LIMNOLOGY OVERVIEW ICE LAKE (1998-1999)

Don’t judge a book by its cover! This little, moderately deep (~15 m), nondescript urban lake proved to be extremely dynamic. Despite its unexpected variations, it remained relatively immune to major changes from a succession of severe thunderstorms, tornado watches and tornado warnings throughout two summers.

The first RUSS unit for WOW was deployed on April 30, 1998, 19 days after ice-out and several weeks earlier than the long-term average for lakes in northeastern Minnesota. In 1999, the unit was deployed on April 30 again, following ice-out on April 9.

 

EXPECTED LAKE BEHAVIORS

Rapid, stable thermal (temperature) stratification: Following ice-out on 4/11/98 and 4/9/99, stable stratification ensued. The thermocline persisted at 4-5 m depth from May through September, decreasing to 9 m by Halloween in 1998.

Cold hypolimnion: In summer the temperature below 8 m was about 5-8°C. Under the ice, the temperature was about 4°C.

Large daily (diel = dye-eel) swings in DO (and to a lesser extent pH) due to algal photosynthesis and respiration during spring and summer. See below:

Fall turnover and complete re-aeration to ~100% DO saturation: The lake slowly turned over between late October and mid-November 1998 and continued to mix until it froze in mid-December. Summary data are presented in the table below:

Note -- An excellent way to see turnover is using the Color Mapper data analysis tool:

1. Launch the Color Mapper applet
2. Make sure that Ice Lake is selected
3. Move the Date Slider to October 1, 1998)
4. Set Color Map to O2 Concentration
5. Set Line Plot to Temperature
6. Click >> to advance automatically through the profiles
7. Slide the SPEED control to FASTER
8. Change "Show Every Profile" if you want to skip over some of the profiles to move through time even faster

Electrical Conductivity (EC) values in the hypolimnion gradually increased during the summer while pH decreased. This occurs because carbon dioxide accumulates in this perpetually dark layer. It’s produced by respiring organisms, particularly bacteria that decompose detritus (dead stuff) and dissolved organic molecules. Without light there is no photosynthesis to remove this CO₂ and it dissolves to form bicarbonate ions (HCO₃^-) and some carbonic acid (H₂CO₃). Basically, the bicarbonate adds to the EC and the acid lowers the pH. The epilimnion has a lower EC in summer due to photosynthetic depletion of bicarbonate and evaporation, and a higher pH due of photosynthetic removal of bicarbonate and carbonic acid (see table below).

Since the RUSS EC sensors are temperature compensated we expect to see increased EC with depth during the summer in stratified systems due to increased respiration in the hypolimnion which produces bicarbonate ion. When the lake turns over and mixes uniformly, surface water readings will then increase relative to late summer and hypolimnion EC would decrease due to it being diluted by epilimnetic water. In the summer epilimnetic EC may increase due to evaporation (this is very noticeable in the arid southwestern US) but may also be affected by direct precipitation (usually low EC) and by groundwater inflows (could be higher or lower than the lake). Also note that many conductivity pens and water quality instruments are NOT temp compensated.

Oxygen typically peaked and was supersaturated just below the thermocline for much of the spring and summer: This layer at about 4-6 m apparently has optimal light (too much can cause "light shock") and sufficient nutrients, which may diffuse up from the hypolimnion- the deeper layer of cold water (shown in blue). At the thermocline (the sharp color change from red to yellow) the water is more viscous which tends to allow algal cells to settle there. Essentially, there is greater O2 production here than is consumed by respiration or lost by diffusion to the mixed layer above. What is curious is that the differences between depths is much greater than the daily swings in DO from day to night which is not what limnology textbooks typically describe. Oxygen was almost completely absent (a condition called "anoxia") by a depth of 8 meters in June because the water below the mixed layer is isolated from the O2 in the air, and because it is too dark in this zone for much O2 to be generated by photosynthesis from algae or aquatic plants. The oxygen that was present in this layer during the period of spring mixing soon after ice-out was consumed primarily by the respiration of bacteria in the water and in the sediment. Oxygen remains near saturation in the uppermost layer throughout the ice-free season due to turbulent diffusion from the atmosphere created by wind mixing.

 

UNEXPECTED RESULTS

Unusually soft water (low salts): Ice Lake had a surface EC value similar to ultraoligotrophic Lake Superior, Lake Tahoe, and Crater Lake, Oregon (~100-115 µS/cm or m mhos/cm).

Incomplete spring turnover! Although the entire water column was nearly isothermal (constant temperature) for several weeks after ice-out in April 1998 and 1999, suggesting complete turnover, the DO below about 8 m in depth in 1998 and 1999 remained at levels < 3 mg/L until fall turnover in November 1998. A chronic DO level < 3 mg/L is a major source of physiological stress to fish, particularly cool and cold water species. Turnover hasn’t happened as of October 6, 1999. This lack of oxygen below the thermocline is an important determinant of the fish community of Ice Lake. It also indicates the sensitivity of the lake’s fishery to potential increases in nutrient loading.

Turbidity peaks at ~5 m and ~9 m: Turbidity peaks persisted through most of the growing season. It appears that both peaks are largely due to algae (see data below). We filtered water from each depth suspecting that the 9 m peak was due to bacteria or resuspended bottom sediment. The chlorophyll data suggests that, although it is anoxic, there are algae settling from upper water that are still intact. Although seemingly dark, there is sufficient light for photosynthesis to occur as deep as about 7-8 m.

THE LAKE’S PRODUCTIVITY: IS IT EUTROPHIC?

Additional data regarding the lake’s trophic status is summarized in the 11 graphs below that show transparency (Secchi depth), algal abundance (chlorophyll-a levels) and nutrient concentrations since 1998. The raw data can be found in the ANCILLARY DATA section of the WOW website.

MAJOR CONCLUSIONS
All figues below can be downloaded via an microsoft word file for better viewing quality.

1. Clarity (Secchi) not very variable, ranging from lows of 2-3 m in the spring to a high of about 4 m.
   
   
   
2. Chlorophyll-a (a measure of algae) was typically <3 µg/L (ppb) and was highest during fall turnover, presumably due to mixing of deep-water nutrients up into the sunlit, near surface water.
   
   
   
3. Concentrations of available phosphorus (TP = total phosphorus) and nitrogen (algae use mostly the ammonium and nitrate fractions) were low in the sunlit (euphotic) zone, and much higher in the hypolimnetic zone. Increased nutrient loading from the watershed would stimulate algal growth, decrease water clarity and accelerate rates of hypolimnetic oxygen depletion.
   

4. The Carlson TSI (Trophic Status Index) for the period May-Sept averaged 48, indicating mesotrophy - a moderate level of algal productivity. The TSI is based on Secchi depth, total-P and chlorophyll concentrations collected from a mid-lake, near-surface sample. Associations of TSI with the beneficial uses of lakes are tabulated below.