Spatial and temporal changes in species composition of submersed aquatic vegetation reveal effects of river restoration

Abstract

We examined temporal changes in spatial patterns of submersed aquatic vegetation (SAV) in response to the restoration of geomorphic habitat in Navigation Pool 8 of the Upper Mississippi River from 1998 to 2016. The frequency of occurrence and species composition of SAV at sampling sites were spatially interpolated for each year to create annual maps. Linear models were fitted to temporal changes in SAV within each map pixel. The frequency of occurrence of SAV (across all species) increased over time in much of the impounded region of the pool, including areas near restored islands. However, impounded areas maintained a relatively consistent species composition over time, with species known to be tolerant of higher flow velocities (>0.10 m/second) and wind fetch distances (>1,000 m) (e.g. Vallisneria americana) being most abundant. In contrast, areas protected by newly constructed islands transitioned from V. americana to species found in other protected backwater habitats and known to be intolerant of high flow velocities and wind fetch distances (e.g. Ceratophyllum demersum). The results suggest that previously reported improvements in water clarity may have improved growing conditions for all SAV species, especially in the lower impounded portion of the pool, while island restoration created more backwater-like habitats and facilitated changes in species composition. Assessing changes in SAV occurrence alone offers only a partial view of local-scale river restoration (e.g. island building), while analyses of species composition are likely to be more indicative of the types of changes (i.e. reduced flow velocity and wind fetch) associated with restoring geomorphic habitat. Implications for Practice• Long-term increases in water clarity promote increases in the frequency of occurrence of submersed aquatic vegetation (SAV) in areas with suitable water depth and substrate conditions. • Assessing changes in the frequency of occurrence of SAV alone offers only a partial view of river restoration projects. Spatial changes in species composition, due to alterations in water depth, flow velocity, and wind fetch, better assess the effects of restoration. • Island construction reduces wind fetch and flow velocity thereby creating backwater-like habitat and facilitates changes in macrophyte species composition from flow-tolerant species (predominantly Vallisneria americana) to species that tend to be more abundant in poorly connected, low velocity areas (predominantly Ceratophyllum demersum). • Our analyses suggest that protected, backwater-like areas may be more temporally persistent in terms of SAV frequency and species composition, as compared to the more open-water and lotic impounded areas.

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

“…Both the environmental niche space and biotic interactions in rivers may lend to structuring riverine macrophytes into discernable vegetation types. Strong habitat gradients in rivers (e.g., sharp changes in water depth and velocity over small spatial scales) create ecological boundaries that can invoke macrophyte responses (Bouska et al, 2022; Carhart & De Jager, 2019). Macrophyte assembly also may be influenced by biotic interactions like facilitation or competition (LeBagousse‐Pinguet et al, 2012).…”

“…This study in the UMR focused on (1) classifying macrophyte vegetation types within multiple study areas, strata, and two phases spanning early and late macrophyte recoveries, (2) identifying indicator species, species abundances, and diversity indices that characterize the vegetation types, and (3) relating the environmental gradients that likely structured the vegetation types during recovery, such as inundation regime, localized habitat, and regional water quality. Hypothesis 1 was that different vegetation types would be distinguishable at two spatial scales: (1) “study areas” (each averaging ~40 km in length) due to slightly different regional species composition and temporal trajectories of species turnover (Bouska et al, 2022), and (2) “strata” (each averaging ~4–15 km length; strata are called floodplain lakes, side channels, and impounded areas) due to strong differences in environmental conditions and disturbance frequency that would reflect lotic and lentic plant assemblages (Carhart & De Jager, 2019). Hypothesis 2 was that the early phase would have few vegetation types, whereas the late phase would have more.…”