Point-Intercept on Steroids

Who would’ve known that an obscure technical report describing a sampling methodology would become a classic in the world of Aquatic Plant Management and be adopted as a standard by lake service providers and government agencies?  Although it was old hat in the world of terrestrial Botany and Forest Ecology, Dr. John Madsen appeared to be the first to make point-intercept a standard tool for aquatic ecologists and lake managers with his Army Corps of Engineers Technical Note No MI-02 published in 1999 entitled “Point Intercept and Line Intercept Methods for Aquatic Plant Management.”

Briefly summarized, point-intercept methodology entails creating a grid of GPS points on a waterbody and traveling to those points and sampling the aquatic plants in those areas typically by throwing a double-headed rake and pulling up whatever it catches (Figure 1).

Figure 1. Contour Innovations Aquatic Biologists Jesse Amo (back) and Ray Valley (front) conduct a point-intercept vegetation survey while logging acoustic data on Orchard Lake, Dakota Co. MN.

The simplest and most objective application of the method is to simply record the presence of each species on the rake.  This does not lend much insight into how abundant each species is at each point and a mat of surface-growing vegetation gets the same weight as a lonely sprig (Figure 2).  To address this short-coming, several adaptations to the method have been made by various practitioners including ranking the abundance of different species on the rake.  Although some may argue it’s a “better than nothing” measure of relative abundance, I would argue, not much.  There is no straightforward way to objectively rank the abundance of 5 different species in a gob of plant matter on a rake like seen in Figure 1.  As a consequence, results are not repeatable and four different investigators could produce four different results for the same sample.  Further a relative ranking lends little biological information about the architectural structure or canopy height of aquatic plants.

Figure 2. Conceptual figure of a point-intercept sampling point in two contrasting environments.  In the pure application of the method, if the rake intercepts the diminutive sprig in panel B, it would be given the same weight as the thick mat in panel A.
Biological processes, water quality, physical habitat and recreational conditions all hinge on the state of aquatic plant ABUNDANCE in a waterbody.  As I have described above, point-intercept or any subjective adaptation is not well suited to address aquatic plant abundance concerns.  Nevertheless, point-intercept has many strengths and one shouldn’t throw the “baby out with the bath water.”  Rather, ciBioBase offers a powerful and efficient way of getting more out of your point-intercept species sampling.
To add biovolume to your point-intercept surveys all you need is a Lowrance HDS depth finder, a $10 SD card from your favorite electronic retailer, and a subscription to ciBioBase (single lake and unlimited pricing are available).  No additional set up is necessary.  No technical mapping experience needed.  Just hit record, and jump from point to point like you’ve done in the past.  The HDS unit will passively record the GPS signal and acoustics the entire time.
After you return from the field, upload the data to ciBioBase, get a cup of coffee and catch up on some email.  Approximately 30-min to an hour later, one of the new emails in your inbox will be an alert from ciBioBase informing you that your plant abundance and bathymetric map is processed and ready for viewing.
Not only does passively logging sonar data while conducting species surveys require no additional work, but you sample important interim areas between points and get understanding of the TRUE coverage of plants (not just the frequency of plants sampled with your rake).
Unleashing the power of Point-intercept by using ciBioBase
Although ciBioBase comes with many analytical tools, its full potential to inform aquatic plant management is realized when the data is exported out of ciBioBase and into GIS for analysis with other data layers (Figure 3).
Figure 3. ciBioBase users have the option to export processed point data along their GPS track (Point) or  the uniform grid created by kriging interpolation (Grid).  Users can then import these files into GIS for further analysis with their point-intercept data layers.
By converting the ciBioBase grid text file into a Raster grid and using a “point on raster” analysis utility available both in ESRI’s ArcGIS and Quantum GIS (an open source GIS program), users can grab the biovolume value for a point-intercept sampling point (Figure 4).

Figure 4. Example of biovolume data (grid of blues, purples, and reds with increasing density or biovolume getting a “hotter” color) imported into GIS and overlain with point-intercept species data (yellow points are northern watermilfoil – a native stand-in for its unwelcome foreign cousin Eurasian watermilfoil).  The Point Sampling Tool in Quantum will extract grid values from one raster layer and attach them to a different point-layer.
In the hypothetical example in Figure 4, anywhere where milfoil is present we can see how dense the vegetation growth was at the sampled point and around it.  By using the Point Sampling Tool in Quantum that captures the biovolume grid cell value for each surveyed point, we found that for all milfoil points, average biovolume was 65% (with many points at 100% or surface growing).  For all other vegetated points, biovolume was only 45% with many less 100% values.
How can information on species abundance lead to better management decisions than presence alone?  It is generally unrealistic to eradicate most invasive species, and often a more realistic objective is to manage the abundance to an acceptable level.   Perhaps the surface growing tendency of milfoil (i.e., biovolume = 100%) is the primary management concern and that reducing “biovolume” to say, 45% with much less surface growth like other native plant species, would be a desirable result.   Presence/absence data from point-intercept surveys alone will not inform whether plant abundance is being managed within desirable levels.
Case Study: Whole-Lake Treatments of Fluridone with Both PI and Biovolume data
Valley et al. (2006) describe results of whole-lake applications of the herbicide fluridone to a nutrient-rich Minnesota Lake (Schutz Lake, Carver Co. MN).  As part of the evaluation, hydroacoustic surveys of vegetation biovolume were conducted before and after the treatments in addition to point-intercept species surveys.
The treatments reduced Eurasian watermilfoil below detection levels, but also directly or indirectly played a role in reducing the other dominant native species in the lake, coontail.  In fact, almost all submersed vegetation disappeared 1-2 years following the treatment; however, one would never get that indication by solely looking at the point-intercept statistics (Figure 5).  

Figure 5. Mean whole-lake percent vegetation biovolume from hydroacoustic surveys (bars) in Schutz Lake, Carver Co. MN from Valley et al. 2006.  Percent frequency of occurrence of all vegetation from point-intercept surveys conducted at the same time (numbers above bars).
What had occurred was a situation that went from Figure 1A to Figure 1B.  To a rake, these environments are the same, to a lake manager and concerned citizen, they are strikingly different.  Evaluating results with Point-intercept frequency sampling alone can mask unintended harm to water quality and lake resilience.
In the 2000’s, point-intercept methods gave resource managers an objective and rapid species assessment tool.  Now, ciBioBase adds a critical third dimension to these surveys with no additional effort or training. By implementing ciBioBase as a part of standard aquatic plant assessments, resource managers and citizens will be better informed about the true state of vegetation growth in a lake and how it’s changing as a result of environmental change and our management responses.

Madsen, J. D. 1999. Point Intercept and Line Intercept Methods for Aquatic Plant Management. APCRP Technical Notes Collection ERDC/TN APCRP-MI-02.
Valley, R. D., W. Crowell, C. H. Welling, and N. Proulx. 2006. Effects of a low-dose fluridone treatment on submersed aquatic vegetation in a eutrophic Minnesota lake dominated by Eurasian watermilfoil and coontail. Journal of Aquatic Plant Management 44:19–25.

Author: biobasemaps

BioBase is a cloud platform for the automated mapping of aquatic habitats (lakes, rivers, ponds, coasts). Standard algorithms process sonar datafiles (EcoSound) and high resolution satellite imagery (EcoSat). Depth and vegetation maps and data reports are rapidly created and stored in a private cloud account for analysis, and sharing. This blog highlights a range of internal and external research, frequently asked questions, feature descriptions and highlights, tips and tricks, and photo galleries.

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