Mixotrophic ciliate dynamics in two zones of a temperate and highly turbid estuary in South America, Argentina

摘要

Mixotrophy is a feeding strategy by which some organisms combine autrotrophic and heterotrophic activities. The seasonal dynamics of the fve mostcommon mixotrophic ciliates were studied monthly in surface layers of the inner and middle zone of the Bahía Blanca estuary, a nutrient-rich, shallowand highly turbid environment in Argentina, from January to December 2009. Temperature, salinity, turbidity and chlorophyll a were recorded, as well asthe abundance and biomass of Strombidium capitatum, Strombidium acutum, Cyrtostrombidium sp., Lohmanniella oviformis and Tontoniaappendiculariformis. The highest mixotrophic ciliate abundance was recorded during the austral winter (June-July) in the inner zone, meanwhile in themiddle zone of the estuary the presence of these ciliates was almost nuil. The highest chlorophyll contribution derived from mixotrophic ciliates to totalchlorophyll a ranged from 6% in the inner zone to 23% in the middle zone, both registered in autumn. The low abundance of mixotrophs in the middlezone of the estuary during the winter, as is usually observed in other coastal ecosystems, could be explained through a higher grazing pressure in thiszone (top-down control) by mesozooplankton (e.g., copepods) in comparison to the inner zone. The secondary bloom of phytoplankton consistentlyobserved during the last summers dominated by small sized diatoms and nanoplankton phytofagelates, could have sustained the high abundance ofmixotrophic ciliates registered in the middle zone in autumn and summer.