Vol. 54, 2015

Seasonal dynamics of the copepod community in a tropical monsoonal estuary and the role of sex ratio in their abundance pattern

G. Vineetha*, N. V. Madhu, K. K. Kusum and P. M. Sooria

CSIR-National Institute of Oceanography, Regional Centre, Dr. Salim Ali Road, Kochi, India

Abstract
Background: As salinity is considered the prime “ecological master factor” governing the zooplankton distribution and abundance in estuaries, a spatio-temporal interactive approach was followed to assess whether the responses of the estuarine copepod community towards the salinity are always direct with a strong positive affinity or whether there exist any complexities in their interrelationship. The study, also for the first time, addressed the roleof sex ratio in governing the abundance and the population structure of copepods in the tropical monsoonal estuarine system.
Results: The ecological scenario in the Cochin estuary revealed that irrespective of the season, higher zooplankton abundance occurred in the mesohaline zone (MSZ; salinity 5–18) of the estuary, despite the pronounced spatial shift of the MSZ from the lower reaches of the estuary to upstream locations, in conjunction with the varying seasonal fluvial influx and marine water intrusion. In the case of the sex ratio of copepods, the seasonal scenario revealed that the dominant copepods had a lower sex ratio during the period of higher abundance, and a negative relation was observed between the abundance and the sex ratio of copepod species during most of the seasons.
Conclusions: The preponderance of the mesohaline and euryhaline species of the copepod community formed the key contributing factor towards the maintenance of higher abundance in the MSZ of the estuary. The bias in the sex ratio towards females favored higher reproductive output, resulting in the higher abundance during the favorable pre-monsoon period.

Key words: Zooplankton; Monsoonal estuary; Mesohaline; Copepod; Sex ratio.

*Correspondence: E-mail: vineetha.varada@gmail.com