CHANGES IN THE VERTICAL MICRODISTRIBUTION OF DIATOMS WITHIN A DEVELOPING PERIPHYTON MAT

We followed periphyton succession for 120 d in an artificial stream sy stem to: 1) document an increase in cell densities and associated decr ease in irradiance to the base of developing periphyton mats; 2) deter mine the vertical location of motile and nonmotile diatom species with in the mat associated with these changes; and 3) determine the viabili ty of diatom cells at the base of the mat compared with those of the s ame species at different vertical locations within the mat. We develop ed a novel method for producing thin cross-sections of intact periphyt on mats to document the vertical distribution of algal species in hori zontal 0.1-mm strata from the base of the mat to the canopy. Results d emonstrated that the adnate nonmotile diatom Achnanthidium minutissimu m exploited the substratum during early succession when the mat was re latively thin and without a canopy, and the population retained live c ells at the base of the mat throughout the 120-d experiment, demonstra ting a high tolerance to low light levels. Gomphonema angustatum also was positioned at the base of the mat and was common during early stag es 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 mu mol quanta m(-2) s(-1)). The only highly motile specie s studied, Nitzschia palea, was distributed throughout the vertical pr ofile of the periphyton mat and its live cell densities did not change throughout the study, suggesting that it moved into resource-rich mic rohabitats and avoided stress. The vertical distribution of a tychopla nktonic chain-forming diatom species appeared to be related to the phy sical architecture of the developing mat and not to resource limitatio ns within the mat. Our data suggest that diatom species differ in thei r tolerance to resource-limiting conditions in developing periphyton m ats, and these differences affect autecological characteristics.