Miroslav Macek

Planktonic ciliates occasionally form brief rapid increases in numbers (blooms) that can be trophically important. Although model simulations and mesocosm studies indicate that blooms occur over 10 to 20 days, field data are rarely suffi ciently detailed to reveal their occurrence and demise. Our data (collected over 57 weeks across a coastal lagoon) offer insights into the population dynamics of a single species, place these in the context of the entire ciliate assemblage, and provide guidance on what should continue to be examined. Specifically, to evaluate population dynamics we examine two species of Cyrtostrombidium, characterise temporal and spatial variation of their abundance, and relate these to abiotic phenomena and biological factors. This is also the first report of Cyrtostrombidium in a tropical coastal lagoon. Collectively our analysis reveals key aspects of the dynamics of this genus: 1) small-scale peaks in abundance are ~30 m in size and can persist for ~10–30 days, reaching a maximum of 100 cells ml–1; 2) these increases are driven by biotic factors (revealed through autocorrelation analysis); 3) long-term trends are driven by the shift between dry and rainy seasons and by the periods of isolation of lagoon from the sea (revealed through multiple regression analysis); 4) blooms may at times control primary production; 5) conjugation, an ecologically important event, may be associated with blooms (at times 9% of population was conjugating); and 6) dinoflagellate parasitism, poorly described in oligotrichs, is potentially important in population demise. These results both reflect on how ciliates may behave in short-term events and should encourage the continued need for detailed observations of field samples at a high taxonomic resolution.
 

The pelagic / anoxic hypolimnion population of Spirostomum teres was investigated as a part of the long-term ciliates’ monitoring (2003–2016) in an oligo- to mesotrophic monomictic hyposaline crater lake Alchichica (Puebla / Veracruz, Mexico), including an analysis of picoplankton (both heterotrophic, HPP and autotrophic, APP) and inorganic compounds of nitrogen (ammonium, nitrite, nitrate), phosphorus (dissolved reactive phosphorus, DRP) and silicon. Additionally, detailed studies of the ciliate vertical distribution and feeding activity measured upon fluorescently labelled APP (picocyanobacteria) were carried out. The results were compared with those from a neighbour freshwater crater lake La Preciosa and with a meromictic karstic lake La Cruz (Cuenca, Spain). The ciliate vertical distribution within the water column was very well defined: During the first decade, the benthic population was frequently found throughout a developing stratification of the lake. The established stratification of the lake turned the conditions favourable for the formation of an oxycline / hypolimnion population, typically, several meters below the deep chlorophyll maximum (formed basically by diatoms); the population preferred the layers without detectable dissolved oxygen. However, an observed gradient of light (PAR) could support both oxygenic and anoxygenic photosynthesis. Late stratification after deepening of the thermocline reduced the layers with S. teres population to a minimum apparently due to the drastic change in physicochemical conditions within a metalimnion, coupled with an oxycline, and limited to 1 to 2 meters; microstratification was found. Last years, the very bottom population disappeared or it was reduced and the late stratification S. teres peaks were smaller or did not appeared. Generally, S. teres oxycline / anoxic hypolimnion population was observed from June through November. Optimum picoplankton numbers in conditions that supported the ciliate growth were found: The ciliate was peaking at APP of 0.6 to 1 × 105 cells mL–1; the optimum of HPP was observed round 1.4 × 106 cells mL–1. S. teres was efficiently feeding upon picocyanobacteria in numbers of 105 cells mL–1 reaching the clearance rate of 2000 nL cell–1h–1, which represented in average 130 to 210 cells cell–1h–1 ingested. Feeding upon purple sulphur bacteria was observed but only during the end of the lake stable stratification when the ciliate population was already dropping. On the other hand, the volume specific clearance of S. teres upon picocyanobacteria (103 h–1) did not support the hypothesis that they could serve as a sole prey. Feeding upon eukaryote phytoplankton (chlorophytes Monoraphidium minutum, diatoms Cyclotella choc tawhatcheeana) could be of higher importance that previously supposed. Additionally, a use of ingested and retained photosynthetic prokaryotes is hypothesized.