The Impact of Episodic Events on Nearshore-Offshore Transport in the Great Lakes:
Sediment Resuspension and Transport

B. Eadie, D. Edgington, B. Lesht, V. Klump, K. Nealson, J. Robbins, and B. Tonner

Within systems, episodic events are essentially the drivers for a major set of processes that control the biogeochemistry of coastal waters. Because of their erratic time dependence and highly variable physical forcing, the role of episodic events on structuring coastal systems, from their basic circulation to their biogeochemical cycles and ecology, is poorly known. The reason is simple, episodic events are inherently difficult to study, and by definition difficult to predict. The Great Lakes, and the Lake Michigan basin in particular, however, provide representative systems in which the study of these types of events are in fact tractable.
Our conceptual biogeochemical/physical model is that new particles enter the lake, primarily from shoreline erosion. Initially, these particles are transported into temporary sinks or repositories within the coastal margin that are non-depositional in the long term (years to decades), and are transported into temporary sinks. Large episodic events like the plume event resuspend and transport these materials from these temporary sinks to more permanent sinks with a small fraction becoming incorporated annually into the sediments of the depositional basins. The majority are stored in transient reservoirs as part of the biogeochemically important and active boundary layer. They are subsequently transformed by numerous biogeochemical processes over an average of 20 years (mean residence time of particle associated with the disappearance of long-lived fallout radionuclides and other tracers).
During this period, these easily resuspendible sediments constitute the major dispersed source of many biogeochemically important materials (BIMS) and contaminants to the pelagic system, however (1) We do not understand the extent, time dependence and dynamics of BIMS in the coastal margin system. (2) The repositories of sedimentary material in the system are not confined to`depositional' environments, but must exist as a concentrated `floc' layer at the benthic boundary throughout much of the system, particularly in the coastal margin. (3) These temporary repositories play a key role in cross-margin sediment transport and in the biogeochemical cycles of coastal systems in general and of the Great Lakes and Lake Michigan in particular, from the dynamics of phosphate supply and primary production to contaminant fate. (4) The residence time of new particles in these repositories depends upon a `conveyor belt' of resuspension and redeposition moving particles both alongshore and offshore. (5) We do not have a good understanding of the nature of the processes occurring at particle surfaces as a result of diagenesis and biogeochemical transformations. This research seeks to address these issues using a variety of in situ and laboratory techniques designed to look at the fate and transformation of biogeochemically active particles in a dynamic coastal system.