Ecological features of zooplankton (copepods) in the perennial oceanic oxygen minimum zone in the Eastern Arabian Sea.

Copepods are the most abundant metazoan/mesozooplankton in the ocean and are adapted to survive in diverse environmental conditions. Exclusive studies on copepods in the Eastern Arabian Sea (EAS) with emphasis on their composition and ecophysiological characteristics, especially from the perennial oceanic Oxygen Minimum Zone (OMZ), are almost absent. Therefore, this work presents the results of a stratum-wise vertical sampling of mesozooplankton over the mixed layer, thermocline, and OMZ along the Indian west coast in the EAS using a Multiple Plankton Net during the Northeast Monsoon (NEM) and Southwest Monsoon (SWM). Mesozooplankton biomass and abundance were considerably higher in the mixed layer during both seasons compared to the thermocline and OMZ. With a contribution ranging from 70 to 95 % across different depth strata, copepods were the most abundant zooplankton taxa. Regardless of depth strata, the study found 170 species of copepods in the EAS, 115 during the SWM, and 152 during the NEM. Unlike the hypoxic OMZ in the Southeastern Arabian Sea (SEAS), the suboxic OMZ of the Northeastern Arabian Sea (NEAS) had a higher abundance of calanoids of the families Lucicutiidae, Metridinidae, Eucalanidae, Scolecitrichidae, Paracalanidae, Calanidae, and Heterorhabdidae. The literature suggests that this is the result of the better adaptation strategies of calanoids to suboxic OMZs, which include dormancy, high lipid storage, sluggish lifestyles, and high enzyme activity. This study also delineated several noteworthy ecological characteristics of copepods, such as (a) the preponderance of cyclopoid Oncaea sp. in all vertical layers, regardless of season, due to their advantageous low respiration rates, omnivorous feeding habits, long periods of inactivity, and low metabolism; (b) the calanoid Lucicutia grandis as the only indicating species of suboxic OMZ in the NEAS; and (c) a significantly higher percentage contribution of females (av. 76.6 ± 1.1 %) in the rest of the area, while the OMZ in the NEAS had a greater proportion of males (av. 44.7 ± 1.4 %). Some earlier studies showed that males are advantageous in stable conditions as was the case in the OMZ in the NEAS, which offers its residents more settling particles to consume from the productive overlying waters and no visual predators. This study summarises that the vertical and zonal fluctuations in the thickness and intensity of OMZ in the EAS result in changes in copepod composition and ecological features.