Effects of pH and feeding regime on methylmercury accumulation within aquatic microcosms

Changes in fish diets and food web mercury bioaccumulation induced by an invasive planktivorous fish

The invasion, boom, collapse, and reestablishment of a population of the planktivorous threadfin shad in Clear Lake, California, USA, were documented over a 20-year period, as were the effects of changing shad populations on diet and mercury (Hg) bioaccumulation in nearshore fishes. Threadfin shad competitively displaced other planktivorous fish in the lake, such as inland silversides, young-of-year (YOY) largemouth bass, and YOY bluegill, by reducing zooplankton abundance. As a result, all three species shifted from a diet that was dominated by zooplankton to one that was almost entirely zoobenthos. Stable carbon isotopes corroborated this pattern with each species becoming enriched in delta13C, which is elevated in benthic vs. pelagic organisms. Concomitant with these changes, Hg concentrations increased by approximately 50% in all three species. In contrast, obligate benthivores such as prickly sculpin showed no relationship between diet or delta13C and the presence of threadfin shad, suggesting that effects of the shad were not strongly linked to the benthic fish community. There were also no changes in Hg concentrations of prickly sculpin. The temporary extirpation of threadfin shad from the lake resulted in zooplankton densities, foraging patterns, isotope ratios, and Hg concentrations in pelagic fishes returning to pre-shad values. These results indicate that even transient perturbations of the structure of freshwater food webs can result in significant alterations in the bioaccumulation of Hg and that food webs in lakes can be highly resilient.

Total mercury and methylmercury residues in river otters (Lutra canadensis) from Wisconsin

The Wisconsin Department of Natural Resources (WDNR) collected trapper-caught river otter (Lutra canadensis) from 3 distinct areas of Wisconsin (north, central, and south). Otter carcasses were collected from a total of 12 counties during the trapping seasons of 2003 and 2004. Liver, kidney, muscle, brain, and fur tissue was collected for mercury (Hg) analysis. Analysis of variance identified collection zone as the significant factor for differences in tissue Hg levels, with a pattern of decreasing Hg concentrations from north to south (p < 0.0001). This trend was apparent in all tissue types analyzed. Strong relationships were observed between Hg concentrations in all tissues. Likewise, highly significant (p < 0.0001) relationships were found to exist between Hg concentrations in otter fur and Hg concentrations of internal organs (brain, muscle, kidney, and liver). Although these data suggest that Hg concentrations are related among tissues, they do not suggest uniform distribution of Hg throughout the tissues. The results suggest that Hg accumulates at higher concentrations in fur followed by liver, kidney, muscle, and brain. Analysis of a subset of samples for methylmercury (MeHg) revealed that MeHg made up a greater percentage of total Hg in brain and muscle compared to liver and kidney tissue. Although a gradient in tissue concentrations was observed from north to south, none of the tissue concentrations reached levels known to cause toxicity in either otter or mink.

Dose-dependent inorganic mercury absorption by isolated perfused intestine of rainbow trout, Oncorhynchus mykiss, involves both amiloride-sensitive and energy-dependent pathways

The trophic transfer and nutritional toxicity of mercury (Hg) in aquatic food chains is well known, but there is limited information on the mechanism of mercury uptake across the gut. In this study, isolated whole gut sacs from rainbow trout were used to identify the regions of the gut involved in Hg absorption, and then perfused intestines were used to investigate Hg uptake. Exposure of whole gut sacs to 100 micromol l(-1) Hg as HgCl2 in the luminal solution caused Hg accumulation primarily in the mucosa (78% or more), with the intact mid and hind gut supporting 59% of the accumulated Hg. Luminal exposure to [Hg] between 0 and 100 micromol l(-1) for 4 h in perfused trout intestines showed a non-linear dose-dependent accumulation with a maximum Hg uptake rate of about 103 nmol g(-1) h(-1), and suggests carrier mediated transport into the gut cells and the blood. Additions of 2 mmol l(-1) amiloride depressed Hg accumulation by the mid and hind gut by 40-50%, whilst additions of the Ca chelator 1 mmol l(-1) EGTA increased Hg levels in the tissue. Symmetrical additions of 10 mmol l(-1) cyanide did not prevent tissue accumulation of Hg, but caused a 3.4-fold decline in net Hg flux to the serosal compartment. We conclude that Hg absorption across the gut is partly carrier mediated and involves both amiloride sensitive, and energy-dependent pathways.

