Aquatic vegetation dynamics in the Upper Mississippi River over two decades spanning vegetation recovery

Kristen L Bouska, U.S. Geological Survey, La Crosse, WI, Danelle M. Larson, US Geological Survey, La Crosse, WI, Deanne C. Drake, Wisconsin Department of Natural Resources, La Crosse, WI, Eric M. Lund, Minnesota Department of Natural Resources, Alicia M. Carhart, Wisconsin Department of Natural Resources and Kyle R. Bales, Iowa Department of Natural Resources

Background/Question/Methods
Aquatic vegetation within large, floodplain rivers provides many important ecological roles like water purification and key forage for waterfowl. Despite the known ecological importance of aquatic vegetation, long-term monitoring is limited. As macrophytes recover in rivers across the world, knowledge of patterns and drivers of species assembly would aid restoration practitioners in devising management strategies to support diverse and resilient aquatic vegetation communities.
We investigated patterns of aquatic vegetation species composition and diversity in thousands of sites in the upper Mississippi River, U.S.A. spanning twenty years of monitoring and a period of vegetation recovery. We relied upon species relative abundance to identify dominant gradients in and local factors associated with the assembly of aquatic vegetation species, assess temporal stability of macrophytes in backwater lakes, and to quantify shared trends in and drivers of community composition and diversity over time at broader scales.
Results/Conclusions
Our results demonstrated site-level differences in aquatic vegetation assemblage structure were associated with water depth and substrate, and a clear gradient of species abundance and diversity was apparent. A common trend in community dissimilarity over time and across sites indicated that community composition changed and diversity increased within the past 20 years with surprising synchrony. Shared trends across the 400 km study reach are symptomatic of widespread, common driver(s); however, hydrologic extremes or turbidity did not explain vegetation community patterns. Following several years of strong changes in community composition and increases in diversity, the vegetation community displayed signs of increasing stability in some pools but not others. Our results have spurred further research aimed at identifying drivers and mechanisms of aquatic vegetation community expansion, assembly, and resiliency, all of which will be applicable to the recovery of aquatic vegetation in floodplain systems worldwide.