Examination of association indices and formulation of a composite seasonal dissimilarity index

1) A composite seasonal dissimilarity index (SD) is developed to compare systematically changing communities. 2) Seven association indices were examined. Percent Dissimilarity was chosen for community comparisons because it retained a large percentile range for scaling dissimilar communities and responded linearly to the degree of overlap between communities. 3) Examination of three consecutive years of Lake Ontario phytoplankton data, using SD, produced reasonable dissimilarity values. Inter-year variability was significantly greater than inter-station variability. 4) SD has great potential in ecology for comparing communities which vary along any gradient.