Physical, chemical, and biological conditions associated with the early stages of the Lake Michigan vernal thermal front

Abstract

An investigation of the thermal front in southeastern Lake Michigan during 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 nearshore waters. Surface isotherms near the front were generally parallel to one another but skewed with respect to shore. Comparison between April 22 and 29 showed that the surface isothern pattern was modulated by wind stress. The pattern from April 29 showed signs of flow instabilities with horizontal scales of 1 km to 5 km. Surface drifter trajectories provided estimates of horizontal convergence at the fron which varied from 7×10⁻⁶ s⁻¹ to 20×10⁻⁶ s⁻¹. Inferred rates of downwelling, which ranged from 9.5 m d⁻¹ to 20.7 m d⁻¹, were sufficient to move a water parcel from the surface to the bottom in 2 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 inshore of the thermal front was characterized by chlorphyll concentrations which exceeded 5.0 μg 1⁻¹ while concentrations offshore were between 1.0 μg 1⁻¹ and 2.0 μg 1⁻¹. A relatively uniform vertical structure in chlorophyll concentrations in the frontal zone was consistent with the observed convergence and inferred downwelling near the front.