Spatial and temporal scaling of periphyton growth on walls of estuarine mesocosms

Although experimental ecosystems are basic and versatile tools widely used in coastal research, periphytic growth on container walls is an intrinsic artifact that must be considered when interpreting results. To better understand how this artifact may confound extrapolation of results from controlled experiments to conditions in natural estuarine ecosystems, we examined how wall periphyton varied with container size and shape in summer and autumn experiments. Replicate (n = 3) cylindrical mesocosms of 3 volumes (0.1, 1.0, 10 m³) were established in both constant-depth (depth = 1 m) and constant-shape (radius/depth = 0.56) series. Mesocosms were initiated with unfiltered estuarine water and homogenized sediments. Periphyton biomass and gross primary production (GPP) per unit of wall area increased with increasing radius (r) or decreasing ratio of wall area (A(W)) to water volume (V) for mesocosms in both series (A(W)/V = 2/r). As a consequence, periphyton biomass and metabolism expressed per unit of water volume increased as a quadratic function of increasing A(W)/V ratio. Results also suggest a secondary scaling effect, whereby wall periphyton growth may be directly related to mesocosm depth, although mechanisms for this effect remain unclear. Significant correlations between periphyton biomass (per m² wall area) and 3 environmental factors (light attenuation coefficient, nutrient concentration, and zooplankton abundance) suggest that these factors may have played important roles in regulating wall growth. Additionally, effects of wall periphyton growth on plankton community dynamics were also indicated by the significant negative relations between periphyton biomass and measures of both phytoplankton and zooplankton abundance. The overall effect of periphyton on the experimental ecosystems was evident in the fact that periphyton accounted for over 50% of total ecosystem GPP and biomass after 2 to 4 wk of these experiments. For mesocosm experiments designed to examine dynamics of planktonic-benthic ecosystems, our results imply that growth of wall periphyton, which is controlled by factors scaling to the radius of experimental ecosystems, tends to dominate major biotic pools and rates within weeks.