Waterbird Egg Mercury Concentrations in Response to 

Wetland Restoration in South San Francisco Bay, California

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2014–1189
Cover: Bird photograph by Abe Borker, 2010. Egg photograph by Michael Kern, 2009.

.
Waterbird Egg Mercury Concentrations in Response to 

Wetland Restoration in South San Francisco Bay, California

By Josh T. Ackerman, Mark P. Herzog, C. Alex Hartman, Trevor Watts, and Jarred Barr

Open File Report 2014–XXXX 

Open-File Report 2014–1189

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Ackerman, J.T., Herzog, M.P., Hartman, C.A., Watts, T., and Barr, J., 2014, Waterbird egg mercury concentrations in 

response to wetland restoration in south San Francisco Bay, California: U.S. Geological Survey Open-File Report 

2014-1189, 22 p., http://dx.doi.org/10.3133/ofr20141189.

ISSN 2331-1258 (online)

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iii

Contents

Executive Summary....................................................................................................................................................... 1

Overview .................................................................................................................................................................... 1

Approach.................................................................................................................................................................... 1

Results....................................................................................................................................................................... 1

Conclusions ............................................................................................................................................................... 2

Introduction.................................................................................................................................................................... 3

Study Area..................................................................................................................................................................... 5

Methods......................................................................................................................................................................... 6

Mercury Determination ........................................................................................................................................... 6

Statistical Analysis.................................................................................................................................................. 6

Results and Discussion ................................................................................................................................................. 7

American Avocets (Recurvirostra americana) ........................................................................................................ 7

Forster’s Terns (Sterna forsteri) ............................................................................................................................14

Management Implications.............................................................................................................................................19

Acknowledgments ........................................................................................................................................................19

Unpublished Data Referenced......................................................................................................................................20

References Cited..........................................................................................................................................................20

Figures

Figure 1. Locations of all American Avocet (Recurvirostra americana) and Forster's Tern (Sterna forsteri) eggs 

collected for this study, south San Francisco Bay, California, during the 2010, 2011, and 2013 nesting seasons ........ 5

Figure 2. Pond site and year differences in egg total mercury (THg) concentrations (in micrograms per gram

fresh wet weight, μg/g fww) for American Avocets (Recurvirostra americana) nesting in South Bay Salt Pond 

Restoration Project area, south San Francisco Bay, California ................................................................................... 11

Figure 3. Total mercury (THg) concentrations (in micrograms per gram fresh wet weight, μg/g fww) in American 

Avocet (Recurvirostra americana) eggs by date in South Bay Salt Pond Restoration Project area before (2010, blue 

dots) and after (2011, red dots; 2013, green dots) management activities associated with restoration of Pond 

A8/A7/A5 Complex, south San Francisco Bay, California, autumn 2010–spring 2011 ................................................ 12

Figure 4. Model-averaged mean and 95-percent credible intervals for total mercury (THg) concentrations (in 

micrograms per gram fresh wet weight, μg/g fww) in American Avocet (Recurvirostra americana) eggs in 

South Bay Salt Pond Restoration Project area before (2010) and after (2011 and 2013) management activities 

associated with restoration of Pond A8/A7/A5 Complex, south San Francisco Bay ,California, autumn 

2010–spring 2011........................................................................................................................................................ 13

Figure 5. Pond site and year differences in egg total mercury (THg) concentrations (in micrograms per gram fresh 

wet weight, μg/g fww) for Forster’s Terns (Sterna forsteri) nesting in South Bay Salt Pond Restoration Project area, 

south San Francisco Bay, California............................................................................................................................ 16

Figure 6. Total mercury (THg) concentrations (in micrograms per gram fresh wet weight, μg/g fww) in Forster’s Tern 

(Sterna forsteri) eggs by date in South Bay Salt Pond Restoration Project area before (2010, blue dots) and after 

(2011, red dots; 2013, green dots) management activities associated with restoration of Pond A8/A7/A5 Complex, 

south San Francisco Bay, California, autumn 2010–spring 2011 ................................................................................ 17

Figure 7. Model-averaged mean and 95-percent credible intervals for total mercury (THg) concentrations (in 

micrograms per gram fresh wet weight, μg/g fww) in Forster’s Tern (Sterna forsteri) eggs in South Bay Salt Pond 

Restoration Project area before (2010) and after (2011 and 2013) management activities associated with restoration 

of Pond A8/A7/A5 Complex, south San Francisco Bay ,California, autumn 2010–spring 2011................................... 18
iv

Tables

Table 1. Egg total mercury concentrations for Forster’s Terns (Sterna forsteri) and American Avocets 

(Recurvirostra americana) nesting in South Bay Salt Pond Restoration Project area before (2010) and after 

(2011 and 2013) management activities associated with restoration of the Pond A8/A7/A5 Complex,

south San Francisco Bay, California, autumn 2010–spring 2011 ................................................................................. 9

Table 2. Model selection results for egg total mercury concentrations (in micrograms per gram fresh wet weight )

in American Avocets (Recurvirostra americana) nesting in South Bay Salt Pond Restoration Project area before 

(2010) and after (2011 and 2013) management activities associated with restoration of Pond A8/A7/A5 Complex, 

south San Francisco Bay, California, autumn 2010–spring 2011 ................................................................................ 10

Table 3. Model selection results for egg total mercury concentrations micrograms per gram fresh wet weight)

in Forster’s Terns (Sterna forsteri) nesting in South Bay Salt Pond Restoration Project area before (2010) and 

after (2011 and 2013) management activities associated with restoration of the Pond A8/A7/A5 Complex, 

south San Francisco Bay, California, autumn 2010–spring 2011 ................................................................................ 15

Conversion Factors, and Abbreviations and Acronyms

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

SI to Inch/Pound

Multiply By To obtain

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

microgram (μg) 0.00000003527 ounce, avoirdupois (oz)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Abbreviations and Acronyms

AIC Akaike’s Information Criterion

dw dry weight

ww wet weight

fww fresh wet weight

Hg mercury

THg total mercury

MeHg methylmercury

NCM New Chicago Marsh

USGS U.S. Geological Survey
1

Waterbird Egg Mercury Concentrations in

Response to Wetland Restoration in South 

San Francisco Bay, California

By Josh T. Ackerman, Mark P. Herzog, Alex Hartman, Trevor Watts, and Jarred Barr 

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. 
2

• 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. 
3

• 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 longterm 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.

