Initial Colonization of Artificial Substrate: Community Development and Structure Studied by Scanning Electron Microscopy

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Abstract
The initial phases of subtidal and intertidal community development were observed using scanning electron microscopy on samples from plastic panels immersed monthly in the St. Lawrence Estuary [Canada, USA]. Bacteria and diatom populations were quantitatively evaluated on samples collected from May-Nov., 1978. The pattern of immersion and/or periodical emersion according to the level greatly influenced the community structure. Subtidal panels (-5.0 m) were rapidly colonized by bacteria while diatoms settled 2-7 wk later, depending on the season. Cocconeis spp. and Amphora spp. were the major diatom colonizers until mid-Aug. In Sept. Synedra tabulata settled on the panels. Until mid-Sept., all dominant species formed well-defined, generally monospecific clumps. In contrast with Cocconeis spp. and Amphora spp., which lie horizontally on the surface, S. tabulata, which is needlelike in shape, formed erect fan-shaped colonies. Other late invaders possessed a mucus stalk, raising themselves from the surface and thus better utilizing the vertical dimension. Clump overlap and increased species interactions occurred with higher cell densities. In the intertidal zone bacteria settled after 8-12 wk while Achnanthes brevipes var. parvula appeared after 20 wk, the only diatom species able to resist semidiurnal emersion. The ability of the panels to retain water through detritus and irregularities is probably the main factor allowing the development of this community. Panels emersed only at spring tides (monthly) were rapidly colonized by bacteria and heavy diatom settlement occurred within 4 wk. Successive monthly emersions eliminated or strongly reduced diatom populations, which were replaced by filamentous (Ectocarpaceae) algae. The 3 types of communities are compared and the strategic advantage of upright forms is discussed in relation to population density, light availability and detrital cover.