conceptual model and working hypotheses of mercury ......dr jay davis, sfei dr. steven schwarzbach,...

Conceptual Model and Working Hypotheses of Mercury Bioaccumulation in the Bay-Delta

Ecosystem and its Tributaries

By

The Delta Biology Group

Dr. Chris Foe, RWQCB Dr Jay Davis, SFEI

Dr. Steven Schwarzbach, USGS Mark Stephenson, CDFG

Darell Slotton, UCD

September 23, 2003

SUMMARY OF MAJOR FEATURES Figure 1 in the Delta Biogeochemistry Synthesis chapter (see Gill et al. this report) depicts a conceptual overview of the transport, biogeochemical cycling, and bioaccumulation of mercury in the Bay-Delta ecosystem and its watersheds. This cycle is based upon recent environmental mercury research; particularly the findings from the Calfed Mercury Project entitled “Assessment of Ecological and Human Health Impacts of Mercury in the Bay-Delta Watershed”. The cycle also includes current working hypotheses describing Hg behavior in the Bay-Delta ecosystem. The major bioaccumulation processes and environmental impacts depicted in the figure are summarized below.

Major internal bioaccumulation cycling processes for mercury include:

• Bioaccumulation of mercury (as monomethyl mercury) into the aquatic food chain with a resulting increase in mercury concentration from the lower to the upper trophic levels.

Primary environmental impacts include:

• Human health concerns from consumption of fish with elevated mercury content

• Reproductive impairment of birds which feed from the aquatic

environment

PRIMARY WORKING HYPOTHESES Our work to characterize the major bioaccumulation processes in the Delta and San Francisco Bay estuary to date have lead us to develop the following working hypotheses. Primary evidence in support of these hypotheses are as follows.

Concentrations of mercury in sport fish from the Delta region represent a potential human health concern.

Mercury Bioaccumulation Conceptual Model - of 7

Fish sampling was performed in the late summer of 1999 and 2000. The primary target species were largemouth bass, white catfish, striped bass, and Sacramento pikeminnow which were analyzed as individuals. Secondary target species were channel catfish, black crappie, Sacramento sucker, common carp, bluegill and redear sunfish, that were sampled as multi-individual composites. Measured concentrations were compared to a screening value for mercury, defined as a concentration in fish or shellfish tissue that is of potential public health concern. The principal conclusions were that largemouth bass, striped bass, Sacramento pikeminnow, channel catfish, and white catfish had mercury concentrations of a high potential human health concern. A majority of samples exceeded the screening value (0.3 ppm wet weight) and many exceeded 1 ppm (Table 1). Three species had mercury concentrations of moderate concern (common carp, black crappie, and Sacramento sucker). Thirty-three to sixty-seven percent of these samples exceeded 0.3 but none were above 1 ppm. Mercury in sport fish appears to be a much lower human health concern in the central Delta than in other portions of the Delta region. A striking contrast was observed in the frequency of screening value exceedances inside and outside of the central Delta in the two years of sampling. Within the central Delta, largemouth bass was the only species to exceed the screening value in a majority of samples (53%), and only one fish of any species was over 1 ppm. Outside the central Delta, high percentages of largemouth (97%), white catfish (76%), channel catfish (89%), black crappie (100%) were above the screening value, and many fish were above 1 ppm (see Davis et al this report) Consistent multi-species methyl mercury tissue concentration patterns exist in the Bay-Delta Estuary. Mercury concentrations were measured in 13 species of fish and shellfish at multiple locations in the Bay-Delta Estuary and in major freshwater tributaries in the Central Valley. The highest tissue concentrations were consistently measured in species and individuals collected in mercury-contaminated tributaries in the Central Valley and immediately downstream in the Delta. Lower values were observed in the central Delta and near the State and Federal export pumps to southern California. Patterns are presented visually for juvenile and adult largemouth bass, white catfish, Asiatic clams, inland silversides, bluegill, signal crayfish, threadfin shad, and


