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Changes in the Vertical Microdistribution of Diatoms within a Developing Periphyton Mat

We followed periphyton succession for 120 d in an artificial stream system to: 1) document an increase in cell densities and associated decrease in irradiance to the base of developing periphyton mats; 2) determine the vertical location of motile and nonmotile diatom species within the mat associated with these changes; and 3) determine the viability of diatom cells at the base of the mat compared with those of the same species at different vertical locations within the mat. We developed a novel method for producing thin cross-sections of intact periphyton mats to document the vertical distribution of algal species in horizontal 0.1-mm strata from the base of the mat to the canopy. Results demonstrated that the adnate nonmotile diatom Achnanthidium minutissimum exploited the substratum during early succession when the mat was relatively thin and without a canopy, and the population retained live cells at the base of the mat throughout the 120-d experiment, demonstrating a high tolerance to low light levels. Gomphonema angustatum also was positioned at the base of the mat and was common during early stages of succession, but viable cell densities declined sharply by day 37 when increasing cell densities reduced irradiance at the mat base by 90% (to ca 8 μmol quanta m-2· s-1). The only highly motile species studied, Nitzschia palea, was distributed throughout the vertical profile of the periphyton mat and its live cell densities did not change throughout the study, suggesting that it moved into resource-rich microhabitats and avoided stress. The vertical distribution of a tychoplanktonic chain-forming diatom species appeared to be related to the physical architecture of the developing mat and not to resource limitations within the mat. Our data suggest that diatom species differ in their tolerance to resource-limiting conditions in developing periphyton mats, and these differences affect autecological characteristics.