Limnological Profiles

Page Updated Apr 17, 2024.

Subpage: 2023 Limnological Profiles & effects of episodic precipitation

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

Vertical profiles of temperature, oxygen, electrical conductivity were obtained at 3 locations on Oct 3, 2017 including the deepest point in the lake, and only at the deepest spot in the lake on Sep 30, 2019, Apr 19, 2021, May 7, 2022 and Aug 22, 2022.

pH values were also observed in 2017 and 2019, not in other years. View Methods.

A completely independently observed profile was obtained by Casey Doucet in August 2021 in connection with her Master of Applied Science thesis research at Dalhousie University; the thesis is available at DalSpace

Beginning on Aug 22, 2022, observations were conducted with equipment and specific directions provided by HRM under their new lake water quality sampling program. This includes a limnological profile at the deepest point with surface and water samples for chlorophyll and phosphorous; and surface readings and water sample at the outlet, both 2x/year (spring and late summer).

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:

View 2019 data file

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

Distinct stratification with a hypolimnion, thermocline (metalimnion) 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.

Without measurements of chloride in the water, it cannot be said with absolute confidence that the higher  EC in the hypolimnion compared to the epilimnion is entirely due to road salts.  Mobilization of iron and manganese (and phosphorus) under anoxia  is another possibility, however, the hypolimnion oxygen values on these two profiles, while low for salmonids,  were still above 2 mg/L, the value generally cited for “hypoxic” water (e.g., from Nurnberg 2004: “2 mg L–1 measured by a DO probe about 1 m above the sediment usually coincides with anoxic conditions at the sediment surfaces located at that depth”. Such mobilization results in EC increases up to 30-40 uS/cm (e.g., Mortimer, 1942; OME 1993); the difference top to bottom in  2017 was about 80 uS/cm, in 2019 about 40 uS/cm.

Some pertinent chemical data are available for 1998:

Samples taken at the surface, in the thermocline at 7 m depth, and at the bottom (18 m) on Sep 2, 1998 illustrate differences in EC and chloride, with equivalent differences between the surface and bottom for both EC and chloride: 34/29*125=146.6, i.e. changes in chloride appear to account for the differences in Electrical Conductivity. At the same time, there is some evidence for some mobilization of iron, manganese and phosphorous in the hypolimnion at an oxygen concentration of 3 mg/Lin early September.

Profile for Apr 19, 2021

3. Apr 19, 2021

(David P and Ed G.) The profiles for temperature, oxygen and EC indicate that a normal spring turnover had occurred and the water column was well and uniformly oxygenated down to 19 m. A thermocline had been formed at 13-17 m (temperature 7 degrees dropping to 5 degrees).  It was difficult to tell precisely when the probe was at the bottom. Separately, we lowered a bucket with rocks down;  the bottom was at approx.  20 m – so the deepest probe sample (21 m) was likely within soft sediment. There was a sharp increase in EC between 20 m ( 172 uS/cm – compared to values of 169-171 uS/cm for 0 to 19 m ) and 21 m (193 uS/cm); oxygen declined from 86.3 to 78.5 mg/L (compared to values of  96 to 89 mg/L, surface to 19 m depth). View Profile19Apr2021.xlsx

3B: Casey Doucet Profile during Aug 16-23 period, 2021
– see Identifying lake water quality trends and effective monitoring strategies in a rapidly urbanizing region by Casey Doucet. Master of Applied Science thesis 2022-12-08, Dalhousie University; and Fig B1 supplement. The Fig for Sandy Lake in the supplement shows Temperature, Dissolved Oxygem (mg/L) and Total P(ug/L) down to approx 19 m. The oxygen profiles exhibits a curious decline in the area of the thermocline with a min value circa 3 mg/l at approx. 6 m; it then increases to close to 5 mg/L at about 8 m, and then declines gradually until 15 m and then sharply between 15 and 19 m reaching a minimum of approx. 0.4 mg/L in the deepest sample at approx. 19 m. (We observed A similar decline of oxygen in the thermocline area in out August 2022 sampling shown below). The Total P sampled at 5 points only (approx.  0, 3, 7, 10 and 18 m is close to 10 ug/L for 0 to 10 m and increases from approx 10 ug/L at 10 m to approx. 22 ug/L at 18 m, suggesting some mobilization of P stimulated by anoxia in the bottom sediments,