Introduction 

Two of the most significant anthropogenic changes in the San Francisco Bay estuary over the 

past 150 years are the loss of more than 85 percent of fringing tidal wetlands (Goals Project, 1999) and 

the contamination of the estuarine food web with mercury (Hg). These effects are particularly 

pronounced in the South San Francisco Bay (South Bay), which, historically, was fringed with extensive 

tidal marshes and which receives drainage from New Almaden, the largest historic Hg mine in North 

America. Extensive wetland restoration in the South Bay aims to return tidal marshes and the important 

ecosystem function that these wetlands provided. However, high rates of methylmercury (MeHg, the 

most toxic and bioaccumulative form of Hg) production, export, and bioaccumulation have been 

associated with wetlands relative to other water bodies (Hurley and others, 1995; Krabbenhoft and 

others, 1999; Waldron and others, 2000; Marvin-DiPasquale and others, 2003). Therefore, Hg 

bioavailability potentially could increase in the South Bay as former salt ponds are restored to tidal 

marsh and other habitats that receive bay water during parts of the year. This is a particularly important 

concern, because Hg concentrations in tissues and eggs of birds in the South Bay exceed toxicological 

thresholds (Ackerman and Eagles-Smith, 2008, unpublished), and there is evidence that Hg may be 

impairing egg hatchability, chick survival, and body condition of birds in San Francisco Bay (Ackerman 

and Eagles-Smith, 2008, unpublished; Ackerman and others, 2008a, 2012a). Thus, any increase in 

MeHg production and subsequent bioaccumulation in waterbirds may further impair bird reproduction, 

as well as other wildlife and associated human health risks. 
4

For the South Bay Salt Pond Restoration Project, one of the first major restoration management 

actions was the breaching of the internal levees that separated Ponds A5, A7, and A8, which was done 

during the winter of 2010–11 in preparation for restoring muted tidal action to the newly formed 

Restored Ponds A8, A7, and A5 (hereafter referred to as “the A8/A7/A5 Complex”), beginning on June 

1, 2011. Tidal exchange was facilitated by the construction of an adjustable 40-ft-wide weir-like notch 

in the southeastern corner of Pond A8 (A8 Notch), which reconnects hydrologic flow between the 

A8/A7/A5 Complex and Alviso Slough for 6 months of the year (about June 1–November 30), with the 

A8 Notch closed during the remainder of the year because of permit restrictions to protect anadramous 

fish. Concerns about the construction of the A8 Notch and the opening of the A8/A7/A5 Complex 

include sediment scour and redistribution of Hg in adjacent Alviso Slough (where total mercury [THg] 

concentrations in sediments were 3 times higher than in the greater South Bay; Marvin-DiPasquale and 

Cox, 2007), and potential changes to MeHg biomagnification in the A8/A5/A7 Complex, Alviso 

Slough, and the larger South Bay Salt Pond Restoration Project area. 

In Pond A8, MeHg concentrations in the biota and sediments are among the highest of any 

measured in wetlands in the entire South Bay (Ackerman and others, 2007a, 2007b, unpublished; 

Ackerman and Eagles-Smith, 2008, unpublished; Miles and Ricca, 2010; Ackerman and others, 2012b, 

unpublished, 2013a, unpublished). It is unclear how Hg cycling within the A8/A5/A7 Complex will

progress with the periodic opening and operation of the Pond A8 Notch. 

During the initial study of the Pond A8 restoration, Ackerman and others (2013a, unpublished) 

noted that egg mercury concentrations in the Forster’s Tern (Sterna forsteri, hereafter referred to as 

“Tern”) increased substantially between 2010 and 2011 at Restored Ponds A8 and A7 (an average 

increase of 74 percent or 1.22 μg/g fww), but were similar between years at Reference Ponds A1 and 

A2W outside of the restoration area (change of -9 percent or -0.04 μg/g fww). For American Avocets 

(Recurvirostra americana, hereafter referred to as “Avocets”), the change in egg mercury 

concentrations between years in Restored Ponds (-3 percent or -0.011 μg/g fww), relative to Reference 

Ponds (-0.4 percent or -0.008 μg/g fww), was small. After the restoration activities, 100 percent of Tern 

eggs and 14 percent of Avocet eggs within Restored Ponds A7 and A8 exceeded the 0.90-μg/g fww 

toxicity threshold. Similarly, the restoration activities between 2010 and 2011 increased mercury 

concentrations in fish in the Restored Pond A8/A7/A5 Complex relative to ambient mercury 

concentrations in the Reference Ponds. 

These increased mercury concentrations in biota occurred in the year immediately following the 

construction actions and initial opening, but it was uncertain whether high mercury concentrations in 

bird eggs would continue in the Pond A8/A7/A5 Complex as this restored habitat was further developed

and the Pond A8 Notch was widened further. The authors suggested that the South Bay Salt Pond 

Restoration Project should develop and implement a long-term monitoring strategy for methylmercury 

exposure to nesting waterbirds to document changes in bird-egg mercury concentrations in the Pond A8 

restoration project as well as throughout the South Bay Salt Pond Restoration Project area as other 

restoration and enhancement projects are constructed. Therefore, the South Bay Salt Pond Restoration 

Project, U.S. Environmental Protection Agency, and State of California Coastal Conservancy funded the 

U.S. Geological Survey in 2013, 2 years after the Pond A8 Notch was opened, to document near-term 

trends in waterbird egg mercury concentrations.
5

Study Area

Within the South Bay Salt Pond Restoration Project boundaries, the main study sites were within 

the Don Edwards San Francisco Bay National Wildlife Refuge and Eden Landing Ecological Reserve 

(fig. 1). The study focused on the Pond A8/A7/A5 Complex and Reference Ponds A1, A2W, AB1, 

N4/N5, E2, SF2, and New Chicago Marsh (NCM). Reference sites were critical to assess baseline 

“ambient” Hg bioaccumulation that was not associated with the restoration activities that occurred in the 

Pond A8/A7/A5 Complex. 

