Background/Question/Methods .  The Great Salt Lake has a salinity near 15% and is critical habitat for more than 200 bird species that account for 85 million bird-use days annually.  The diet of many of these birds is dependent on the food webs developing on stromatolite biostromes that grow profusely at depths < 3m and cover 23% of the lake’s littoral zone.  These reef-like structures are the only solid substrate in the lake and they are consequently the dominant area where periphyton and benthic invertebrates grow.  We investigated this community to understand its importance for production processes that support the bird assemblage and to assess whether they are an important vector for the extremely high mercury levels that have been found in three duck species that utilize the lake.

Results/Conclusions . The periphyton community growing on (and building) the biostromes is >99% colonial cyanobacteria (Aphanothece sp.).  Periphyton chlorophyll levels averaged 800 mg/m²or about seven times that of the lake’s phytoplankton.  Lake-wide estimates of chlorophyll suggest that production on the stromatolites rivals that of the phytoplankton.  The stromatolites are the principal habitat for brine fly (Ephydra gracilis) larvae and pupae that are fed upon by many of the birds utilizing the lake.  A pumped-bucket sampler operated by SCUBA divers was used to quantify densities and the size structure of larvae and pupae.  There was no significant difference between larval abundance at shallow (1 m) and deep (2.5 m) sites (p = 0.27), and densities increased from 7000/m² in June to over 20,000/m² in December.  Preliminary estimates of total mercury concentrations in the periphyton, fly larvae, fly pupae, and adult flies were 170, 166, 344, 551 ug Hg / kg dry weight, suggesting that biomagnification is relatively limited in this short food web.  However, the dominant bird feeding heavily on the brine fly larvae, the goldeneye duck (Anas clypeata), had concentrations near 60000 ug Hg / kg dry weight (Vest et al. 2008), indicating either very high biomagnification or that the ducks obtained their mercury loads elsewhere.  The relatively unstudied stromatolite community apparently is extremely important in the lake’s food web dynamics.  However, additional work is needed to understand the transport of mercury, selenium and other contaminants into ducks and shorebirds utilizing the lake.