Periphyton-nutrient dynamics in a gradient-dominated freshwater marsh ecosystem.
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Scott, J. Thad (Jefferson Thad).
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In this study, the factors influencing the development of a nutrient availability gradient in a wetland ecosystem and the subsequent controls to, and feedbacks from, the structure and function of the periphyton community were investigated. Field surveys, field experiments, and a laboratory experiments were conducted over a three year period in the Lake Waco Wetlands, a created wetland system near Waco, Texas. Results of these studies indicated that nitrogen (N) retention/removal always exceeded phosphorus (P) retention/removal along the flow path of water. Over 90% of nitrate (NO3-) entering the wetland was generally retained by the system and 50% of that retention was a result of influx into sediments. Intact sediment core experiments revealed that virtually all available NO3- in the sediments was denitrified. Inorganic P entering the wetland was less well retained (up to 50%), but the wetland was sometimes a source of P as well. The disproportionate loss of inorganic N resulted in a distinct gradient of nutrient availability where N was relatively more abundant near the inflow, but became decreasingly less abundant than P as distance from the inflow increased. Experimental N and P enrichments at the inflow often resulted in an increase in periphyton biomass accumulation. However, only N alone stimulated biomass accumulation in downstream areas. Floating periphyton mats, or "metaphyton", appeared to overcome N deficiency on a seasonal basis by fixing, and efficiently retaining, large quantities of atmospheric N2. In fact, average N-specific metaphyton production throughout the entire wetland was inversely correlated with average metaphyton phosphatase activity (r2 = 0.78; p = 0.0015). These results suggest that fixed N2 may supply a sufficient quantity of N to offset periphyton N limitation and initiate P limitation. The response to inorganic N additions is maintained because use of NO3--N is less energetically expensive to utilize than N2. Furthermore, results of laboratory experiments suggest that some mechanisms may exist by which bacteria and photoautotrophs compensate the supply of P and fixed N2, respectively, to one another within the community.