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Nova Scotia Dept. of Agriculture and Fisheries Lake Hypolimnion Project (2002)
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Soil & Water Conservation Society of Metro Halifax (SWCSMH)

March 04, 2007      Nova Scotia Lake Hypolimnion Projects      Salutations to the NSFA and the T.E.A.M.


Contents:



Nova Scotia (Canada) Lake Hypolimnion Project

(cf. Brylinsky, M. 2002. Nova Scotia Lake Hypolimnion Project., Acadia Centre for Estuarine Research, Wolfville, NS for NS Dept. Agriculture and Fisheries. 41p.)

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data on water temperatures (Inland Fisheries Division, Nova Scotia Department of Agriculture and Fisheries-DOF, unpublished data) suggests that the waters of many lakes, as well as many of the Province's rivers, warm to levels unsuitable for brook trout during the late summer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

There are about 6,700 lakes in Nova Scotia having a surface area larger than 1 hectare. Many of these lakes are deep enough to stratify (generally >3m for brown-water or dystrophic lakes and >6m for clear-water lakes) and potentially contain coldwater within and below the thermocline that could serve as refugia from brook trout during summer.

The Nova Scotia Department of Agriculture and Fisheries (DOF) has conducted surveys on about 15% of the lakes in the Province. The surveys are typically conducted during a one or two-day period, in most cases during the summer months. Although the surveys provide a good assessment of the habitat conditions for brook at one point in time during the summer, changes in habitat conditions over the summer period are not assessed, and there is no indication of the degree of increase in temperature, or decline in dissolved oxygen level, within the hypolimnion once water column stratification is established.

In order to determine the degree of change that may be occurring in the thermal and dissolved oxygen characteristics during late summer, 20 lakes, widely distributed over 4 geographic areas within the Province, were selected from a database containing 1,080 lakes. The initial selection criteria were:
  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
A total of 300 (16.7%) of the 1,080 lakes surveyed met this criteria. These lakes were then divided into 93 `good' and 207 `poor' lakes based on the level of DO saturation in the bottom waters, `good' lakes being those having values ≥50% and `poor' lakes having values <50%. Of these 300 lakes select number within the HRM area have been listed in the Table.

Of the 300 lakes, 20 were selected to be re-surveyed, 5 from each of the following areas: southern Cape Breton, Eastern Shore, central Nova Scotia, and southwestern Nova Scotia. 8 of the lakes were classified as `poor', and 11 were classified as `good' with respect to the presence of coldwater habitat.


Results

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.

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):

To avoid erroneous conclusions concerning trophic state, the precedent setting international OECD studies caution the following: lakes with high inputs of allochthonous organic matter or lakes where water color is over 10 pt. units, should not be used for oxygen deficit calculations.

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

It would be premature, based on the results of this study, to conclude that few lakes in Nova Scotia contain habitat suitable for coldwater species. The criteria used to select lakes for this study should be reviewed, especially with respect to lake morphology and the relative proportions of epilimnion and hypolimnion volumes. It is likely a key parameter, and one not fully appreciated when the selection criteria for the lakes to include in this study was established. (also see Shalom Mandaville's comments)

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

Only lakes in HRM and/or adjacent to HRM of immediate interest to us have been reproduced here; for the extensive report, cf. Brylinsky, 2002.

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.

