The article uses exploratory and change-point methods to investigate changes in the level and spatial pattern of size-fractionated primary productivity in Lake Erie during the summers of 1992 and 1996. In July 1992 and 1996 primary productivity measurements were made at 44 and 34 sampling stations, respectively, and separated into three size classes (<2 μm, 2 to 20 μm, >20 μm). Spatially, the overall productivity increased gradually from east to west, with the medium size class showing the highest rate of increase. The 1996 productivity was higher than that of the 1992 in almost all size classes. The 1996 level appears to be nearly proportional to the 1992 level for the medium and larger size classes. For the small size class, the increase occurs only in the western region of the Lake. These findings were supported graphically and by statistical modelling. Using geographical coordinates of sampling locations as explanatory variables, change-point analysis is used to separate the lake into regions such that each region has its own regression regime. The findings indicate that the characteristics of the east basin extend beyond its traditional physical boundaries and into the central basin. This analysis provides a more accurate characterization of the lake than the traditional practice of assuming that lake is divided into three homogeneous basins. Here the lake is divided into regions where the concentration within each region is allowed to vary but according to its own regression model.

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