mosquitofish (Figures 1-9). Sufficient samples of largemouth bass were collected to estimate the mean mercury concentration at a 350 mm standard length by analysis of covariance. The results demonstrate significantly elevated tissue concentrations around the periphery of the Delta (Suisun Bay, and the Feather, lower Sacramento, Cosumnes, and San Joaquin Rivers) and lower values at all sites in the Central Delta (Figure 10). Significant mercury accumulation occurs in food webs on both sides of the Valley and in both the Sacramento and San Joaquin watersheds, in spite of the different forms of mercury sources (cinnabar vs elemental mercury) and different water chemistry in these regions (see Davis et al., Foe et al. and Slotton et al. this report). The mercury concentrations in fish and clams did not correlate significantly to sediment mercury and methyl mercury concentrations. Reasons for the lower biotic concentrations in the central Delta as opposed to the tributaries include; lower concentrations of mercury in water (see Foe et al. this report), fewer trophic levels in the food chain in the central Delta, and biodilution. At this time there is little evidence to suggest which of these reasons is most important. Mercury tissue concentrations in striped bass have not changed in the last 30 years. One hundred and two striped bass were collected in the Bay-Delta in the early 1970s for mercury analysis. This is the only fish species for which a long term data set exists. Fifty-five bass had concentrations above 0.5 ppm wet weight. This resulted in a human health advisory recommending that pregnant women and children not consume striped bass. The 1970 values were compared with results obtained between 1994 and 2000 (Figure 11). No change in tissue concentration is apparent over the 30-year interval (see Davis et al. this report).

Methyl mercury primarily accumulates in the base of the food web in the spring and early summer. Composite fish and Asiatic clam samples were collected from the Delta and from its freshwater tributaries in 1999 and 2000. Positive correlations were observed between methyl mercury tissue concentrations in clams and in four of the nine fish species examined (largemouth bass, redear sunfish, Sacramento suckers, and inland silversides; (Table 2). The results suggest that the clam may be useful as a model for studying methylmercury uptake at the base of the Delta food web.


Two transplant studies were undertaken with the filter feeding clam to measure methyl mercury tissue concentration changes. The first was a one-year study (September 2000-September 2001) in Putah Creek and in the Sacramento River at Rio Vista. The second was a seven-month study at six locations in and around the Estuary (February and October 2001). Change in tissue weight (gm dry weight-clam-1 month-1) and methylmercury body burden (ng methylmercury-clam-1 month-1) was measured in both studies. Gains in both tissue weight and body burden predominately occurred in the spring and in early summer in the Delta (March to June; Figure 12). No methylmercury uptake occurred in winter coincident with the highest annual aqueous methylmercury concentrations (see Foe et al. this report). Methylmercury tissue concentrations in biota track aqueous methyl mercury and phytoplankton concentrations in the spring and early summer. Four thousand clams were collected in February 2001 and transplanted into replicate cages at six locations in the Delta and in the freshwater tributaries. Changes in clam methylmercury body burden, tissue weight, and a suite of water quality parameters were followed monthly at each site. Change in clam methylmercury tissue concentration can be expressed as: Tissue Concentration = ∆ Body Burden Dry Tissue Weight Where the units in equation 1 for ∆ tissue concentration are ng-methylmercury-gm tissue weight-1, ∆ body burden is ng-methylmercury-clam-1, and ∆ dry tissue weight is gm-clam-1. The rate of change in clam methylmercury body burden per month was positively correlated to unfiltered methylmercury divided by the sum of the chlorophyll and phaeophytin concentration in ambient water (R2 = 0.42, P = 6 X 10-5; Figure 13) while change in tissue growth was positively related to the sum of the chlorophyll and phaeophytin concentration (R2 = 0.46, P<0.0; Figure 13). Substituting the two field relationships into the above equation and simplifying results in: Tissue Concentration = Methylmercury Concentration (Chlorophyll + Phaeophytin)2

The results suggest that change in methylmercury tissue concentrations in clams is driven by changes in the unfiltered aqueous methylmercury


concentration divided by the square of the sum of the chlorophyll and phaeophytin concentration. Tissue correlations between clams and fish suggest that methylmercury uptake in clams is being transferred up the food chain (see Foe et al. this report).

