This study reports spatial patterns of mesozooplankton and related hydrographic variables across the Northeastern Arabian Sea (NEAS) during the spring intermonsoon (March 2019). The work forms part of an in-house marine research project of the Zoological Survey of India and reports field sampling carried out on board FORV Sagar Sampada at 14 stations spanning coastal (>50 m) and oceanic (<1000 m) waters between 15°N and 21°N latitude.
Mesozooplankton were collected using a Bongo net. Hydrographic parameters measured concurrently included sea surface temperature (SST), sea surface salinity (SSS), dissolved oxygen (DO) and chlorophyll a, all recorded using a SEABIRD CTD profiler. The abstract provides summary ranges and averages for these parameters and for plankton biovolume, numerical abundance and taxonomic composition. Detailed station coordinates, tow depths, mesh sizes, sample processing protocols and statistical analyses were not reported in the abstract.
Sea Surface Temperature and salinity both showed a clear latitudinal pattern, decreasing toward the northern transects in both coastal and oceanic waters. Coastal SST ranged from 24.33°C to 27.79°C; oceanic SST ranged from 25.13°C to 28.68°C. Coastal SSS varied from 35.25 to 36.62, while oceanic SSS ranged from 35.51 to 36.65.
Dissolved oxygen exhibited an increasing trend toward the northern transects. Coastal DO values were 4.55–5.24 ml/l and oceanic DO values 4.79–5.15 ml/l.
Measured chlorophyll a concentrations were higher in coastal regions than in oceanic waters. Coastal chlorophyll a ranged from 0.18 mg/l to 2.31 mg/l (average 1.45 mg/l). Oceanic chlorophyll a ranged from 0.02 mg/l to 2.02 mg/l (average 0.36 mg/l).
Average mesozooplankton biovolume (explicitly stated to exclude large gelatinous components) was substantially greater in coastal waters than in oceanic waters, with a roughly threefold difference. Coastal biovolume averaged 1.39 ml·m⁻³ compared with an average of 0.38 ml·m⁻³ in oceanic regions.
Numerical abundance also differed between coastal and oceanic stations. Coastal abundance averaged 605 individuals·m⁻³ (range 82–1354 No.·m⁻³). Oceanic abundance averaged 524 individuals·m⁻³ (range 108–1069 No.·m⁻³). These values indicate higher mean counts inshore, although ranges overlapped between habitat types.
A total of 21 mesozooplankton taxa (including holo- and meroplankton) were recorded across the survey. Coastal waters contained 19 recorded taxa, while oceanic waters contained 21.
Copepods were the dominant taxon overall: coastal average contribution 61.07% and oceanic average contribution 56.69% of the assemblage. Secondary dominant groups varied by habitat. In coastal waters, the next most abundant groups were Chaetognaths (14.76%), Cladocerans (9.39%), Decapod larvae (5.69%), Lucifer (2.63%), Stomatopod larvae (2.30%) and Salps (1.37%). In oceanic waters, Ostracods comprised 22.93% and Salps 12.51% as notable secondary contributors, followed by Chaetognaths (2.84%) and Decapods (1.23%).
The authors observed prominent gelatinous blooms, specifically salps and siphonophores, in coastal waters near approximately 17°N, 18°N and 19°N latitudes. The abstract notes these blooms as prominent features of the coastal samples; quantitative contribution to total biovolume when large gelatinous organisms were included was not provided in the abstract, and large gelatinous components were explicitly excluded from the reported average biovolume metric.
Field sampling in March 2019 across 14 NEAS stations found latitudinal decreases in SST and SSS toward the north and increasing DO toward northern transects. Chlorophyll a concentrations and mesozooplankton biovolume were higher in coastal than in oceanic waters, with a roughly threefold higher average biovolume inshore. Numerical abundance was higher on average in coastal waters, and copepods dominated the community in both habitats. Gelatinous blooms (salps and siphonophores) were locally prominent at several coastal latitudes.
The abstract summarizes key ranges, averages and percent contributions by major groups, but does not include full methodological detail, precise station coordinates, statistical tests or the complete taxonomic list and numeric tables; those details may be present in the full manuscript but were not reported in the abstract provided here. The authors declare no competing interest.