Quantifying a within-lake gradient of wave energy in Gillfillan Lake, Nova Scotia

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Abstract
The zonation patterns of shoreline plants vary greatly within lakes and exposure to waves may be an important cause of this variation. The principal objective of this study was to find a method for ranking lakeshores from low to high exposure. Two methods for ranking were tried. The first was simply the percent of the substrate composed of silt and clay and the second was calculated based on fetch values from aerial photographs and wind data from a nearby weather station. Field data were collected from 30 points chosen at random along the shoreline of Gillfillan Lake in Yarmouth County, Nova Scotia. Each was sampled for the proportion of silt and clay in the substrate and zone widths occupied by shoreline plants. Exposure values were calculated for all 30 points, using 16 different methods of calculation. The proportion of silt and clay in the substrate was negatively correlated with exposure, whichever method of calculation was used. Principal components analysis was then used to summarize the zonation patterns of 64 plant species. The positions of transects on the first principal component (PC1, 18.7% of variation) were not correlated with any measure of exposure; they appear to reflect 2 types of glacial till found in Gillfillan Lake. The positions of transects on PC2 and PC3 (12.9 and 9.6% of the variation, respectively), were strongly correlated with the percent of the substrate consisting of silt and clay but not significantly with the calculated exposure measure. Thus, percent silt and clay is a more appropriate axis for gradient analysis of zonation patterns. Species such as Calamagrostis canadensis, Carex stricta and Pontederia cordata had zone widths which were negatively correlated with exposure, whereas the widths of the zones occupied by Aster novi-belgii, Agrostis perennans and Aster tradescanti were positively correlated with exposure. While exposure affects the zonation patterns of some species, these patterns are apparently superimposed upon those produced by different types of glacial till.