A FLUCTUATING WATER-LEVEL CHAMBER FOR BIOGEOCHEMICAL EXPERIMENTS IN TIDAL MARSHES

An experimental chamber is described which is closed to the water column but encompasses a selected section of the marsh surface. The microcosm is connected to a subsurface collapsible reservoir which functions as a flood water source. Head differences due to tidal fluctuations force reservoir water into and out of the microcosm, so that over complete tidal cycles the net exchanges of materials can be quantified. For this study, seasonal patterns of ammonium and phosphate dynamics in a tidal freshwater marsh were determined using four experimental chambers. On average, phosphate was removed from the water column during the spring, and released to the water column during summer and early fall. Seasonal patterns of ammonium dynamics were less clear, but the marsh removed ammonium from the water column on two of three summer sampling dates. Ammonium and phosphate removal from replicated chambers in vegetated sections of the marsh complex was greater than from unvegetated sections, indicating spatial heterogeneity of nutrient processing. The use of chambers in tidal marshes creates the opportunity to run controlled experiments in situ without imposing artificial hydrologic regimes.