Mercury concentrations in avian eggs vary greatly within the Bay-Delta system, were highest in fish eating birds, and varied spatially with concentrations in other media Mercury concentrations in 328 bird eggs in the Bay-Delta varied by two orders of magnitude, from less than 0.02 to 3.33 ppm on a fresh wet weight basis. In the Delta, degree of piscivory strongly influenced species egg mercury patterns as double crested cormorants consistently had more mercury than great egrets, which had more mercury than great blue herons, when two or more of these species were sampled from the same locations. Within the Bay the highest mean concentrations were consistently found in Caspian terns and Forster’s terns, which are among the most piscivorous species sampled in the Bay. Some fish eating birds, such as Brandt’s cormorants nesting at Alcatraz had lower mercury concentrations in eggs, but these were likely reflective of the lower mercury concentrations in their foraging range near the Golden Gate. Egg mercury in great blue heron eggs illustrated a within-Delta pattern of mercury bioavailability that was similar to that found in sport fish and silversides where concentrations in the San Joaquin River and Central Delta region were significantly lower than other locations such as Suisun Bay, Prospect Slough, and the Sacramento River. Great blue heron egg mercury was significantly correlated with silverside mercury results at five Delta locations (R2 = 0.78, p = 0.046). Within San Francisco Bay findings of elevated egg mercury were not restricted to piscivorous birds. Species nesting in more mercury contaminated habitats like snowy plovers and black-necked stilts nesting near south Bay salt ponds and California clapper rails nesting in tidal marshes also had elevated mercury concentrations (see Schwarzbach et al. this report). Mercury concentrations in eggs of some bird species in the Bay-Delta were above embryotoxic thresholds Feeding studies in birds have established two toxic thresholds (0.5 ppm in pheasants and 0.8 ppm in mallards) for mercury in bird eggs sufficient to impair reproduction. Among species sampled in the Delta (double-crested cormorant, great blue heron, great egret) only double-crested cormorants at


Wheeler Island in 2000 had a location mean concentration (0.54 ppm fww) above a known toxic threshold. Among species sampled in San Francisco Bay, three species had location means above the mallard toxic threshold of 0.8 ppm. These were the Caspian tern, which had location means ranging from 0.7 to 1.2 ppm, the Forster’s tern, which had location means between 0.5 and and 1.63 ppm, and the California clapper rail, which had a mean of 0.82. Two other species, the snowy plover and black-necked stilt, had a location mean concentration just below 0.5 but had some eggs between the 0.5 and 0.8 thresholds. The egg injection work of Heinz seems to indicate the cormorant is less sensitive than the mallard so the threshold exceedance in this species is probably not indicative of a mercury problem for cormorant hatchability. Heinz’s work also indicated clapper rails were more sensitive than both the mallard and the pheasant and the high concentrations found in fail-to-hatch rail eggs were likely embyrotoxic. Preliminary data of Heinz in tricolored herons suggested they may also be sensitive to methylmercury, however, more data are needed for herons and egrets to provide confidence in interpreting egg mercury concentrations in these species. There are not yet data available to provide species specific interpretation of concentrations in the stilts, plovers, and terns, but concentrations over 1 ppm in fish eating birds and over 0.5 ppm in non-piscivorous species should probably be considered elevated. The reproductive implications of very high mercury in tern eggs in the Bay should be considered a high research priority (see Heinz this report and Schwarzbach et al. this report).


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Figure 9. Average mercury concentrations in adult largemouth bass from each samplinglocation, 2000. All fish are 305 – 438 mm total length.

Figure 16. Average mercury concentrations in white catfish from each samplinglocation, 1999. All fish are 229 - 330 mm total length.

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UC DAVIS COLLABORATIVE DELTA STUDY: WETLAND RESTORATION EFFECTS ON MeHg PRODUCTION AND BIOACCUMULATION D.G. SLOTTON et al.

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Figu

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Figure 10 (Davis) Spatial comparison of largemouth bass mercury concentrations estimatedat standard length of 350 mm (mean and 95% confidence interval) by thepolynomial regression ANCOVA method of Tremblay et al. (1998).Locations are listed in north (top) to south (bottom) order. Locations withnon-overlapping intervals are considered significantly different.

