Chara brittonii T. F. Allen ex C. B. Robinson is a freshwater green macroalga in the family Characeae. Although treated as a variety of the apparently cosmopolitan Chara zeylanica in the most recent global monograph of the family (Wood and Imahori 1965), this taxon was originally described as a distinct species (Allen 1889, Robinson 1906) and molecular phylogenetic studies have supported its recognition at the species level (Karol et al. 2018). The species is now widely recognized as distinct (Karol et al. 2018, Scribailo and Alix 2010).

Chara brittonii has a known area of occurrence (AOO) of 36 km² and is present in nine extant known locations in the Midwest U.S.A, in the states of Indiana, Michigan, and Wisconsin. These consist of five lakes, one bog, two fens, and one group of interdunal ponds. It has undergone a decline in AOO (from 68 km² to 36 km²) and number of locations (from 15 to 9), but this decline is considered historic, as the last documented loss was recorded in 1982. Chara brittonii is classified as Near Threatened because it has a limited geographic range (AOO of 36 km²) and is only known from nine locations, making it close to qualifying as Endangered under Criterion B (B2ab(ii,iii,iv)). However, at present, quantitative habitat and population data, including population size, composition and genetic diversity, are lacking to assess contemporary population decline, and information to confirm any aspect of continuing decline in its habitat area and quality, population size, or distribution, are lacking. Threats to this species include human recreation, climate change, invasive plants and herbicides.

Further research in these areas could provide evidence that would reclassify C. brittonii into a threatened category.

Chara brittonii (Britton's Chara) currently occurs in the Midwest U.S.A., where it is known to be extant in the states of Indiana, Michigan, and Wisconsin. As of 2018, 24 vouchered localities from nine different locations scattered throughout these states were known. Historically, Britton's Chara was also known from New Jersey (type locality) and Ohio, but these localities have since been revisited without finding extant subpopulations (Karol et al. 2018). C. brittonii is not known to exist in any other locations worldwide. Within each extant location C. brittonii is typically restricted to the very shallow margins of wetlands. In deeper water it is out-competed by other more robust charophytes and larger aquatic vascular plants. Consequently, the area occupied by C. brittonii at a given location could be measured in a few square meters. It should be noted that hundreds of other locations within the possible range of C. brittonii have been surveyed over the last 20+ years without detecting additional populations (Sleith et al. 2015, Hall and Karol 2016, Karol et al. 2017, Karol et al. 2018, Sleith et al. 2018).

Only nine localities of Chara brittonii are known, and though it is considered quite rare (Karol et al. 2018), no quantitative data about population size, composition, or genetic diversity are available. C. brittonii occupies a specialized marginal freshwater habitat and so plants are only found in a small portion of the larger habitat. On the other hand, these habitats are not often studied, which may contribute to apparent rarity.

The biology and life history of C. brittonii may contribute to local abundance and long-term viability in the geographically small and isolated populations where it is found. C. brittonii is a physically small species – measuring less than a few inches tall – so a subpopulation of a few square meters may contain tens if not hundreds of individuals. Each individual may produce dozens of spores through sexual reproduction. Although the long-term viability of C. brittonii spores has not been tested, spores have been demonstrated to be viable for several years in other species of Characeae (Casanova and Brock 1999). Consequently, the extant locations probably derive from a multi-year spore bank in each locality. These spore banks would mitigate some of the risks from normal seasonal and annual environmental variability. On the other hand, the restricted size of the preferred shallow habitat likely limits the dispersal of spores to new habitats because it would be unlikely that a vector (most likely waterfowl) would ingest and later deposit the spores in another suitable habitat.

This species grows in a specialized habitat at the margins of alkaline freshwater lakes and wetlands, as well as shallow intradunal ponds. It is often slightly emergent, avoiding desiccation by capillary action and incurving branchlets that retain moisture. It has been reported from the shallow edges of lakes and ponds and in heavily saturated soils in bogs and fens in New Jersey and in the Midwest U.S.A. (Robinson 1906, Wood and Imahori 1965, Karol et al. 2018). One subpopulation is known from a series of interconnected calcareous dune ponds along the southern shore of Lake Michigan (Karol et al. 2018). Many of the localities are characterized by the presence of cold-water calcareous upwelling from underground seeps.

