Periphyton communities in experimental marine ecosystems: Scaling the effects of removal from container walls

Although it is widely held that growth of periphyton communities on walls of marine mesocosms creates artifacts which bias experimental results, there are surprisingly few studies that directly quantify such 'wall effects'. To test the hypotheses that the magnitude of wall effects is related to experimental system dimensions and that these effects can be controlled by routine periphyton removal, we conducted a series of studies using cylindrical mesocosms of 5 different sizes and shapes with treatments involving removal of wall periphyton. Results indicate that periphyton biomass and production were consistently and significantly reduced in containers receiving twice-weekly wall-cleaning treatment, but not in systems cleaned at weekly intervals. Whereas partitioning of ecosystem properties (total primary production and algal biomass) among wall, plankton, and sediment habitats revealed large and significant changes with wall-cleaning, treatment effects on the mean values of these and other ecosystem-level properties (e.g. nutrient concentrations) were small to negligible and generally inconsistent. Changes in biomass of wall periphyton and phytoplankton induced by wall-cleaning were significantly related to mesocosm radius. Reductions in wall periphyton with cleaning treatment were generally compensated by parallel increases in algal biomass in other habitats, with enhanced sediment microalgae only present in shallower systems. It appears that wall-cleaning also affected the abundance of benthic macrofauna and bacterioplankton, as well as phytoplankton taxonomic composition. Although periphyton dislodged through wall-cleaning were retained within the mesocosms in this study, we estimate that removal of this material from the experimental systems would have caused a severe nitrogen loss, equivalent to that required for supporting total ecosystem primary production.