Figure 1. Locations of all American Avocet (Recurvirostra americana) and Forster's Tern (Sterna forsteri) eggs 

collected for this study, south San Francisco Bay, California, during the 2010, 2011, and 2013 nesting seasons. 

Restored Ponds A8, A7, and A5 are highlighted in blue. Reference Ponds A1, A2W, AB1, N4/5, E2, SF2, and New 

Chicago Marsh (NCM) are highlighted in white. 

2010 2011

2013

Avocet

Tern
6

Methods

We monitored MeHg concentrations in randomly collected Tern and Avocet eggs at more than 

four colonies per species per year in 2010, 2011, and 2013 (fig. 1). Colony locations for Tern and 

Avocet egg collections were selected to include two primary nesting colonies in the restored area (Ponds 

A8 and A7) and two nesting colonies outside of the immediate vicinity of the Pond A8 restoration area 

to act as reference sites (Ponds A1 and A2W in the Moffett Salt Pond Complex). For Avocets, reference 

nesting colonies also were included in five additional pond units (NCM and Ponds AB1, E2 [not in 

2013], N4/N5 [not in 2013], and SF2). We randomly sampled one egg from as many as 15 nests per 

colony for each species during the 2010, 2011, and 2013 nesting seasons. We refrigerated collected eggs 

until laboratory processing, at which time we measured egg size and volume, dissected and opened each 

egg, removed all egg contents and placed them in a polypropylene jar, and froze the egg at -20 oC until 

THg analysis. 

Mercury Determination

The mean percentage of THg in the MeHg form in bird eggs is 96 percent (Ackerman and 

others, 2013b); therefore, THg concentrations were used to estimate MeHg concentrations in bird eggs. 

We processed and analyzed all egg samples for THg at the U.S. Geological Survey, Dixon Field Station 

Environmental Mercury Laboratory, using a Milestone DMA-80 Direct THg Analyzer (Milestone Inc., 

Shelton, Connecticut) following U.S. Environmental Protection Agency Method 7473 (U.S.

Environmental Protection Agency, 2000). THg concentrations in eggs were determined on a dry-weight 

basis and then converted to a fresh wet weight egg concentration using egg moisture content and a 

species-specific egg volume and egg density coefficient developed by J.T. Ackerman (U.S. Geological 

Survey, unpub. data, 2009). Quality-assurance measures included analysis of two certified reference 

materials (either dogfish muscle tissue [DORM-3 or DORM-4], dogfish liver [DOLT-3 or DOLT-4], or 

lobster hepatopancreas [TORT-2 or TORT-3] by the National Research Council of Canada, Ottawa), 

two system and method blanks, three continuing calibration verifications, and two duplicates per batch. 

Recoveries (mean ±SE) were 101.2 ±0.5 percent (N=125) for certified reference materials, 98.7 ±0.4 

percent (N=178) for calibration verifications, and 96.9 ±0.5 percent (N=87) for matrix spikes. Absolute 

relative percent differences were 2.8 ±0.3 percent (N=111) for duplicates and 2.1±0.3 percent (N=43) 

for matrix spike duplicates. 

Statistical Analysis

To test for changes in egg THg concentrations associated with restoration actions in the Pond 

A8/A7/A5 Complex, we performed linear mixed modeling (Pinheiro and Bates, 2000) to test for 

differences among wetlands and years for Terns and Avocets. Each species was analyzed separately and

egg THg concentrations were log-transformed prior to analyses. 

For Terns and Avocets, we tested the effects of the restoration actions by examining the 

differences in egg THg concentrations between (1) Restored Ponds and Reference Ponds, and (2) among 

years (2010, 2011, and 2013). In addition to Pond Type (Restored Ponds compared to Reference Ponds)

and Year, we also included Nest Initiation Date (standardized as day of the year), the quadratic form of 

Date (Date2; for example, Eagles-Smith and Ackerman, 2009), and all two-way interactions (Pond 

Type×Year, Pond Type×Date, Year×Date), yielding a total of 31 models including the null model 
7

(intercept and variance only). We did not include the Before or After the Pond A8 Notch opening test 

because we did not find it to be important for THg concentrations in Tern eggs in 2010 and 2011, and, 

in 2013, few data were available after the Pond A8 Notch was opened. We used 180 Tern and 224 

Avocet eggs in the modeling effort. In all models, we incorporated pond site (pond identification) as a 

random effect. We assessed model performance using model inference and Akaike’s Information 

Criterion (specifically, the second-order metric, AICc; Burnham and Anderson, 2002). 

All predictions are model-averaged predictions, based on 1,000 simulations of each model 

weighted by the AICc weight of each model. The overall mean is the mean of these 1,000 simulations. 

We also present 90 percent credible intervals (hereafter referred to as “90 percent CI”) between the 5th

and 95th percentiles of the 1,000 simulations. We then backtransformed the results to provide estimates 

within the same scale as the observed data.

Results and Discussion

We sampled 60 Forster’s Tern and 60 American Avocet eggs during this 2013 study. Together 

with the historical dataset from 2010 and 2011 (Ackerman and others, 2012b, unpublished, 2013a, 

unpublished), the total sample size of eggs included in this report was 180 Tern and 224 Avocet eggs. 

We monitored THg concentrations in as many as 15 randomly collected Tern and Avocet eggs from 

four or more nesting colonies per species, including Ponds A8 and A7 in the restoration area, as well as 

Reference Ponds outside of the immediate restoration area (fig. 1).

American Avocets (Recurvirostra americana) 

Across all ponds and years, egg THg concentrations in Avocets ranged from 0.02 to 1.99 μg/g 

fww, with a mean of 0.32 μg/g fww (N=224 eggs; table 1). Overall, 3 percent of randomly sampled 

Avocet eggs in 2013 exceeded the 0.90 μg/g fww toxicity threshold developed for Forster’s Terns in 

San Francisco Bay (Ackerman and Eagles-Smith, 2008, unpublished). 

