Stauroforma exiguiformis

This taxon has a circumboreal distribution. Populations illustrated with SEMs in North America have been reported from Cape Cod (Siver et al. 2005), and New Jersey (Siver and Hamilton 2011). Reported to occur in lakes, ponds, rivers and streams (Bahls 2021).

In the Great Lakes National Parks (Edlund et al. 2011, 2012, 2013), this taxon is common (1-10% relative abundance) in sediment core material from lakes in Apostle Islands (Outer Island Lagoon), Voyageurs (Shoepack, Brown, Ryan lakes), and more rare at St. Croix National Scenic Riverway (Lake St. Croix; Edlund et al. 2009).

This taxon is generally reported to be acidophilous (Flower et al. 1996, Siver et al. 2005, Buczko et al. 2012), though some sources report it to be alkaliphilous (Bahls 2021). Interestingly, Valieva et al. (2022) reported this taxon to occur in core material from the South Urals along with what was otherwise an alkaliphilic community. Buczko et al. (2012) conducted a principal component analysis which identified populations from Romania as acidophilic and oligotrophic. Novais et al. (2014) report this taxon to be occasionally aerophilic, neutrophilic, oligotrophic, and an endangered species.

Many research groups have used this taxon as an indicator organism and published regional environmental optima and tolerances including:

Flower et al. (1996): Reported a pH optima of 5.7 for “Fragilaria exiguiformis”, with low abundances below pH 5.0 and above pH 7.0 for populations from lakes in Britain, Sweden, and Norway. The authors noted a bimodal distribution suggesting hidden diversity.

Camburn and Charles (2000): pH – 6.41 +/- 0.78, Acid Neutralizing Capacity – 62 +/- 76 µeq/L, Total Aluminum – 71 +/- 135 µg/L, Dissolved Organic Carbon – 249 +/- 152 µmol/L, Total Phosphorus (TP) – 6.61 +/- 5.65 µg/L, for lakes from the Northeast US; however, the morphological species concept being applied would have encompassed both S. exiguiformis and S. inermis; without SEM images, it is not possible to verify the identity of the specimens presented in the light micrographs.

Siver et al. (2005): pH – 5.9 +/- 0.8, TP – 14 +/- 12.6 µg/L for populations from slightly acidic ponds in Cape Cod. The authors lumped S. exiguiformis and S. inermis in their counts due to challenges associated with distinguishing the two species via light microscopy.

Antoniades et al. (2008): Dissolved organic carbon – 6.10 +/- 1.54 mg/L for populations from Quebec. Color – 23 +/- 3.1 Pt units, Alkalinity – 34 +/- 2.3 µeq/L for populations from Labrador.

Siver and Hamilton (2011): pH – 6.1 +/- 0.8, TP - 13.5 +/- 11.7 µg/L for populations from the Atlantic Coastal Plain.

Bahls (2021): pH – 7.9, Conductivity – 1300 µS/cm, Temperature – 17.9 °C, Total Nitrogen – 530 µg/L, TP – 105 µg/L for populations from the Rocky Mountains, Great Plains, Cascades, Sierras and Nunavut. Though Bahls presents light micrographs of specimens identified as S. exiguiformis and S. inermis, it is unclear on what he based this distinction; no SEM images were included.