Influence of water-level disturbances on the performance of ecological indices for assessing human disturbance: A case study of Georgian Bay coastal wetlands

In this study, we compare the performance of three ecological indicators (Water Quality Index (WQI), Wetland Macrophyte Index (WMI) and Wetland Fish Index (WFI)), to assess the impact of human activities on ecosystem health of coastal marshes in eastern and northern Georgian Bay (Lake Huron) over two decades (1999–2019), when there had been a minor change in human population (increase of 7%), but a marked difference in the pattern of water-level fluctuations. Lake Huron-Michigan is known to have 8 and 12-year oscillations in water levels, but between 1999 and 2019, water levels remained 0.5 m below the long-term mean for 14 years, and then abruptly rose nearly 1 m, remaining high for the next five years. We compared index scores of wetlands surveyed during 2003–2013 (Period 1; low-water years) with those surveyed during 2014–2019 (Period 2; high-water years). In Wilcoxon signed rank pairwise comparisons, mean WQI scores increased significantly from 1.50 to 1.96 between Periods 1 and 2, respectively (p < 0.0001); by contrast, WMI scores remained numerically and statistically the same (3.38 vs 3.38, p = 0.42), while WFI scores dropped slightly, but not significantly (3.65 vs 3.59, p = 0.15). We hypothesize that WQI scores increased because of diluting effects from increased volume of water in wetlands due to higher water levels. Given the unpredictable influences of climate change on the pattern of Great Lakes water levels, index scores based on water-quality variables must be cautiously interpreted when they are used to compare sites across different water-level scenarios.