Population dynamics of oxiclinal species in lake Arcas-2 (Spain)

Abstract

Oxiclinal' rotifer species show high concentrations just above the oxic-anoxic interface in the hypolimnion of some lakes . The stratification of their populations is best shown by sampling at close depth intervals and quantifying their densities by the Utermohl technique . With this technique we were able to count males which otherwise pass through filters and more accurately count egg production . We evaluated female, male and egg numbers of the two main oxiclinal species of lake Arcas-2 : Filinia hofmanni and Anuraeopsis fissa, during two annual cycles (1990-91) . F hofmanni was an exclusive oxiclinal species . It had an exponential growth phase at the onset of stratification giving a distinct spring peak . The population then maintained a high density during summer, but was almost absent the rest of the year. This cycle is repeated annually but population density can vary among years, depending on winter-spring circulation . Sexuality was always observed when the animal was present in the samples, with a maximum of males and resting eggs at the peak of the population . Resting eggs were always inside females . The annual cycle of A . fissa is displaced with respect to that of F hofinanni : A . fissa attained greatest densities during summer, until the autumn overturn . Mixis in A . fissa was restricted to the end of the stratification period . Moreover, A . fissa occured throughout the vertical profile and secondarily occupied the oxicline .

Diapause in Monogonont Rotifers

“…Very high mictic ratios (close to 100%) seem to be rare in natural populations; they have been reported in B. plicatilis (Aparici et al, 1996), in B. urceolaris (Buchner, 1992), and in B. calyciflorus (Pourriot, 1965). More often, mictic ratios are around 20-30% Miracle & Armengol-Dı´az, 1995). Field data indicate that mixis already occurs at much lower density levels than the laboratory data would suggest as threshold levels Schro¨der, 2001).…”

Vertical Distribution of Zooplankton in a Strongly Stratified Hypertrophic Lake

“…This pattern was apparently a result of avoiding the predation pressure of fish and Chaoborus larvae, as well as of the abundance of food. Converging of metazooplankters in a distinct zone of the metalimnion has been described earlier for rotifers (Miracle & Armengol-Diaz, 1995), as well as for cladocerans (Williamson et al, 1996). Anoxic conditions were established in the depth of 2 m in 1998 (the water level was lowered by 0.7 m during the reconstruction of shores for swimming pool) and 3-3.5 m in 1999 (Fig.…”

4. Rotifera

“…In stratified lakes, the vertical distribution of rotifers is strongly related to the season, and the thermal and oxygen gradients, with some species also showing diurnal vertical migrations (e.g., Larsson 1971, Ruttner-Kolisko 1980b, Armengol-Díaz et al 1993, Miracle & Armengol-Díaz 1995. In stratified lakes, the vertical distribution of rotifers is strongly related to the season, and the thermal and oxygen gradients, with some species also showing diurnal vertical migrations (e.g., Larsson 1971, Ruttner-Kolisko 1980b, Armengol-Díaz et al 1993, Miracle & Armengol-Díaz 1995.…”