The most parsimonious model describing Avocet egg THg concentrations included pond type, 

year, date, and the year×date interaction, and had an Akaike weight of 0.24 (table 2). Three other 

models containing year, date, and the year×date interaction plus either date2, pond type, or pond 

type×date interaction had a change in AICc value of less than or equal to 2.0 from that of the most 

parsimonious model (ΔAICc≤2.0; table 2). All models containing year, date, pond type, and the 

year×date had a cumulative Akaike weight of 0.65. We estimated the relative importance of individual 

variables (calculated as the sum of all model weights where the variable was present) and determined

that the data strongly supported effects of year (relative variable importance=1.00), date (0.95), and the 

year×date interaction (0.87). Pond type (0.75) had a moderate level of support, and date2 (0.35), pond 

type×date interaction (0.25), and pond type×year interaction (0.11) had little support. To further 
8

determine the importance of variables in the best model, we compared the best model to the same model 

structure, but omitted one of the variables of interest. Using this evidence-ratio approach, we estimated

that the best model that included year was 14,673 times more likely than the same model without year. 

Similarly, the best model was 8.7 times more likely than the same model but without the date variable 

and 1.7 times more likely than the same model but without the pond type variable. The arithmetic mean 

of egg THg concentrations for Avocets by year for each pond is shown in figure 2. The raw and modelpredicted (partial residual, which statistically accounts for the other variables in the model) egg THg 

concentrations for each Avocet egg by date for Reference and Restored Ponds is shown in figure 3. 

Model-predicted (at mean nest initiation day of year=129) mean egg THg concentrations in 

Avocets decreased by 4 percent between 2010 and 2011 at Restored Ponds (2010, 0.35 μg/g fww; 2011, 

0.33 μg/g fww), and also decreased slightly by 4 percent between years at Reference Ponds (2010, 0.19 

μg/g fww; 2011, 0.18 μg/g fww (fig. 4). Thereafter, mean egg THg concentrations in Avocets decreased 

by 39 percent between 2011 and 2013 at Restored Ponds (2011, 0.33 μg/g fww; 2013, 0.20 μg/g fww), 

and decreased by 40 percent between years at 

Reference Ponds (2011, 0.18 μg/g fww; 2013, 0.11 μg/g fww) (fig. 4). The end result of this 3-year 

comparison was that model-predicted mean egg THg concentrations in Avocets decreased

between 2010 and 2013 by 42 percent at Restored Ponds and by 43 percent at Reference Ponds (fig. 4).

Mean Avocet egg THg concentrations in the Restored Ponds still were 92 percent higher than 

concentrations in the Reference Ponds in 2013 (Restored Ponds, 0.20 μg/g fww; Reference Ponds, 0.11 

μg/g fww), similar to 2010 baseline THg concentration differences where Restored Ponds were 87 

percent higher than Reference Ponds (Restored Ponds, 0.35 μg/g fww; Reference Ponds, 0.19 μg/g 

fww) (fig. 4). Pond-specific percent changes in the arithmetic means of Avocet egg THg concentrations, 

based on raw data, are shown in table 1. 

Therefore, the restoration project has not changed the baseline Hg concentration trajectory in 

Avocet eggs at the Restored Ponds, and Ponds A7 and A8 still were considered to be Hg “hotspots” for 

Avocet eggs in 2013, similar to the finding of our prior research (Ackerman and others, 2007a, 2007b, 

unpublished, 2008a). For example, the arithmetic mean of Avocet egg THg concentrations in Restored 

Pond A8 was 0.57 μg/g fww in 2013, which is 420 percent higher than the highest primary reference 

site mean THg concentration in Avocet eggs at Pond A2W (0.11 μg/g fww; table 1). Moreover, 11 

percent of Avocet egg Hg concentrations in the Restored Ponds in 2013 were higher than concentrations 

associated with reproductive impairment. Importantly, New Chicago Marsh (NCM), another highly 

contaminated site, also showed substantial decreases in egg THg concentrations in 2013. 
9

Table 1. Egg total mercury concentrations for Forster’s Terns (Sterna forsteri) and American Avocets (Recurvirostra americana) nesting in South Bay Salt Pond 

Restoration Project area before (2010) and after (2011 and 2013) management activities associated with restoration of the Pond A8/A7/A5 Complex, south San 

Francisco Bay, California, autumn 2010–spring 2011. 

[Restored Ponds included Ponds A7 and A8. Reference Ponds included Ponds A1, A2W, AB1, E2, N4/5, SF2, and New Chicago Marsh. Abbreviations: SD, standard 

deviation; Min, minimum; Max, maximum; %THG, percent total mercury; μg/g fww, micrograms per gram fresh wet weight (mercury concentrations); -, no data]

Forster's Tern American Avocet

Site Year

Number 

of Eggs

Arithmetic 

mean 

(μg/g fww)

SD 

(μg/g 

fww)

Min 

(μg/g 

fww)

Max 

(μg/g 

fww)

THg 

change 

(2011–

2010)

THg change 

(2011-2010 μg/g 

fww)

THg 

change 

(2013–

2011)

THg 

change 

(2013–

2011 μg/g 

fww)

Number 

of Eggs

Arithmetic 

mean 

(μg/g fww)

SD 

(μg/g 

fww)

Min 

(μg/g 

fww)

Max 

(μg/g 

fww)

THg 

change 

(2011–

2010)

THg 

change 

(2011-

2010 μg/g 

fww)

THg 

change 

(2013–

2011)

THg 

change 

(2013–-

2011 μg/g 

fww)

A7 2010 15 1.74 0.67 0.82 2.77 +78% +1.35 -57% -1.76 7 0.50 0.18 0.30 0.81 +18% +0.09 -41% -0.24

2011 15 3.09 1.50 1.76 7.33 7 0.59 0.42 0.25 1.49

2013 15 1.33 0.47 0.63 2.29 5 0.35 0.16 0.10 0.50

A8 2010 15 1.69 0.60 0.78 2.63 +66% +1.11 -66% -1.85 15 0.41 0.25 0.03 0.91 +16% +0.07 +20% +0.09

2011 15 2.80 0.87 1.64 4.45 15 0.48 0.44 0.08 1.72

2013 15 0.95 0.43 0.44 1.88 13 0.57 0.43 0.11 1.53

A1 2010 15 1.51 0.51 0.80 2.39 -10% -0.15 -33% -0.45 14 0.13 0.06 0.05 0.24 +37% +0.05 -53% -0.09

2011 15 1.37 0.55 0.56 2.11 8 0.17 0.12 0.09 0.44

2013 15 0.92 0.31 0.46 1.71 4 0.08 0.02 0.07 0.11

A2W 2010 15 1.56 0.52 0.86 2.81 +8% +0.13 -43% -0.73 10 0.16 0.09 0.05 0.31 +58% +0.09 -55% -0.14

2011 15 1.70 0.59 0.92 3.24 15 0.25 0.48 0.06 1.97

2013 15 0.97 0.27 0.62 1.50 15 0.11 0.11 0.02 0.44

AB1 2010 ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ 10 0.22 0.11 0.08 0.44 -62% -0.14 -14% -0.01

