At Contour Innovations we’ve long argued the importance of objectively assessing submersed aquatic vegetation (SAV) abundance to better inform management decisions. Our last blog post discussing a recent controversy over the role of herbicides in indirectly affecting fisheries declines in Wisconsin reinforces why this is so important. When we talk abundance per se, we need a metric that is quantitative, yet is intuitive. The percent of the water column taken up by vegetation growth (i.e., percent “biovolume”) represents such a metric and is the primary variable that is mapped in ciBioBase. Zero means no growth (blue). 100% represents growth all the way to the surface (red; Figure 1).
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| Figure 1. SAV Biovolume map (left), boat tracks (red lines), boat location (red dot), and sonar chart of vegetation growing to the lake surface on Orchard Lake, MN. |
Zero is undesirable in lake environments where vegetation growth is natural or where an artificial lake is managed for vegetation-dependent fisheries (e.g., largemouth bass or northern pike). No vegetation growth can also cause and be an effect of water quality impairments as discussed here). In contrast, 100% is undesirable from an aquatic recreation standpoint because props get tangled up and it’s difficult to navigate your boat through surface mats of vegetation (Figure 2).
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| Figure 2. Aquatic Vegetation (100% Biovolume) growing all the way to the water surface on Orchard Lake, MN and impediments to motorized recreation. |
If no plant growth is bad (0%), but plant growth all the way to the surface (100%) is bad, then good MUST be somewhere in between. Indeed! From a Fisheries standpoint, 40-60% average biovolume is good because there is habitat for vegetation-dependent species like largemouth bass, bluegill, northern pike, and indicator species like blackchin shiners that are sensitive to vegetation loss (Figure 3).
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| Figure 3. Probability of sampling blackchin shiners as a function of increasing SAV % biovolume in Square Lake, MN (Adapted from Valley et al. 2010 Hydrobiologia 644:385-399) |
From a water quality standpoint, 40-60% biovolume is sufficient to anchor sediments and will promoting better water clarity than if nothing was growing. Finally, 40-60% biovolume means that most growth is below the depth of your outboard prop and thus you generally won’t encounter the situation as seen in Figure 1.
A case study in MN, WI, NC, and FL lakes
CI is currently involved in a collaborative research project where acoustic data with Lowrance HDS was passively collected while conducting point-intercept surveys. Acoustic data (.sl2 files) were uploaded to ciBioBase and the Biovolume value for each species survey point was extracted from the exported raster grid (“Extract Value From Point” in the Spatial Analyst Toolbox in ArcGIS or see our Point-Intercept on Steroids blog). Figure 4 displays a wealth of information about the status of plant growth and management in the surveyed lakes. With on-the-fly data entry for the plant species surveys and uploading of the .sl2 file to ciBioBase, a similar graph could be produced within hours of finishing a survey, and thus facilitating informed and rapid decision making.
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| Figure 4. Biovolume at invasive species sample points and native sample points free of invasive species. Non-vegetated sites are not included in the analysis. Lakes range from intermediate nutrient levels, Mesotrophic (M), to high nutient levels, eutrophic (E). Berry, Gibbs, Swan, Wingra, and Round are in WI; Gray’s, Gideon’s, and St. Alban’s Bays are bays of Lake Minnetonka, MN; Waccamaw is NC; Tracy, Kissimmee, Istokpoga are FL lakes. All MN and WI lakes are infested with Eurasian watermilfoil. All NC and FL lakes are infested with Hydrilla. Waccamaw is bog stained and the hydrilla is a recent infestation |
Specifically this graph tells us the following:
- Invasives grow closer to the surface of lakes than natives and growth seems to be highest in lakes of intermediate productivity (meso-eutrophic)
- Natives appear to grow at the 40-60% biovolume level regardless of productivity.
- Native growth can be an objective benchmark from which to judge the success of invasive management in non-eradication management regimes.
- Aquatic Plant management was successful at bringing down invasive growth to the level of natives in Gray’s Bay of Lake Minnetonka, Kissimmee, and Istokpoga