Trophic transfer and biomagnification potential of contaminants in aquatic ecosystems

This review summarizes information obtained from published literature to determine to what degree biomagnification of organic compounds and metals occurs in freshwater and marine food webs. This review was conducted by: (1) examining data from studies conducted in laboratory experiments to establish body burden ratios between trophic levels (trophic transfer coefficients; TTCs); (2) comparing laboratory-derived TTCs with data obtained from field studies; and (3) comparing biomagnification predictions described by published aquatic food-web models with data obtained in this review. It was determined that: (1) the majority of chemicals evaluated (both organic and metals) do not biomagnify in aquatic food webs; (2) for many of the compounds examined, considerable trophic transfer does occur in aquatic food webs; (3) DDT, DDE, PCBs, toxaphene, methyl mercury, total mercury, and arsenic have the potential to biomagnify in aquatic systems; (4) the lipid fraction of receptors directly influences biomagnification of lipophilic compounds; (5) the food web model reviewed provided similar estimates for most of the organic compounds examined (log Kow values between 5 and 7), with model predictions falling within the range of values of all compounds except dieldrin; (6) for many organic compounds, lack of information precludes assessing the relative importance of biomagnification for these contaminants; and (7) even those compounds for which evidence for biomagnification is strongest show considerable variability and uncertainty regarding the magnitude and existence of food-web biomagnification in aquatic systems.

Metal bioavailability and toxicity to fish in low-alkalinity lakes: A critical review

Fish in low-alkalinity lakes having pH of 6.0-6.5 or less often have higher body or tissue burdens of mercury, cadmium, and lead than do fish in nearby lakes with higher pH. The greater bioaccumulation of these metals in such waters seems to result partly from the greater aqueous abundances of biologically available forms (CH(3) Hg(+), Cd(2+), and Pb(2+)) at low pH. In addition, the low concentrations of aqueous calcium in low-alkalinity lakes increase the permeability of biological membranes to these metals, which in fish may cause greater uptake from both water and food. Fish exposed to aqueous inorganic aluminum in the laboratory and field accumulate the metal in and on the epithelial cells of the gills; however, there is little accumulation of aluminum in the blood or internal organs. In low-pH water, both sublethal and lethal toxicity of aluminum has been clearly demonstrated in both laboratory and field studies at environmental concentrations. In contrast, recently measured aqueous concentrations of total mercury, methylmercury, cadmium, and lead in low-alkalinity lakes are much lower than the aqueous concentrations known to cause acute or chronic toxicity in fish, although the vast majority of toxicological research has involved waters with much higher ionic strength than that in low-alkalinity lakes. Additional work with fish is needed to better assess (1) the toxicity of aqueous metals in low-alkalinity waters, and (2) the toxicological significance of dietary methylmercury and cadmium.

Metabolic rate and uptake and loss of cadmium from food by the fish Noemacheilus barbatulus L. (stone loach)

Fish, Noemacheilus barbatulus (stone loach), of different body weights were used to study rates of uptake and loss of cadmium during and after dietary exposure. Fish were kept singly in a continuous-flow system, and fed tubificid worms. The worms had a range of cadmium levels, but all levels were below that needed to cause acute lethal toxicity in the fish. Body weight affected both the maintenance ration and the amount of food consumed ad libitum, but the exponent for body weight (0.78+/-0.04), relating body weight to food consumption, was unaffected by either temperature or the size of feeding ration. The cadmium content of the worms did not affect the size of the maintenance ration. Metal burden in fish changed rapidly both during and after exposure. After exposure, the cadmium burden of starved fish usually declined more rapidly than in fed fish. A 58-fold increase in cadmium content in the food produced a 28% increase of body burden in the fish, and there was no evidence for biomagnification. Maintenance ration and ration ad libitum and rates of uptake and loss of cadmium increased with temperature within the range 8-18 degrees C, but exposure to cadmium at 16 degrees C yielded a higher asymptotic body burden than either 8 degrees C or 18 degrees C. Rate constants for loss of cadmium after exposure appear to be lower than for loss during exposure. Rates of uptake and loss of cadmium vary with metabolic rate. A maximum in the rate of oxygen consumption was measured at 16 degrees C, above which the rate dropped, presumably due to stress. The exponent for body weight was unaffected by activity or temperature. Body weight of fish appeared to affect both the rates of uptake and loss of cadmium, and feeding rations and respiration to the same extent: body weight exponents were not dissimilar.