US Long-Term Ecological Research Network
Monitoring the spatial distribution of aquatic macrophytes: A consideration of plant associations in Allequash Lake, Wisconsin
Accurate assessment of aquatic vegetation is fundamental to detecting temporal changes in the waterscape due to disturbance or changes in water quality. Often central to vegetation assessment is the use of aerial photography using color infrared film. Color infrared aerial photography acquired in the Spring and Fall of 1986 supported the examination of macrophyte distributions the shallow lower basin of Allequash Lake in Vilas County, Wisconsin. Photographs acquired with a 70mm format camera at a scale of 1:10,000 as well as National High Altitude Photography (NHAP) at a scale of 1:58,000 were scanned on an Optronics Photomation System P-1700 drum scanning densitometer resulting in digital imagery that had pixel resolutions of 0.5 meters squared and 1.5 meters squared, respectively. Unsupervised classifications for species identification in both data sets were performed resulting in classification accuracies ranging from 82\% - 100\% for the NHAP data and 75\% - 99\% in the 70mm data. Spatial aspects of macrophyte distributions related to seasonal changes and changes in resolution including patch shape, size, adjacency, and changes in patch shape with respect to both distance from shore and lake bathymetry were investigated. Adjacency analyses indicated that each class had only one or two dominant neighbors and that the same classes exhibited similar relationships regardless of depth. The higher resolution data indicated that relationships between plants of structural similarity may exist. Results from the aggregated version of this data set indicate that such relationships become more apparent later in the growing season and may not be affected by changes in resolution. Patch shape analyses were based on development factor (DF) values. Mean DF values for the NHAP data were larger than those in the 70mm data sets. Comparison of results from the NHAP data and an aggregated version of the 70mm data having the same resolution as the NHAP data indicate that this may also be a function of the growing season as the area and perimeter values were an order of magnitude larger in the early season NHAP data. Depth analyses for the NHAP data indicate that mean DF decreased with increased water depth, as did mean area and perimeter values, with the exception of Nuphar. Depth analyses for the 70mm data indicate that there is less of a tendency for DF values to decrease with increased water depth. Results from depth analysis of aggregated 70mm data agreed with the NHAP data indicating that resolution may affect the measurement of patch shape. Regardless of resolution, regression analysis did not indicate a relationship between DF, area and perimeter values and the macrophytes’ distance from the shoreline. ANOVA and the Tukey HSD test used to evaluate the significance of class DF values suggest that sampling strategies used in this research may have affected the measurement of shape.
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