) profiler (Seacat, Seabird Electronics, USA)
recorded vertical distribution of temperature and salinity. Standard
Winkler’s method13 was used to estimate the dissolved
oxygen concentration from the surface, subsurface, and bottom waters in
the first sampling during 2010-2011 and from many prefixed standard
depths (1 m, 5 m, 10 m, 20 m, and 40 m) during the second sampling in
2018. We also used the Regional Ocean Model System (ROMS) outputs of the
vertical distribution of relevant environmental variables for the
Southwest Monsoon period, obtained from the Indian National Centre for
Ocean Information Services (INCOIS), Hyderabad, India
(www.incois.gov.in) 14,15. Vertical sections of four
cross-shore transects (T1-T4) obtained from ROMS beginning from the
southern tip of India (Cape Comorin) towards the GoM were used to
comprehend how best the model projection follows the environmental
signatures evident in the in-situ observations14,15. Univariate ANOVA identified the
significance of the spatial difference in hydrographical parameters
between transects sampled in during 2018 (SP1-SP5). Before the ANOVA
test, the homogeneity and distribution of the environmental variables
were analysed using XL Stat PRO software. Tukey’s HSD post hoc test was
performed for pairwise data comparisons. The distribution of the
ecological variables was analysed in XL stat Pro (License information –
Academic version, Purchased by CSIR-National Institute of Oceanography,
Regional centre, Kochi). ANOVA was analysed in PASTUiO software Version
4.02 (Free software downloaded from -https://folk.uio.no/ohammer/past/).
Principle Component Analysis (PCA) was also performed to visualise the
spatial differences in the distribution of relevant environmental
variables. Correlation matrix-based scaling was used in PCA Biplots,
having the first two principal components. The dissolved oxygen values
were overlaid (dotted line) on the Biplots to indicate the spatial
variation. PCA was performed using CANOCA 4.5 software purchased by the
CSIR- National Institute of Oceanography, Regional centre, Kochi.