Seasonal dynamics of free and attached heterotrophic nanoflagellates in an oligomesotrophic lake

Abstract

1. The seasonal development of heterotrophic nanoflagellates (HNF), bacteria, rotiferans and crustacean zooplankton was studied in the epilimnion of Lake Pavin, an oligomesotrophic lake in the Massif Central of France. 2. HNF abundance varied from 0.1 to 2.5 × 103 mL–1. Free‐living HNF reached their highest density in spring when the copepod Acanthodiaptomus denticornis dominated the metazooplankton. They were present in low numbers when rotifers and cladocerans were numerous. 3. Attached HNF, consisting of bicoecids and choanoflagellates, were fixed to large diatoms and to the colonial cyanobacterium Anabaena flos‐aquae. The abundance of attached HNF was significantly correlated to bacterial abundance, which fluctuated between 1.1 and 2.7 × 106 mL–1. Highest abundance of these epiphytic protists was recorded when free‐living heterotrophic nanoflagellates declined. 4. The comparison of the dynamics of heterotrophic nanoflagellates, bacteria, and the impact of zooplankton grazing suggested that prey abundance, the presence of suitable attachment sites and limited competition from the free‐living forms were the main factors controlling the development of the epiphytic flagellate protists. In contrast, the low abundance of free‐living forms during the period of rotiferan and cladoceran development suggests the prevalence of a top‐down control by predation of the metazoopankton.

Phagotrophic ciliates and flagellates in an oligotrophic, deep, alpine lake: contrasting variability with seasons and depths

“…In total, numbers of HF in Traunsee were within the range reported for oligo-to mesotrophic water bodies (e.g. Carrias et al 1998b, i.e. between 40 and 2900 ml -1 for the present study.…”

“…Spumella spp., are known to be efficient bacterivores, able to consume around half of the bacterial production (Weisse 1990, 1991, Simon et al 1998. A coincidence of bacterial and HF densities and the predominance of HF < 5 µm in length has already been reported for Lake Constance (Weisse 1990, 1991, Cleven & Weisse 2001, Mondsee (Salbrechter & Arndt 1994) and Lake Pavin (Carrias et al 1998b). However, the relationship between HF and bacteria seems to be stronger in systems with low bacterial abundance compared to systems with higher bacterial numbers (Gasol & Vaqué 1993).…”

“…Free-living ciliates, namely Pelagohalteria viridis, halteriids and rimostrombidiids were most likely able to resist grazing pressure by their jumping behaviour (Gilbert 1994, 203 review of Jürgens 1994, Wickham 1995. In contrast, epiphytic ciliates gained an advantage by their attachment to diatom colonies, thus becoming inedible for zooplankton predators (Carrias et al 1998b, detailed discussion see below (Weisse & Frahm 2002). Then again B. planctonicum increased in abundance in July and had its maximum in autumn.…”

Stuck in the mud: suspended sediments as a key issue for survival of chrysomonad flagellates

“…Colorless chrysophytes are an often dominating component of the microbial food web (Salbrechter & Arndt 1994, Carrias et al 1998, Finlay & Esteban 1998. Suspended clays and silts are also common in many lakes worldwide (Dokulil 1979, Gliwicz 1986, Threlkeld 1986, Hart 1988, Knowlton & Jones 1995.…”

“…Annually, on average, 20 to 50% of the pelagic heterotrophic nanoflagellate (HNF) biomass in freshwaters is formed by small heterokont taxa, mainly colorless chrysophytes (i.e. chrysomonads) and bicosoecids (Salbrechter & Arndt 1994, Carrias et al 1998.…”