Understanding constraints on submersed vegetation distribution in a large, floodplain river: the role of water level fluctuations, water clarity and river geomorphology

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Aquatic vegetation is a key component of large floodplain river ecosystems. In the Upper Mississippi River System (UMRS), there is a long-standing interest in restoring aquatic vegetation in areas where it has declined or disappeared. To better understand what constrains vegetation distribution in large river ecosystems and inform ongoing efforts to restore submersed aquatic vegetation (SAV), we delineated areas in  ~1200 river Km of the UMRS where the combined effects of water clarity, water level fluctuation,  and bathymetry appeared suitable for establishment and persistence of SAV based on a 22-year data set for total suspended solids (TSS), water surface elevation and aquatic vegetation distribution.  

We found a large increase in suitable area downstream of a large natural riverine lake near the northern end of the UMRS (river Km 1230) that functions as a sink for suspended material. Below river Km 895, there was much less suitable area due to decreased water clarity from tributary input of suspended material, changes in river geomorphology, and increased water level fluctuation.  A hypothetical scenario of 75% reduction in TSS resulted in only minor increases in suitable area in the southern portion of the UMRS, indicating limitations by water level fluctuation and/or bathymetry (i.e. limited shallow area). These results improve our understanding of the structure and function of large river systems by illustrating how water clarity, fluctuations in water level, and river geomorphology interact to create complex spatial patterns in habitat suitability for aquatic species and may help to identify locations most and least likely to benefit from management and restoration efforts.

 

Map depicting the estimated area suitable in hectares for submersed aquatic vegetation in pool 4

  Estimated area suitable (ha) for submersed aquatic vegetation (SAV); A) compared to area suitable under a 75% TSS reduction scenario (B) for Pool 4, Upper Mississippi River. Suitability refers to the area with suitable light and depth conditions > 50% of years (1993-2014, green on map). Note minor changes in Lake Pepin and lower Pool 4, and more response in upper Pool 4 (additional 1,635 hectares of suitable area)

(Credit: Alicia Carhart, Wisconsin Department of Natural Resources. Public domain.)

 

Information generated by this project will advance our understanding of how water level fluctuations and water clarity constrain submersed aquatic vegetation distribution within the Upper Mississippi River System and aid in the habitat rehabilitation and enhancement project design and selection process. Of particular interest for management will be the identification of areas where light and water level fluctuation conditions appear suitable, but submersed aquatic vegetation remains scarce.

 

Link to manuscript

Link to model output spatial data sets -  https://doi.org/10.5066/P9TWZXVZ

Alicia M. Carhart1, John E. Kalas1, James T. Rogala2, Jason J. Rohweder2, Deanne C. Drake1, and Jeffrey N. Houser2

1Wisconsin Department of Natural Resources, La Crosse Field Station, 2630 Fanta Reed Road, La Crosse, WI, USA

2U.S. Geological Survey, Upper Midwest Environmental Sciences Center, 2630 Fanta Reed Road, La Crosse, WI, USA

 

Model output spatial data sets are also available for viewing within the Upper Mississippi River System - Systemic Spatial Data Viewer.  The specific data layer group within the viewer containing these data sets is titled “UMRS Submersed Aquatic Vegetation Suitability”.