Nutrient cycling in the epilithon of running waters

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Abstract
The development and physiology of microcommunities established on glass slides was investigated in the Matamek River, Quebec, during the summer of 1985. Microorganisms loosely attached to the substrate (overstory) and those strongly adhering to the substrate (understory) were considered separately. Community biomass accumulation resulted from increases in the overstory. Cell numbers remained relatively unchanged in the understory. As the community developed, the uptake of glucose, inorganic carbon, ammonia, and phosphate increased, mainly because of autotrophic organisms in the overstory. Organisms in the understory, especially bacteria, appeared to contribute significantly to the assimilation of glucose. The spiralling concept of nutrient reutilization in running waters was assessed in view of the changing physiology of the microorganisms comprising a developing biofilm. Release, downstream translocation, and subsequent biological assimilation of recently fixed bicarbonate, glucose, and orthophosphate was measured. The translocation of bicarbonate and glucose release products was similar in both poorly developed and well-developed communities, whereas orthophosphate metabolites were reassimilated more rapidly in well-developed assemblages. The apparent conservation of phosphate by the epilithic community was considered to be a physiological adaptation in response to the low nutrient concentration of the waters. Organic carbon, conversely, was probably not limiting and therefore was not rapidly reutilized. We conclude that the intensity at which compounds are reutilized is directly proportional to their degree of limitation in the epilithic community.