Annual Summer Submersed Macrophyte Standing Stocks Estimated From Long-Term Monitoring Data in the Upper Mississippi River

Abstract

System-scale restoration efforts within the Upper Mississippi River National Wildlife and Fish Refuge have included annual monitoring of submersed aquatic vegetation (SAV) since 1998 in four representative reaches spanning ~440 river km. We developed predictive models relating monitoring data (site-scale SAV abundance indices) to diver-harvested SAV biomass, used the models to back-estimate annual standing stock biomass between 1998 and 2018 and compared biomass estimates to previous abundance measures. Two morphologically distinct groups of SAV with differing sampling efficiencies were modeled and estimated separately: the first category included only wild celery Vallisneria americana, which has long, unbranched leaves and dominates lotic environments, while the second category included 17 branched morphology species (e.g., hornwort Ceratophyllum demersum and Canadian water weed Elodea canadensis) and dominates lentic environments. Wild celery accounted for approximately half of total estimated total biomass in the four reaches, combined branched species accounted for half, and invasive species (Eurasian watermilfoil Myriophyllum spicatum and curly-leaf pondweed Potamogeton crispus), a fraction of the branched species, accounted for <1.5%. Site-scale SAV estimates ranged from 0 to 535 g m-2 (dry mass). Increases in biomass were observed in most areas between 1998 and 2009 and substantial increases (e.g., from <10 g m-2 to ~125 g m-2) in wild celery observed in extensive impounded areas between 2002 and 2007. Analyses also indicate a transitional period in 2007-2010 during which changes in biomass trajectories were evident in all reaches, and included the start of a nine-year, ~70% decrease in wild celery biomass in the southernmost impounded area. Biomass estimates provided new insights and illustrated scales of change that were not previously apparent using traditional metrics. The ability to estimate biomass from LTRM monitoring data improves conservation efforts through better understanding of changes in habitat and food resources for biota, improved goal setting for restoration projects and improved quantification of SAV-mediated structural effects such as anchoring of sediments and feedbacks with water quality.

Aquatic vegetation types identified during early and late phases of vegetation recovery in the Upper Mississippi River

“…We were surprised that wild rice, Zizania aquatica L., was not its own vegetation type given its rapid expansion and high abundance in the late recovery phase in some UMR study areas, except in Pool 13 (Larson, DeJong, et al, 2022; Larson, Lund, et al, 2022). Interestingly, invasives (like P. crispus and Myriophyllum spicatum ) were present in low abundance in all vegetation types but were not indicator species (Figure 3) because aquatic plant invasives rarely dominate in this system (Appendix S1: Table S1; Drake et al, 2022; Larson, DeJong, et al, 2022, Larson, Lund, et al, 2022). However, our river experience is that a new and prolific invader as of year 2020, Butomus umbellatus L., may soon become a novel vegetation type.…”

“…Hypothesis 2 was that the early phase would have few vegetation types, whereas the late phase would have more. In the UMR, the late phase of recovery (years 2015-2019, 15 years after the early phase) was characterized by the greatest macrophyte prevalence and relative stability (Figure 1b, and see Bouska et al, 2022;Drake et al, 2022;Larson, Lund, et al, 2022). Thus, we expected the early phase would have only a few turbid-tolerant and cosmopolitan species (Lougheed et al, 2001), such as Stuckenia pectinata (L.) Borner (POPE6), Potamogeton crispus L. (POCO3), Ceratophyllum demersum L. (CEDE4), and Elodea canadensis Michx.…”