Heterogeneous Spatial Distribution of Periphytic Diatoms on Vertical Artificial Substrata

Abstract

Traditional techniques for algal enumeration on substrata may ignore variation along microenvironmental gradients. To examine microdistributional patterns of diatoms on such surfaces, topographically simple, nutritionally inert artificial substrata were positioned in a duckweed-covered canal and in the littoral zone of a prairie lake. After a period of colonization, substrata were sampled using a surficial peel technique along a short section suspected to traverse sharp microenvironmental gradients of irradiance and nutrient concentration. At both sites, significant horizontal and vertical microheterogeneity in diatom species abundance was observed on a single substratum. A zonation of diatom species occurred with depth in the duckweed mat; Achnanthes hungarica, an apparently host-specific taxon, was distributed mostly in the leaf zone of the mat. Join-count analysis of a peel sample from the lake demonstrated that the major diatom taxa were significantly more abundant on some parts of the substratum than others. Refined nearest-neighbor analysis of mapped populations of Cocconeis diminute and Epithemia turgida in these peels showed that they were significantly aggregated in the distance range of a few cell lengths, suggesting that their progeny were only weakly motile after cell division. A relatively motile diatom, Achnanthes hungarica, was distributed randomly at low cell density; a more dense A. hungarica population was distributed regularly. These results illustrate the importance of examining microdistributional patterns of diatoms on substrata as the genesis of ultimate community development and macrodistribution.