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Temporal trends in water quality and biota in segments of Pool 4 above and below Lake Pepin, Upper Mississippi River: indications of a recent ecological shift

Popp, W. A., R. M. Burdis, S. A. DeLain, and M. J. Moore. 2014. Temporal trends in water quality and biota in segments of Pool 4 above and below Lake Pepin, Upper Mississippi River: indications of a recent ecological shift. Upper Mississippi River Restoration Long Term Resource Monitoring Program Completion Report 2010D6 submitted to the U.S. Army Corps of Engineers, Rock Island, Illinois. 47 pp.

Abstract

High suspended sediment loads from the heavily agricultural Minnesota River Basin and the sediment trapping efficiency of the 34 km long Lake Pepin have together created a navigation pool unlike any other in the Upper Mississippi River, with upper and lower segments that are sharply and uniquely dissimilar. This study sought to better understand the relationships among the biota, hydrology, and physical/chemical habitats in Upper and Lower Pool 4 and to present evidence indicating a recent ecological shift in both segments using 19 years of Long Term Resource Monitoring Program data. Upper Pool 4, above Lake Pepin, is turbidity impaired and sparsely vegetated. Lower Pool 4 is a heavily vegetated mosaic of secondary channels and backwaters whose primary characteristic is the clear water that flows from Lake Pepin. Decreases in discharge, water elevation and total suspended solids during the period 2005 through 2011 drove changes in the submersed macrophyte and fish communities in both Upper and Lower Pool 4. Lower Pool 4 exhibited a 29% increase in frequency of submersed vegetation during this period and the upper pool showed a 36% increase, with even greater increases in the backwaters. A more diverse flora also developed in the upper pool during this period relative to the earlier high flow period of 1998-2004. Relative frequency of most centrarchid species increased as the frequency of emerald shiners, an open water species, decreased in both upper and lower pool during the low flow period (2005-2011). There was a large increase in abundance of fish species with an affinity for vegetation in the lower pool, such as weed shiners, yellow perch, largemouth bass, and young-of-year bluegills, during this low flow period. Likewise, largemouth bass and bluegills also increased in abundance in the upper pool. The results observed in this study may be temporary and driven by hydrology, but they demonstrate that degraded habitat and biota can be ameliorated through long-term improvements in water quality, water level management to restore a more natural summer hydrograph, and through projects that enhance and restore degraded habitat.

Keywords

Upper and Lower Pool 4, Lake Pepin, ecological shift, hydrology, water quality, submersed aquatic vegetation, fish community