Quantification of floating ephippia in lakes: a step to a better understanding of high dispersal propensity of freshwater plankters

Planktonic cladocerans of the genus Daphnia are an example of organisms whose ability to disperse among aquatic habitats is limited to dispersal of dormant eggs, encapsulated in protective structures called ephippia. In the present study, we aimed to quantify the production of floating and sinking ephippia in lakes to test the hypothesis that, even if they eventually sink, most of them are first floating at the surface. In addition, we checked the egg content status of the ephippia. The results of this study revealed numbers of ephippia floating at lakes surface reaching thousands per square metre at the time of ephippia production, and constituting substantial share of that production. In studied shallow lowland lakes and in a deep mountain lake, most ephippia were first floating at the water surface, while in deep lowland lakes the proportion was reversed. Approximately half of ephippia that appeared initially at the water surface sank during the six-week sampling period. The egg content status did not differ between floating and sinking ephippia. High numbers of ephippial females staying in the surface layers at night in a mountain lake, and a laboratory experiment with ephippial females indicated that ephippia are actively oviposited to the water surface by gravid females.