Scaling considerations of mesocosm wetlands in simulating large created freshwater marshes

Abstract

To explore the effects of experimental scale on ecological functions in wetlands, flow-through mesocosm wetlands (1 m 2) were compared over the first two growing seasons to a large, created, flow-through wetland (10 000 m 2) over four growing seasons. Hydrology was generally similar with mean hydraulic loading rates of 7.8 cm day −1 for the large wetland (excluding an extensive flooding year of 1995) and 6.3 cm day −1 for mesocosms. Mean hydraulic retention time was 2.1 days for the large wetland and 1.7 days for mesocosms. Temperature of surface water decreased slightly from inflow to outflow in mesocosms, while it increased in the large wetland. Conductivity of water in mesocosms showed no significant changes from inflow to outflow, while it decreased significantly in the large wetland. Phosphorus was retained effectively in the large wetland for 3 of 4 years and was retained in the mesocosms during the first of 2 years. Phosphorus was exported in the second year in the mesocosms, when dissolved oxygen (DO) and redox potential dropped significantly. Net aboveground primary productivity was similar between mesocosm wetlands (∼353 g m −2 year −1) and the large wetland (∼380 g m −2 year −1). Extensive shading with no open space may have led to cooler water temperatures and lower water column productivity in the densely vegetated mesocosms than in the large wetland in the second year. Less surface turbulence in the mesocosms due to less fetch affected DO too. These conditions may have stimulated development of reduced conditions in mesocosm soils more rapidly than in the large wetland, thereby causing the release of phosphorus. Scale of experiments and mesocosm artifacts must be considered before the results from mesocosm studies are generalized to large field-scale wetlands.