Limnological Profiles

Page Updated, more profile data added on Dec 13, 2020

For some background info on lake stratification, see this page.

Vertical profiles of temperature, oxygen, electrical conductivity, and pH were obtained at 3 locations on Oct 3, 2017, and one location (at precisely the deepest spot in the lake) on Sep 30, 2019. View Methods.

1. Oct 3, 2017

Profiles of temperature, oxygen, and electrical conductivity were obtained for 3 sites in Sandy Lake on Oct 3, 2017, as shown below.
The depths for sites 1, 2 and 3 were 17.5, 10 and 5.5 m.

Profiles at Sites 1 and 2 were similar with a distinct surface layer in the top 7 m, and then continuous declines in temperature and oxygen, and increases in conductivity to the bottom. It’s likely that in the summer when surface water temperature want up to 25 degrees C or higher, there were distinct surface and deep layers (epilimnion and hypolimnion) with a thermal gradient between them (the metalimnion), and that on Oct 3, that 2 -layer systems was in the process of breaking down.

Two features to note:

(i) There is a distinct gradient in conductivity (a measure of dissolved ions).

(ii) For the deepest profile (Site 1), the oxygen content of the deeper layers (2.25 mg/L)  is below guidelines for both salmonids and more generally, aquatic life.  The 2017 value is less than half of the  value obtained on Aug 30 1971  when the lake was still in the oligotrophic range (that’s the only comparable historical data for Sandy Lake).

Click on image above for the source document


Location of three sites at which limnological profiles were obtained on Sandy Lake; bathymetric map at right . (Bathymetric map from P. Mandell, 1994. The effects of land use changes on water quality of urban lakes in the Halifax/Dartmouth region. MSc thesis, Dalhousie University.
Click on image for larger version


View 2017 data file

2. Sep 30, 2019
These data posted Dec 13, 2020

Using the same equipment and procedure, Ed Glover obtained these profiles at the deepest point in the lake on Sep 30, 2019:

These observations were at a deeper point than sampled at nearby site 1 in 2017.

Distinct stratification with a hypolimnion, thermocline and hypolimnion  are evident.

The overall patterns are similar to those for site #1 above with low O2 and saltier water in the deepest layer (hypolimnion).

The deepest water in 2019 was warmer (7.3 degrees at 17 meters; 7.2 at 21 m) than in 2017 (5.7 at 17.5 meters degrees, the deepest point) and less salty (195 uS/cm at 21 m in 2019 compared to 246 uS/cm at 17.5 m in 2017). Surface temperatures and EC values were more similar (17.1 degrees/169 uS/cm in 2017; 16.9 degrees, 159 uS/cm in 2019).

It also appears that the profile on Sep 30, 2019 was still in its summer, full stratification mode, while on Oct 3, 2017 it appeared to be breaking up.

View 2019 data file