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Brine fly
Ephydra gracilis
NatureServe conservation status
Global (G-rank): GNR
State (S-rank): SNR
External links
Phenology
The exact timing of brine fly emergence depends on local climate conditions, though at the Great Salt Lake (GSL) E. gracilis typically begin to hatch when the lake level recedes in May/June, continuing through October/November. Multiple generations can occur in a single year (Wurtsbaugh et al., 2011); larvae can hatch within one week of egg laying, moving through the larval and pupal stages over a period of several weeks to emerge as adults, which typically live less than a week (Collins, 1980). The period of larval development is flexible and can range greatly depending on environmental factors such as food availability and salinity (Collins, 1980; Wurtsbaugh et al., 2011).
Species range
Brine flies are distributed across saline lakes in North America, with notable occurrences in Texas, California, and Mexico (Milne & Milne, 1980). Though methods of dispersal are unknown, this species can also be found colonizing man-made saline habitats in states such as Ohio and even as far as Hawaii (Steinly, 2002; Wirth, 1947). In Utah, they are abundant around the Great Salt Lake (GSL). Ephydra gracilis occurs in association with the complex microbial communities that colonize GSL microbialites; the microbialites themselves cover approximately 20% of the lake bottom, limited to shallower waters at the lakes edge (Baskin, 2014). Following the construction of the railroad causeway across the lake in the 1950s, the north arm of the lake became uninhabitable for E. gracilis, due to rising salinity and the disruption of the microbial communities the fly relies on for survival (Baxter & Butler, 2020). A further road causeway isolating Farmington Bay from the rest of the lake has further disrupted fly habitat and caused them to disappear there.
Habitat
This species is closely associated with saline and alkaline environments, particularly salt lakes and marshes. In the Great Salt Lake (GSL), E. gracilis shows a strong preference for microbialite reefs that occur in the shallow margins of the lake (Collins, 1980). A “microbialite” is a unique type of sedimentary structure that occurs in some lake and ocean settings as the result of microbial activity: over long periods of time, microbial mats trap sediments and facilitate the precipitation of minerals from the water. The rocky, layered structures that result from this process are host to a diverse microbial community consisting mainly of archaeal and algal species (Post 1977); this community is responsible for a large portion of photosynthetic activity in the lake and creates the ideal conditions for brine fly development. Though E. gracilis lays its eggs on the water surface, resulting in a haphazard distribution owing to water currents, Collins (1980) observed that larvae of E. gracilis often concentrate on microbialites, showing a strong preference compared to sandier substrates; he also observed adults emerging from reef sites were significantly larger than those emerging from other sites, indicating better food availability and nutrition for developing flies. Microbialites also provide physical support structures for larvae, allowing them to physically attach to the substrate and grow a pupae and undergo metamorphosis (Baxter & Butler, 2020). These findings, as well as further studies demonstrating the biological productivity of microbialites in the GSL (Wurtsbaugh et al., 2011), highlight the importance of these deposits and their associated microbial communities in the conservation of E. gracilis.
Food habits
Adult brine flies feed on the periphyton of the water bodies they inhabit, and the larvae feed mainly on diatoms in the microbialite communities in which they occur (Collins 1980). Larvae in the Great Salt Lake (GSL) exhibit increased swimming activity at dusk, during which time they will seek out microbialites to feed. Larvae present on microbialites tend to remain there through their development into adults. Adults are short-lived and may remain on the water surface without feeding; however, some adults are observed to opportunistically feed on algae and other detritus, even submerging themselves to feed underwater at times (Collins, 1980).
Ecology
Brine flies play a crucial ecological role as a food source for numerous bird species, including migratory shorebirds and waterfowl. Their presence supports the Great Salt Lake's (GSL's) rich avian biodiversity. The GSL plays host to a large number of shorebirds and waterfowl every year, and represents irreplaceable habitat for many of them. For example, over 50% of North America’s Wilson’s phalaropes (a Utah SGCN) can be found in the GSL each fall (Roberts, 2013). The brine fly population at the GSL serves as an important food resource for these avian species, in part due to their great abundance. A 2013 review of the known food resources of the most abundant bird species found at the GSL found brine fly adults and larvae present in the diets of all species (Roberts, 2013); the studies used in this review reported up to 100% of avian diet consisting of brine fly adults (in the case of the American Avocet and Wilson’s phalarope) and up to 94% of diet consisting of brine fly larvae (in the case of the Red-necked phalarope). These high percentages indicate the importance of E. gracilis in the continued survival of avian species. They also highlight the vulnerability of these avian species should brine flies, or indeed the microbialites on which they rely, disappear. Though some omnivorous avian species may be able to adapt to a reduced abundance of brine flies, it is unlikely that all avian species could adapt to such a change. A study of red phalaropes at a similar saline lake in California found that the birds are not able to survive on brine shrimp alone, and brine flies are a critical component of avian diet in order to survive and gain weight (Rubega and Inoye, 1994). Findings such as these underscore the importance of understanding E. gracilis population dynamics, particularly as they relate to ongoing environmental change.
Threats or limiting factors
Despite its abundance around the Great Salt Lake (GSL), E. gracilis’s range is limited to a single ecosystem in Utah. The species is considered an SGCN for the key role it plays in the GSL and the consequences its reduced abundance would have on the conservation of other wildlife species. One of the largest and most obvious threats limiting the abundance of this species is the effect of human transportation networks on the GSL’s water chemistry. The construction of a railroad causeway across the lake in the 1950s dramatically reduced the habitable area of the lake for E. gracilis. The causeway bisected the lake into a north arm and a south arm, with the north arm receiving far fewer freshwater inflows than the south arm. Despite the extreme salt tolerance of the species, water in the north arm of the lake has become so saline as to near its saturation point, creating intolerable conditions for many lake species. The result has been the elimination of most of the species present in the original food-web from the north arm of the lake, including E. gracilis (Baxter & Butler, 2020). Since the construction of the railroad causeway, a second causeway has been constructed that separates Farmington Bay from the rest of the south arm of the lake. As this has reduced the mixing of inflowing freshwater with the rest of the lake, this reach of the lake has become less saline than the rest of the south arm. The result for brine flies, interestingly, has been the same as extreme salinity in the north arm: now lacking the ideal conditions for which they are adapted, E. gracilis is absent in Farmington Bay. In their place is a community of organisms adapted to lower salinity. Previous studies have shown the relative abundance of E. gracilis has decreased as salinity is diluted in lakewater (Welker & Havertz, 1973), but the lack of long-term studies of brine flies limits current understanding of how their populations change with varying salinity levels (Baxter & Butler, 2020). Further threats include water diversions for agriculture and other uses, as well as climate change and data gaps that limit our understanding of this species’s population dynamics.
The conservation of E. gracilis is tied closely to the continued existence of the microbialites on which the fly depends. Unfortunately, a number of factors presently threaten these microbial communities. Research has shown that heightened salinity in lake water causes a loss of microbial biomass within the mats that make up the microbialites (Lindsay et al., 2019). At levels above 20% salinity, it is expected that this reduction in microbial productivity will have cascading effects in the GSL ecosystem, reducing the number of brine flies and other organisms that ultimately serve as the primary food source for migrating birds (Baxter and Butler, 2020). Declining lake levels resulting from climate change and worsened by stream diversion projects also threaten the microbialites, which are distributed near the shoreline and are therefore at risk of becoming exposed, dramatically reducing their biological productivity (Baxter & Butler, 2020).
Because of the role the microbialites play in the survival of E. gracilis, as well as their role in the greater GSL ecosystem, continued monitoring and protection of these microbial communities should be a high priority.
