WHAT'S SO SPECIAL ABOUT THE ST. CROIX — A SEMI-QUALITATIVE LOOK AT RARE LOTIC AQUATIC INVERTEBRATES AND HABITAT CHARACTERISTICS OF THE ST. CROIX AND OTHER WATERSHEDS IN WISCONSIN

William Smith, Wisconsin Department of Natural Resources

While widely perceived to be a special or even a unique river system, it seemed worthwhile to dig into what exactly might make the Saint Croix so. Fago and Hatch, 1996 reported on the aquatic resources of the Saint Croix River basin, but new information is available which allows a more detailed look with a different focus. Data used for this analysis are restricted to Wisconsin, meaning the Minnesota half of the Saint Croix watershed is not reflected. Datasets used are the WI Natural Heritage Inventory Program Invertebrate Atlas and the Great Lakes Aquatic Gap files for WI. Also, this analysis is limited to 123 rare aquatic invertebrate species (Species of Greatest Conservation Need-from WI's Wildlife Action Plan and WI NHI working List species).

The WI Invertebrate Atlas is a large working database maintained in ACCESS to capture all known sampling efforts for aquatic invertebrates and their results in WI including border streams. Published reports, gray literature, biotic inventory results, museum specimen data, web reports, and unpublished research efforts comprise this database. To date over 23,000 sample efforts are recorded resulting in over 146,000 taxa records. Nearly 1900 water bodies were sampled yielding 921,000 specimens. This database is maintained by the WI NHI Program.

The WI Aquatic Gap Database, the other key dataset utilized for this project, originated from the USGS Aquatic Gap Program (Brenden et al. 2006). This dataset consists of GIS coverage of WI stream segments from the National Hydrogeology Dataset (NHD) plus it individually attributes each of the component 40,247 stream segments with modeled environmental values using GIS. These segments represent 96,030km of stream channel in Wisconsin including border streams. WI DNR Science Services created several additional variables for this dataset utilizing GIS modeled procedures. I created yet additional variables by combining existing variables in several categories. Generally, these 225 variables addressed features of the landscape thought to be determinants of lotic animal distribution (Brenden et. al. 2006). Variable values were populated for one to four geographic scales. Table 1 below describes variable categories and scales at which they were populated.

NHI Conservation Status simply refers to GRANKS assigned by NatureServe and SRANKS assigned by the WI NHI Program. These ranks are based on a standardized methodology evaluating each species' range extent, area of occupancy, population size, number of occurrences, number viable occurrences, environmental specificity, long term and short term trends, threats, and intrinsic vulnerability (Master, et al., 2009). A 5 point scale is used with G(Global)1 and S(State)1 being the rarest and G5 and S5 being the most secure.

Comparisons are made between the Saint Croix watershed and 23 other WI watersheds with rare target species. The Saint Croix Basin was paired individually with each of the 23 other WI watersheds in Classification and Regression Tree runs (CART) and important Aquatic Gap variables for each run recorded in order to determine what about the watersheds might be different. The Saint Croix and Namekagon Rivers are compared with other WI streams harboring target species in terms of rare species diversity and important habitat characteristics (Aquatic Gap Variables). GRANKS and SRANKS of target species are looked at with respect to stream size to answer the question of what kind of stream harbors the rarest species. Predominant taxonomic groups of rare species present in large streams like the Saint Croix are compared to that of smaller streams. Finally I looked at uniqueness of the rare species represented by each stream. Specifically, does the Saint Croix harbor species that few or no other stream does, what habitat characteristics are important for these species, and how unique are these streams in a global sense?