Contrasting trajectories in macrophyte community development after shoreline restoration: water level obscures trends

Abstract

• Macrophytes were evaluated to determine shoreline restoration success. • Epiphytic algae were surveyed to evaluate their potential impact on macrophytes. • Macrophytes were influenced by slope, wind and wave exposure among habitats. • Hydrologic changes impacted macrophyte metrics, but algal biomass had no impact. • Restoration improved habitat quality but water level change conflated assessment. Macrophytes are critical components of biologically productive lake littoral zones. Sensitivity to environmental factors such as sediment content and light availability makes macrophytes potential bioindicators of anthropogenic stress. The industrial past of Muskegon Lake (Michigan, USA) has severely disturbed the system, resulting in shoreline hardening and sediment contamination. Shoreline restoration during 2010 and 2011 presented an opportunity to use macrophytes as indicators of pre-restoration (2009–2010), shorter-term (2011–2012) and longer-term (2018) post-restoration ecosystem status. Macrophytes were sampled along transects perpendicular to the shoreline in two restored and one reference habitat, and predicted to experience post-restoration density and richness increases. Epiphytic algae were surveyed in 2018 only. Restored habitat quality, based on macrophyte species composition, minimally improved during post-restoration and there was no clear pattern in macrophyte richness and density. Water level was the strongest environmental driver of macrophyte community change, especially in 2018, when emergent macrophyte richness declined. Epiphytic algal dry weight was not sufficient to negatively impact macrophytes, as wave exposure lowered algal densities. Post-restoration habitat quality remained greater at the reference transect than at the two restored transects, although the restored habitats exhibited contrasting trajectories due to differences in slope (%), wave exposure, and disturbance.