Montana Field Guide

Morphological Category - Asymmetric biraphid.

A large, robust, bottle-shaped diatom with capitate apices. Axial area narrow, linear. Central area rounded. Raphe lateral; proximal ends bulbous, deflected slightly towards stigmata; distal fissures hooked towards side away from stigmata. Striae composed of prominent areolae, radiate, becoming parallel towards the apices.

Size Range
Length 65–160 µm. Width 35–40 µm. Striae in 10 µm 7–9. Stigmata 2–5.

Useful Link:
Diatom Glossary [Diatoms of North America website]

Distinguished in the field by the golden-brown, felt-like mats and under the microscope by the large, robust, bottle-shaped valves and translucent stalks.

Didymosphenia is more closely related to the cymbelloid genus Cymbella, especially Cymbella mexicana and Cymbella janischii, than to other gomphonemoid (bottle-shaped) genera (e.g., Gomphoneis and Gomphonema).

Didymo is found in benthos of rivers and creeks and wash zones of lake shores.

In Glacier National Park Didymo has predominately been found in flowing waters with fewer occurrences in standing water based on samples taken from 1976 to 2007 (Bahls 2007; see Range Comments section). Sites that supported substantial numbers in 2007 were exposed to openings within the forest canopy, of which several locations were in flowing waters below waterfalls or lake outlets. Sites that supported small numbers were found at higher elevation creeks.

Cells attach to stream bottoms by polysaccharide stalks, forming thick, golden-brown, felt-like mats. In older colonies, stalks may grow up to 1 meter in length and cells become detached, leaving just the white stalks. Mats typically persist for a short period of time, but under changing conditions can become large and persistent (see Management section).

WATER CHEMISTRY
Cool, alkaline, low-nutrient waters with low to moderate conductivity. Abundance-weighted means: pH = 8.3; conductivity = 144 µS/cm; temperature = 10.2 degrees Celsius.

MACROINVERTEBRATES
Where colonies develop large and thick mats, there can be a decrease in the density of large-bodied macroinvertebrates, particularly Mayflies (Ephemeroptera), Stoneflies (Plectoptera), and Caddisflies (Trichoptera), and an increase in small-bodied taxa such as Midges (Chironomidae) (Coyle 2015).

There is a misconception that Didymo is inherently invasive in streams (Coyle 2015; Bothwell et al. 2014; Taylor and Bothwell 2014). Research is demonstrating that the development of large mats is not a sign of ‘invasion’ as much as a signal that water quality in the water body is changing (Coyle 2015). Nuisance mats are defined as mats that extend more than 1 kilometer and persist for several months of the year (Spaulding and Elwell 2007). Unlike algal responses, Didymo forms nuisance mats during winter in cold rivers with low nutrient concentrations, high light availability, and stable rock (Coyle 2015; Bothwell et al. 2014; see Habitat section). These conditions occur below dams in the tailwaters of impounded streams, at lake outlets, along lake shores that have significant wave action, or in other similar microhabitats. Research on the Kootenai River in Montana have found nitrogen concentrations increasing since about 2006 as a result of upstream anthropogenic activities in the watershed (Coyle 2015). Downstream of Libby Dam on the Kootenai River, extensive mats have been found extending for 32 miles below the dam with patchy mats occurring for another 49 miles downstream and persisting for 10 months out of the year. Where mat coverage is the greatest, the average soluble reactive phosphorus (SRP) concentration was found to be below the detectable measure of 0.5 ug/L while the combined nitrate/nitrite concentrations were greater than 200 ug/L.

Colonies may become a nuisance and impair aesthetics and habitat for fish and other aquatic life (Schweiger et al. 2011). Large blooms of Didymo can create thick mats, making stream substrates more slippery. Large mats can snag fishing gear and alter the community of macroinvertebrates (Coyle 2015; see Ecology section).

Didymo may be a potential indicator of thermal and hydrological modifications associated with climate warming, particularly in fairly pristine ecosystems, such as in portions of Glacier National Park (Schweiger et al. 2011). In Glacier National Park, Didymo is anticipated to increase in abundance as a response to global warming and reduced stream flows while Distrionella incognita is expected to decrease in abundance; both of which could serve as indicators of environmental change (Bahls 2007).

Didymo is native in the USA, but is considered invasive in New Zealand and South America.

In Canada persistent nuisance mats were reported in 1989 on Vancouver Island and associated with popular fishing areas (Cothwell et al. 2009). Research found that Didymo could remain viable on damp, felt-soled waders indicating that fisherman could disperse this diatom (Kilroy et al. 2007). The development and persistence of mats seems to be linked more with site conditions and water quality (Coyle 2015; Bahls 2007).