A Measure of Plankton Community Similarity Utilizing Variable Weighting of Total Density and Taxonomic Proportionality

Four commonly used formulae for measuring percentage similarity ( PS ) of biological communities were tested for their usefulness in relating to two plankton community properties, species proportional differences and total density differences. The formula best combining species proportionality and total density in the expression of PS is new:\documentclass{article}\pagestyle{empty}\begin{document}$$ PS = 1/2[\sum\limits_i {\min \,(x_i ,\,y_i )] + 1/2\left[ {100 - \frac{{100}}{{\sum\limits_i {z_i } }}\,|\,\sum\limits_i {x_i } - \sum\limits_i {y_i \,|} } \right]} $$\end{document}where min (xi,yi) is the lesser percentage (doubly standardized) of a species in two samples X and Y and where 2 q, 2xi and 2yi are the total quantities of all species in samples 8,X and Y, where \documentclass{article}\pagestyle{empty}\begin{document}$ \sum\limits_i {z_i } ,\,\sum\limits_i {x_i } \,and\sum\limits_i {y_i } $\end{document} are the total quantities of all species in samples Z, X and Y, respectively. Sample 2 contains the highest density of all species in the set; \documentclass{article}\pagestyle{empty}\begin{document}$ \sum\limits_i {z_i \, > \,(\sum\limits_i {x_i ,\,} \sum\limits_i {y_i } )} $\end{document} . The new expression of PS is simple to use and has the additional advantage of offering the analyst an unlimited choice of weighting factors or importance values for proportionality of species content and total density. The method has been applied to data from Gravenhurst Bay (Ontario) and effectively demonstrates the consequences of phosphorus loading reductions for phytoplankton communities.