2011 ̶ ̶ ̶ ̶ ̶ 2 0.08 0.05 0.04 0.12

2013 ̶ ̶ ̶ ̶ ̶ 1 0.07 NA NA NA

E2 2010 ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ 7 0.13 0.06 0.07 0.22 +38% +0.05 -- --

2011 ̶ ̶ ̶ ̶ ̶ 4 0.17 0.12 0.10 0.35

2013 ̶ ̶ ̶ ̶ ̶ 0 NA NA NA NA

N4/N5 2010 ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ 7 0.27 0.09 0.15 0.45 -20% -0.05 -- --

2011 ̶ ̶ ̶ ̶ ̶ 3 0.22 0.11 0.12 0.34

2013 ̶ ̶ ̶ ̶ ̶ 0 NA NA NA NA

NCM 2010 ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ 6 0.74 0.49 0.26 1.52 +30% +0.22 -73% -0.71

2011 ̶ ̶ ̶ ̶ ̶ 15 0.96 0.50 0.30 1.99

2013 ̶ ̶ ̶ ̶ ̶ 15 0.26 0.14 0.10 0.71

SF2 2010 ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ 3 0.10 0.03 0.07 0.13 -7% -0.01 +47% +0.04

2011 ̶ ̶ ̶ ̶ ̶ 15 0.09 0.09 0.02 0.37

2013 ̶ ̶ ̶ ̶ ̶ 8 0.13 0.08 0.06 0.31

Total 180 1.63 0.93 0.44 7.33 224 0.32 0.36 0.02 1.99
10

Table 2. Model selection results for egg total mercury concentrations (in micrograms per gram fresh wet weight ) 

in American Avocets (Recurvirostra americana) nesting in South Bay Salt Pond Restoration Project area before 

(2010) and after (2011 and 2013) management activities associated with restoration of Pond A8/A7/A5 Complex, 

south San Francisco Bay, California, autumn 2010–spring 2011.

[Pond site was included as a random effect in all models. Model structure: The + denotes an additive effect and the × 

denotes an interaction. k: The number of parameters in the model, including the intercept and variance. AICc: Akaike's 

Information Criterion (AICc). ∆AICc: The difference in the value between AICc of the current model and the value for the 

most parsimonious model. Akaike weight t (wi):The likelihood of the model given the data, relative to other models in the 

candidate set (model weights sum to 1.0). Cumulative model weight: The cumulative weight of evidence for the top models 

(model weights sum to 1.0). Evidence ratio: The weight of evidence that the top model is better than the selected model, 

given the candidate model set.]

Model structure N k -2LogL AICc ∆AICc

Akaike 

weight 

(wi)

Cumulative 

model 

weight

Evidence 

ratio

Pond Type + Year + Date + Year×Date 207 8 436.94 453.66 0.00 0.24 0.24 1.00

Year + Date + Year×Date 207 7 440.13 454.70 1.03 0.14 0.38 1.68

Pond Type + Year + Date + Date2 + Year×Date 207 9 436.00 454.92 1.25 0.13 0.51 1.87

Pond Type + Year + Date + Pond Type×Date+ Year×Date 207 9 436.37 455.28 1.62 0.11 0.61 2.25

Pond Type + Year + Date + Date2 + Pond Type×Date+ 

Year×Date 

207 10 434.77 455.89 2.23 0.08 0.69 3.05

Year + Date + Date2 + Year×Date 207 8 439.25 455.97 2.31 0.08 0.77 3.17

Pond Type + Year + Date + Pond Type×Year + Year×Date 207 10 436.08 457.20 3.54 0.04 0.81 5.87

Pond Type + Year 207 5 447.69 457.99 4.33 0.03 0.83 8.71

Pond Type + Year + Date + Pond Type×Year + Pond

Type×Date+ Year×Date 

207 11 434.85 458.20 4.54 0.02 0.86 9.67

Pond Type + Year + Date 207 6 445.93 458.35 4.69 0.02 0.88 10.43

Pond Type + Year + Date + Date2 + Pond Type×Year + 

Year×Date 

207 11 435.34 458.70 5.03 0.02 0.90 12.39

Pond Type + Year + Date + Date2 + Pond Type×Year + Pond 

Type×Date+ Year×Date 

207 12 433.30 458.91 5.25 0.02 0.92 13.80

Year 207 4 451.09 459.29 5.63 0.01 0.93 16.66

Year + Date 207 5 449.06 459.36 5.69 0.01 0.95 17.22

Pond Type + Year + Date + Date2 207 7 444.96 459.52 5.85 0.01 0.96 18.68

Pond Type + Year + Date + Pond Type×Date 207 7 445.01 459.58 5.91 0.01 0.97 19.23

Pond Type + Year + Date + Date2 + Pond Type×Date 207 8 443.20 459.92 6.26 0.01 0.98 22.87

Year + Date + Date2 207 6 448.13 460.55 6.89 0.01 0.99 31.30

Pond Type + Year + Pond Type×Year 207 7 447.57 462.14 8.47 0.00 0.99 69.13

Pond Type + Year + Date + Pond Type×Year 207 8 445.80 462.53 8.87 0.00 1.00 84.23

Pond Type + Year + Date + Pond Type×Year + Pond 

Type×Date

207 9 444.72 463.64 9.97 0.00 1.00 146.53

Pond Type + Year + Date + Date2 + Pond Type×Year 207 9 444.79 463.70 10.04 0.00 1.00 151.18

Pond Type + Year + Date + Date2 + Pond Type×Year + Pond 

Type×Date

207 10 442.94 464.06 10.40 0.00 1.00 181.01

Pond Type + Date + Date2 + Pond Type×Date 207 6 458.56 470.98 17.31 0.00 1.00 5744.27

