Microphytobenthos Accumulation and Current.

Abstract

Processes regulating microphytobenthos accumulation in two Michigan streams were studied under different current and substrate surface conditions. Algal immigration, growth, emigration, and death rates were monitored in five experimental flow regimes. The flow regimes were habitats where current velocity varied from 27 cm/sec eddies to 33 cm/sec zones of high shearing stress. A mathematical model was developed to simulate algal accumulation as a function of immigration, growth, and death rates. Immigration was a hyperbolic function of current velocity as a result of the balance between cell impingement rates, adsorptive forces between substrate and organisms, and shearing forces removing organisms from the substrates. Immigration rates increase as silt, s and and organisms collect on substrates and interrupt flow. Immigration is also enhanced by either natural or experimental conditions which increase adsorptive forces between organisms and substrates. Flow interruption and augmenting adsorptive forces between organisms and the substrate caused as much as a 15-fold increase in immigration rates. Immigration rates were generally higher in low velocity flow regimes and reached a maximum of 1300 cells/cm('2)/day. Substrate surface conditions significantly changed immigration rates of specific algal populations. There was no correlation between the relative abundance of algal populations in potamoplankton and immigration assemblages. High accumulation rates in low velocity flow regimes generated a 6-fold difference between algal abundance in low and high velocity regimes after thirty-two days of colonization. The high rate of algal accumulation in low velocities was attributed to immigration because no differences in algal growth were observed. Emigration and grazing were not evident under the experimental conditions used. Species diversity of the community gradually decreased during the 32-day colonization period. Species richness was highly correlated to immigration rate. Evenness of the species frequency distribution decreased as algal growth became important during the last two weeks of accumulation and growth of some populations exceeded growth of others.