Biomonitoring selenium, mercury, and selenium:mercury molar ratios in selected species in Northeastern US estuaries: risk to biota and humans

Abstract

The mutual mitigation of selenium and mercury toxicity is particularly interesting, especially for humans. Mercury is widely recognized as a pantoxic element; all forms are toxic to all organisms. Less well known is that selenium in excess is toxic as well. The high affinity between these elements influences their bioavailability and toxicity. In this paper, we use selected species from Barnegat and Delaware Bays in New Jersey to examine variations in levels of selenium and mercury, and selenium:mercury molar ratios between and within species. We report on species ranging from horseshoe crab eggs (Limulus polyphemus), a keystone species of the food chain, to several fish species, to fish-eating birds. Sampling began in the 1970s for some species and in the 1990s for others. We found no clear time trends in mercury levels in horseshoe crab eggs, but selenium levels declined at first, then remained steady after the mid1990s. Concentrations of mercury and selenium in blood of migrant shorebirds directly reflected levels in horseshoe crab eggs (their food at stopover). Levels of mercury in eggs of common terns (Sterna hirundo) varied over time, and may have declined slightly since the mid2000s; selenium levels also varied temporally, and declined somewhat. There were variations in mercury and selenium levels in commercial, recreational, and subsistence fish as a function of species, season, and size (a surrogate for age). Selenium:mercury molar ratios also varied as a function of species, year, season, and size in fish. While mercury levels increased with size within individual fish species, selenium levels remained the same or declined. Thus selenium:mercury molar ratios declined with size in fish, reducing the potential of selenium to ameliorate mercury toxicity in consumers. Mercury levels in fish examined were higher in early summer and late fall, and lower in the summer, while selenium stayed relatively similar; thus selenium:mercury molar ratios were lower in early summer and late fall than in midsummer. We discuss the importance of temporal trends in biomonitoring projects, variations in levels of mercury, selenium, and the molar ratios as a function of several variables, and the influence of these on risks to predators and humans eating the fish, and the eggs of gulls, terns. Our data suggests that variability limits the utility of the selenium:mercury molar ratio for fish consumption advisories and for risk management.

Keywords: Birds; Fish; Fish consumption; Horseshoe crabs; Humans; Mercury; Selenium; Selenium:mercury molar ratio.

Figure 1.

Simplified food web for Delaware and Barnegat Bays (New Jersey), illustrating the species discussed in this paper to examine mercury and selenium levels, and selenium:mercury molar ratios in biota.

Figure 2.

Map of showing Barnegat Bay and Delaware Bay in New Jersey, United States (after Burger and Gochfeld 2016).

Figure 3.

Mean ± SE levels (ng/g, wet weight) of selenium, mercury, and selenium:mercury molar ratios in eggs of horseshoe crabs collected from Delaware Bay from 1993 – 2019. Crab eggs are one of key bases of food webs in east coast beaches and estuaries (after Burger et al. 2014, unpubl. data).

Figure 4.

Mean ± SE levels (ng/g, wet weight, n=8–20) of selenium, mercury, and selenium:mercury molar ratios in eggs of common terns collected from nests on Barnegat Bay islands from 1971 to 2019. Common terns prey on small bait fish, and tern eggs are eaten by many predators, including people in some countries (after Burger and Gochfeld 2016, unpubl. data).

Figure 5.

Mean ± SE levels (ng/g, wet weight, n=25–88) of selenium, mercury, and selenium:mercury molar ratios in several small “bait” fish species collected from Barnegat Bay that are prey for common terns and other birds, as well as for larger fish (after Burger and Gochfeld 2016, unpubl. data). The prey fish were brought back to the nest to feed chicks (2001–2014; small SE indicates little variation over the years).

Figure 6.

Mean ± SE levels (ng/g, wet weight, n=20–258) of selenium, mercury, and selenium:mercury molar ratios in several fish species that were collected from people fishing recreationally along the Jersey shore (after Burger and Gochfeld 2011, , unpubl data).

Figure 7.

Mean ± SE levels (ng/g, wet weight) of selenium, mercury, and selenium:mercury molar ratios in bluefish (n=206) and striped bass (n=178) collected from people fishing recreationally along the New Jersey coast, as a function of size (which relates to age) (after Burger 2009; Gochfeld et al. 2012, unpubl. data).

Figure 8.

Mean ± SE levels (ng/g, wet weight) of selenium, mercury, and selenium:mercury molar ratios in bluefish and striped bass collected from people fishing recreationally along the Jersey coast, as a function of season (after Burger 2009; Gochfeld et al. 2012, unpubl. data)..