Lake Restoration and Applied Limnology
(A rare opportunity for ecologist to study whole system manipulations)
 

Several potential problems. For example: 
1. Sedimentation

2. Pathogens -Fecal coliform bacteria
 
3. Toxins in sediment, water, and fish tissue

4. Taste and order in water treated for drinking (geosmin and MIB)

5. Sport fisheries

6. Invasive species


However, the two most common problems deal with eutrophication (though not unrelated to the above problems):

1) DEEP LAKES WITH HIGH PHYTOPLANKTON PRODUCTIVITY

 

What are the negative effects associated with eutrophication?

  Oligotrophic Mesotrophic Eutrophic
Conditions
  • few nutrients
  • few phytoplankton, clear
  • not stratified
  • oxygen from top to bottom
    		•
    Intermediate

    conditions
  • many nutrients
  • many phytoplankton, turbid
  • orders and scum
  • stratified
  • no oxygen in deeper waters
    		•
    Effects for human users
  • supports most uses
    		•  supports most uses
  • diminished sport fishery
  • increase cost of treating drinking water
  • loss of recreation
  • loss of aesthetics 
    		•


    A. Determining that eutrophication has reached a problematic stage; Methods of estimating trophic state:

    1. Direct measurements of phytoplankton productivity

    3. Less-traditional indirect indicators of of productivity  
    -phytoplankton: Biomass can be estimated by linear measurement and volume calculation.  Taxonomic indicators are universal on a gross scale (diatoms good, cyanobacteria bad) or species/genus measures including diversity metrics, pollution tolerance indicator taxa, and similarity indices with reference lakes (but metrics must be developed on a regional level).

    -zooplankton: vertical tow nets or plankton traps. Poor indicators of trophic condition because diversity tends to be a function of lake size, body size (hence taxa) dependent on presence/absence of predators, and pollution tolerance tends not to be taxon-specific.

    -benthic animals: dredges.  Low oxygen tolerant taxa (e.g. tubificid oligochaetes) indicate hypoxia as do diversity and abundance.  Also EPT and EPO used  (but metrics must be developed on a regional level).
       

      B. Determining the limiting nutrient

    -Algal growth potential test autoclaved lake water innoculated with single species of algae and 4 treatments (P, N, PN, and control).


    C. Narrowing down the sources of nutrients

    -Point versus non-point sources  (correlation between nutrient concentration and stream discharge where a positive relationship is associated with non-point sources and a negative relationship is associated with point sources).
    -hand sampling at multiple times
    -automated storm event monitoring


    - External and internal loading
      -direct measurements of sediment phosphorus exchange using sediment chambers
    What is the advantage and disadvantage of this method?

    -nutrient budgets:

    Can the nutrients in the lake water be accounted for from the rate at which nutrients are entering and leaving the lake.

    First, the amount of water coming into and going out of the lake must be determined as well as the volume of the lake (i.e. a hydrologic budget):
      Volume = sum of h * (a1 + a2 + sqrt(a1a2)) / 3 where h=height of cone segment,
    a1= area at top of segment,
    a2= area at bottom of segment

    Volume estimates might also be used to estimate rates of sedimentation if past measurements are available (alternatively cores and probes)
     
     
     

         Discharge of inflows and outflow must be also be determined (current meter and stream cross-sectional area) What determines rates of inflow and outflow?


     Retention Time or Hydraulic Residence Time can also be calculated, the length of time a parcel of water stays in the lake (or the inverse, Water Replacement Rate or Flushing Rate which is the proportion of water in the lake replaced per unit time).
    Retention time = volume of lake  /  tributary discharge
     

    where:
    a positive value indicates phosphorus is moving out of the sediment ('internal loading' of lake water from the lake bottom sediment).

    a negative value indicates phosphorus is moving into the sediment from the lake water.
    Analogy: If your bank account (lake water) is increasing more slowly than is accounted for  by income (load in inflow) relative to withdraw (load in outflow), then somebody (the sediment) in the bank must be stealing money from your account:  -$5 = $45 - ($100 - $50)



      D. Choosing a restoration strategy for a lake with excessive phytoplankton productivity based on the above information.


    Lake Washington (Seattle) - wastewater divervision
    stats    
    Sewage from Seattle during first half of 1900's resulted in algal blooms by the mid 1950's  (Oscillatoria rubescens).   Sewage effluent diverted to  Pugeut Sound (Pacific Ocean) eliminated  99% of nutrient inflow to the lake resulting in decline of phosphorus levels in the lake (from 70 to 16 µg/L), and increased water clarity , and declines in algal biomass  (from 35 to 4 µg/L).
     
     

    Lake Shagawa (MN)   Wastewater treatment

     Reduction in wastewater phosphorus did not result in major changes in chlorophyll levels. Why?

    What are the disadvantages of this strategy?
     


     



     
  • Artificial Circulation/Hypolimnetic aeration


    • Why might this work?
     
     
     To calculate the energy needed to breakdown stratification, RTRM should be calculated.

    What are the disadvantages?
     
     
     

    What are the disadvantages?
    Other chemical and biocides used to reduce phytoplankton directly, though how they work and how effective they are may not be clearly understood:

     
     
     
     

    2) SHALLOW WEED CHOKED LAKES  (Macrophyte dominated lakes)
    One example of potentially problematic aquatic macrophytes: Eurasian watermilfoil (Myriophyllum spicatum
     

    Nutrient reduction does not necessily lead to macrophyte reduction and sometimes increases macrophytes. Why?
     
     

    Sedimentation causing increased littoral zone may be the driving force in small lake eutrophication.

    Restoration strategies for macrophyte dominated lakes:
    Addition of tripliod grass carp.

    What are the disadvantages?
     
     
     
     

  • Lake Level Drawdown

    • Expose rooted plants to hot or freezing. Results variable

     

    What are the disadvantages?
     


     
     
     

    Controlling macrophytes have often increased phytoplankton.  Why?

     
    Many solutions to macrophytes create phytoplankton problems and vice-versa
     
     
     

    SOLUTIONS THAT MAY ADDRESS BOTH PHYTOPLANKTON AND MACROPHYTE PROBLEMS

  • Sediment Removal


    • What are the disadvantages?
     
     
     

  • Watershed protection


    • What are the disadvantages?

    Conclusions:

    Effectiveness depends on funds available, risks willing to be taken, goals, and the lake system.
    (from Soil & Water Conservation Society of Metro Halifax at http://www.chebucto.ns.ca/Science/SWCS/lakerest.html)
     

    Overall problems complex.  Every lake is different and must understand the basic limnology to evalute the effectiveness of a possible treatment.

    Lake Allatoona

    Lake Acworth