Water-level regulation and coastal wetland vegetation in the upper St. Lawrence River: inferences from historical aerial imagery, seed banks, and Typha dynamics

Abstract

We evaluated habitat changes of tributary (drowned river mouth) wetlands in the upper St. Lawrence River with analysis of pre-and post-regulation water levels and historical vegetation reconstruction from aerial photographs. In addition, the germination response of transplanted wetland soil was compared to understand responses to moist versus saturated hydrology. Typha stem density was sampled in reference sites under the influence of water-level regulation by the International Joint Commission (IJC) and compared to treatment sites where installed control structures held levels (<0.7 m) higher. Major hydrologic changes due to regulation included a reduction of inter-annual variability with a reduction in peak levels and periodic lows, leading to a dampening of 30–40 year water-level cycles. Wetland photo interpretation indicated that flooded and mixed habitat categories were apparent at all sites for pre-regulation in 1948, but post-regulation photos (1972 and 1994) showed encroachment of robust emergent (Typha angustifolia, T. latifolia, and T. x glauca) in these habitats. Vegetation surveys (7 years, 2001–2007) indicated that reference and treatment sites were dominated by Typha, but mean stem densities were not statistically different. Typha stem density, however, declined in response to decreased summer water level. Periodic summer low water levels coupled with higher winter levels (that promote muskrat activity) were hypothesized to have the greatest effect on reducing Typha density. Seed-bank analysis indicated that a greater diversity of plant species germinated in mesic (moist) conditions than in the saturated treatment (flooded), where Typha was the dominant seedling component. Altered hydrologic regimes and invasive Typha have had a substantial effect on habitat structure within coastal wetlands and inferences from local management of levels provide useful guidance for future system-wide regulation.