Climate change and nuisance (or invasive) species pose serious threats to the structure and function of aquatic ecosystems worldwide (Parmesan and Yohe 2003). Over the past 100 years, annual mean air temperatures in Glacier National Park, Montana, have increased twice as much as global temperatures, resulting in declining snowpack, increasing fire frequency, altered hydrology, and loss of the park’s iconic glaciers (Pederson et al. 2010; Hall and Fagre 2003). Changes in the hydrological cycle will warm perennial streams, thereby threatening the stability of aquatic ecosystems and potentially increasing the spread of aquatic nuisance species (Rahel and Olden 2008).

Didymosphenia geminata (hereafter “didymo”; fig. 1) is a diatom native to mountain habitats of North America and Europe (Blanco and Ector 2009). In recent years didymo has expanded into lower elevations, latitudes, and new regions of the globe (Kumar et al. 2009). In Montana, didymo was first reported in 1929 at Flathead Lake (Prescott and Dillard 1979) and has likely been present in the northern Rockies since at least the end of the last ice age, about 10,000 years ago (Bahls 2007). Didymo can form extensive mats (or blooms), which can be several centimeters thick and up to 20 km (12 mi) in length (Blanco and Ector 2009). Larger blooms can inhibit growth of other algal species, change the composition of aquatic communities, decrease the amount of suitable spawning habitat for fish, and cause changes in stream chemistry (Spaulding and Elwell 2007). Blooms of didymo also greatly decrease the aesthetic appeal of streams—an important consideration for a tourist destination like Glacier National Park (fig. 1). For these reasons, understanding the causes and consequences of didymo blooms is a high priority for the park, especially for predicting and, where possible, managing its spread.

Because of the lack of data, speculation exists on why there has been a change in distribution of didymo and whether it is linked to climate warming (Bothwell and Spaulding 2008). Recent research suggests that didymo is associated with high mean summer temperature, a stable base flow index (less variation in streamflow), and is more abundant in nutrient-poor systems (Kumar et al. 2009). Here we present NPS monitoring data collected from 2007 to 2009 throughout Glacier National Park to estimate the distribution of didymo and better understand some of the environmental factors associated with its spread in a relatively pristine aquatic system.