Nova Scotia Lake Hypolimnion Project
SUMMARY
A major limiting factor for book trout (Salvelinus fontinalis) in Nova Scotia is the presence of suitable cold-water habitat during summer. In an attempt to develop a model that could be used to predict the type of lake most likely to contain cold-water fish habitat during summer, 20 lakes distributed over a wide geographic area within Nova Scotia were surveyed during July and August 2001. The lakes were chosen on the basis of data contained in the Province’s lake survey database, and included 11 lakes that contained cold-water habitat and 8 lakes in which cold-water habitat was absent during the time of the original survey. Suitable cold-water habitat was defined as water temperature ≤15 °C and dissolved oxygen saturation ≥50 %. The major parameters measured during the 2001 survey were indices of trophic state (total phosphorus concentration, chlorophyll a concentration and Secchi Disk depth), and water temperature and dissolved oxygen depth profiles.
The results indicate that there was relatively little difference between the July and August surveys in determining the status of a lake with respect to the presence of cold-water habitat. There was, however, considerable difference between the status of the lakes based on the original survey data, some of which was collected more than 25 years ago, and the data obtained during the 2001 surveys. Only two of the lakes surveyed during 2001 contained suitable cold-water habitat. It was not possible to determine conclusively, based on the data available, if these changes are a result of changes in trophic state or other factors.
The two lakes that contained suitable cold-water habitat during 2001 were the deepest lakes surveyed. This suggests that an important factor in determining the presence of cold-water habitat is the relative proportions of the epilimnetic and hypolimnetic volumes, a factor that was not fully appreciated when this study was initiated.