Diurnal variations in the vertical distribution and abundance of zooplankton in the continental shelf waters off Cochin during April 1991

Selvaraj, G S D and Mathew, K J and Varghese, Molly (1996) Diurnal variations in the vertical distribution and abundance of zooplankton in the continental shelf waters off Cochin during April 1991. In: Second workshop on scientific results of FORV Sagar Sampada, 15-17 February 1994, Kochi.

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Abstract

The present study is based on zooplankton samples collected by vertical hauls from 0-30, 30-60 and 60-90 m depths from a 100 m depth station in the continental shelf (09° 44' N, 75° 42*8) off Cochin for two days at three hourly interval. The mean displacement volume of zooplankton biomass at these depth zones were 6.24, 6.70 and 6.07 ml/100 m water while their numerical counts were 14490,13355 and 11290 no/100 m respectively. The overall mean volume (ml) and number per 100 m in the endre water column (0-90 m) were 6.34 and 13045 respectively and were distributed as 5.89 ml and 11715 no/100 m^ in the daytime and 6.79 ml and 14376 no/100 m^ during night hours respectively. The biomass was dominated by copepods (75.6%), decapod larvae (12.2%), chaetognaths (6.5%) and appendicularians (2.4%) in April. In general, copepods, decapod larvae and appendicularians showed increasing trend in number from the bottom (60-90 m) to the surface layer (0-30 m); chaetognaths, medusae, pteropods, heteropods, salps and doliolids were distributed more in the middle zone (30-60 m); while ostracods and planktonic polychaetes were more in the bottom zone (60-90 m). Distribution of amphipods and ostracods indicated significant diurnal vertical migration. The phenomenon of mid- night sinking was noticed in the case of copepods, chaetognaths, appendicularians, medusae, siphonophores, salps and doliolids.

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