Seasonal variation of vegetative stages and cysts of planktonic ciliates in a temperate coastal bay

Abstract

Abstract Marine planktonic ciliates serve as a key component of the plankton food web. Their vegetative stages have frequently been studied, whereas their cyst stages have largely been ignored. Seasonal encystment and excystment of ciliates has great implications for population persistence, maintenance of biodiversity and stability of ecosystem processes. Here, we investigated seasonal variation of planktonic ciliate cysts in surface sediments of Bohai Bay, and explore the relationships between cysts and environmental factors. Eight types of aloricate ciliate cysts were identified and only two species of tintinnid cysts were recognized, namely, Favella sp. and Helicostomella longa. Abundance of ciliate cysts was lowest in winter, and highest in summer. There was a significant positive correlation between abundance of ciliate cysts and vegetative cells, which supported the viewpoint that encystment could be induced by high population density. The species composition of ciliate cysts exhibited obvious seasonal variation, and water temperature was the key driving factor. The natural encystment event of ciliates may be triggered, enhanced and terminated by a rapid variation of temperature. Our study exhibited that ciliate cysts—served as a “seed bank” for vegetative populations—were common in temperate coastal waters, and seasonal changes of ciliate cysts are crucial to vegetative population dynamics.