Invasive Dreissena Mussel Coastal Transport From an Already Invaded Estuary to a Nearby Archipelago Detected in DNA and Zooplankton Surveys.

Coastal waters of Lake Superior are generally inhospitable to the establishment of invasive Dreissena spp. mussels (both Dreissena polymorpha and Dreissena bugensis). Dreissena have inhabited the Saint Louis River estuary (SLRE; largest commercial port in the Laurentian Great Lakes) for over three decades, but only in the last few years have small colonies been found in the Apostle Islands National Lakeshore (APIS, an archipelago situated 85 km to the east of SLRE) A 2017 survey determined a low abundance Dreissena spatial distribution in APIS, with the largest colonies on the north and west islands which suggested potential veliger transport from the SLRE via longshore currents. Our objective in this study was to determine if Dreissena veligers are transported by currents at low densities along the south shore of Lake Superior from the SLRE to APIS. To do so, we used both eDNA (water and passive substrate samples) and zooplankton collection methods at eight sites evenly spaced between the SLRE and APIS with three sampling times over five weeks. Dreissena veligers were consistently detected along the south shore, although at low abundances (veligers per m3 range = 0-690, median = 8), and for every 1 km increase in distance from the SLRE, both veliger counts and water eDNA copy numbers decreased on average by 5 and 7%, respectively. D. polymorpha (suited to estuary habitats) was detected two times more than D. bugensis (better suited to deep-lake habitats). There was not a trend in the veliger size distribution along the south shore, and temperature and calcium concentrations fluctuated around the threshold for Dreissena veliger and adult development, averaging 11.0°C and 14.8 ppm, respectively. Three zooplankton taxa representative of the estuary community-Daphnia retrocurva, Diaphanosoma birgei, and Mesocyclops copepodites-decreased as the distance from the SLRE increased mirroring Dreissena veliger abundance patterns. Findings represent multiple sources of evidence of a propagule "conveyor belt" for Dreissena along the south shore of Lake Superior. We conclude that veligers are functioning as a propagule, using coastal currents to spread from the point of invasion, thereby traversing coastal habitat previously reported as inhospitable to distant habitats suitable for colonization.

Dreissena veligers in western Lake Superior - inference from new low-density detection.

The notion that Lake Superior proper is inhospitable to dreissenid mussel survival has been challenged by recent finds on shipwrecks and rocky reefs in the Apostle Islands region. Motivated by concerns surrounding these finds, we conducted an intensive sampling campaign of Apostle Islands waters in 2017 to understand Dreissena prevalence and distribution. The 100-site effort combined random and targeted sites and collected zooplankton, benthos, video, environmental DNA, and supporting water quality data. We did not find settled Dreissena in any video footage or benthos samples, and quantitative PCR applied to eDNA samples was negative for Dreissena. Dreissena veligers were found in almost half the zooplankton samples but at orders of magnitude lower densities than reported from other Laurentian Great Lakes. Veligers were most prevalent around the western islands and associated with shallower depths and slightly higher phosphorus and chlorophyll, but did not spatially match known (still very localized) settled Dreissena colonies. This is the first study to conduct veliger-targeted sampling in western Lake Superior and the first to report consistent detection of veligers there. We speculate that these Apostle Islands veligers are not a new locally-spawned component of the zooplankton community, but instead are transported from an established population in the St. Louis River estuary (~100 km away) by longshore currents; i.e., low-density propagule pressure that may have been present for years. Small-mesh zooplankton data collected along a gradient from the Apostle Islands to the St. Louis River estuary and enumerated with thorough veliger searching would help elucidate these alternatives.

A reference inventory for aquatic fauna of the Laurentian Great Lakes.

The Laurentian Great Lakes encompass an expansive and diverse set of freshwater ecosystems that contain a concordantly large and diverse vertebrate and invertebrate fauna. Although numerous publications exist concerning the composition and distribution of this fauna, there is at present no single readily available resource that brings all this information together. Here, we present and describe the compilation process for a comprehensive Great Lakes aquatic fauna inventory covering fishes, reptiles, amphibians, zooplankton, mollusks, annelids, insects, mites, and various other aquatic invertebrates. Inventory entries were developed via an extensive search of literature and internet sources and are attributed with detailed nomenclature information, general lake and habitat occurrences, and supporting citations and links to life history and genetic marker information. The inventory scope is the Laurentian Great Lakes proper and their connecting rivers, and their fringing coastal wetlands and lower tributaries. Over 2200 unique taxa are contained in the inventory -- 85% resolved to species and 14% to genus. The listing substantially expands previous richness estimates for invertebrates in the Great Lakes, but taxonomic resolution and spatial distribution information for them remains quite uneven. Example pattern analyses for fauna in this inventory show that aquatic vertebrates are generally more widely distributed than invertebrates, and that biodiversity is concentrated in the coastal margins. The inventory is being packaged into a public, searchable database that showcases the biodiversity of the Great Lakes aquatic fauna and can assist the research and management community in their biological investigations.