Propaguies exist in a world dominated by viscous physical forces because of their very small size and relatively slow swimming speeds. The planktonic and benthic environments are heterogeneous physically, chemically, and biologically when viewed on the spatial scale of a macroalgal propagule. Macroalgal propaguies coexist in the plankton with a wide diversity of other microscopic organisms but we are limited largely to speculation in matters concerning the importance of various biological interactions in the planktonic microenvironments. The layer of water near the bottom where propaguies attach is called the boundary layer and is physically, chemically, and biologically distinct from the water above it. Many physical and chemical properties of the boundary layer environment are consequences of the metabolic or secretory activities of the organisms which inhabit the microbial film, but many properties are also influenced by abiogenic factors. Nutrient concentrations in some patches are likely to be much higher than those measured in the water column using standard techniques. Chemical morphogens or toxins may also be produced by bacteria and other microorganisms in the boundary layer. Once a macroalgal propagule firmly attaches to the bottom, competition with other algae becomes more likely than in the plankton and interspecific competition involving germlings has the potential to be very important in regulating macroalgal assemblages. Also, recently settled macroalgal spores and germlings are potential prey for a diverse array of benthic grazers. Motile propaguies have the potential ability to actively influence their position in the microenvironment through behaviours such as phototaxis, chemotaxis, settlement stimulation, and active selection of surface depressions. © 1992 The British Phycological Society.