Response of micro- and mesozooplankton in the southwestern Bay of Bengal to a cyclonic eddy during the winter monsoon, 2005

Abstract

A cyclonic eddy (CE) in the southwestern Bay of Bengal (SWBoB; 10-15° N; 81-87° E) during winter monsoon 2005 and associated changes in the open ocean hydrography and productivity patterns were studied using satellite observations and in situ measurements. Analysis of the satellite-derived sea surface height anomaly (SSHA) indicated the existence of a large eddy (10-15° N; 81-87° E) from November to January, with its core centered at 13° N and 82° E. The large positive wind stress curl (~1.8 × 10(-7) N m(-2)) and resultant Ekman pumping (~3 × 10(-2) cm s(-1)) were identified as the prominent forcing mechanisms. In addition, the cyclonic storms and depressions experienced in the region during the study period seem to have served to maintain the strength of the CE through associated Ekman pumping. The cold (~26.6 °C), nutrient-enriched (NO3 > 2 μM, PO4 > 0.73 μM and SiO4 > 3 μM) upwelled waters in the upper layers of the CE enhanced the biological production (chl. a > 0.56 mg m(-3)). Dissolved oxygen in the surface waters was > 200 μM. The phytoplankton and zooplankton biomass recorded during the season was comparable or perhaps higher than the peak values reported from the northeastern Arabian Sea (chlorophyll a concentrations of 0.2 to 0.4 mg m(-3) and zooplankton biovolume 0.6 ml m(-3)) during winter. Occurrence of a higher mesozooplankton biovolume (0.8 ml m(-3)) and relatively low abundance of microzooplankton indicates the prevalence of a short food chain. In conclusion, high biological production, both at primary as well as secondary level, suggests the prevalence of an efficient food web as a result of physical forcing and subsequent ecological interactions evident up to tertiary level in an oligotrophic basin like BoB.

Seasonal variation of community composition of zooplankton in the Palk strait, (9‐10 °N, Arabian sea, India).

“…The results about species assemblages of zooplankton agree with literature descriptions about Arabian Sea, due high productivity, although there are marked seasonal density fluctuations due monsoons (Vineetha et al 2015;Vishnu Radhanan et al 2015;Gupta et al 2016). Within the zooplankton species, Copepods by their sheer and diversity constitute the most important group in any zooplankton community (Anbazhagan, 1988;Jayalakshmi et al 2015;Vineetha et al 2015), because they occupy an important position in the trophic structure and play a major role in the energy transfer of any aquatic environment. Because of their predominance, copepods form the chief index of utilization of biotope at secondary level (Madhupratap & Haridas, 1975;Madhupratap, 1979;Gupta et al, 2016).…”

Insignificant effects of eddies and typhoons on the biogeochemistry of the tropical northwest Pacific Ocean

“…One exception was a slight decrease in DO concentration (~5 mmol kg -1 ) in the cyclonic eddy around 12 -13 September 2020 (Figures 6E, F). This decrease is in contrast with previous studies reporting that DO concentrations in the cyclonic eddies were higher than in the outer areas of the cyclonic eddies (Sukigara et al, 2014;Jayalakshmi et al, 2015). The studies ascribed the increase in DO concentration to upward nutrient supply resulting in enhanced biological production.…”

Quantifying Tropical Cyclone's Effect on the Biogeochemical Processes Using Profiling Float Observations in the Bay of Bengal

“…From the ship‐based in situ observations, Naik et al () have reported an enhancement of 5 μM in the nitrate concentration in the near‐surface layer after the passage of a TC in the Arabian Sea. Similarly, Jayalakshmi et al () have also reported the increase of 3.8 μM in nitrate concentration at a cyclonic eddy region in the southwestern BoB during northern winter.…”