Effects of atypical water-level fluctuations on macrophyte species composition, ecological structure, and identification of water-level Indicator Species for coastal wetlands

• Long-term dataset assessing macrophyte response to atypical water levels. • Prolonged low water levels increased dominance of emergent and meadow taxa. • Subsequent high water levels led to increased unrooted submergent taxa. • Return of high water levels led to formation of a novel dead-tree zone. • Traits grouping macrophytes ecologically may predict future community changes. Unlike the historical 8-year period oscillations (octennia) of high and low water levels (WLs), Lakes Huron-Michigan recently experienced two octennia of sustained-low WLs (1999 to 2014), followed by an octennium of continuously high WLs. We used macrophyte presence data collected between 2003 and 2021 across 20 sub-regions in Georgian Bay to assess the long-term effects of this atypical pattern of WL fluctuations on the plant communities of 58 wetlands (119 wetland-years). There were significant differences in species composition between Period 1 (2003–2006; low WL) and Period 2 (2015–2021; high WL) (perMANOVA; R 2 = 0.057; p = 0.001), with a significant decrease in α-diversity (17.65 ± 0.72 vs 15.63 ± 0.53, respectively; Wilcoxon signed rank test; p = 0.012), but a significant increase in β-diversity (11.06 ± 0.28 vs 11.86 ± 0.30; Wilcoxon signed rank test; p = 0.002). Indicator Species Analysis identified 10 species that were strongly associated with low WLs and four species with high WLs. We classified these Indicator Species into nine Macrophyte Ecological Groups that reflected their functional roles in wetlands. Low WLs were dominated by nearshore emergent and rosette basal species, whereas unrooted submergent and floating species were more prevalent during high WLs. An unprecedented development of a dead tree zone during Period 2, attributable to these atypical WL fluctuations, potentially prevented nearshore species from colonizing. Such shifts in the wetland plant community will likely have pervasive and cascading effects on the wetland fish community that rely on certain plant species to provide habitat structure.