HETEROGENEOUS SPATIAL-DISTRIBUTION OF PERIPHYTIC DIATOMS ON VERTICAL ARTIFICIAL SUBSTRATA

Traditional techniques for algal enumeration on substrata may ignore v ariation along microenvironmental gradients. To examine microdistribut ional patterns of diatoms on such surfaces, topographically simple, nu tritionally inert artificial substrata were positioned in a duckweed-c overed canal and in the littoral zone of a prairie lake. After a perio d of colonization, substrata were sampled using a surficial peel techn ique along a short section suspected to traverse sharp microenvironmen tal gradients of irradiance and nutrient concentration. At both sites, significant horizontal and vertical microheterogeneity in diatom spec ies abundance was observed on a single substratum. A zonation of diato m species occurred with depth in the duckweed mat; Achnanthes hungaric a, an apparently host-specific taxon, was distributed mostly in the le af zone of the mat. Join-count analysis of a peel sample from the lake demonstrated that the major diatom taxa were significantly more abund ant on some parts of the substratum than others. Refined nearest-neigh bor analysis of mapped populations of Cocconeis diminuta and Epithemia turgida in these peels showed that they were significantly aggregated in the distance range of a few cell lengths, suggesting that their pr ogeny were only weakly motile after cell division. A relatively motile diatom, Achnanthes hungarica, was distributed randomly at low cell de nsity; a more dense A. hungarica population was distributed regularly. These results illustrate the importance of examining microdistributio nal patterns of diatoms on substrata as the genesis of ultimate commun ity development and macrodistribution.