Spatial Sedimentation and Phosphorus Dynamics Trends Along Wild Rice Beds
University of Wisconsin--Stout
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Plants need phosphorus to grow. The Red Cedar Watershed has plenty of excess phosphorus in the summer months, which promotes the growth of the green layer of cyanobacteria that residents have learned to see and smell so well. Why not use this phosphorus to promote the growth of a different plant, one that adds cultural significance to the water, rather than to fuel the growth of a slimy, green layer? Wild rice, a native aquatic plant to Wisconsin, should be this plant (among others). Wild rice would add value to the Red Cedar Watershed as a natural and ecological approach to phosphorus mitigation. Wild rice has the potential to thrive and support native wildlife in the Red Cedar Watershed due to its historic presence. To better understand the dynamics of a wild rice ecosystem, my team and I went straight to the source. We found and surveyed five sites in the Red Cedar and surrounding watersheds that had healthy wild rice stands. These visits alone were enough to show that wild rice can both grow and thrive in Northwestern Wisconsin, so why not throughout Red Cedar Watershed, too? Before jumping right into planting and seeding of this plant, it is important to understand how wild rice growth affects phosphorus in the water and sediment. To study how wild rice growth impacts phosphorus concentration and deposition along the bed, we took samples from the start to end of the wild rice bed. There was a noticeable difference in the sediment as we sampled throughout the zones just by the appearance of the muck as it oozed out of the corer. The sediment before the bed was barely penetrable sand, while throughout the stand the sediment was a black-brown, fibrous muck. There was an obvious difference in the sediment along the wild rice bed- a complex environment connected with wild rice that was waiting to be explored. To quantify the apparent differences in the sediment, my team and I took to the lab where we analyzed phosphorus concentrations and texture of the sediment from the samples. The phosphorus was measured as soluble reactive phosphorus (SRP), a form of phosphorus readily available to plants. There was an increase in SRP concentration in the downstream direction of the bed. Increasing SRP levels are likely related to high levels of nutrient sediment as well as decaying plant matter from previous years. In addition, the sediment with the highest sand content was found before the bed began and the sediment with the highest content of fine particles, like clay and silt, was found in the most upstream portion of the bed. This expected sedimentation from coarse to fine particles in the downstream direction is most likely caused a decrease in flow velocity due to the presence of wild rice. Since phosphorus is attached to particles floating in the water, sedimentation allows for the settling of phosphorus. The increasing SRP levels in the sediment along the length of the wild rice bed- and evidence of sedimentation- provide initial data to phosphorus dynamics among the plant, water, and sediment. An increasing concentration of phosphorus in the sediment may be sourced from the water column. A decrease in phosphorus in the water column would lead to decreased phosphorus available for algae growth. To further understand the transport of SRP at the sediment-water interface, water measurements of SRP concentration along wild rice beds should be investigated. Data that revealed a decrease in SRP in the water column- as the SRP in the sediment decreased- would better support our hypothesis that wild rice removes phosphorus from the water column. Wild rice nutrient uptake throughout the growing season is also important to understand because the plant takes up and releases varying levels of phosphorus throughout the year to support growth. For instance, wild rice holds the most nutrients in the sediment in mid-July to August so that the nutrients are ready to support the later stage of grain formation in late-August to September. Since the maximum nutrient uptake of wild rice occurs at the same time of algae blooms, wild rice may be an optimal plant for eutrophication mitigation. Further studies may measure SRP concentrations in the sediment throughout the year to understand phosphorus intake by the growing plant and phosphorus release by the decaying plant. This study provides the initial evidence that wild rice affects phosphorus levels at the water-sediment interface and can be a part of the solution to decreasing algae blooms. Members of the community have asked me, “Why not just throw some scum suckers on the lake to clean up the mess?” While this machine and other forms of artificial technology may seem like the desired quick fix for algae blooms, they are by no means a long-term solution. Human activity and change to the landscape of Dunn County has led to the disruption of the lakes, but a natural process like plant growth can be used to balance the nutrients in the water bodies. It is time for us to rely on the restoration of wild rice, within a larger ecosystem restoration approach, to return the lakes their historic, clean state.
Civil Engineering at The Cooper Union