Mercury (ppm wet) at 350 mm (mean and 95% CI)

0.0 0.5 1.0 1.5

SJR Landers

SJR Crow's

Stanislaus

SJR Vernalis

SJR Naval

Mildred

Big Break

Frank's

Sherman

White Sl

SJR Potato

Mokelumne

Cosumnes

SacR Isleton

Cache Sl

SacR RM44

Putah

Feather Nicolau

Feather Yuba FEATHER

CENTRAL DELTA

COSUMNES

SACRAMENTO

PUTAH

SAN JOAQUIN

Figure 11. a) Mercury concentrations in striped bass from the Estuary in studies from 1970 to 2000. Data from CSDPH (1971),Fairey et al. (1997), Davis et al. (2002), Greenfield et al. (2002), and this study.

Figure 20. a) Mercury concentrations in striped bass from the Estuary in studies from 1970 to 2000. Data from CSDPH (1971),Fairey et al. (1997), Davis et al. (2002), Greenfield et al. (2002), and this study.

0

0.5

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0 200 400 600 800 1000 1200

Length (mm)

Mer

cury

(ppm

wet

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1970-1973 CDFG1994 RMP1997 RMP2000 RMP1999/2000 CALFED

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Figure 12 (Foe). Seasonal changes in individual clam weight and methyl mercury body burden around the Estuary in 2001.

Cache Creek at Road 102

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0.6

0.8

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Methyl Hg

Prospect Slough

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San Joaquin River at Vernalis

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F M A M J J A S O N

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Figure 12 (Foe) (continued)

Sacramento River at Greene's Landing

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Figure 13 (Foe). Change in transplanted clam methyl mercury body burden (ng/clam/mo) as a function of the raw methyl mercury concentration of the water divided by the sum of the chlorophyll and phaeophytin concentration.

y = 907.4x + 10.383

R2 = 0.42

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Raw Methyl Mercury (ng/l) /(Chlorophyll + Phaeophytin conc (µg/l))

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Figure 14 (Foe). Caged clam tissue growth in the Sacramento-San Joaquin Delta as a function of water temperature and phytoplankton concentration.

y = 0.2097Ln(x ) - 0.0625

R2 = 0.46

y = 0.045Ln(x ) - 0.0148

R2 = 0.49

0.0

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16-22 degrees C 22-28 degrees C

DRAFT

Page 27

Table 1. Combined statistics from 1999 and 2000 on screening value exceedances.

Species Sample Type #

Analyzed # Fish

Represented# over 0.3

ppm % over 0.3 ppm

# over 1 ppm

% over 1 ppm

Largemouth bass Individuals 326 326 262 80% 54 17% White catfish Individuals 142 142 77 54% 7 5% Striped bass Individuals 36 36 31 86% 6 17% Sacramento pikeminnow Individuals 43 43 25 58% 13 30% Channel catfish Composites 11 38 8 73% 1 9% Black crappie Composites 6 27 4 67% 0 0% Sacramento sucker Composites 17 78 6 35% 0 0% Common carp Composites 9 41 3 33% 0 0% Bluegill Composites 33 159 3 9% 0 0% Redear sunfish Composites 20 96 1 5% 0 0% TOTAL 643 986 420 65% 81 13%

27

Table 2. (Foe). Pierson correlation coefficients and associated probability value for methyl mercury tissue concentrations in composite fish and clam samplescollected in the Sacramento-San Joaquin Delta Estuary and in its tributaries in 1999 and 2000. Statistically significant relationships are indicated in bold. Datafor all species are in ppm wet weight except for Silversides and Asiatic clams which are dry weight. Striped

Bass White Catfish

Channel Catfish

Blue Gill

Redear Sunfish

Sacramento Suckers

Pike Minnow

Largemouth Bass

Inland Silversides

Asiatic Clams

r =0.52 n=11 p=0.1

r=0.27 n=10

p=0.44

r =0.79 n=6

p=0.06

r=0.39 n=11

p=0.23

r =0.73 n =11

p=0.01

r =0.84 n =8

p=0.01

r =0.05 n=7

p=0.91

r =0.63 n =33

p=0.0001

r =0.56 n =23

p=0.006

conceptual model and working hypotheses of mercury ......dr jay davis, sfei dr. steven schwarzbach,...

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