A study of the freshwater dinoflagellates Ceratium hirundinella and Ceratium furcoides with special reference to their taxonomy and recent history in the Lake District
Motile cells and cysts of Ceratium were studied usinglight and scanning electron microscopy. Two species, C. hirundinella and C. furcoides, were distinguished on the basis of the cell length to breadth ratio, the shape of the epithecaand the arrangement of apical plates. Cysts differed in shape and in the length of the horns. Analytical scanning electronmicroscopy demonstrated the presence of silicon in the multi-layered granular wall of the cysts of both species.The vertical distribution of viable cysts of C. furcoides and C. hirundinella was studied in 8 cm cores taken over threeseasons from Esthwaite Water, Cumbria. The diatom Stephanodiscus parvus was used as a marker species, to attribute a time scale to the cores. Some agreement was demonstrated between cyst numbers and past populations of Ceratium spp., although the majority of cysts occurred in the upper 4 cm of the cores. A study of the proportion of C. hirundinella to C. furcoides cells from 1946-1986 showed that the ratio of each species changed markedly over this period. It was concluded that the relative numbers of each species were determined by the proportion of cysts germinating in the spring, with parasitism an important factor in controlling cyst viability.Germination of the cysts of both species was induced in the laboratory. The excystment of C. furcoides was achieved down to a depth of 5.5 cm, from cysts with an equivalent age of approximately 7 years. Cysts from sediment which had been leftto dry out failed to germinate.The Ceratium populations of several southern sites were also studied. The number of Ceratium cells was shown to decline when the water column was disturbed, either by the input of water through high velocity jets, or the failure to form a stable thermocline. C. hirundinella was observed to be the more numerous of the two species in the reservoirs studied, but in Virginia Water Lake C. furcoides occurred in greater numbers. <p
