WET AND DRY SLACKS: THE RELATIONSHIP BETWEEN LAKE MICHIGAN WATER LEVELS AND THE HYDROLOGY/ECOLOGY OF AN INTERDUNAL WETLAND/SLACK ON THE SOUTHEAST COAST OF LAKE MICHIGAN

Interdunal wetlands or slacks are an imperiled ecosystem along the Great Lakes.  We initiated a geohydrological/ecological study of a large slack near Saugatuck, Michigan. The slack is located between the arms of a large parabolic dune approximately 200m inland from Lake Michigan. We reviewed historic aerial photographs together with the Lake Michigan water level curve to examine the relationship between slack features and the hydrology of the lake. In the earliest photo, 1938, three pools occurred along the northernmost edge and emergent wetland vegetation was noted in the northeast corner and western edge. During rising Great Lakes water levels in the early 1950s, the area of the northern ponds and the emergent vegetation significantly increased and pools appeared within the areas of emergent vegetation. Lake levels dropped in the late 1950s and stayed low until 1965. All wetland ponds except the three on the northernmost edge disappeared. In 1965 water levels began to recover, remaining mostly above average until 1998. Larger pond areas occurred in years with higher water levels, while smaller pond areas occurred in years with lower water levels. Water levels dropped below average in 1998 and remained low until 2014. The areas of emergent wetland vegetation shrank significantly and in many places were replaced by upland dune vegetation. Even the northernmost pools were dry during a portion of this time. Thus, water levels and the nature of the vegetation in the slack appear to be tied to water levels in Lake Michigan. Lake levels have been above average since 2014. The emergent vegetation areas have reappeared and wetland species now dominate in areas previously occupied by upland dune species. Different plant communities have been mapped in the different pools and lobes of this slack. A network of monitoring wells has been installed in the slack and surrounding dunes to determine groundwater flow within the larger slack and coastal dune system.