Ecological strategies of benthic diatoms for nutrient competition

Diatoms are an important component of stream benthic communities and depend on nutrients and light, which are often limiting resources. The effects of different nutrient concentrations on benthic diatoms were examined. In the experiment, pristine stream water was supplied continuously to artificial streams, and nutrients were manipulated to assess effects of nitrogen (N) and phosphorus (P) additions on algae in near-natural stream conditions. Many diatoms were able to grow at very low N and P concentrations. Of the 14 most common species the same species grew fastest in low N and low P conditions. No evidence indicated that there were tradeoffs in species abilities (measured as growth rate) to compete for N and P at low resource levels, or that there were any tradeoffs for highest performance in low and high resource conditions.; Intra- and inter-specific competition of clonal populations of Achnanthidium minutissimum and two potential competitors ( Cocconeis placentula var. lineata and Cymbella cistula) were examined in laboratory experiments. In circular artificial channels, four treatment combinations of nutrients and light were created. When light was limiting, nutrient addition did not increase density. Biomass (as total cell surface area) of A. minutissimum increased when both nutrients and light were added. Intraspecific competitive effect rather than interspecific competitive effects regulated A. minutissimum growth rates. Cocconeis placentula var. lineala grew very slowly in cultures, while Cymbella cistula grew well. Facilitation by C. cistula stalks, providing substratum for colonization, may have countered interspecific competition on A. minutissimum.; Diatom species abundance depends on suitability of ecological conditions and species' ability to disperse. A positive correlation was observed between diatom abundance and distribution of diatom species. Achnanthidium minutissimum occurred at all sites and had the highest relative abundance. Relative abundance of the most common species in pristine streams decreased with increasing nutrients and chloride concentrations, however they persisted in habitats with high human disturbance. Taxa requiring high nutrients were more diverse than were taxa from pristine streams, but even the high-nutrient taxa with widest distributions occurred at low abundance. Evolutionary strategies of persistence of species adapted to low nutrients were evident in their wide-spread regional distribution.