Limits of the animal and vegetable kingdoms

Abstract

Diatom-dominated biofilm communities which developed in one-month periods over two years were monitored in tributaries of a small eutrophic prairie stream with agricultural watershed subject to extensive soil erosion. Site A, which was spring fed, had a year-round flow, the highest silica and nitrate-nitrogen levels, and a distinct diatom community regardless of the season with generally less trapped sediments in the early spring than sites downstream. All communities (Sites A-E) had at least two tiers existing side by side on the primary glass substrates: a prostrate tier of pennate diatoms, attached or motile, with their valve surfaces parallel to the substrate; and a second tier of pennate species attached by a proximal apical pole perpendicular to the primary substrate. At all sites, but particularly the downstream sites, the communities were surrounded and/or covered by sediments held by mucilages of dominant and codominant diatom species and bacteria of the first two tiers. Second tier (perpendicular) life-forms included rosettes of Surirella ovata, individual Navicula lanceolata, clumps or rows of Gomphonema parvulum, rosettes of Synedra ulna, clumps or rosettes of Eunotia curvata, and fan-like aggregates of Meridion circulare forming hemispherical colonies. Some diatoms of this tier were not attached directly to the artificial substrate but to other diatoms or to bound particulates. Filamentous bacteria were particularly important in tying together clumps of organisms and sediments in tiers 1 and 2, at sites C and D. A third tier consisted of filamentous organisms, e.g. Oedogonium sp. (Chlorophyceae), a stalked diatom Gomphonema sp., and protozoans including stalked peritrichs, which developed intermingled with the first two tiers and directly attached to the glass substrate. Secondary attachment of Gomphonema and Synedra to filaments of the third tier occurred, but this tier was never sufficiently populated or stable enough to trap and hold a layer of sediments. A species of diatom could occupy only one tier (Achnanthes lanceolata, tier 1; Synedra ulna, Synedra acus tier 2), or more than one tier if its growth habit included structural and/or functional heteromorphy. Diatom life-forms are illustrated, as are bacteria in their associations with the diatom-dominated communities.