Discontinuity Bar in a Wetland on Lake Huron's Saginaw Bay

ABSTRACT

A 500 m transect was established in a large wetland on an exposed shoreline in Saginaw Bay. It extended from shore to just beyond the limit of emergent vegetation. During the spring of the year before development of vegetation, water along the transect had homogeneous characteristics. This was not the case when concentrations of electrolytes, electric conductivity, turbidity, planktonic chlorophyll a and periphytic chlorophyll a were measured in mid-summer. At that time, submersed and emergent vegetation was at annual maximum biomass and a discontinuity bar existed where parameters changed rapidly with distance along the transect. Location of the bar changed from time-to-time within a zone 250–350 m from shore as water surged shoreward and then receded during seiches. Currents associated with these changes were measured in the vicinity of the bar. They ran perpendicular to the shoreline and were generally on the order of 2–7 cm/s. During a severe storm depth of water in the wetland increased more than 50 cm in one hour with flow rates rising to about 10 cm/s. Mixing between near-shore and offshore water in the wetland occurred during this event, but the discontinuity was not broken down. Temperature and solute-related density differences in water on opposite sides of the discontinuity were always small. Aquatic plants appeared to dampen currents and mixing enough to allow these and other differences to persist as prominent mid-summer features of this wetland.