Waterbird Egg Mercury Concentrations in Response to Wetland Restoration in South San Francisco Bay, California

Executive Summary

Overview

• The conversion of 50–90 percent of 15,100 acres of former salt evaporation ponds to tidal marsh habitat in the south San Francisco Bay, California, is planned as part of the South Bay Salt Pond Restoration Project. This large-scale habitat restoration may change the bioavailability of methylmercury. The South Bay already is known to have high methylmercury concentrations, with methylmercury concentrations in several waterbirds species more than known toxicity thresholds where avian reproduction is impaired.
• In this 2013 study, we continued monitoring bird egg mercury concentrations in response to the restoration of the Pond A8/A7/A5 Complex to a potential tidal marsh in the future. The restoration of the Pond A8/A7/A5 Complex began in autumn 2010, and the Pond A8 Notch was opened 5 feet (one of eight gates) to muted tidal action on June 1, 2011, and then closed in the winter. In autumn 2010, internal levees between Ponds A8, A7, and A5 were breached and water depths were substantially increased by flooding the Pond A8/A7/A5 Complex in February 2011. In June 2012, 15 feet (three of eight gates) of the Pond A8 Notch was opened, and then closed in December 2012. In June 2013, 15 feet of the Pond A8 Notch again was opened, and the Pond A8/A7/A5 Complex was a relatively deep and large pond with muted tidal action in the summer.
• This report synthesizes waterbird data from the 2013 breeding season, and combines it with our prior study’s data from 2010 and 2011.

Approach

• We tested the effect of the Pond A8 restoration project by examining changes in waterbird egg mercury concentrations over time (2010, 2011, and 2013) at the Restored Ponds (Pond A8/A7/A5 Complex) in relation to Reference Ponds that were outside the restoration area.

Results

• We sampled 60 Forster’s Tern (Sterna forsteri) and 60 American Avocet (Recurvirostra americana) (hereafter referred to as “Tern” and “Avocet,” respectively) eggs during the 2013 study. Together with our historical dataset from 2010 and 2011, the total sample size of eggs included in this report was 180 Tern and 224 Avocet eggs.
• Egg mercury concentrations averaged 1.63 micrograms per gram fresh wet weight (μg/g fww; range of 0.44–7.33 μg/g fww) in Terns and 0.32 μg/g fww (range of 0.02–1.99 μg/g fww) in Avocets across all 3 years.
• Mercury concentrations in Tern eggs increased by 69 percent between 2010 and 2011 at the Restored Ponds after the restoration actions, compared to a slight decrease in Tern egg mercury concentrations (10 percent) between years at the Reference Ponds.
• Between 2011 and 2013, Tern egg mercury concentrations decreased by 59 percent at the Restored Ponds, compared to a decrease of 23 percent at the Reference Ponds.
• The end result of this 3-year comparison was that Tern egg mercury concentrations decreased between 2010 and 2013 by 31 percent at both the Restored and Reference Ponds.
• Avocet egg mercury concentrations in Restored Ponds followed a pattern similar to the concentration trajectory for the Reference Ponds, with egg mercury concentrations decreasing by 4 percent between 2010 and 2011, and further decreasing by about 40 percent between 2011 and 2013. The end result was that Avocet egg mercury concentrations decreased between 2010 and 2013 by 42 percent at the Restored Ponds and 43 percent at the Reference Ponds.
• Regardless of the changes between years, Avocet egg mercury concentrations in the Restored Ponds still were 92 percent higher than concentrations in the Reference Ponds in 2013, similar to 2010 baseline conditions where mercury concentrations in the Restored Ponds were 87 percent higher than concentrations in the Reference Ponds. Similarly, Tern egg mercury concentrations in Restored Ponds were 6 percent higher than concentrations in the Reference Ponds in 2013, similar to 2010 baseline conditions where mercury concentrations in the Restored Ponds were 7 percent higher than concentrations in the Reference Ponds.
• In the Restored Ponds in 2013, most Tern egg mercury concentrations (70 percent) and a smaller percentage of Avocet egg mercury concentrations (11 percent) still remained higher than concentrations associated with reproductive impairment (>0.90 μg/g fww). For reference, 90 percent and 100 percent of Tern eggs in the Restored Ponds exceeded this threshold in 2010 and 2011, respectively.

Conclusions

• Despite the significant increase (2010–2011) and correspondingly large decrease (2011–2013) in Tern egg mercury concentrations at the Restored Ponds, both Tern and Avocet egg mercury concentrations in the Restored Ponds still are similar to concentrations that would have been expected without the restoration actions.
• Ambient egg mercury concentrations within the south San Francisco Bay have decreased since 2010, but the restoration actions apparently did not change the baseline trajectory of egg mercury concentrations within the Restored Ponds. In particular, Ponds A7 and A8 still are mercury “hotspots” for both Tern and Avocet eggs compared to other ponds in 2013, even though it has been 2 years since most construction activities have ceased and the Pond A8/A7/A5 Complex has been opened to the flow of bay water.
• It will be important to document if egg mercury concentrations continue to decrease in the future as the Restored Ponds continue to develop, the Pond A8 Notch is opened for more of the year, the gates are opened earlier (March 6 in 2014), and more gates are opened (currently [2014] only three of eight gates are being opened). With further decreases in egg mercury concentrations relative to the Reference Ponds, the Restored Ponds possibly could attain lower egg mercury concentrations than would have been expected without any restoration actions, but, as of 2013, this goal for the South Bay Salt Pond Restoration Project has not been met.
• Continued monitoring of waterbird egg mercury concentrations in the restoration project area, relative to reference sites, is warranted over a period of multiple years, given the large fluctuations in waterbird egg mercury concentrations observed in these studies.
• The data indicate that the South Bay Salt Pond Restoration Project develop and implement a long-term monitoring strategy for methylmercury exposure to nesting waterbirds at several project sites. This monitoring network could build on the existing and robust dataset of methylmercury concentrations in eggs of key waterbird species that breed within the South Bay Salt Pond Restoration Project boundaries, including Forster’s Terns, American Avocets, and Black-necked Stilts (Himantopus mexicanus). These data would allow restoration managers to document changes in methylmercury bioaccumulation in taxa most sensitive to methylmercury exposure and would guide multiple restoration actions occurring at several sites within the region.

First posted September 9, 2014