36ertical distribution of planktonic rotifers in a karstic meromictic lake
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Hydrobiologia255/256:381-388, 1993.J. J. Gilbert, E. Lukens & M. R. Miracle (eds), Rotifer Symposium VI.
381© 1993 Kluwer Academic Publishers. Printed in Belgium.
Vertical distribution of planktonic rotifers in a karstic meromictic lake
Javier Armengol-Diaz, Angeles Esparcia, Eduardo Vicente & Maria Rosa Miracle
Area of Ecology, Universitat deValencia, 46100-Burjassot, (Valencia)Spain
Key words:rotifers, meromixis, vertical distribution, oxycline, karstic lake
Abstract
Vertical distribution of planktonic rotifers is described in relation to temperature and oxygen in Lake
La Cruz, a single-doline, closed karstic lake (121 m diameter and 25 m maximum depth) which showsiron meromixis. Samples were taken by peristaltic pumping at 10 cm depth intervals in the oxycline zonefrom June 1987 to September 1988. A model of rotifer vertical structure in stratified lakes is proposed.Rotifers concentrate their populations at the depths with intense gradients. As stratification develops
some rotifer populations show a downward migration following the thermocline and some others showan upward migration following the oxycline. The production-respiration balance in the lake, and so theposition of the oxycline with respect to the thermocline and the layer of maximum production, depends
on meteorological conditions. A shift in the dominance of congeneric or related species can occur inconsecutive years. In Lake La Cruz, mixing conditions and subterranean inflow in spring were much
more intense in 1988 than 1987, and the distance between production and decomposition depths wassmaller in 1988.Anuraeopsis miraclei,an oxycline-bound species with high abundance in 1987, wasdisplaced byA. fissain 1988.A. fissa,which was a metalimnetic species during early summer, reached
peak densities (3 x 104ind 1-1)at the oxycline, equaling the abundance ofA. miracleithe preceeding year.
Introduction
The planktonic rotifer community in highly strat-
ified lakes shows a distinct vertical structure,which is very much related to the thermal and
oxygen concentration gradients (Larsson, 1971;Ruttner-Kolisko, 1980; Hofmann, 1987; Miracle
& Vicente, 1983; Mikschi, 1989). In this paper we
compare the vertical distribution of rotifers in a
meromictic, karstic lake in two consecutive butmeterologically different years. We discuss howthe different mixing conditions in spring overturn
and the differences in subterranean inflow from
year to year can affect the rotifer populations ofthis highly stratified lake. The observed shift of
dominance in these years between congenericspecies, occurring in the oxic/anoxic boundary,indicates the importance of interannual metereo-
logical variations to planktonic rotifer assem-
blages.
The lake
Lake La Cruz (UTM 30 SWK 962272) is a small
dissolution lake, located at 1000 m altitude, inan important limestone karstic area near the
town of Cuenca (Spain). It is a closed lake sunkin a circular depression with a mean diameter of
121 m and a maximum depth in the center of24.5-25.5 m, depending on the season. The lakelevel and flow of water into the lake depends
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on the water table. The lake is believed to havea long water renewal time, which favors stratifi-
cation.
Its morphometry, i.e. small surface/depth ratio,
and its location inside a dissolution basin havingsteep vertical walls rising 20-30 m above the
water surface, produces an important reductionof wind mixing. The stability of the water layers
is also enhanced by density gradients due prima-
rily to dissolved ferrous bicarbonate and other
ions common to dissolution lakes. Iron mero-
mixis has been described in this lake (Vicente &
Miracle, 1988). A sharp redoxcline was estab-
lished at 14 to 20 m, depending on the season,with a completely anoxic bottom layer of water.
Methods
Water samples were collected in the center of thelake with a peristaltic pump connected to a thin
layer inlet sampler (Miracle & Vicente, 1985) at
different depths, indicated in Fig. 2 by horizontal
lines in the vertical profiles. The oxyclinal zone
was sampled at 10 cm depth intervals. Zooplank-
ton was concentrated by filteringin situ2 to 4 li-
ters of water through 30 um nylon mesh and pre-
served in 4 % formaldehyde. All the organisms in
each sample were then counted with an invertedmicroscope at 200 x magnification. Water sam-
ples were also taken with 2.7 liters transparent
Van Dorn bottles from the epilimnion to completethe data. No significant differences in rotifer den-
sities were observed between these bottle samples
and those collected by pumping. To confirm that
rotifers were alive in microaerobial layers, fresh
samples were also observed. Temperature and
