Differentiation of submersed aquatic plants may be more than meets the eye…

Successful aquatic plant management can be broken down into three main components: 1) Identify, survey, and quantify, 2.) exert management and 3).  assess the efficacy of management efforts.  Arguably the most important part of a successful aquatic plant management plan is the accurate identification and quantification of the species of interest.  More often than not, surveys of submersed aquatic plants require extensive, time-consuming survey to get an adequate “picture” of what lies beneath the surface.  Traditional techniques such as point intercept and quadrat sampling give managers an idea of species composition and distribution, yet these technique require a great deal of time (and personnel) to accomplish, especially over the often large expanse of the areas being managed.  What if there were a way however, to differentiate between species without the use of such labor intensive surveys?  A group of scientists are taking a more in depth look at the reflective properties of submersed aquatic plants.  More specifically, the use of remote sensing as a means to achieve critical differentiation between species.

Photo Credit: Amy Poturnicki

Photo Credit: Amy Poturnicki

In their article entitled “Evaluation of hyperspectral reflectance data for discriminating six aquatic weeds”, Everitt et al. looks specifically at the potential use of hyperspectral remote sensing for species differentiation among six common submersed plants:  Hydrilla, Curlyleaf pondweed, Eurasian watermilfoil, Northern milfoil, Hybrid milfoil, and Parrotfeather.  Many of these species are widely distributed across the United States, sometimes existing in the exact same water body.  Differentiation between species is important as appropriate management hinges on the identification of the target species as well as other existing species which may be affected.  Satellite and aerial remote sensing have been widely and successfully used to quantify emergent and floating communities of aquatic plants, however submersed plants like the six mentioned, have proven more difficult to detect due to spatial and spectral limitations and their positioning beneath the water’s surface.  The use of newly available hyperspectral sensors may provide the separation needed to tell the difference between these species.  The group uses a spectroradiometer to determine the reflectance of each species on three separate dates.  In theory, the different reflectance values of each species will lend to the researchers ability to tell the difference between species in a water body based on those, hopefully different values.

Surveys often require extensive sampling across large areas.

Surveys often require extensive sampling across large areas.

Everitt et al. shows that although reflectively similar along many bands, there were several bands that led to discrimination among species during the three different sample dates.  During the two May sampling periods, 9 and 10 bands from blue to the Near-infrared portion of the electromagnetic spectrum were used (mostly in the green-red edge, red and Near IR), whereas 7 bands from the red-edge to near infrared could be used on the July date.  The specifics of each date are further discussed in the article.

Spectral signature of four species during July collection.

Mean percent reflectance of four species during July collection.

The group of researchers “shed light” on the potential use of hyperspectral sensors for differentiation among submersed species, particularly during the times that the species were assessed.  Their hope is that the spectral library developed in this study will be expanded upon and eventually applied to satellite and aerial mounted hyperspectral systems. It should be noted that target plants were at the waters surface or “topped out” when assessed.  The properties of light beneath the waters surface still tend to inhibit the ability to remotely sense submersed plant species, and the researchers suggest that this be taken into consideration when applying their work.

To read the article in it’s entirety, click here.

To learn more about the Aquatic Plant Management Society, visit our website.

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The APMS Blog is prepared by Dr. Brett Hartis, NC State University
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