Mesozooplankton Distribution in the Northeastern Arabian Sea During the Spring Intermonsoon

Mesozooplankton is a vital component of pelagic food webs, serving as a key trophic link between primary producers and higher marine organisms, and driving coastal and oceanic biogeochemical cycles. This study investigated the abundance, biovolume, community structure, and spatial distribution of mesozooplankton in relation to hydrographic variables in the Northeastern Arabian Sea (NEAS 15 ^◦ N − 21 ^◦ N) during the Spring Intermonsoon (March 2019) on board FORV Sagar Sampada . Sampling across 14 stations comparing coastal (> 50 m) and oceanic (< 1000 m) waters revealed distinct environmental gradients.

Both Sea Surface Temperature (SST: 24.33 ^◦ C − 28.68 ^◦ C) and Sea Surface Salinity (SSS: 35.25 − 36.65) exhibited a declining trend toward northern latitudes, whereas Dissolved Oxygen (DO: 4.55 − 5.24 ml ⋅ L ⁻¹ ) increased northward. Chlorophyll concentrations were significantly higher in coastal regions (mean 1.45 mg ⋅ m ⁻³ ) than in oceanic waters (mean 0.36 mg ⋅ m ⁻³ ). Correspondingly, mesozooplankton biovolume (excluding large gelatinous taxa) was threefold higher in coastal waters (mean 1.39 mL ⋅ m ⁻³ ) compared to oceanic waters (mean 0.38 mL ⋅ m ⁻³ ). Mean numerical abundance was also higher coastally (605 ind. ⋅ m ⁻³ ) than oceanically (524 ind. ⋅ m ⁻³ ).

A total of 21 mesozooplankton taxa were identified. Copepods dominated the community structure across both regimes (coastal: 61.07%; oceanic: 56.69%). In coastal waters, Chaetognatha (14.76%), Cladocera (9.39%), and Decapod larvae (5.69%) were the secondary drivers, with localised salp and siphonophore swarms observed at 17 ^◦ − 19 ^◦ N. In oceanic waters, Ostracoda (22.93%) and Salpidae (12.51%) were major secondary contributors. These results indicate that during the Spring Intermonsoon, the NEAS exhibits moderate productivity representative of a receding winter cooling regime.