COS 130-2 - Re-evaluating Dreissena composition and distribution in the St. Louis River estuary and Lake Superior

Abstract: Dreissena are an invasive genus of mussels that have a planktonic early stage of life (veligers) which is passively transported by currents. There are two species in the genus, zebra mussel (D. polymorpha) and quagga mussel (D. bugensis), with zebra being generally more inshore/littoral and quagga being more offshore/profundal. Dreissena are established in the lower Great Lakes but have made only small inroads into Lake Superior, most notably in the St. Louis River estuary (SLRE) where previous surveys found almost all bottom-settled Dreissena to be zebra mussels. Older surveys were not able to identify veligers to species, but we can do so now thanks to development of a species-level qPCR marker; qPCR methods are also better suited to detection of low-abundance veligers than morphological zooplankton ID. In 2019, the first year we had this qPCR marker, our Lake Superior nearshore veliger monitoring survey showed an unexpectedly high proportion of quagga mussel eDNA at the mouth of the SLRE. Which was both unexpected given the older SLRE data and concerning since quagga are potentially better suited to colonizing open Lake Superior. Therefore in 2022 we collected new samples in the SLRE to reevaluate species composition of Dreissena along a lake to upriver gradient. Sample collection took place late July to mid-September, when temperatures are suitable for veliger production, and yielded 165 eDNA samples, 24 zooplankton samples, and 90 benthic trawl samples. We also obtained 187 zooplankton samples from across Lake Superior (112 offshore and 75 nearshore) that are being analyzed for presence of veligers using qPCR methods. The bottom trawls have already been evaluated for Dreissena presence -- 60% of them contained mussels and the spatial extent has expanded relative to previous surveys and qPCR analyses of the zooplankton and eDNA samples are actively underway. Using these data, we will present on the distribution of eDNA and veligers of the two Dreissena species within the SLRE and report on the extent to which Dreissena propagules are reaching open Lake Superior.