Daphnia galeata in the deep hypolimnion: spatial differentiation of a “typical epilimnetic” species

Abstract

Daphnia galeata is traditionally regarded to be a non-migratory species, which lives in warm epilimnetic waters. Depth segregation or vertical migration is usually attributed to other Daphnia species such as D. hyalina or D. longispina. In a two-year study, we found that in a deep, dammedvalley reservoir (Ř ímov Reservoir, Czechia) the majority of the population of D. galeata lives in the warm epilimnetic waters during the summer months, but some specimens of this species could be always found in the deep strata as well. This hypolimnetic subpopulation stays in the cold hypolimnetic water and does not migrate. The abundance of hypolimnetic D. galeata does not exceed one specimen per litre and usually shows seasonal variation (minimal densities in early spring, maximal in late summer). Using allozyme electrophoresis, we found that the subpopulation from the deep hypolimnion was clearly genetically differentiated from the population in the epilimnion. We found significant differences in both allele and multilocus genotype frequencies; the F ST values at most sampling dates exceeded 0.05. However, the spatial segregation between the epilimnetic and hypolimnetic subpopulations is not permanent. The reservoir is dimictic and hence, at least twice per year, all vertically segregated parts of the population are mixed together. Our results suggest that the deep hypolimnetic subpopulation is repeatedly re-established in spring by deepwater ''colonists'', at least some of which seem to be ecologically specialised for the hypolimnetic conditions, and dominate the hypolimnion by the end of the season. The genetic differentiation is likely the result of both the different depth preferences of various D. galeata clones and different selective pressures in the epilimnion and hypolimnion.

Sinusoidal swimming in fishes: the role of season, density of large zooplankton, fish length, time of the day, weather condition and solar radiation

“…This assumption is well supported by Vašek & Kubečka (2004) who have described circadian feeding activity of adult common bream and roach with clear afternoon peaks and distinct nighttime declines in gut fullness. Sinusoidal swimming is restricted to daylight hours and to the epilimnion of the reservoir (Č ech & Kubečka, 2002, this study) where there are appropriate light (Č ech et al, 2005), temperature, oxygen (Bohl, 1980;Prchalová et al, 2008;Vašek et al, 2008) and feeding conditions (Hudcovicová & Vranovsky, 2006;Seďa et al, 2007). Only 51 individuals out of 1,875 sinusoidally swimming fish performed this behaviour outside daylight hours.…”

Pelagic underyearling communities in a canyon-shaped reservoir in late summer

“…For investigation of the relationship between zooplankton biomass and fry density only zooplankton retained by the 0.71 mm sieve was used, because this size fraction, which consisted almost entirely of large cladocerans (Daphnia, Leptodora), contained the main dietary components of the dominant species of fry in Římov Reservoir during late summer (Vašek et al 2006). Note that nearly all the zooplankton biomass, although expressed per square metre, was actually concentrated in the upper 5 m layer in the 40 meters deep reservoir during the summer (Seďa et al 2007). …”

Daphniahybridization along ecological gradients in pelagic environments: the potential for the presence of hybrid zones in plankton

“…Vertical gradients are characteristic of thermally stratified water bodies such as temperate lakes (figure 1). Hybridizing Daphnia taxa may indeed show different spatial distributions within these gradients ( Weider & Stich 1992;Seda et al 2007b), and intraspecific genetic differentiation on a vertical scale has been documented as well (Seda et al 2007a). The vertical segregation of taxa may be caused by variable selection pressures in different depths or by individual spatial preferences.…”