SPATIAL PARADIGMS OF LOTIC DIATOM DISTRIBUTION: A LANDSCAPE ECOLOGY PERSPECTIVE

Passy, S. I. Department of Biology, Rensselaer Polytechnic Institute c/o US Geological Survey, 425 Jordan Rd., Troy, NY 12180‐8349 USA On August 27, 1999, diatoms were sampled and current velocity and depth were measured on a regular square sampling grid at 81 locations in an unshaded, cobble‐bottom reach of White Creek, NY. Diatom relative abundance was subjected to detrended correspondence analysis. The first axis (DCA1) explained 51% of the variance and separated the samples according to current regimes. The spatial autocorrelation of DCA1 sample scores was determined by Moran's I statistic, which showed that communities' patch length was> 3.1 m while their patch width was 0.5 to 1 m. Achnanthes minutissima and Fragilaria capucina, the two species with the highest relative abundance, displayed spatially structured patches of low abundance and comparatively random patches of high abundance, suggesting broad scale abiotic control of species performance in low abundance regions and finer scale biotic control of high abundance areas. The spatial complexity in low vs. high velocity transects was determined by calculating the respective fractal dimension (D). D of DCA1 was higher in the higher current velocity transects, demonstrating that spatial complexity of diatom communities increased in faster currents, probably due to impeded immigration. Partial canonical correspondence analysis on diatom, environmental and spatial data revealed that 38% of the variance of species data was explained exclusively by current velocity, while space alone contributed only 10%. This result indicates that 1) current velocity was the major factor controlling diatom distribution and 2) there were other spatially dependent variables, most likely biotic, but their role in shaping diatom communities was minor.