Pond Type 207 3 465.45 471.57 17.91 0.00 1.00 7734.08

Pond Type + Date + Date2 207 5 462.18 472.48 18.81 0.00 1.00 12177.19

Pond Type + Date 207 4 464.65 472.85 19.19 0.00 1.00 14672.54

Pond Type + Date + Pond Type×Date 207 5 462.79 473.08 19.42 0.00 1.00 16486.31

Null (Intercept Only) 207 2 469.19 473.24 19.58 0.00 1.00 17858.89

Date + Date2 207 4 465.72 473.92 20.25 0.00 1.00 25005.32

Date 207 3 468.23 474.34 20.68 0.00 1.00 30952.11
11

Figure 2. Pond site and year differences in egg total mercury (THg) concentrations (in micrograms per 

gram fresh wet weight, μg/g fww) for American Avocets (Recurvirostra americana) nesting in South Bay 

Salt Pond Restoration Project area, south San Francisco Bay, California. The horizontal black bar 

represents the arithmetic mean of egg mercury concentrations. The error bar represents the standard 

deviation of the data. The gray box indicates the maximum egg mercury concentration observed. The 

white circles show the actual mercury concentration for each individual egg. The red dashed line indicates 

the toxicity threshold of 0.90 μg/g fww, where bird reproduction may be impaired (Ackerman and EaglesSmith, 2008, unpublished).
12

Figure 3. Total mercury (THg) concentrations (in micrograms per gram fresh wet weight, μg/g fww) in 

American Avocet (Recurvirostra americana) eggs by date in South Bay Salt Pond Restoration Project area 

before (2010, blue dots) and after (2011, red dots; 2013, green dots) management activities associated 

with restoration of Pond A8/A7/A5 Complex, south San Francisco Bay, California, autumn 2010–spring 

2011. Pond A8 Notch was opened on June 1, 2011, and June 6, 2013, corresponding to a potential 

exposure to eggs by June 8, 2011 (day of year=159) and June 13, 2013 (day of year=164). The top panels 

indicate the raw data, and the bottom panels show the partial residuals from the model.
13

Figure 4. Model-averaged mean and 95-percent credible intervals for total mercury (THg) concentrations 

(in micrograms per gram fresh wet weight, μg/g fww) in American Avocet (Recurvirostra americana) eggs 

in South Bay Salt Pond Restoration Project area before (2010) and after (2011 and 2013) management 

activities associated with restoration of Pond A8/A7/A5 Complex, south San Francisco Bay ,California, 

autumn 2010–spring 2011.

-43% -42%

-4%

-40%

-4%

-39%
14

Forster’s Terns (Sterna forsteri) 

Across all ponds and years, Tern egg THg concentrations ranged from 0.44 to 7.33 μg/g 

fww, with an arithmetic mean of 1.63 μg/g fww (N=180 eggs; table 1). Overall, 60 percent of 

randomly sampled Tern eggs in 2013 exceeded the toxicity threshold developed for Forster’s 

Terns in San Francisco Bay (0.90 μg/g fww; Ackerman and Eagles-Smith, 2008, unpublished). 

The most parsimonious model describing Tern egg THg concentrations included pond 

type, year, date, and the interactions pond type×year and year×date, and had an Akaike weight of 

0.27 (table 3). Furthermore, all models containing year, pond type, and pond type×year 

interaction had a cumulative Akaike weight of 0.99. Two other models containing these three 

variables plus either date or date2, had a change in AICc value of less than or equal to 2.0 from 

that of the most parsimonious model (ΔAICc≤2.0; table 3). We estimated the relative importance 

of individual variables and determined that the data strongly supported effects of year (relative 

variable importance=1.00), pond type (0.99), date (0.94), and the pond type×year interaction 

(0.99). In contrast, year×date interaction (0.48), date2 (0.29), and pond type×date interaction 

(0.23) had little support. Using evidence ratios, we estimated that the best model that included 

the pond type×year interaction was 3,295 times more likely than the same model without this 

interaction. Similarly, the best model was 9,930 times more likely than the same model but 

without the pond type variable, and 2.29×1017 times more likely than the same model but without 

the year variable. There was some support for an increase in Tern egg THg concentrations with 

date, especially in 2011 after the Pond A8 Notch was opened. However, the influence of date on 

egg THg concentrations was relatively small relative to the effects of other variables. The 

arithmetic mean of egg THg concentrations for Terns by year for each pond is shown in figure 5. 

The raw and model-predicted (partial residual) egg THg concentrations for each Tern egg by 

date for Reference and Restored Ponds is shown in figure 6. 

The overriding importance of year, pond type, and pond type×year interaction indicated 

that changes in Tern egg THg concentrations between years were not consistent among pond 

types (Restored Ponds compared to Reference Ponds). Specifically, model-predicted (at mean 

nest initiation day of year=156) mean THg concentrations in Tern eggs increased by 69 percent

between 2010 and 2011 at Restored Ponds (2010, 1.60 μg/g fww; 2011, 2.71 μg/g fww), but 

mean egg THg concentrations decreased slightly by 10 percent between years at Reference 

Ponds (2010, 1.50 μg/g fww; 2011, 1.35 μg/g fww) (fig. 7). Thereafter, mean Tern egg THg 

concentrations decreased by 59 percent between 2011 and 2013 at Restored Ponds (2011, 2.71 

μg/g fww; 2013, 1.10 μg/g fww), and decreased by 23 percent between years at Reference Ponds 

(2011, 1.35 μg/g fww; 2013, 1.04 μg/g fww) (fig. 7). The end result of this 3-year comparison 

was that model-predicted mean egg THg concentration in Terns decreased between 2010 and 

2013 by 31 percent at both Restored and Reference Ponds (fig. 7). Thus, Tern egg THg 

concentrations in 2013 returned to equivalent baseline concentrations measured in 2010 before 

any of the restoration actions were conducted. Importantly, Tern egg THg concentrations in the 

Restored Ponds still were 6 percent higher than concentrations in the Reference Ponds in 2013 

(Restored Ponds, 1.10 μg/g fww; Reference Ponds, 1.04 μg/g fww), similar to the 2010 baseline 

THg concentration trajectory in Tern eggs, where Restored Ponds had concentrations 7 percent

higher than Reference Ponds (Restored Ponds, 1.60 μg/g fww; Reference Ponds, 1.50 μg/g fww) 

(fig. 7). Pond-specific percent changes in the arithmetic means of Tern egg THg concentration, 

based on raw data, are shown in table 1. 
15

Table 3. Model selection results for egg total mercury concentrations micrograms per gram fresh wet 

weight) in Forster’s Terns (Sterna forsteri) nesting in South Bay Salt Pond Restoration Project area before 

(2010) and after (2011 and 2013) management activities associated with restoration of the Pond A8/A7/A5 

Complex, south San Francisco Bay, California, autumn 2010–spring 2011.

