The recent invasion of Dreissenid mussels in there North American Great Lakes,especially quagga mussels(Dreissena bugensis)in Lake Michigan,has resulted in a profound shift in nutrient and carbon dynamics of the nearshore zone.With enhanced nearshore nutrient cycling,proliferation of the filamentous green alga Cladophora is evident in many areas,especially nearshore zones on the western and northeastern regions of the lake.Under turbulent conditions and during senescence,Cladophora detach,and in many cases,accumulate and decay both on the shore and in nearshore depositional zones.This shift in carbon flow in the nearshore zone has significant impacts on mercury(Hg)cycling,most notably in production of methyl mercury(MeHg).Two study sites were investigated--a nearshore beach and shallow water site,and a nearshore depression that serves as a Cladophora deposition zone.Solid phase deposits of decaying plant material on beaches were elevated in MeHg and were seasonally dynamic,with 10-50%of total Hg(HgT)as MeHg.Pore water(which most likely is transported advectively to Lake Michigan)collected under the decaying Cladophora mats ranged from 1.9 to 5.3 ng/L MeHg(64-76%of HgT as MeHg).A transect parallel to the shore showed increased aqueous MeHg levels near cladophora-rich zones,exceeding offshore zones by a factor of 2 to 10,with highest concentrations in the beach wash zone.At the second site,the Cladophora deposition zone,solid phase surface sediments were enriched with MeHg(10-20%of HgT as MeHg),with pore water and overlying water suggesting strong diffusive gradients from the deposited Cladophora.Using the quagga mussel as a biosentinel,we also observed about a twofold enrichment of MeHg content in nearshore organisms over their offshore counterparts.Previous work on Lake Michigan had identified deep sediment fluxes and riverine inputs as major drivers of MeHg cycling.Here we have identified a new source of nearshore MeHg production in Lake Michigan,driven by invasive species,that represents a potential zone for significant bioaccumulation.