Spatial Heterogeneity of Macrophytes and Selected Invertebrates in Narragansett Bay (RI, U.S.A.) as Revealed by Principal Component and Cluster Analyses

Principal component (PCA) and cluster analyses (CA) were applied to data matrices on measurements of percentage cover for 15 groups of macrophyte species and three invertebrates at 18 stations (10 sites) in Narragansett Bay, RI, U.S.A. For each season, 50 field measurements were taken at all of the 10 selected stations in the intertidal zone and 25 measurements were taken in the subtidal zones of eight of these stations. Principal component plots and cluster dendrograms were generated for each of the four seasons to identify similarities among stations and between the intertidal and subtidal zones within each station. The separation of intertidal stations from subtidal stations on PCA plots was defined by a diagonal axis for the spring and a horizontal axis for autumn, winter and summer. Intertidal and subtidal stations formed loose associations to their respective sides of an axis for any season. Across the seasons, CA revealed a consistent pattern of dissimilarity among stations; one loose cluster with several outliers. Overall, stations differed more from each other than within one site over the year. It is concluded that the stations were heterogeneous based on the presence and abundance of macrophytes and selected invertebrates. Future efforts to monitor these species would be better put into examining many sites in one time interval rather than fewer sites over several seasons.