[Pond site was included as a random effect in all models. Model structure: The + denotes an additive effect and the 

× denotes an interaction. k: The number of parameters in the model, including the intercept and variance. AICc:

Akaike's Information Criterion (AICc). ∆AICc: The difference in the value between AICc of the current model and 

the value for the most parsimonious model. Akaike weight t (wi):The likelihood of the model given the data, 

relative to other models in the candidate set (model weights sum to 1.0). Cumulative model weight: The 

cumulative weight of evidence for the top models (model weights sum to 1.0). Evidence ratio: The weight of 

evidence that the top model is better than the selected model, given the candidate model set]

Model structure N k -2LogL AICc ∆AICc

Akaike 

weight (wi)

Cumulative

model 

weight

Evidence 

ratio

Pond Type + Year + Date + Pond 

Type×Year + Year×Date

178 10 131.20 152.51 0.00 0.27 0.27 1.00

Pond Type + Year + Date + Pond 

Type×Year

178 8 136.11 152.96 0.44 0.22 0.49 1.25

Pond Type + Year + Date + Date2 + 

Pond Type×Year

178 9 134.99 154.06 1.55 0.12 0.61 2.17

Pond Type + Year + Date + Date2 + 

Pond Type×Year + Year×Date

178 11 131.01 154.60 2.09 0.10 0.71 2.84

Pond Type + Year + Date + Pond 

Type×Year + Pond Type×Date + 

Year×Date

178 11 131.20 154.79 2.27 0.09 0.79 3.12

Pond Type + Year + Date + Pond 

Type×Year + Pond Type×Date 

178 9 136.09 155.16 2.65 0.07 0.87 3.76

Pond Type + Year + Pond Type×Year 178 7 140.78 155.44 2.92 0.06 0.93 4.31

Pond Type + Year + Date + Date2 + 

Pond Type×Year + Pond Type×Date 

178 10 134.99 156.31 3.80 0.04 0.97 6.68

Pond Type + Year + Date + Date2 + 

Pond Type×Year + Pond Type×Date + 

Year×Date

178 12 131.01 156.90 4.39 0.03 1.00 8.97

Pond Type + Year + Date + Pond 

Type×Date + Year×Date

178 9 146.58 165.65 13.14 0.00 1.00 712.44

Pond Type + Year + Date + Date2 + 

Pond Type×Date + Year×Date

178 10 146.01 167.32 14.81 0.00 1.00 1,644.83

Pond Type + Year + Date + Pond 

Type×Date 

178 7 153.92 168.58 16.07 0.00 1.00 3,087.35

Pond Type + Year + Date + Year×Date 178 8 151.86 168.71 16.20 0.00 1.00 3,295.00

Pond Type + Year + Date + Date2 + 

Pond Type×Date 

178 8 151.89 168.74 16.23 0.00 1.00 3,339.54

Pond Type + Year + Date + Date2 + 

Year×Date

178 9 151.24 170.31 17.80 0.00 1.00 7,318.32

Year + Date + Year×Date 178 7 156.05 170.71 18.19 0.00 1.00 8,930.19

Year + Date + Date2 + Year×Date 178 8 155.67 172.53 20.01 0.00 1.00 22,165.28

Pond Type + Year 178 5 162.52 172.87 20.36 0.00 1.00 26,362.35

Pond Type + Year + Date + Date2 178 7 159.42 174.08 21.57 0.00 1.00 48,175.65

Pond Type + Year + Date 178 6 162.51 175.00 22.49 0.00 1.00 76,547.58

Year + Date + Date2 178 6 164.75 177.24 24.73 0.00 1.00 234,486.76

Year 178 4 169.37 177.61 25.09 0.00 1.00 281,087.29

Year + Date 178 5 169.37 179.71 27.20 0.00 1.00 806,721.40

Pond Type + Date + Date2 + Pond 

Type×Date 

178 6 215.91 228.40 75.89 0.00 1.00 3.01×1016

Pond Type + Date + Date2 178 5 219.25 229.60 77.09 0.00 1.00 5.49×1016

Pond Type + Date + Pond Type×Date 178 5 219.75 230.10 77.59 0.00 1.00 7.05×1016

Date + Date2 178 4 222.82 231.05 78.54 0.00 1.00 1.13×1017

Pond Type + Date 178 4 224.23 232.46 79.95 0.00 1.00 2.29×1017

Date 178 3 230.01 236.15 83.64 0.00 1.00 1.45×1018

Pond Type 178 3 247.28 253.41 100.90 0.00 1.00 8.14×1021

Null (Intercept Only) 178 2 253.90 257.97 105.46 0.00 1.00 7.94×1022
16

Figure 5. Pond site and year differences in egg total mercury (THg) concentrations (in micrograms per 

gram fresh wet weight, μg/g fww) for Forster’s Terns (Sterna forsteri) nesting in South Bay Salt Pond 

Restoration Project area, south San Francisco Bay, California. The horizontal black bar represents the 

arithmetic mean egg mercury concentrations. The error bar represents the standard deviation of the data. 

