Effects of shade and nutrient manipulation on periphyton growth in a subtropical stream

Abstract

We used artificial substrates in an open pasture stream in south-east Queensland, Australia, to test (a) the relative importance of shade vs. nutrients on periphyton growth, and (b) whether nitrogen or phosphorus (or both) are limiting. Nutrient-diffusing substrates consisting of agar solutions with three nutrient treatments (N, P, N + P) were positioned in three shade treatments (0, 50%, 90%) and replicated at five sites. Periphyton communities sampled from substrates of all treatments after four and seven weeks were dominated by diatoms. Chlorophyll a and ash-free dry mass values of periphyton in both shaded treatments were similar to those in open sites. However, the addition of nitrogen resulted in significantly higher chlorophyll a levels compared with phosphorus-enriched substrates and control substrates. After seven weeks, lowest chlorophyll a values were found on phosphorus-enriched substrates. Nutrient additions did not have any measurable effects on periphyton ash-free dry mass values. Nitrogen rather than shade appears to be the primary factor limiting periphyton growth in this open pasture stream.