Mercury and drought along the lower Carson River, Nevada: II. Snowy egret and black-crowned night-heron reproduction on Lahontan Reservoir, 1997–2006

Abstract

Mercury concentrations in the floodplain of the Carson River Basin in northwestern Nevada are some of the highest ever reported in a natural system. Thus, a portion of the basin including Lahontan Reservoir was placed on the U.S. Environmental Protection Agency's Natural Priorities List for research and cleanup. Preliminary studies indicated that reproduction in piscivorous birds may be at risk. Therefore, a 10-year study (1997--2006) was conducted to evaluate reproduction of snowy egrets (Egretta thula) and black-crowned night-herons (Nycticorax nycticorax) nesting on Gull Island in Lahontan Reservoir. Special attention was given to the annual flow of the Carson River, the resultant fluctuation of this irrigation reservoir, and the annual exposure of snowy egrets and night-herons to methylmercury (MeHg). The dynamic character of the river due to flooding and drought (drought effect) influenced snowy egret and night-heron reproduction more so than did MeHg contamination of eggs. During an extended drought (2000--2004) in the middle of the study, snowy egret nests containing eggs with concentrations of MeHg (measured as total mercury [THg] approximately 100% MeHg) > or =0.80 microg THg/g, ww, all failed, but in 1997 and 2006 (wet years with general flooding), substantial numbers of young were produced (but fewer than at nests where eggs contained <0.80 microg/g). Thus, a variable reproductive threshold of tolerance to MeHg may be associated with habitat quality (food type and abundance). Clearly, drought was the most important factor affecting snowy egret annual productivity. In contrast to snowy egrets, night-herons generally had fewer nests meeting the 0.80 microg THg/g criterion, and those above the criterion were less sensitive to mercury than were snowy egrets. Furthermore, night-herons appeared more tolerant of drought conditions than snowy egrets because they nested earlier, selected more protected nesting sites, and had a more generalist diet that provided additional food options including terrestrial organisms, which also reduced exposure to MeHg. A putative biological effect threshold of 2.0 microg THg/g in whole blood for young of both species was evaluated, which was frequently exceeded, but with no evidence, while still in the colony, of an association with direct mortality. An evaluation of physiological associations with blood residues and post-fledging survival will be presented in future reports in this series.

Reproduction in mallards exposed to dietary concentrations of methylmercury

“…The 17th eggs of females fed 1, 2, 4, or 8 lg/g mercury contained means of 1.6, 3.7, 5.9, and 14 lg/g mercury on a wet-weight basis, respectively (Table 4). These mercury concentrations in eggs from our study more than encompassed the range encountered in wild bird eggs from mercury-contaminated areas (Burgess and Meyer 2008;Evers et al 2008;Hill et al 2008;Schwarzbach et al 2006;Thompson 1996;Scheuhammer et al 2007). Unlike our experimental diets where methylmercury chloride was dissolved in corn oil, the methylmercury in the natural foods a wild bird might eat is likely bound to the sulfhydryl groups of proteins (Wiener et al 2003).…”

“…When compared to results from mallards, where about 1 lg/g caused very little effect on hatching and about 5.5 lg/g mercury in eggs was needed to cause about a 55% decrease (Heinz 1974), pheasant embryos are clearly more sensitive than are mallard embryos. In a recent field study, Hill et al (2008) reported that snowy egrets (Egretta thula) can suffer serious reproductive failure when concentrations of mercury in eggs reached 0.8 lg/g on a wet-weight basis, suggesting that they also are more sensitive to the reproductive effects of methylmercury than are mallards.…”

Oxidative stress response of Forster's terns (Sterna forsteri) and Caspian terns (Hydroprogne caspia) to mercury and selenium bioaccumulation in liver, kidney, and brain

“…Methylmercury (MeHg) bioaccumulates in aquatic ecosystems with adverse effects to piscivorous birds [1][2][3][4][5][6][7]. Methylmercury is thought to be one of the possible causes for reduced reproduction of long-legged wading birds (Ciconiiformes) in the Florida Everglades in recent decades [8][9][10] and toxic responses to MeHg have been documented for piscivorous birds within the lower Carson River system of Nevada, USA, where evidence of cellular damage (i.e., histopathologic and physiologic, including oxidative stress) in the nervous, immune, hepatic, and renal systems of young snowy egrets (Egretta thula), black-crowned night-herons (Nycticorax nycticorax), and double-crested cormorants (Phalacrocorax auritus) were reported [11][12][13]. Additionally, common loons (Gavia immer) in northeast North America have been shown to exhibit impaired reproduction, due in part to MeHg [14][15].…”

Teratogenic effects of injected methylmercury on avian embryos

“…When adult breeding females become contaminated with methylmercury, they deposit some in their eggs [4,5], and in birds the embryo seems to be the life stage most sensitive to methylmercury [1,2,6,7]. In highly contaminated areas, the methylmercury transferred from the mother to her eggs can reach concentrations associated with impaired reproduction in species such as the common loon (Gavia immer) [8,9], common tern (Sterna hirundo) [10], snowy egret (Egretta thula) [11], and California clapper rail (Rallus longirostris obsoletus) [12].…”