EEGLE: Episodic Events - Great Lakes Experiment

The Impact of Episodic Events on the Nearshore-Offshore Transport and Transformation of Biogeochemically Important Materials in the Great Lakes

B. Eadie, D. Schwab, V. Klump, W. Gardner

Program Summary

This proposal is submitted in response to the NSF/OCE and NOAA/COP Joint Announcement of Opportunity for Coastal Studies in the Great Lakes (NSF Publication 97-38). This proposal has been developed to focus on a critical theme that was common to two workshops (NOAA 1992; Klump et al. 1995): the importance of episodic events on nearshore-offshore transport and subsequent ecological consequences. Each of the components of this program is being led by a team of scientists with proven experience and long-term interest in coastal research. The episodic events study described in these proposals provides a unique opportunity to combine their talents in a comprehensive program directed toward the NSF-NOAA goals as defined in the Announcement of Opportunity:

  1. to determine what processes control the cross-margin (inshore to offshore) transport of biological, chemical, and geological materials in the coastal margins of the Great Lakes, and
  2. to develop and test scientific strategies for assessing, quantifying, and predicting the impacts of multiple stressors, both natural and anthropogenic, in the Great Lakes or selected sub-regions.
Issue: A tight coupling between contaminated sediments and overlying water exists in lakes and coastal ecosystems through the process of sediment resuspension. Recent satellite observations of suspended sedimentary material in Lake Michigan illustrate a unique opportunity to investigate an annually recurrent major episode of nearshore-offshore transport, a 10 km wide plume of resuspended material extending over 200 km along the southern shores of the lake (Fig. 1). The plume appears to be initiated by a major late winter storm after the melting of surface ice, and it eventually veers offshore along the eastern shore of the lake, coincident with the area of highest measured sediment accumulation in the lake. The inventory of particulate matter in the plume, on April 2, 1996, is approximately equal to the total annual load of fine sediments into the southern basin. Preliminary evidence indicates that this episodic event may be the major mechanism for cross-margin sediment transport in Lake Michigan. We believe this type of event is ideal for studying internal recycling of biogeochemically important materials (BIMS), ecosystem responses, and one of the major processes controlling cross-isobath transport in the Great Lakes. While we are focusing on a particular episodic process in southern Lake Michigan, the program results will be applicable to similar events in many coastal areas.
Significance: The episodic resuspension and subsequent transport of surface sediments profoundly influences biogeochemical processes in coastal ecosystems. Resuspension and transport of the large inventories of nutrients and contaminants deposited over the past few decades (e.g. P, 137Cs, PCBs), presently results in much greater fluxes to the water column than from all external inputs. In addition, control of biological processes can occur as a result of effects on light and substrate availability and the introduction of meroplanktonic species. The magnitude and episodic nature of these processes in the Great Lakes has been poorly described from a few point measurements or as the residual term in mass balance models. This multi-disciplinary project will employ a comprehensive measurement and modeling approach to examine and compare effects of episodic physical forcing in relation to more persistent long-term (i.e., seasonal meteorological) forcing on nutrient inventories, fluxes and distributions, and on biological distributions and rate processes. The results of this proposed research will improve our understanding of critical processes and support the development of a resource management-oriented information and modeling system.
Program Goal: The purpose of this program is to create an integrated observational program and numerical modeling effort to identify, quantify, and develop prediction tools for the winter-spring resuspension event and to assess the impact of this event on the transport and transformation of BIMS and on lake ecology. Three fundamental hypotheses focus this program:
  1. that the plume is a result of the first winter-spring storm after ice-out and represents the resuspension of particulate materials (and associated constituents) that have been stored in the lake as surface sediment "floc" for a distribution of times, during which they have undergone differential diagenesis,
  2. that the forced, two-gyre vorticity wave response of the lake to episodic wind events, occasionally modified by stratification, is a major mechanism for nearshore-offshore transport of particulate matter and associated constituents in the Great Lakes, and
  3. that physical processes, (e.g. resuspension, turbulence) associated with the plume event are important in determining the nutrient and light climate, and in structuring the biological communities throughout the spring isothermal period, and in setting the conditions for the critical `spring bloom' period.