The independent and interactive effects of light, current and nutrients on benthic algal structure and function

This research investigated the independent and interactive effects of light intensity, current velocity and nutrient concentration on benthic algal structure and function at the community (Chapter Two) and taxon (Chapter Two) level. The study was designed as a 23 factorial, repeated-measures experiment. Algae were exposed to two levels of current velocity (5 cm sec−1and 35 cm sec−1), light intensity (1,860 μE m−2 sec−1 and 158 μE m−2 sec−1) and nutrient (19.2 μg L−1 nitrate-nitrogen, 3.7 μg L −1 phosphate-phosphorus and 194.2 μg L−1 nitrate-nitrogen, 38.7 μg L−1 phosphate-phosphorus) concentration. Samples were collected every three days for a total of nine days.; Community level response variables included several measures of biomass, per capita “mat” nutrients and water chemistries. Current was the primary factor influencing community structure and function and the influence changed with time. Initial biomass was always lower in low current treatments; however, the rate of accumulation was higher for the high current treatments. Thus, final biomass was indistinguishable between treatments. There was some evidence of a density-dependent response to nutrient additions. Initial uptake of nitrogen declined and the proportion of phosphorous in the mat increased relative to nitrogen. The response to light also occurred near the end of the experiment when biomass was high and probably reflected self-shading. All measures of community biomass responded to the interaction of light and nutrients. This was also a density-dependent effect caused by light and diffusion limitation as community biomass increased.; Autecological responses to current velocity, light intensity and nutrient concentration were examined in Chapter Three. In general, six taxa responded to the manipulations. Although absolute abundance increased for all taxa over time for all treatments, the relative contribution of A. minutissima declined with time for all treatments. The relative abundance of A. deflaxa, C. cistula, S. ulna, and N. acicularis either increased or remained fairly constant through time for all treatment. Mougeotia spp. consistently responded to the low current treatment and N. acicularis consistently responded to light, regardless of biomass. The response of other taxa changed as biomass increased and indicated the onset of resource limitation.