Profiles for May 7, 2022

4. May 7, 2022
Ed G. & Derek S). The profiles for temperature, oxygen and EC indicate that a normal spring turnover had occurred. On May 7, a thermocline is developing at 5-7 m. EC values are close to uniform  top (222 to bottom (224 uS/cm). Oxygen shows a sharp decline 12-21 m; however these values are a bit uncertain as the meter gave quite different values going down versus coming up. View file: SL_Data. The EC values are well above the values for Apr 19, 2021, likely reflecting higher salt loading over the winter in 2022 compared to 2021.

5. Aug 22, 2022
(Ed G, Derek S. David P. Secchi Disk: 2.5 m
These observations were conducted with equipment and specific directions provided by HRM under their new lake water quality sampling program. There were some oddities issues with the oxygen values – note curious dip in oxygen in the thermocline –  however, the observations are consistent between the 2 sets  of measurements in showing very low oxygen at the deepest point/not in the sediment (1.36 and 1.09 mg/L).

Profiles for Aug 22, 2022

Image of Excel File. Click on image for larger version

Comments by David P in Letter to HRM
Hi Emma,

A few notes:

It was a windy day, and we anticipated issues with the boat moving, detecting the bottom etc.

We attached the probe to a rope with bucket with rocks in it so all of the tension was on the rope. (see pic, attached). So you might add 30 cm to the observed depth values.

Ed navigated to the spot you gave as the location; we did it twice (with some anchoring issues); it was clear the bottom was at 15 meters, so we decided to go to the spot where we had sampled before when Ed was with us – (Sep 30, 2019, Apr 19, 2021, May 7, 2022), bottom at 21 m –

In the course of events we lost Bruce’s large Danforth Anchor.. fortunately we were able to get another anchor from Bruce and anchored firmly on the desired second site. Observations are those as the we lowered the probe. In the first set we allowed only 5-10 secs at each level. Because of peculiar oxygen mid-way, we repeated with a second set, this time leaving the probe until the % oxygen had stabilized or was changing very slowly, that was often 1 minute plus

In both sets, we observed an exceptionally low O2 at 5 m, the value was lower in the second set than in the first.
The nearer the surface value s were higher in the second set than in the first, the values 15 m and deeper were more similar.

So I am wondering (i) the O2 probe/membrane might need to be replaced; (ii) regardless it seems there was something peculiar at ~4.5 to 6 meters or so which is also where the temperature started to drop.

For the the very last measurement in the second set, I allowed the rope to sink into the bottom (after the bucket had hit bottom) there is a jump in electrical conductivity which is what I have observed before then you allow the probe to go right onto the bottom (so EC values may provide some indication of where the bottom is when a bucket or other bottom-detecting device is not used; I did not do that with the first set and there is no jump in the final ECvalue).

Ed I am thinking it would be helpful if you send Emma your lake bathymetry map with both locations shown; please add additional comments as you might think appropriate.

— David P

Thx to Bruce Sarty for anchors!

Comparison of  May and Aug 22, 2022 profiles

Of particular notice: Drop in EC in surface waters and some increase in EC of deeper waters in Aug compared to May; oxygen minimum at 5 m in Aug,

Total P, chloride and Total P values on May 7 and Aug 22, 2022
Our observations of May and Aug 22, 2022 were conducted in collaboration with HRM’s LakeWatcher program; the profile monitoring equipment was provided by HRM.

In addition,  HRM provided a Van Dorn bottle for taking water samples and the top and bottom at the deepest site and at the outlet.

The water samples were analyzed for Total P, and Chlrophyll-a, whihc are measures of the tropic status of a lake (see fig at right).


