USING REMOTE SENSING TO MONITOR HISTORIC AND CURRENT ECOLOGICAL CHANGES IN A GREAT LAKES SLACK/INTERDUNAL WETLAND

We used historic infrared (IR) USDA NAIP and 3-band IR drone images (2017, 2018) to examine ecological changes occurring in response to fluctuating lake levels in a secondary interdunal wetland or slack on Lake Michigan’s eastern shore at Saugatuck Harbor Natural Area. Secondary slacks in Great Lakes coastal dunes are created where wind scours the sand to the water table within a dune. The studied slack is >1 ha in size with ridges and pools creating a diverse vegetation mosaic. Its water table is tied to that of Lake Michigan which has quasi-periodic water level cycles of ~30 years. The 2010 photo shows several pools (black) surrounded by bright red colors indicating vigorously growing or wetland vegetation. In contrast dune vegetation is light pink in color. Pools are not noted in the 2012 photo. Areas previously shown in red are now light red to pink, suggesting a decrease in plant vigor and a shift to dune/upland vegetation. This shift corresponds to a significant lake level drop from 2010 which dried the slack. Pools reappear in 2014 as lake levels rise from below average to average. The slack vegetation exhibits green, tan and pink to medium red colors, suggesting a mixture of plant vigor, density, and dune/upland and wetland vegetation in response to conditions changing from dry to wet. The 2016, 2017 photos show larger pool areas and dull to bright red colors dominating the slack, suggesting more vigorous or wetland plant growth in response to rising water levels. The pools greatly expand in 2018 while slack vegetation shows dull red, indicating vigorous or wetland plants. Vegetation quadrat sampling in 2016‒2018 confirmed dominant wetland vegetation throughout the slack with different species creating the red color variations. Vegetation indices (2017, 2018 data) provide greater detail on ecological and species changes. Hence, these methods provide the means to evaluate past and to monitor current ecological changes due to fluctuating water levels.