Mapping Changes in Great Lakes Nearshore Substrate Distributions

Abstract

Nearshore substrate characteristics such as texture (grain size) and composition, available surface area, pattern, distribution, and relative stability are critical parameters that determine how biological organisms (and communities) utilize those substrates during different life stages. Geo-referenced sidescan-sonar data are used in combination with traditional sampling techniques and Geographic Information System (GIS) technologies to map substrate contacts and aquatic habitat distributions within Great Lakes nearshore areas. New methods are developed that includes area change analysis and centroid analysis to quantitatively assess net change in substrate area and evaluate relative substrate stability. An Area Change Ratio (ACR) can be calculated for individual substrate polygons (patch analysis) or groups of substrate polygons (area analysis) of the same substrate type. Areas of relative stability are represented by ACR values near zero. Areas of moderate stability are represented by values ranging between 0.2 and 0.5 and highly unstable areas are represented by values of 0.5 or greater. Substrate centroids or “centers of mass” are also calculated and can be used to quantitatively integrate change in substrate area and the location of individual substrate polygons or groups of polygons within a survey area. Comparison of centroid movement provides a quantitative measure of the magnitude and direction of change in substrate area through time. These techniques address many of the spatial and temporal limitations associated with more traditional approaches, e.g., shore-normal profiling, detailed bathymetric surveys, and repetitive sampling of bottom materials, and yet can be combined with these traditional approaches to develop a more detailed understanding of nearshore habitat distribution and stability.