The gray box indicates the maximum egg mercury concentration observed. The white circles show the 

actual mercury concentration for each individual egg. The red dashed line indicates the toxicity threshold of 

0.90 μg/g fww where bird reproduction may be impaired (Ackerman and Eagles-Smith, 2008, unpublished).
17

Figure 6. Total mercury (THg) concentrations (in micrograms per gram fresh wet weight, μg/g fww) in 

Forster’s Tern (Sterna forsteri) eggs by date in South Bay Salt Pond Restoration Project area before (2010, 

blue dots) and after (2011, red dots; 2013, green dots) management activities associated with restoration of 

Pond A8/A7/A5 Complex, south San Francisco Bay, California, autumn 2010–spring 2011. Pond A8 Notch 

was opened on June 1, 2011, and June 6, 2013, corresponding to a potential exposure to eggs by June 8, 

2011 (day of year=159) and June 13, 2013 (day of year=164). The top panels indicate raw data and the 

bottom panels display the partial residuals from the model.
18

Figure 7. Model-averaged mean and 95-percent credible intervals for total mercury (THg) concentrations 

(in micrograms per gram fresh wet weight, μg/g fww) in Forster’s Tern (Sterna forsteri) eggs in South Bay 

Salt Pond Restoration Project area before (2010) and after (2011 and 2013) management activities 

associated with restoration of Pond A8/A7/A5 Complex, south San Francisco Bay ,California, autumn 

2010–spring 2011. 

-10%

-23%

+69%

-59%

-31% -31%
19

Management Implications

These results indicate that, although there was a significant short-term influence of the 

restoration actions on increased Hg bioaccumulation in Tern eggs the following year, this effect 

appeared to be short-lived. However, the restoration project apparently has not changed baseline 

mercury (Hg) conditions in eggs at the Restored Ponds—Ponds A7 and A8—are still Hg 

“hotspots” for bird eggs (especially in Tern eggs compared to other ponds in 2013) 2 years after 

the restoration action. For example, the arithmetic mean of Tern egg total mercury (THg) 

concentrations in Restored Pond A7 was 1.33 μg/g fww in 2013, which is 38 percent higher than 

the highest reference site at Pond A2W (0.97 μg/g fww) (table 1). Moreover, most Tern egg Hg 

concentrations in the Restored Ponds in 2013 (70 percent) still remain higher than 

concentrations associated with reproductive impairment. Egg THg concentrations at these 

concentrations previously have been shown to reduce hatching success and nest survival, to 

increase the likelihood of embryos being malpositioned within eggs, to suppress baseline 

corticosterone concentrations in juvenile birds, to increase adult demethylation rates in bird 

livers, and to reduce adult body condition (Ackerman and Eagles-Smith, 2008, unpublished, 

Ackerman and others, 2008a,b,c, Eagles-Smith and others, 2009a,b, Herring and others, 2010, 

Ackerman and others, 2009, Ackerman and others, 2011, Ackerman and others, 2012a, Herring 

and others, 2012). 

Despite the significant increase and correspondingly large decrease in THg 

concentrations in Tern eggs at the Restored Ponds, both Tern and Avocet egg THg 

concentrations in the Restored Ponds still are similar to concentrations that would have been 

expected without the occurrence of the restoration actions. Although ambient egg THg 

concentrations in the south San Francisco Bay have decreased since 2010, the restoration actions 

apparently did not change the baseline Hg concentration trajectory in eggs within the Restored 

Ponds. In particular, Ponds A7 and A8 still are Hg “hotspots” for both Terns and Avocets 2 

years after the restoration actions. It will be important to document whether egg THg 

concentrations continue to decrease in the future as the Restored Ponds continue to develop, the 

Pond A8 Notch is opened for a longer portion 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 relative to the Reference Ponds, the Restored Ponds possibly 

could achieve lower egg THg concentrations than would have been expected without any 

restoration actions, but the South Bay Salt Pond Restoration Project has not achieved this goal as 

of 2013. These data suggest that continued monitoring of waterbird egg THg concentrations in 

the restoration project area, relative to reference sites, is warranted over a period of multiple

years.

Acknowledgments 

This study was funded and supported by the U.S. Environmental Protection Agency, the 

State of California Coastal Conservancy, and the South Bay Salt Pond Restoration Project. 

Logistical support was kindly provided by Cheryl Strong, Eric Mruz, Laura Valoppi, John 

Bourgeois, and John Krause, and the staffs of the Don Edwards San Francisco Bay National 

Wildlife Refuge and Eden Landing Ecological Reserve. We thank the following U.S. Geological 

Survey Western Ecological Research Center field and laboratory technicians who helped with 

this research: Ashley Casey, Rosielyn DeGuzman, Michelle Boyles, Laura Young, and Erik 

Nass. 
20

Unpublished Data Referenced

Ackerman, J.T., and Eagles-Smith, C.A., 2008, A dual life-stage approach to monitoring the

effects of mercury concentrations on the reproductive success of Forster’s Terns in San 

Francisco Bay—Annual report: U.S. Geological Survey, Western Ecological Research Center, 

Davis, California, 44 p.

Ackerman, J. T., Eagles-Smith, C.A., Heinz, G.H., Wainwright-De La Cruz, S.E., Takekawa, 

J.Y., Adelsbach, T.L., Miles, A.K.,Hoffman, D.J., Schwarzbach, S.E., Suchanek, T.H., and 

Maurer, T.C., 2007b, Mercury in birds of the San Francisco Bay-Delta—Trophic pathways, 

bioaccumulation and Eco toxicological risk to avian reproduction: 2006 Annual Report to 

CALFED, U.S. Geological Survey, Western Ecological Research Center, and U. S. Fish and 

Wildlife Service, Environmental Contaminants Division, 41 p.

Ackerman, J.T., Herzog, M.P., and Hartman, C.A., 2012b, The South Bay Mercury Project—

Using biosentinels to monitor effects of wetland restoration for the South Bay Salt Pond 

Restoration Project (waterbird mercury component): U.S. Geological Survey, Western 

Ecological Research Center, Davis, California, 79 p.

Ackerman, J.T., Marvin-DiPasquale, M., Slotton, D., Eagles-Smith, C.A., Herzog, M.P., 

Hartman, C.A., Agee, J.L., and Ayers, S., 2013a, The South Bay Mercury Project—Using 

biosentinels to monitor effects of wetland restoration for the South Bay Salt Pond Restoration

Project: U.S. Geological Survey, Western Ecological Research Center, Davis, California, 

227 p.

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Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

 Director, Western Ecological Research Center

U.S. Geological Survey

3020 State University Drive East

Sacramento, California 95819 

http://werc.usgs.gov/
Ackerman and others—Waterbird Egg Mercury Concentrations in Response to Wetland Restoration in South San Francisco Bay, California—Open-File Report 2014–1189

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20141189