Historically, Chara brittonii was known from a hillside seep near a spring-fed creek in Indiana as well as an alkaline fen in Ohio. These populations are no longer extant. The type specimen of C. brittonii was collected from an unspecified locality two miles southwest of Sparta, New Jersey. There are no known populations in New Jersey. Currently, this species is known to be extant in Wisconsin at Cedarburg Bog, Ennis Lake, Bass Lake, Bohn Lake, and Pickerel Lake Fen; in Michigan at Lawrence Lake and Fish Lake; and in Indiana at the West Beach intradunal ponds and Beaver Dam Lake Fen. Within these habitats, C. brittonii only occurs in very shallow water – often less than one inch – meaning that the total area available in any habitat is limited. The environmental factors controlling the distribution of this species are not clear. However, it has been found in cold calcareous waters, often where an upwelling of ground water occurs, and seems to prefer sandy or marl locations with low nutrients and few competitors.

Due to the specificity and small size of its habitat, Chara brittonii is threatened by invasive plants that colonize wetland margins, water pollution from a variety of contaminants, human disturbance, and any fluctuations to the water level. These threats are a growing concern for freshwater ecosystems across the United States. While little information is known about the historic localities of this species, it is assumed that habitat loss and/or degradation caused by any number of these factors played a role in the shrinking of its historic range. Establishment of other species in the specialized areas where C. brittonii grows can be a threat as well, even if there is no direct competition involved. Increasing amounts of organic matter from the decay of vascular macrophytes over time can allow larger species to move into the area. This is a likely explanation for the extinction of this species from some locations, such as Cabin Creek bog in Indiana. The combination of too many emergent plants, an increase in organic matter, and excessive growth of other more aggressive grasses and sedges likely contributes to this.

Invasive species, especially invasive aquatic and emergent plants that encroach wetland margins (i.e., Frangula alnus, Phragmites australis) are a threat to all extant locations where this species occurs, where they can potentially outcompete and/or shade out C. brittonii. Four of the 9 total locations are found in protected areas that utilize long-term management practices to curb invasive plant species. These include prescribed burns, manual plant removal, and herbicide treatments. These management practices are currently implemented without considering the potential negative effects on C. brittonii and its specialized habitat. Runoff from nearby farmland, industrial areas, and/or golf courses poses a potential threat to 8 out of the 9 locations. Early-season algae blooms fed by runoff can impede oospore germination when they sink and blanket the bottom.

Recreational lake use poses many threats, especially if the lake margin is damaged or disturbed to allow convenient access. Potential introduction of invasive species and pollutants caused by boating is another threat. In addition to direct threats to the species and its preferred habitat, C. brittonii may also be imperilled by changes to local or regional hydrology due to human intervention or climate change. The establishment of larger stands of emergent species also can alter hydrology dramatically through excessive evapotranspiration rates in warmer weather. Any activity or alteration that would raise or lower the water table may affect the habitat where C. brittonii occurs. Further research is needed to determine the extent to which these threats are impacting this species.

There is no known utilization and/or trade for Chara brittonii.

There are no current conservation actions in place for this species, although four of the nine locations are located in protected areas. Chara brittonii is found in three State Natural Areas in Wisconsin (Cedarburg Bog, Ennis Lake, and Bohn Lake), and one National Park in Indiana (Indiana Dunes National Lakeshore).

Conservation efforts could include public awareness and education as well as measures to protect this species and its unique and fragile habitat. Restricting public access from the wetland and lakeside margins where C. brittonii is known to occur should be considered. Limiting the use of herbicides to control invasive emergent vegetation and instead employing manual removal techniques for these species should also be considered.