Spatial paradigms of lotic diatom distribution: A landscape ecology perspective

Spatial distributional patterns of benthic diatoms and their relation to cu rrent velocity were investigated in an unshaded cobble-bottom reach of Whit e Creek (Washington County, NY). On 27 August 1999, diatoms were sampled an d current velocity and depth were measured on a regular square sampling gri d with a grain size of 0.01 m(2), interval of 0.5 m, and extent of 16 m(2). The relative abundance of the 18 common diatom species enumerated in the 8 1 samples was subjected to detrended correspondence analysis (P)CIA). The f irst axis (DCA1) explained 51% of the variance in diatom data and separated the samples according to current regimes. The spa tial autocorrelation of DCA1 sample scores in deposition and erosion regions of White Creek was det ermined by Moran's I statistic to indicate patch size. In White Creek the p atch length of all diatom communities was more than 3.1 m, whereas the patc h width was 1m in the deposition region and 0.5 m in the erosion region. Th ere were 5 dominant diatom taxa, Achnanthes minutissima Kutz. et vars, Frag ilaria capucina Dezmazieres et vars, F. crotonensis Kitt., Diatoma vulgaris Bory, and Synedra ulna (Nitz.) Ehr. ef vars. The patch length of the domin ant species varied from 1 to more than 4.1 m, whereas the patch width, if d efined, was 0.5 m. Achnanthes minutissima and F. capucina, the two diatom s pecies 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 abun dance regions and finer scale biotic control of high abundance areas. Anoth er objective of this study was to test the hypothesis that higher current v elocities, which generally impede immigration, would increase randomness an d complexity (i.e. homogeneity of diatom distributional patterns). The spat ial complexity in low versus high velocity transects was determined by calc ulating the respective fractal dimension (D) of DCA1 scores. D of DCA1 was higher in the higher current velocity transects, suggesting that spatial co mplexity and homogeneity of diatom communities increased in faster currents . Partial canonical correspondence analysis was conducted on diatom, enviro nmental, and spatial data to assess how much of the variance in species dis tribution could be attributed to environmental (current velocity and depth) versus spatial factors. The variance of species data, explained by the env ironment (exclusively current velocity), was 38%; whereas space alone contr ibuted only 10%, indicating that 1) current velocity was the major factor t hat controlled diatom distribution in streams and 2) there were other spati ally dependent variables, most likely biotic, but their role in shaping dia tom communities was minor.