Effects of Substrate and Water Depth on Lotus Nelumbo lutea and Wild Rice Zizania aquatica Frequency in Mississippi River Lower Pool 4, 1998-2015

Abstract

Observable trends in MN DNR data indicate Lake Pepin is gradually filling in, increasing sedimentation in lower pool 4, a 15-milestretch of river extending from the bottom of Lake Pepin to Lock and Dam No. 4 at Alma, WI. Lake Pepin acts as an enormous filter, slowing water and allowing sediment to drop out. Vegetation levels and water clarity rise dramatically below the lake, especially during low-flow years. High-flow years, such as the Great Floods of 1993 and 2001, result in extremely low vegetation levels due to limited sunlight and the inability of plants to establish a root system in deep and turbid water. Historically, high water levels invariably drown out wild rice and are linked to poor yields. Locations in our sampling area that have historically been lotus beds appear to be shifting toward emergent-type vegetation such as wild rice. From 1998-2010, wild rice had nearly disappeared in lower Pool 4. A threshold level of plants was able to establish a solid root system during recent low flow years and the population now appears to be thriving. This study examined the frequency of lotus and wild rice in lower pool 4 backwater and side channels from 1998-2015, which until now have not been systematically studied. It was hypothesized that over time, frequency of lotus decreased and frequency of wild rice increased. It was our objective to verify the hypothesis and place these frequency changes in the context of water depth and substrate type.