The Impact of Episodic Events on the Cycling of Biogeochemically Important Materials in the Great Lakes: Potential Impact on Contaminants.

Brian J. Eadie. NOAA-GLERL

Seminar at USEPA GLNPO, Chicago. June 24, 1997

Abstract: A tight coupling between contaminated sediments and overlying water has been shown to exist in all of the Great Lakes ecosystems through the process of sediment resuspension. 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. 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.

Recent satellite observations of suspended sedimentary material in Lake Michigan illustrate a unique opportunity to investigate an annually recurrent major sediment resuspension episode, a 10 km wide plume of resuspended material extending over 200 km along the southern shores of the lake. 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. NSF CoOP and NOAA-COP have just begun a large multi-disciplinary project to employ a comprehensive measurement and modeling approach examining and comparing 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.

Based on EPAs long range strategic plans, it would appear that the Great Lakes COP program has many parallel objectives with EPA-GLNPO. The purpose of this briefing is to provide an overview of the Great Lakes COP objectives and early plans in order to promote cooperation with GLNPO and to encourage them to consider a supplemental research effort covering contaminants of interest to them (e.g., PCB, Toxaphenes, Hg).

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.