Assessing Climate Vulnerability and Adaptation Opportunities for Apostle Islands Wetlands (DRAFT)

Cooper, Matthew; Johnson, Sarah; Elias, Joan

The Apostle Islands National Lakeshore supports several types of coastal wetlands including lagoons, bogs, freshwater estuaries, marshes and peatlands that provide important ecological functions, increase diversity, and provide habitat for a variety of plant and wildlife species. This project was the most comprehensive look at the park’s wetlands to date, including water quality, hydrology, geomorphology, plants, birds, amphibians, and fish. Our aim with this project was to provide important information on which wetlands, wetland species, and wetland communities are most at risk. We provide evidence here that suggests that a hydrologic connection to Lake Superior and the geomorphology of each wetland are important factors in how these wetlands will respond to changing conditions. Coastal wetlands in the park exhibit three types of connections to Lake Superior: wetlands with semi-permanent sand barriers that exchange water through the barrier; transient barriers that form and break multiple times throughout the year, generally in response to wetland water levels rising high enough to breach the barrier or when lake waves cause sufficient erosion to the barrier; and open connected wetlands that maintain a surface water connection throughout the year. While the ecology of all three types is influenced principally by Lake Superior water levels, transient barrier wetlands are also subject to changes in precipitation patterns, which may alter the frequency of connection. Open connected wetlands may be particularly vulnerable to changes in Lake Superior wave intensity. Semi-permanent barrier wetlands are protected from Lake Superior waves, but the interaction of high lake levels and intense wave energy increases the vulnerability of these barriers to breaching, which would cause a transition of these wetlands to open embayments. We determined through bathymetric analysis of the park’s coastal wetlands that six of the seven wetlands surveyed contained significant area with benthic elevation that is lower than the record low water level for Lake Superior of 599.5’, the lowest recorded level since record-keeping began in 1917. This represents a measure of inherent climate resilience in Apostle Islands wetlands, as aquatic refugia would persist even at extremely low lake levels. Our bathymetric analyses revealed consistently sloping bathymetric surfaces of unconsolidated sediment that extended well above record high Lake Superior elevations. This also represents climate resilience, as wetland vegetation is conceivably able to expand upslope during periods of high lake level. Water quality in the Lakeshore’s coastal wetlands remains very good with low concentrations of dissolved and particulate nutrients and low concentrations of other dissolved ions. Through nutrient amendment studies, we determined that benthic algal productivity during the summer is limited by the availability of reactive nitrogen. This is a natural condition for healthy coastal wetlands in the Great Lakes. Accordingly, coastal wetlands in the park remain vulnerable to atmospheric nitrogen loading as well as nitrogen inputs from Lake Superior, which has relatively high and rising nitrate levels. Breaching of wetland barriers or wave splash over during periods of high lake level are two mechanisms that could lead to nitrogen loading to the wetlands from Lake Superior. This is particularly relevant for the Long Island interdunal swales and Michigan Island lagoon where we observed evidence of wave-induced mixing of lake water into the wetlands.