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A Submersed Macrophyte Index of Condition for the Upper Mississippi River
Citation:
MOORE, M. J., H. A. LANGREHR, AND T. R. ANGRADI. A Submersed Macrophyte Index of Condition for the Upper Mississippi River. ECOLOGICAL INDICATORS. Elsevier Science Ltd, New York, NY, 13(1):196-205, (2012).
Impact/Purpose:
Portions of the Upper Mississippi River are listed as impaired for aquatic life use under section 303(d) of the United States Clean Water Act by the State of Minnesota’s Pollution Control Agency and Wisconsin’s Department of Natural Resources for exceeding turbidity and eutrophication standards. To help regulatory agencies address Clean Water Act requirements, we developed a submersed macrophyte index (SMI) for the Upper Mississippi River. Aquatic macrophytes are a critical driver of the ecology of the Upper Mississippi River. Because they are rooted, they integrate the effects of long and short term physical and chemical disturbance. Fifty-four submersed macrophyte-based attributes were evaluated for inclusion in the index by testing their range, responsiveness to a published multimetric stressor gradient, scope of impairment among least impaired samples, and redundancy. Four attributes were ultimately selected as metrics for the SMI including (1) percent frequency of occurrence, (2) 95th percentile of maximum depth of submersed macrophyte occurrence, (3) total rake score, a measurement of abundance quantified by the proportion of rake tines filled by aquatic macrophytes, and (4) 95th percentile of submersed species richness. The SMI separated least and most impaired samples. Using the validation dataset, the stressor gradient explained 36% of the variation in the SMI. There was a significant downriver increase in SMI scores mostly due to the effect of Lake Pepin, in the middle of Pool 4 of the Upper Mississippi River, which serves as a settling basin for transported suspended solids, increasing water clarity and improving conditions for submersed macrophyte growth in areas downstream of the lake. Proximity to a navigation dam also affected SMI scores, which consistently decreased from the sample immediately above a dam to the sample immediately below. The individual metrics within the SMI appear to be driven by two principal forces: light penetration and hydrologic conditions created by navigation dams. The correspondingly low SMI scores where listed impairments occur substantiate the use of our SMI to detect such impairments. A significant relationship between the stressor gradient and our SMI further confirms our index’s ability to detect anthropogenic stresses on the Upper Mississippi River. Our index is the first developed in North America using aquatic macrophytes for assessing large river ecosystem condition.
Description:
Portions of the Upper Mississippi River are listed as impaired for aquatic life use under section 303(d) of the United States Clean Water Act by the State of Minnesota’s Pollution Control Agency and Wisconsin’s Department of Natural Resources for exceeding turbidity and eutrophication standards. To help regulatory agencies address Clean Water Act requirements, we developed a submersed macrophyte index (SMI) for the Upper Mississippi River. Aquatic macrophytes are a critical driver of the ecology of the Upper Mississippi River. Because they are rooted, they integrate the effects of long and short term physical and chemical disturbance. Fifty-four submersed macrophyte-based attributes were evaluated for inclusion in the index by testing their range, responsiveness to a published multimetric stressor gradient, scope of impairment among least impaired samples, and redundancy. Four attributes were ultimately selected as metrics for the SMI including (1) percent frequency of occurrence, (2) 95th percentile of maximum depth of submersed macrophyte occurrence, (3) total rake score, a measurement of abundance quantified by the proportion of rake tines filled by aquatic macrophytes, and (4) 95th percentile of submersed species richness. The SMI separated least and most impaired samples. Using the validation dataset, the stressor gradient explained 36% of the variation in the SMI. There was a significant downriver increase in SMI scores mostly due to the effect of Lake Pepin, in the middle of Pool 4 of the Upper Mississippi River, which serves as a settling basin for transported suspended solids, increasing water clarity and improving conditions for submersed macrophyte growth in areas downstream of the lake. Proximity to a navigation dam also affected SMI scores, which consistently decreased from the sample immediately above a dam to the sample immediately below. The individual metrics within the SMI appear to be driven by two principal forces: light penetration and hydrologic conditions created by navigation dams. The correspondingly low SMI scores where listed impairments occur substantiate the use of our SMI to detect such impairments. A significant relationship between the stressor gradient and our SMI further confirms our index’s ability to detect anthropogenic stresses on the Upper Mississippi River. Our index is the first developed in North America using aquatic macrophytes for assessing large river ecosystem condition.