EXPERIMENTAL STUDY ON THE ECOLOGY OF LONGIROSTRIS (O. F. MÜLLER) ().

BY

W. R. BHAJAN 1) & H. B. N. HYNES Department of Biology, University of Waterloo, Ontario, Canada

During a study of the zooplankton of Sunfish Lake, Waterloo County, Ontario, Canada, which has been described by Duthie & Carter (1970), it was found that numbers of Bosmina longirostris (0. F. Miller) fluctuated greatly and that the species was sometimes absent from samples for months. However, during the four years 1966 to 1969 populations were always present during May and June, and they rose to densities of about 10/litre. During most of the summers the were scarce and resting eggs (ephippia) could usually be found; and during the summer of 1967 the animals were very rare. They were similarly scarce during the winters, except for that of 1966 to 1967 when they were abun- dant (20-50/1). Such irregular occurrence of Bosmina has been observed elsewhere (Hutchin- son, 1967), but several workers cited by him and others also (e.g. Smyly, 1957; Patalas, 1956; Mleczko, 1965) record spring maxima. Yet other investigators record maxima in both spring and fall (Ischreyt, 1926; Chandler, 1940; Davis, 1962), spring and winter (Rylov, 1935), spring and summer (Borecky, 1956; Hrbacek & Novotnd-Dvofdkovd, 1965) or only summer (Komarovsky, 1959). Such variations in the findings are puzzling and the reasons for them are cer- tainly complex. However, our field work in Sunfish Lake also indicated that in their vertical distribution, which changed with the seasons, the occurred mostly where the water was cool and well oxygenated. Indeed, studies made on vertical migration during 24-hour periods during June 1967, and May and June 1968 showed that Bosmina remained in cool water in or below the thermocline where temperatures did not exceed 9, 10, or about 11 °C on the three occasions. Indications were therefore that temperature is at least one important factor in the life of this species, and it was decided to investigate this in the laboratory. The species had not before been brought into culture and many attempts to do

1) Present address: Department of Health, San Juan, Puerto Rico. 134 so previously have failed. This is because the animals adhere very readily to the surface film, but the development by Greve (1968) of his Planktonkreisel seemed to offer possibilities.

MATERIALSAND METHODS

The original stock of animals obtained from the lake was identified as Bosmina longirostris (0. F. :Nlüller). There is, however, so much uncertainty as to the status of this species (Hutchinson, 1967) that we should add that our material appeared to be B. 1. sirnilis Lilljeborg, and is typical of the North American type described by Austin (1942). Our version of Greve's apparatus is illustrated in fig. 1 and was made of glass,

Fig. 1. A Planktonkreisel unit with the buried portions seen as if the gravel and sand were transparent.

an air-stone, quartz pebbles and sand. Bubbles from the air-stone raised water in the inner tube by suction into the funnel through the sand and gravel. The water then returned via the small tube to the sand surface. In this way a slow circulation with a wide downward trend was maintained. We had 20 such units which were variously maintained in a refrigerator (6°C), a cold room (11°C) and room temperature (ca 21 °C). Light periods were normally 12 hours light 12 dark, except as described below. The water was filtered lake water, 1.5 1/unit, and in most experiments the animals were fed by the addition every three days of about half a million cells of a mixture of Chorella vulgaris and C. ellipsoidea together with detritus centrifuged from cultures and resuspended in 1 ml of deionized water.