Lou, J., NOAA/GLERL
Schwab, D.J., NOAA/GLERL, david.schwab@noaa.gov
Beletsky, D., CILER, dima.beletsky@noaa.gov

A quasi-3D suspended sediment transport model was developed to study bottom sediment suspension and transport in Lake Michigan. The model used results from a three-dimensional circulation model and a wind wave model as the driving force. The nonlinear wave-current interactions were considered in calculations of turbulence intensity and bottom shear stresses. The sediment entrainment and suspension were empirically established and parameterized by laboratory and field data. The model was applied to the 1994-1995 Lake Michigan Mass Balance Study period as well as the turbidity plume event in March 1998. The model results were compared with field measurements and satellite imagery. The model was able to reproduce the general patterns of high turbidity, and gave a reasonable depiction of sediment erosion/deposition in the lake. The separate effects of waves and currents on sediment resuspension and onshore-offshore transport in Lake Michigan are investigated and discussed, as well as the combined effect of waves and currents. Finally, the limitations and possible improvements of the model are discussed.

Lesht, B. M. Argonne National Laboratory, lesht@anler.er.anl.gov

Previous observations of near-bottom conditions and sediment resuspension in Lake Michigan have concentrated on areas of high sediment deposition in the southeastern part of the lake and on Indiana Shoals in the southwestern part. No previous observations have been made in the western areas of the lake adjacent to the Wisconsin bluffs, primarily because these waters were considered to be non-depositional and thus limited in the amount of resuspendable sediment available for transport. During 1998, the first EEGLE field year, we used an instrumented tripod to make measurements intended to test the hypothesis that the recurrent winter-spring resuspension event (misnamed "plume") includes new material eroded from the Wisconsin near-shore. The results show that the local sediment, a sandy cohesive clay, is indeed resuspended in response to the combined action of currents and surface waves. Net horizontal sediment flux in April was almost directly parallel to the shore and toward the south. In November, however, the net horizontal sediment flux was onshore, perhaps representing the temporary storage of a more easily resuspendable pool of sediment.

Lesht, B. M., Argonne National Laboratory, lesht@anler.er.anl.gov
Leshkevich, G. A. NOAA/GLERL, george.leshkevich@noaa.gov

During early March 1998, a major spring storm caused a widespread near-shore sediment resuspension event in southern Lake Michigan. One result of this event was a spectacular eddy feature, thought to represent high concentrations of suspended sediment, that was observed off the eastern shore of the lake north of South Haven, Michigan, on March 12. Remnants of this feature could be detected with visible imagery across the lake weeks later. We have used AVHRR visible, near-infrared, and thermal data, as well as SeaWiFS visible data, to examine temporal changes in southern Lake Michigan associated with the spring sediment resuspension event. We attempted to track the movement and evolution of the disturbance fields by calculating both temporal and spectral changes in successive registered images of the lake. The results suggest that the SeaWiFS images might be useful for qualitative estimation of temporal changes in the particle size distribution of the suspended material.

Beletsky, D., CILER, dima.beletsky@noaa.gov
Schwab, D.J., NOAA/GLERL, david.schwab@noaa.gov
Lou, J., NOAA/GLERL

In this presentation, we apply numerical models of coastal circulation and wind-waves to the March, 1998 turbidity plume event in Lake Michigan to investigate the role of wind- induced circulation in the offshore transport of sedimentary material in Lake Michigan. According to satellite imagery, the strongest sediment resuspension occurred in the southern lake. This is consistent with wave model results that predicted larger waves in southern Lake Michigan due to the dominant northerly wind during the two storm events. Hydrodynamicmodel results showed that circulation in Lake Michigan is highly episodic since it is almost entirely wind-driven in early spring. The characteristic wind-driven circulation pattern in the lake consists of two counter-rotating gyres, a counterclockwise-rotating gyre to the right of the wind and a clockwise-rotating gyre to the left. The gyres are separated by a convergence zone along the downwind shore with resulting offshore flow and a divergence zone along the upwind shore with onshore flow. This two-gyre circulation pattern was especially clearly seen during two northerly wind events in March in southern Lake Michigan.

Johengen, T.H., CILER, johengen@umich.edu
Vanderploeg, H.A., NOAA/GLERL, henry.vanderploeg@noaa.gov
Ruberg, S.A., NOAA/GLERL, steve.ruberg@noaa.gov
Lang, G.A., NOAA/GLERL, gregory.lang@noaa.gov

A major sediment resuspension event occurred in Southern Lake Michigan in March of 1998. The resulting turbidity plume covered approximately 300 km of coastline and was evident from shore out to depths of around 35m. Surveys for suspended solids, nutrients, and plankton were conducted before, during, and after the plume event along nearshore-to-offshore transects distributed from Racine, WI to Muskegon, MI. During the plume, total suspended solids concentration increased from 1 to 33 mg/l and total phosphorus concentrations increased from 5 to 45 ug/l. Smaller increases in dissolved phosphorus and silica concentrations were also observed within the plume. A plankton survey system was tow-yoed throughout the water column to define fine scale distributions of particles and plankton. We were not able to completely characterize these distributions within the nearshore region of the plume because the extreme particle densities blinded the optical plankton counter and light attenuation meter. In general, particles and plankton were uniformly distributed throughout the water column and outer edge of the plume was fairly sharp. The plume was visible for over 6 weeks but a large fraction of the particles settled within a few weeks. Additional sampling with a modified survey system will occur in 1999 and results between years will be contrasted.

Bogdan, J.J., U of Iowa, jjbogdan@engineering.uiowa.edu
Hornbuckle, K.C, U of Iowa, kchorn@engineering.uiowa.edu

Atmospheric deposition is an important component of the whole-lake cycling of many organic compounds in Lake Michigan. We hypothesize that the magnitude of atmospheric deposition of persistent organic compounds (POPs) is at least partly dependent on the partitioning behavior of the compounds between water and sediment in the water column as well as seasonal meteorological/hydrological events. To test this hypothesis, we have collected air, water and sediment trap samples before (Jan. 1998) and during (March 1998) a major sediment plume event in Lake Michigan. These samples were collected in coordination with EEGLE (Episodic Events-Great Lakes Experiment), a large multidisciplinary study in its pilot year designed to assess the biogeochemical importance of the plume event. The air, water, and sediment trap samples were analyzed for a suite of PAHs and PCBs. Concentrations of these compounds were highly variable in space during the two sampling periods. For example, total dissolved phase PAHs ranged from 407.82 to 27,550.57 ng/m³. Some of this variation may be attributed to the plume event.