LakeNTIS map sheetN.S. Mapbook CodeHABITATVolume
m3
Max. depth (m)Surf. area (ha)Bottom temp (°C)DO
(mg/l)
DO sat. (%)Date
m/d/yr
Alma11D1528C1Good5,400,00020.0440.09.09.279.307/06/90
Anderson11D1224A4Good-26.061.78.08.067.608/21/71
Bare Rock11D1528B2Good1,011,70011.027.49.07.060.407/31/75
Bayer11D1424D3Poor1,104,2008.036.09.05.043.108/22/74
Bennery11D1324A2Poor1,691,37315.043.59.55.043.607/11/84
Big11D1221D1Poor746,90310.027.19.03.025.908/01/79
Big Connor11D1220C4Good551,24712.010.36.38.972.807/24/57
Bluff11D1221E1Good82,0006.04.312.08.880.807/15/71
Brine11D1220C5Poor769,90410.030.08.01.08.507/17/79
Chocolate11D1220E5Poor273,07513.07.111.31.412.708/05/71
Cockscomb11D1320D2Good17,000,00033.0145.58.09.076.107/05/78
Conrod11D1424C4Good7,330,40027.0119.510.58.071.107/08/74
Cooks11E324C1Good865,0768.044.013.57.066.407/18/84
Coon11D1528D2Good-22.019.010.06.052.807/07/73
Cox11D1220D4Poor5,480,00014.0100.78.05.849.007/08/74
Cranberry11D1220D5Poor378,4038.029.19.01.08.607/18/79
Dauphinees Mill21A920B5Good15,263,28025.0287.07.69.075.507/17/86
Dollar11D1424C2Good24,923,20034.0215.19.57.464.507/03/83
Eagle11D1124B4Poor4,382,20012.088.69.04.034.508/12/74
East11D1424B4Good9,253,41844.074.16.410.081.908/08/84
Eastern11D1221D1Poor140,3658.06.49.02.017.208/01/79
East Pine Island Pd.11D1225A1Poor173,1839.06.810.52.017.808/16/79
Fenerty11D1320E3Poor1,847,1128.063.012.81.514.007/26/84
Five Island11D1220D5Poor4,556,35211.0133.512.03.027.507/11/79
Five Mile11D1320C3Good22,999,72026.0505.39.07.060.407/21/86
Fraser11D1221C1Poor741,92414.012.56.05.040.707/16/79
Frasers11D1220D5Poor5,550,00020.070.77.16.049.808/09/83
Governor11D1220E5Poor1,889,88814.040.08.81.815.508/10/83
Governor11E227C4Poor-13.0651.512.22.018.408/20/73
Grand11D1424D2Poor9,287,74930.099.57.55.041.907/24/78
Grey (Gray)11D521E2Poor236,1079.07.212.03.027.508/19/79
Halfway11D1220D4Good1,000,00016.014.04.57.256.907/21/78
Hatchet11D1221E1Poor5,047,49322.068.38.05.042.307/26/79
Horseshoe11D1528A2Good352,5008.025.99.08.069.007/18/75
Hubley Big11D1220D5Poor7,730,46914.0255.311.05.044.908/29/79
Hubley Mill11D1220D5Poor587,5878.020.211.52.018.207/09/79
Jones11D1528A2Poor429,3008.018.88.04.033.807/23/75
Kearney11D1220E5Good5,658,92026.061.56.57.057.508/11/83
Lake Echo11D1124B4Poor4,642,62110.0163.87.12.016.607/27/83
Lay11D1424D2Poor1,109,00011.034.08.04.538.008/05/88
Lewis11D1220D5Poor1,303,67013.024.610.04.035.207/04/79
Little11D521E2Poor175,1688.04.811.53.027.207/31/79
Long Bridge (Bridgend)11D1424D4Good3,830,60012.069.612.57.065.008/29/74
Long Canal11D521C2Poor94,3757.03.611.55.045.408/20/79
Loon Pond11D1528A2Good2,038,10020.028.410.06.052.807/30/80
Lower Beaver11E228B1Good-8.029.111.17.063.007/09/73
Martin11D1124B4Poor553,5009.019.15.02.016.008/15/74
Maynard11D1224A5Good351,0257.07.48.57.059.807/16/71
McCabe11D1320D4Poor7,797,97015.0163.411.11.09.007/30/84
Mill11D1220C4Poor551,2588.027.314.04.038.407/22/85
Moody11D1221E1Poor1,531,68711.057.811.02.018.008/08/79
Newcombs11D1528B3Poor1,760,4008.070.312.02.018.408/11/75
Nowlin11D1628E1Good3,599,56916.058.111.87.064.007/13/81
Oathill11D1224A5Poor185,9539.04.912.04.036.708/19/71
Otter11D1220E5Poor3,670,87612.087.812.40.21.908/10/83
Otter11D1424B4Good128,13412.05.015.05.049.107/18/78
Otter11D1524A1Poor1,060,40012.027.77.58.067.007/02/75
Pearl11D1528A3Poor280,4607.011.110.12.017.607/29/80
Penhorn11D1224A5Good124,5259.04.314.56.058.208/04/71
Petpeswick11D1424D4Good12,506,60020.0256.810.59.080.008/24/74
Philip11D1528B2Good578,20011.019.89.07.060.406/3075
Pigott11D1320D2Poor5,120,08017.090.59.92.017.607/24/80
Porters11D1424C4Poor-23.01651.16.71.08.207/15/67
Power Pond11D1225A1Poor319,2069.09.712.02.018.408/04/81
Porcupine11E727C3Poor485,94612.016.311.00.10.908/14/91
Puddle11D1220C5Poor420,25711.018.58.15.042.407/10/85
Pug Hole11D1528A3Poor348,5908.010.09.31.210.407/28/80
Rocky11D1224A4Poor3,214,97511.0141.610.21.08.807/23/84
Sandy (Rafter)11D1220C4Poor11,106,01024.0175.39.24.034.608/07/85
Sawler21A920B5Poor2,539,62727.053.06.86.049.508/26/85
Scott (Murphy)11D1221D1Poor261,8727.012.913.01.09.408/08/83
Scraggy11D1528A2Poor-13.0644.510.05.044.007/15/75
Second (Keough)11D1320E3(4?)Poor3,359,16012.090.39.05.043.107/27/83
Seloam11E227D4Poor-12.0291.413.23.734.908/14/73
Settle11D1224B5Poor125,7187.06.68.00.21.707/12/90
Seventeen Mile11E227D4Good-7.016.212.26.055.307/06/73
Shaw Big11D1424E2Poor2,810,00011.078.713.01.816.908/09/88
Shaw Little11D1424E2Poor440,6297.020.815.00.98.808/06/91
Sheldrake11D1220D5Poor356,5447.012.910.01.08.807/05/79
Southwest11D1528C2Good11,504,50016.0203.18.06.050.708/07/75
Spider11D1224A4Poor1,443,61510.064.72.03.224.208/22/89
Stillwater11D1220D4Poor2,266,96517.050.67.03.024.908/27/79
Tangier11D1528C3Poor4,253,60011.0164.513.03.028.107/29/75
Taylor11D1220D4Poor2,019,94119.046.38.05.042.308/15/85
Ten Mile11E227C5Poor6,682,80514.0202.614.02.019.207/04/91
Thomas11D1324A3Poor3,766,16315.0113.31.34.029.907/03/84
Tomahawk11D1320D3Good4,700,0009.0124.515.06.462.807/19/78
Three Mile11D1324A3Poor588,69411.016.48.00.43.407/09/84
Whites11D1221D1Poor1,610,45011.043.710.02.017.607/30/79
Witherod11D1221D1Good261,62413.09.99.89.078.908/26/83
Wrights (Marr)11D1220D4Good12,662,07118.0268.19.76.052.507/13/84


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