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Nova Scotia Dept. of Agriculture and Fisheries Lake Hypolimnion Project (2002)

January 01, 2016            Salutations to the NSFA and the T.E.A.M.

Contents:

• Habitat
• Nova Scotia (Canada) Lake Hypolimnion Project
  • Introduction
  • Approach
  • Results
    • Shalom Mandaville's comments
  • Discussion
  • Table: Lakes selected for possible inclusion in the survey

• Study suggests we're losing quality summer trout habitat; The Halifax Herald Limited, February 17, 2005

  But "hypolimnetic oxygen levels do not seem to be decreasing on a long-term, regional scale" (cf. Lawlor, L. J. 2006. Chironomidae as Hypolimnetic Oxygen Indicators in Brook Trout Lakes of Nova Scotia; A Paleolimnological Investigation, Queen's University, Canada)
  

Nova Scotia (Canada) Lake Hypolimnion Project

Note: All of the following info are select excerpts from the above report:

Introduction

The Nova Scotia sport fishery is estimated to be worth approximately $57 million per year in direct and indirect expenditures. Particularly important in this respect is the stocking of hatchery reared brook trout by the Nova Scotia Department of Agriculture and Fisheries. In 2000, more than 300,000 fingerling and 195,000 yearling brook trout were stocked into about 400 freshwater systems.

Brook trout require cool, well-oxygenated water to survive. The ideal temperature range is 12-14°C and they tend to avoid waters warmer than 20°C. Dissolved oxygen levels must also be high and brook trout are seldom found in waters having dissolved oxygen levels less than 5.0 mg/l which at 20°C is equal to about 50% dissolved oxygen saturation.

Existing data on water temperatures (Inland Fisheries Division, Nova Scotia Department of Agriculture and Fisheries-DOF, unpublished data) suggests that the waters of many lakes, as well as many of the Province's rivers, warm to levels unsuitable for brook trout during the late summer months.

In these instances, in order to survive, brook trout must migrate to areas of cooler water, typically located in small shaded tributaries or in the deep water (hypolimnion) of stratified lakes. Identification of these cold-water habitats, as well as their status in terms of temporal change, is important for both the maintenance and enhancement of brook trout within the Province. This is especially true with respect to the lakes chosen for stocking of fingerlings, since it is necessary that cold-water refugia be available if these are to survive to the adult stage.

Approach

1. the survey had to be carried out during either July or August (this is the time at which water column stratification is strongest and hypolimnium DO concentrations are the lowest)
2. the maximum depth of the lake had to be ≥6m (to ensure sufficient hypolimnetic volume to serve as cold-water refugia) and
3. the bottom water temperature had to be ≤15°C

Results

However, if the classification of lakes is extended to include the presence of cold-water habitat anywhere in the lake, such as within the thermocline, results of surveys carried out in July are not as useful as those carried out in August.

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 coldwater 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 were the deepest lakes surveyed. This suggests that an important factor in determining the presence of coldwater habitat is the relative proportions of the epilimnetic and hypolimnetic volumes, a factor that was not fully appreciated when this study was initiated.

Shalom Mandaville's comments (cf. OECD):

In addition, only lakes with a well-defined thermocline (>1 °C/m) at the end of the summer stratification are to be considered, and the hypolimnium was defined as beginning downwards from the depth of the inflection point during the two months preceding the onset of the fall overturn. In addition, only lakes where the hypolimnetic to epilimnetic volume ratio is atleast 1.5 were considered.

Measurement of DIC increment yields a more comprehensive estimate of total hypolimnetic respiration than oxygen consumption rate.

Discussion

Alternatively, it may be possible to use a surrogate variable to determine the relative proportions of the epilimnion and thermocline. Two parameters often employed to characterize the morphology of a lake are `development of volume' and `relative depth' (Wetzel, 1983). Both of these parameters are used to determine if the lake is either cone-shaped or saucer-shaped. Provided they are deep enough to stratify, saucer-shaped lakes will have a higher epilimnion:hypolimnion ratio than a cone-shaped lake of equal depth.

Another approach would be the surface area of the lake to that of the area at the depth of the thermocline.

A potentially useful approach to identifying factors important in determining whether or not a lake contains summertime coldwater habitat would be to carry out a detailed multivariate analysis on the lake survey database. Various classification and ordination procedures, such as cluster, hierarchical and correspondence analysis have proved useful in numerous studies dealing with limnological data (Matthews, 1991).

Table: Lakes selected for possible inclusion in the survey

The N.S. Mapbook code was based on the edition at the time of the study and may not be applicable to the latest book since there were major revisions made in the latest edition.

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