A NEW SOURCE OF BIOGENIC SEDIMENT IN THE GREAT LAKES: CONTRIBUTIONS OF QUAGGA MUSSELS (DREISSENA ROSTRIFORMIS BUGENSIS) TO THE BEACH SEDIMENTS OF LAKES ONTARIO AND ERIE

The invasive mussel species Dreissena polymorpha (zebra mussels) and D. rostriformis bugensis (quagga mussels) have had a profound impact on the benthic communities, ecology, and physical limnology of the Great Lakes. To this list, we now add a sedimentologic impact – the production of bioclastic sediment. Valves and fragments of valves of these mussels are making a substantial contribution to beach sediments and even appear in coastal dune sands in the eastern Great Lakes.

Shallow surface samples (1-5 mm depth) with fragments of Dreissena sp. were collected from the swash zone at Selkirk Shores State Park (SSSP) and from dunes at Sandy Island Beach State Park (SIB) on the eastern end of Lake Ontario (New York State). Whole mussels and complete valves were identified as D. rostriformis bugensis (quagga mussels). Concentrations of shell fragments range from 9.2% to 25.6% in the surf zone at Selkirk Shores and 1.4% to 2.9% in deflation lags in dunes at Sandy Island Beach (random point counts on strews; n = 95 to 142). Weight percents, derived from acid digestion, range from 1.9% to 2.3% from the SIB dunes and 3.0% to 13.6% in the swash samples from SSSP. High concentrations of Dreissena debris in the swash zone were observed also at Point Gratiot beach in Dunkirk, New York (Lake Erie) but were not sampled.

Modal sizes of Dreissena bioclasts in samples range from coarse sand to small pebbles (whole valves and articulated shells). Fragments of shells are typically one Wentworth grade coarser than the accompanying siliciclastic sediments. Occurrence of these bioclasts in the swash zone with smaller siliciclastic sediments indicates hydraulic equivalence, resulting from the lower density (2.4-2.5 g/cm³) of the shell fragments. Shape factors likely play a role in hydraulic equivalence as well.

Valves of D. rostriformis bugensis have smaller, weaker byssal attachments than D. polymorpha, which makes these mussels easier to dislodge and transport. We hypothesize that quantities of Dreissena bioclasts have increased since D. rostriformis bugensis has outcompeted D. polymorpha. Current levels of CaCO₃ saturation and pH in lakes Erie and Ontario suggest that Dreissena fragments are chemically stable and likely to persist. Over time, we expect quantities of Dreissena sp. bioclasts to increase in the coastal sediments of the Great Lakes.