Summary
The submerged macrophyte community of Lake Opinicon was sampled by a combination of quadrats and line transects in 1972 and 1973. Mean organic biomass was 248 g -m2 with a range of 1 to 1 154 g -m2 : biomass decreased significantly with depth. The most frequent plants were Chara globularis, Lemna trisulca, Vallisneria americana, Potamogeton robinsii, P. friesii, Myriophyllum spp., Najas flexilis, N. guadalupensis and Ceratophyllum demersum, which were all positively associated. N. guadalupensis and C. demersum occurred in deeper waters. Time to flowering varied considerably from species to species. A comparison with the 1947 survey of Curran showed no major changes (Myriophyllum spicatum has however increased since 1974). A checklist of the macrophytes of both the deep and the shallow water is appended.
Methodology
Fifteen samples were collected for estimation of biomass in 1972, and 22 in 1973; the sampling positions are shown in Figure 1. Scuba divers collected the plants from a 25 x 25 cm quad rat in a plastic bag, the plants were then oven-dried and their organic weight determined, according to the method of Craig (1976).
Two sampling methods were used in studying frequency, abundance, and distribution. The first was the collection of samples of vegetation from points arranged in a grid pattern throughout the lake. All plants inside a 25 x 25 cm quadrat were collected in a plastic bag by a diver, brought to the surface, identified, counted and measured. At the same sites sediment samples were collected, and depth, temperature and Secchidisc levels were measured. These sampling points under-represented the shallow-water area of the lake, — only 10% of the samples were in 0-1,5 m depth whereas 45% of the lake bottom is at that depth.
The second method used in analysing the community was a form of point transect. This was adopted because it provided a basis for the comparison of sites at different times and because it gave a high return of information for diving time, and could be used in opaque conditions when the divers depended on the transect line to locate them selves.