The deep-water P values compared to surface values in the late summer provide some indication whether “internal P-loading” is occurring – as documented by C. Doucet in her Aug 2021 sampling of Sandy Lake cited above. Deep water P was not elevated according to these data, in 2022:

6. April 21, 2023

Profiles for April 21, 2023


Ed S. with YSI instrument Right) and probe with weight (left)

Notes: As on Aug 22, 2022, these observations were conducted with equipment and specific directions provided by HRM under their new lake water quality sampling program. We also sampled water at the outlet. In the past, we have attached the probe to a separate line with a weighted bucket at the end of it to facilitate our sensing when the probe/bucket reaches the bottom. This year Bruce S. made special weight  that goes directly on the probe (there was already a threaded groove on the bottom of the probe). This worked very well. The 19.5 m sample above was with the probe sitting freely on the bottom.

Samplimg sites

It’s notable that the temperature at 5 m and above were approx, 2 deg warmer than they were on April 19, 2022; also it appears the water column has turned over and is developing a thermocline at ~ 5 to 7.5 m. Comparison with the two spring profiles obtained in 2022 (Apr 19 and May 7) suggests faster warming and earlier turnover. Ice cover of Sandy Lake in 2022/2023 was much more restricted than in 2021/2022.

For comparison of YSI Specific Conductivity values with values we measure with the pocket Aquapro Water Tester (see Seasonal Monitoring), I made measurements as below:

Location   Water Temperature (degC)      EC(uS/cm)
Outlet (Surface water)   12.1   207
At Deep Spot Surface   11.8   198
At Deep Spot.V-D bottle at 0.5 m   10.5   205

7. Aug 14, 2023

Profiles for Aug 14, 2023


Comment: We had very high rainfall events in July and August with flash floods associated with the rainfall on July 21 & 22, and Aug 5 affecting the Sandy Lake/Bedford area, the latter much less so than the earlier date (Weather Network Aug 5, 2023; Reuters July 24, 2023).  Sandy Lake was impacted by overflow from a sewage pumping station on Farmers Dairy Lane: “A wastewater treatment facility in Bedford, N.S., has flooded due to heavy rainfall in the area over the weekend. Its pump and electrical systems have failed. Halifax Water said in a statement Sunday night its wastewater pump station on Farmer’s Dairy Lane is now offline and untreated waste water is overflowing directly into nearby Sandy Lake.” (CTV news July 24, 2023) That overflow was contained by July 26 (HalifaxWater message July 26, 2023).  The water level in Sandy lake went 5-6ft higher (B. Sarty, lakeside resident). The surface EC was 182 uS/cm on the June 7th sampling, and 101 uS/cm on Aug 2 (see Seasonal Monitoring page)

Comparison of  Apr 21 and Aug 14, 2023 profiles

Of particular note: Freshening of entire water column in Aug.

Comparison of  mid-August profiles 2022 and 2023

In 2023 compared to 2022: (i) weakening of thermal stratification; (ii) “freshening” of entire water column & increased salt stratification; (iii) Hypolimnion oxygen increased; Metalimnion Oxygen Minimum at 5 m retained;  there is a second minimum  at 12 m  – is that the “old” 5 m minimum pushed deeper, or a new one?

Also see this subpage for more discussion of the 2023 results: 2023 Limnological Profiles & effects of episodic precipitation

All Report Files received from the HRM LakeWatchers Program for analyses of water samples from Aug 14, 2023 (titles inserted) :
Results in Excel File Saved as PDF.pdf 
Results in Excel File.xlsx
CCME Guidelines for Chloride .pdf
CCME Guidliens for dissolved-oxygen.pdf
CCME Guideleins for phosphorus.pdf

& this link given to help in interpretation: A MONITOR’S GUIDE TO WATER QUALITY

Ed G, Chris K (foreground), photo by Derek S

8. Apr 15. 2024
Courtesy of Sandy Lake volunteers Ed G. Derek S., and Lake Watchers Halifax’s Chris K.

Normal spring turnover!


Literature Cited Above

Quantified Hypoxia and Anoxia in Lakes and Reservoirs
Gertrud K. Nürnberg. 2004  The Scientific World Journal 4, 42–54

The Exchange of Dissolved Substances between Mud and Water in Lakes
Clifford H. Mortimer 1942. Journal of Ecology 30: 147-201

Chesley Lake 1991 Water Quality Assessment
Ontario Ministry of Environment, 1993

On the Metalimnion Oxygen Minimum:
– Surface Waters/ Lit & Links: Metalimnetic Oxygen Minima
2023 Limnological Profiles & effects of episodic precipitation