Summary
A mass encystment of Mallomonas pseudocoronata cells was observed in a polymictic lake from Southern Ontario, Canada. Scanning electron microscopy was used to describe its statospore, which is oval in shape, covered by short spines, and has a short cylindrical collar.
Methodology
Site description. Lake Opinicon (44"33'N, 76'26'W) is a 9.2 km long, shallow (mean depth = 3.0 m, max. depth = 9.1 m), meso-eutrophic lake in Southern Ontario, Canada. The approximately 60 km2 catchment basin is underlain by crystallized limestone to the north and metamorphosed granite, primarily gneisses and schists, to the south. A small outcrop of Ordovician limestone is present on the east shore (Wynne-Edwards 1959). Due to the low re- lief of the surrounding area and the shallowness of the basin, the lake is polymictic throughout its ice-free period. The limnological data, at a depth of 1 m and re- corded on 29 Sep were: water temperature = 16.5'C; specific conductance = 150 pS/cm2; pH = 8.0; alkalinity = 65 mg CaCOJl; dissolved oxygen = 9 mg OJI; and secchi depth = 3.5 m. Because the lake is polymictic, these values do not change significantly throughout the water column. The phytoplankton was dominated by blue-green algae (e.g. Microcystis aeruginosa, Anabaena spp.) and diatoms (Asterionella formosa, Fragilaria crotonensis). Dinobryon bavaricum was also common. Sparling & Nalewajko (1970) and the Ontario Water Resources Commission (Anon. 1971) included Lake Opinicon in their water quality surveys of Ontario lakes. As summarized in Crowder et al. (1977), these in- vestigations noted the following ranges in chemical properties for the lake water at a depth of 1 m: conductivity = 182-202 pS/cm2; pH = 7.7-8.5; dissolved oxygen = 10.7-15.2 mg/l; free CO, = 0-2.4 mg/l; alkalinity = 79.5-92.5 mg CaCO,/I; total kjeldahl nitrogen = 0.39-0.43 mg/l; total phosphorus = 0.014-0.026 mg/l; potassium = 11.7 mg/l; sodium = 2.5 mg/l; calcium = 192 mg/l; magnesium = 24.3 mg/l; and chloride = 4-8.2 mg/l. Materials and methods. The samples reported on in this paper were collected from the middle of Lake Opinicon on 29 Sep, 2 Oct, 8 Oct, 1979. A tow net (50 pm mesh), pulled behind a slow moving boat, was used to concentrate a large quantity of cells. The collections were immediately fixed with Lugol’s preservative. For light microscopy, an aliquot of the algal sample was rinsed in distilled water and a few drops were dried onto glass cover slips that had been placed on a drying table set at low heat. When dry, the cover slips were mounted on glass slides using Hyrax, a permanent mounting medium with a high refractive index. A total of 20 slides were prepared in this manner. Each slide was first scanned at low power. Once the postition of a statospore was recorded, it was re-examined at x 1 500. A total of 65 statospores were studied, 53 of which were lying in side view and could be measured for length and width determinations. The processing of samples for scanning electron microscopy essentially followed Cronberg (1980). In sum- mary, a few drops of the algal sample were dried on a glass cover slip, mounted on a specimen stub, and gold plated with a Polaron sputter coater. The statospores were then studied using a Hitachi-S-450 SEM.