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Nutrient uptake by marshes and seagrasses: Ecosystem functions and spatial variability in the provision of this ecosystem servoce
Citation:
Koch, E., C. Stevenson, AND T. H. DEWITT. Nutrient uptake by marshes and seagrasses: Ecosystem functions and spatial variability in the provision of this ecosystem servoce. Presented at US EPA Ecosystem Services Research Program annual meeting, Las Vegas, NV, October 19 - 21, 2010.
Impact/Purpose:
Nitrogen entering coastal wetlands undergoes several important transformations involving oxidation and reduction; some ends up in the atmosphere, while much of it remains in the sediments of healthy marshes and seagrass beds – the rest passes into receiving waters.
Description:
Nitrogen entering coastal wetlands undergoes several important transformations involving oxidation and reduction; some ends up in the atmosphere, while much of it remains in the sediments of healthy marshes and seagrass beds – the rest passes into receiving waters. Variability in nutrient cycling and nutrient uptake rates results from temporal variation in nitrogen concentrations in the water column as well as spatial variations in plant biomass distribution. For example, denitrification is highly dependent on the concentration of nitrate which varies as a function of in situ nitrification and/or upland runoff. Since land runoff most often enters coastal fringe-marshes at the upland interface, it is not surprising that the high marshes have elevated rates of dentrification compared to low marshes. Additionally, the biomass distribution of plant communities determines the extent to which water and plants interact. At the edge of seagrass beds where biomass is relatively sparse or in areas where seagrass canopies “open” and “close” every few seconds due to the orbital motion of waves, there is more interaction between plants and water leading to the continuous replenishment of nutrients adjacent to the leaf surface (in the diffusive boundary layer) and within the seagrass canopy. As a result, nutrient uptake occurs at or close to saturating levels. In contrast, in dense salt marshes and seagrass beds, water flow is restricted and residence time is high which allows the plants to draw water column nutrient concentrations down to levels that are no longer saturating and, as a result, nutrient uptake rates from the water column decline. When nutrient concentrations are relatively low in the water column, seagrasses are capable of taking up nutrients from the sediments thereby sustaining their growth and existence but decreasing their contribution to the ecosystem service of water purification. These spatial and temporal variations in ecosystem functions related to nutrient cycling make the prediction of the provision of nutrient uptake by marshes and seagrasses rather complex. Our goal is to identify common processes in nutrient uptake that can be used when transferring findings from one ecosystem to another. It appears that water flow and spatial biomass distribution are strong candidates for the evaluation of the ecosystem service of water purification provided by marshes and seagrasses.