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 role of 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.
Impact of microplastic pollution on the marine environment and its biota is a major concern globally. Gulf of Mannar (GoM) and Palk Bay (PB) are two important biodiversity hotspots along the south-east coast of India. However, in the recent years the intense tourism and fishery activities have made the ecosystem and biota of these two ecologically significant coastal zones vulnerable to microplastic contamination. Hence, a comparative study on the seasonal distribution of microplastics in the surface waters, sediments, and in commercially important fishes were conducted to evaluate the threats imposed by microplastics on the environment and biota of both these coastal ecosystems. Microplastic distribution in the surface waters and sediments of GoM and PB exhibited conspicuous seasonal variation and showed positive correlation with the seasonal current patterns. In the surface waters of GoM, microplastics were abundant during the South West Monsoon whereas in PB it was high during the Early Winter Monsoon period. In the sediments of PB, microplastics was more during Spring Inter-monsoon and South West Monsoon whereas in GoM it was high during Late Winter Monsoon. Fragments, of size 1 to 5 mm formed the abundant microplastic type in the surface waters whereas in the sediments, fragments of size < 1 mm dominated. Of the gut content of the 613 fishes belonging to 12 families examined, microplastics were more in pelagic than demersal fishes with higher incidences in the family Clupeidae (42%). Among the fishes studied, microplastic ingestion was more in Selaroides leptolepis (27.77%), Sphyraena sp. (14.28%), Pelates quadrilineatus (12%), Caranx sp. (10.34%), and Sphyraena barracuda (10%).
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