Seasonal dynamics, composition and feeding patterns of ciliate assemblages in oligotrophic lakes covering a wide pH range

Abstract

Seasonal changes in the structure of ciliate assemblages in eleven oligotrophic mountain lakes at different altitudes, covering a wide pH range from 4.9 to 6.9, were studied. Seven mountain lakes lay above the timberline (0vre NeÅdalsvatn and Stavsvatn, Norway; Lochnagar, Scotland; Starolesnianske pleso and Nizne Terianske pleso, Slovak Republic; Chuna ozero, Russia; Lago Paione Superiore, Italy) and four acidified lakes (Cerne, Certovo, Plesne and Prasilske jezero, Sumava Mountains, Czech Republic) in the mountain forest. Additionally, thr tropical high latitude Lago de Alchichica (Mexico) was analysed. Ciliate taxons were identified using the quantitative protargol staining approach, and feeding patterns were tentatively detected by using fluorescence microscopy methods. Nano- to microphytoplankton hunters and/ or mixotrophic ciliates prevailed in all acidic lakes. Numbers of ciliates were very low (seasonal lake mean below 200 cells/I) except in two Tatra lakes (to 21000 cells/I). Prostomes of the genera Urotricha, Holophrya and Prorodon dominated in most of the samples, both numerically as well as in biomass (seasonal lake mean from 13.6 to 100% and 4.6 to 99.7%, respectively), particularly in acidified water lakes (Starolesnianske, Certovo and Prasilske jezero). Among mixotrophs, oligotrichs of genera Pelagostrombidium and Limnostrombidium were the most prominent (up to 49.9 and 64.6%, respectively). The picoplankton-feeders (minute oligotrichs, peritrichs and scuticociliates) were found to be an important component even in the oligotrophic environment but this ecological type never dominated within the water column (up to 22.2 and 30.3 %, respectively). Major food sources of gymnostomes, prevailing only in Lochnagar (82.5 and 87.5 %, respectively; Mesodinium sp.), remained unclear; large Askenasia spp. were apparently mixotrophic. Using a cluster analysis, the lakes were grouped according to the total numbers of ciliates and contributions of distinct ecological groups with different feeding patterns. Within low acidic lakes, Lochnagar differed from all others. The rest of the lakes was divided into two subgroups: remote pristine lakes of the Northern transect (the Norwegian lakes and Chuna ozero), and the others, geographically affiliated to the south transect (Pyreneans, Alps, Tatra Mts.). Such a differentiation is in good agreement with the clustering based on chemical parameters reported for the lakes. No marked differences in the ciliate distribution were detected when the above timberline- and forest surrounded mountain lakes were compared.