Timing of Wave Disturbance and the Resistance and Recovery of a Freshwater Epilithic Microalgal Community

Microalgal communities on clay tiles in Lake McConaughy (Nebraska, USA) were subjected to simulated wave disturbance after 6, 12, 18, or 24 d of development to examine the effects of community age on resistance and recovery in these communities. Six-day communities were less resistant than older communities; diatom standing crops in 6-d communities were reduced 47.6% by disturbance. Resistance of older communities was apparently a function of increased mat stabilization by diatom mucilages and overlying Oscillatoria surface layers. Diatom density reductions in 6-d communities were due to differential removal of large, chain-forming species of Fragilaria, and solitary, motile Navicula and Nitzschia. Sediment-trap data showed that Fragilaria and Navicula resettled quickly, but that once removed, many Nitzschia were not readily replaced through immigration. Differential removal of diatom taxa by disturbance at 6 d, as well as differential settling of taxa following disturbance affected community recovery patterns. Simulated wave disturbance also caused short-term increases in growth rates of four common taxa in 6-d communities. After 24 d, phaeophytin content of control communities exceeded chlorophyll a concentrations, indicating senescence of algal cells. Communities disturbed after 18 or 24 d had chlorophyll a concentrations similar to controls, but significantly lower phaeophytin content, suggesting that wave disturbance delayed senescence within algal mats. We conclude that wave disturbance can affect resistance and recovery of epilithic algal communities in lentic systems and that timing of disturbance influences community response.