Episodic particle transport events controlling PAH and PCB cycling in Grand Traverse Bay, Lake Michigan

To evaluate the influence of episodic events on particle and hydrophobic organic contaminant (HOC) cycling in the Great Lakes, we deployed sequencing sediment traps at two locations in the western arm of Grand Traverse Bay, Lake Michigan. The traps collected integrated samples of settling particles every 2 weeks from May 1997 to September 1999. The total polycyclic aromatic hydrocarbon (t-PAH) and total polychlorinated biphenyl (t-PCB) settling fluxes from the surface waters in the southern site were significantly greater than those from the northern site. In addition, there were more frequent brief increases in the mass flux to the southern site than to the northern site. These episodic events, which occurred only 20% of the time, accounted for 65% of both the mass flux and t-PAH flux. The t-PCB flux was not influenced by these episodic events, and only 18% of the t-PCB flux occurred during these events. PAHs and PCBs appear to be tracing different types of particles in the water column. Several large mass flux events characteristic of seiches were observed simultaneously in the benthic nepheloid layer (BNL) at both the northern and the southern sites. The particles settling as a result of these resuspension events had lower t-PCB and t-PAH concentrations than particles settling at other times. This suggests that the material settling into the traps on the high mass flux days is composed of a mixture of the less contaminated underlying resuspended sediment and the "regular" contaminant-rich particles settling into the BNL.