LIMNOLOGY OVERVIEW LAKE INDEPENDENCE (1998-1999)

Historical monitoring and other studies conducted by the Hennepin County Parks (HCP) give Lake Independence a much richer data set than either Ice Lake or Grindstone Lake. A complete analysis of the entire data set is in progress in collaboration with HCP as part of the related Lake Access project. The following comments highlight the major observations made on Lake Independence in 1998 and 1999 through the WOW study.

Ice-out occurred on March 29 in both 1998 and 1999, several weeks earlier than the long-term average for lakes in this region. The RUSS was deployed on June 19, 1998 and again on May 14, 1999. Logistical and technical difficulties made the data sets for 1998 and 1999 on Independence Lake more fragmented than for Ice Lake. The lake behaved similarly both years with respect to the data collected.

EXPECTED LAKE BEHAVIORS

Complete spring mixing: Unlike Ice Lake which stratified thermally within weeks of ice-out, Lake Independence mixed completely to the bottom in the spring and continued mixing until mid-May. This extended mixing is due to the lake’s large size and fetch. It resulted in a complete re-saturation of the water column with O₂ following winter stagnation and O₂ depletion.

Water column warmed to 9°C prior to stratification: The warmed water produced a cool rather than cold hypolimnion throughout summer.

Stable thermal stratification: Stable temperatures and a thermocline at 4-5 m depth that persisted from mid-May through late September.

Hypolimnion rapidly became depleted in oxygen in both 1998 and 1999: Oxygen at 8m in the upper hypolimnion decreased from about 7.5 mg/L in mid-April to < 1.0 mg/L (the level of hypoxia) by mid-May.

EC increased and pH decreased in hypolimnion: During stratification CO₂ accumulates in this perpetually cold layer that is too dark for photosynthesis. The CO₂ dissolves to form mostly bicarbonate ions (HCO₃^-) and carbonic acid (H₂CO₃) causing EC to increase (new ions) and pH to decrease (new acid). 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.

Large daily (diel = dye-eel ) swings in DO (and to a lesser extent pH): Dramatic swings in DO were a regular feature of lakes Independence and Minnetonka as relatively large and productive algal populations photosynthesized and respired (see table below). Both lakes are more highly buffered than Ice Lake and so exhibit smaller changes in pH.

Fall turnover and complete re-aeration to ~ 100% DO saturation: Turnover occurs much earlier in Lake Independence than it does in Ice Lake. In 1998 and 1999 Lake Independence mixed enough to become nearly uniform in temperature (isothermal) by the last week in September whereas Ice Lake slowly turned over between late October and mid-November 1998 and as of this writing (October 13) has not mixed below about 5 meters! Complete re-oxygenation in 1999 took an additional week.

Large changes in turbidity with depth and time were recorded during summer: Vertical "blobs" of higher turbidity within a water column probably indicate patches of higher algae density. Some of the dominant species in summer are blue-green algae (cyanobacteria ) that can regulate their buoyancy and float and sink over the course of a day (See Algae section of the Lake Ecology Primer). Changes over days and weeks reflect blooms and collapses of algae associated with nutrient availability, climatic (light and turbulence) conditions, and possibly zooplankton feeding. These data may be easily visualized with the profile plotter, the color mapper, or perhaps most effectively with the DXT tool that displays the water column throughout the ice-free season.

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 Lake Independence is selected
3. Move the Date Slider to August 15, 1998
4. Set Color Map to O2 % Saturation
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

 

UNEXPECTED RESULTS

Difficulty maintaining the quality assurance of data collected continuously: We underestimated how difficult it would be to maintain some of the sensors, particularly DO and turbidity. The growth of attached algae and other microorganisms, which was higher in fertile lakes like Independence, fouled some sensors. Even a maintenance schedule of 2 weeks was sometimes insufficient to maintain accurate calibrations. We "parked" the units as deep as we could between profiles to light-limit algal growth but had to be careful to avoid prolonged exposure to high sulfide anoxic water below the thermocline (especially in the METRO area lakes).

 

THE LAKE’S PRODUCTIVITY: IS IT EUTROPHIC?

Additional data regarding the lake’s trophic status is summarized in the 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. Fall 1999 data will be posted as soon as it passes QA/QC evaluation.

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

1. Clarity (secchi) was quite variable, ranging from lows of <1 m in spring and summer to >3 m immediately after ice-out and in late fall before freeze-up.
   
   
   
2. Chlorophyll-a (a measure of algae) in surface water was highly variable, ranging from < 5 µg/L (ppb) soon after ice-out to ~ 30 µg/L during a mid-summer bloom and again during mixing during fall turnover in 1998, presumably due to mixing of deep-water nutrients up into the sunlit surface water. Midsummer blooms in 1999 yielded chlorophyll-a concentrations >45 µg/L.
   
   
   
3. Concentrations of available phosphorus (TP = total phosphorus) were high both years (median = 49 µgP/L in 1998 and ~58 for 1999). Levels of available nitrogen (algae use mostly the ammonium and nitrate fractions defined as dissolved inorganic nitrogen, or DIN) declined to <50 µgDIN/L in the sunlit (euphotic) zone. These are conditions that are ideal for the proliferation of nitrogen fixing species of scum forming blue-green algae.
   
   
   
4. Concentrations of nutrients in the anoxic summer hypolimnion were extremely high with TP exceeding 100 µgP/L and ammonium-N exceeding 1000 µgN/L. It is likely that following severe summer windstorms, algal blooms are stimulated or enhanced by the injection of some this high nutrient water into the upper, sunlit epilimnion.
   
   
   
5. Increased nutrient loading from the watershed will exacerbate an already deteriorated situation of poor water clarity and obnoxious levels of algae, and accelerate rates of hypolimnetic oxygen depletion.
   
   
6. The Carlson TSI (Trophic Status Index) for the period May-Oct averaged 58 for both years, indicating the lake was moderately eutrophic; it had a relatively high 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 as designated by the Minnesota Pollution Control Agency are tabulated below.