Citizens all over the globe love their lakes and go to great lengths and spend lots of money to protect and manage them. In the US, the Environmental Protection Agency supports a multitude of State, Local, and citizen efforts to monitor water quality in lakes and has implemented a rigorous National Lakes Assessment. Despite these efforts, lakes across the nation continue to be impacted from runoff pollution and invasive species proliferation under our noses. How does this happen?
Unfortunately, despite well-intentioned efforts to monitor various lake parameters, established monitoring methods such as water sampling from the middle of the lake or presence/absence surveys of aquatic plants often do not change significantly until there has been a fundamental shift in a lake’s ecology. We blogged about this concept in more depth last year.
The risks of not monitoring sensitive indicators are high. First, once a lake has “tipped” into a new regime, it’s difficult if not impossible to restore the lake to its original condition. Second, a lot of money is on the table for either watershed protection/restoration or lake management (e.g., herbicide or other remediation costs). This reality demands that citizens and lake managers monitor lake parameters that respond quickly to environmental change and lake management interventions such that environmental or economic costs are kept minimal.
Aquatic plant abundance in lakes is responsive to change, frequency of occurrence is not
We discussed an interesting case study near the end of our Point-Intercept on Steroids blog last October about a moderately nutrient-polluted lake in Minnesota infested with non-native Eurasian watermilfoil. A whole-lake herbicide treatment was applied to target and kill the Eurasian watermilfoil. Unfortunately, there was little else growing in the lake that was not vulnerable to the herbicide. The herbicide wiped out almost all submersed vegetation. This had negative effects on the water clarity and fish habitat in the lake. In glacial lakes, aquatic plants are often critical components of healthy lakes.
The fact that the herbicide “wiped out almost all vegetation” would have been nothing more than the desperate cries of a Fisheries manager or a concerned citizen who saw it “with their own eyes” if it was not for a hydroacoustic assessment of plant abundance that occurred before and two years following the treatment (Valley et al. 2006; Figure 1). Concurrently occurring rake surveys of frequency of occurrence, although important for determining what species were growing at the time, did not detect the almost complete loss of submersed vegetation in the lake (Figure 1). Figure 2 demonstrates why rake frequency surveys are so insensitive to changes in abundance. To put it concisely, rake surveys are not abundance surveys, they are species occurrence surveys. Large changes to the lake must occur before change is reflected in species frequency data.
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| Figure 1. Frequency of occurrence of all plants estimated using the point-intercept method (numbers above bars) in a eutrophic Minnesota lake (Schutz) treated with a whole-lake herbicide in June 2002. The bars represent the whole-lake biovolume of aquatic vegetation in Schutz and a nearby reference lake (Auburn). Average biovolume declined from 35% in 2002 before the treatment to 1.5% the year following the treatment. Biovolume was assessed over the entire waterbody and more closely reflects the true changes in abundance than percent frequency or any other adaptation that uses qualitative estimations of abundance (e.g., abundance on a scale of 0-3). Figure adapted from Valley et al. 2006 |
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| Figure 2. Two drastically different environments that get the same data value (present) if a rake picks up the sprig in panel B. Weighting by a rank (e.g., A = “3” or Abundant and B = “1” or Sparse) is only moderately more informative about true abundance since no quantitative judgement can be made by the ranks |
Passive logging of acoustics by citizens to rapidly detect change in lakes
Lowrance™ HDS log up to 20 data points (pings) per second. A GPS report of your location is automatically logged approximately every second. Spend a couple of hours driving back and forth on your lake (like you may already do normally) and now you have a full system map incorporating 144,000 data points on plant abundance on a lake all summarized nicely in a map and summarized statistical reports (Figure 3). By repeating this process multiple times throughout the year with other lake citizens (see our wisdom of the crowd blog) over several years, you will measure the “heartbeat” of your lake and begin to notice when an irregular rhythm shows up and what might be causing it.
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| Figure 3. Sample output of vegetation abundance (red = vegetation near the surface, green = vegetation near the bottom, blue = no vegetation) and GPS tracks in a 235 acre Minnesota Lake. Bathymetry, vegetation, and bottom hardness maps were created by simply logging acoustic data from Lowrance™ HDS, driving back and forth for 3 hours, and then uploading the data to BioBase |
