Summary
The exotic macrophyte Myriophyllum spicatum spread widely in Lake Opinicon, Ontario, after 1974, dominating the 2.0- to 3.5-m depth zone which had previously been largely open water. In the shallows it formed beds within the species-rich native Potamogeton–Vallisneria community. This paper compares fish and prey-invertebrate abundance and distribution (i) before and after the milfoil invasion for a series of depth zones (1972 relative to 1979–1980), and (ii) in communities of M. spicatum relative to native macrophytes in the littoral zone shallows in the summer of 1980. At a depth of 2.0–3.5 m (major M. spicatum zone), there were significantly fewer bluegill but more black crappie and golden shiner in 1979–1980 compared with 1972, and significantly fewer isopods, chironomid and trichopteran larvae, and ephemeropteran nymphs. Differences were greatest in late summer. Inshore and offshore from the M. spicatum zone, differences were minor, suggesting that the invader, not time, was responsible. At a 1.0- to 1.3-m depth in the summer of 1980, three to four times as many fish occurred during the daytime feeding period in the Potamogeton–Vallisneria community than in the M. spicatum area. In the benthos, beneath the native beds, five major taxa of prey invertebrates were three to seven times more abundant. The foliage of Potamogeton plus Vallisneria supported twice as many invertebrates per square metre in late summer than did M. spicatum. Insect emergences over the summer (May–September) were twice as high. Up to the present, the net impact of the habitat change on the fish populations of Lake Opinicon has not been great. If in the future M. spicatum were to replace the rich native macrophyte beds in the shallows, this would be cause for concern.
Methodology
The study was carried out in Lake Opinicon, Leeds County, Ontario, latitude 44'30' N longitude 76'30' W. This eutrophic lake, in the central part of the Rideau system, has an area of 890 ha, a maximum depth of 11 m, a diversified shoreline, and extensive shallow areas. Since it has previously been the site of extended fish investigations (e.g., Keast 1965, 1978a), it was a good candidate for the present study. Cow Island Bay, on the north side of the lake and adjacent to the Queen's University Biological Station, formed the study site (Fig. 1). Water depths were determined by map contours and lead lines. Plant species composition and dry weight biomass (grams per square metre) were determined on the basis of series of 10 quadrats each 1 m2. Divers gathered the material and brought it to the surface in plastic bags. It was washed, identified to species, and the roots and foliage were separated and dried for 24 h at 60°C. Species diversities were determined using a modified Simpson index (Rotenberry 1980). The substrate material was sampled by divers equipped with Ekman grabs. Particle sizes and organic content was determined by the method of Brady (1974). The United States Department of Agriculture particle size classification was used. However, the substrate beneath M. spicatum beds proved to be of fine suspended matter that did not lend itself to this method of particle size analysis. Subsurface water temperatures at a depth of 0.4 m were taken throughout the season in 1980 (using a Wm. H. Lambrecht continuously recording thermometer mounted on a stand 2 m from the shoreline) for a comparison of plant growth and insect emergences. Temperature and dissolved oxygen (parts per million) profiles were taken from the surface down to 7.0 m. They were based on a series of five determinations at each depth. They covered areas of M. spicatum as well as open water. A Yellow Springs Instruments temperature and oxygen probe was used.