Fluctuations in the rotifer population of a tropical estuary in Kerala concurrent to the century’s largest deluge

Abstract

Rotifers, because of their small size, slow motility, and high calorific value forms an important live feed in aquaculture. The present study was conducted to elucidate the impact of the massive flood that occurred in Kerala during August 2018 on the estuarine hydrography and rotifer community of Cochin estuary, the largest tropical monsoonal estuary along the southwest coast of India. Sampling conducted in seven stations from the inlets to the upstream during the pre-flood, flood, and post-flood periods revealed conspicuous changes in the zooplankton community of the estuary, especially the rotifers. The decline in the rotifer abundance from pre-flood (17.6 ± 26.3 ind 100 m−3) to flood (1.3 ± 2.6 ind 100 m−3) followed by a sharp increase in the post-flood period (86 ± 129.8 ind 100 m−3) indicated the impact of the flood on the rotifer population. A conspicuous change was also noticed in their community structure with the genus, Keratella dominating during the low saline, pre-flood, and flood periods and the genus, Brachionus and specifically the species, Brachionus plicatilis dominating during the relatively high saline post-flood period. The species, Filinia longiseta var. acaudata (Filinia saltator, Gosse, 1886), hitherto not recorded from the study area, was observed exclusively after the flood. Cochin estuary (CE), being a monsoonal estuary where the high freshwater discharges associated with monsoon results in the washing out of a majority of the rotifer population, this scenario was much more aggravated by the massive flood that occurred during August 2018. However, with the reduction in the floodwater discharges, the concomitant increase noticed in the rotifer abundance indicates their rapid adaptation and proliferation strategies towards favourable hydrographic conditions in the estuary.