PHYSICAL, CHEMICAL, AND BIOLOGICAL CONDITIONS ASSOCIATED WITH THE EARLY STAGES OF THE LAKE-MICHIGAN VERNAL THERMAL FRONT

An investigation of the thermal front in southeastern Lake Michigan du ring April 1988 revealed a dynamic physical, chemical, and biological environment. The front was observed approximately 4 km from the coast as a distinct gradient separating cold open-lake waters from warmer ne arshore waters. Surface isotherms near the front were generally parall el to one another but skewed with respect to shore. Comparison between April 22 and 29 showed that the surface isotherm pattern was modulate d by wind stress. The pattern from April 29 showed signs of flow insta bilities with horizontal scales of 1 km to 5 km. Surface drifter traje ctories provided estimates of horizontal convergence at the front whic h varied from 7 x 10(-6) s-1 to 20 x 10(-6) s-1. Inferred rates of dow nwelling, which ranged from 9.5 m d-1 to 20.7 m d-1, were sufficient t o move a water parcel from the surface to the bottom in 2 d to 6 d at the front. Convergent circulation was observed on both sampling dates despite contrasts in wind stress. Concentrations of chloride, soluble silica, and chlorophyll, which were always higher inshore, were 5% to 82% larger than offshore mean values. The aquatic environment just ins hore of the thermal front was characterized by chlorophyll concentrati ons which exceeded 5.0 mug l-1 while concentrations offshore were betw een 1.0 mug l-1 and 2.0 mug l-1. A relatively uniform vertical structu re in chlorophyll concentrations in the frontal zone was consistent wi th the observed convergence and inferred downwelling near the front.