The Effect of Dietary Exposure to Coal Ash Contaminants within Food Ration on Growth and Reproduction in Daphnia magna.

Coal ash contains numerous contaminants and is the focus of regulatory actions and risk assessments due to environmental spills. We exposed Daphnia magna to a gradient of coal ash contamination under high and low food rations to assess the sublethal effects of dietary exposures. Whereas exposure to contaminants resulted in significant reductions in growth and reproduction in daphnids, low, environmentally relevant food rations had a much greater effect on these endpoints. Environ Toxicol Chem 2020;39:1998-2007. © 2020 SETAC.

Stoichiometric controls of mercury dilution by growth.

Rapid growth could significantly reduce methylmercury (MeHg) concentrations in aquatic organisms by causing a greater than proportional gain in biomass relative to MeHg (somatic growth dilution). We hypothesized that rapid growth from the consumption of high-quality algae, defined by algal nutrient stoichiometry, reduces MeHg concentrations in zooplankton, a major source of MeHg for lake fish. Using a MeHg radiotracer, we measured changes in MeHg concentrations, growth and ingestion rates in juvenile Daphnia pulex fed either high (C:P = 139) or low-quality (C:P = 1317) algae (Ankistrodesmus falcatus) for 5 d. We estimated Daphnia steady-state MeHg concentrations, using a biokinetic model parameterized with experimental rates. Daphnia MeHg assimilation efficiencies (approximately 95%) and release rates (0.04 d(-1)) were unaffected by algal nutrient quality. However, Daphnia growth rate was 3.5 times greater when fed high-quality algae, resulting in pronounced somatic growth dilution. Steady-state MeHg concentrations in Daphnia that consumed high-quality algae were one-third those of Daphnia that consumed low-quality algae due to higher growth and slightly lower ingestion rates. Our findings show that rapid growth from high-quality food consumption can significantly reduce the accumulation and trophic transfer of MeHg in freshwater food webs.

Accumulation of inorganic and methylmercury by freshwater phytoplankton in two contrasting water bodies.

Phytoplankton concentrate mercury from their aqueous surroundings and represent the primary entry point for Hg in aquatic food webs. We used 203Hg to compare the uptake of inorganic mercury, Hg(II), and methylmercury, MeHg, in four phytoplankton species (a diatom, a chlorophyte, a cryptophyte, and a cyanobacterium) in two waters containing different concentrations of dissolved organic carbon (DOC). At steady state, volume concentration factors (VCFs) for Hg(II) in the four species were similar and ranged from 0.5 to 5 x 10(4) for both water types, whereas VCFs for MeHg exceeded those for Hg(II) and ranged from 1.3 to 14.6 x 10(5). The VCFs for MeHg in the three eukaryotic cells in the high DOC water were 2-2.6 times greater than those in the low DOC water, but the VCFs for the prokaryote were similar in both waters. Higher cell surface area to volume ratios correlated with increased MeHg concentrations but not with Hg(II). In both water types, VCFs of Hg(II) were similar for living and heat-killed cells, but the VCFs of MeHg were 1.5-5.0 times greater in living cells, suggesting an active uptake component for MeHg. Hg(II) and MeHg were entirely bound to cell surfaces of the dead cells, whereas 59-64% of the MeHg and 9-16% of the Hg(II) in living cells entered the cytoplasm.

Contrasting uptake routes and tissue distributions of inorganic and methylmercury in mosquitofish (Gambusia affinis) and redear sunfish (Lepomis microlophus).

High Hg concentrations in freshwater fish are a concern for human health, yet we lack a clear understanding of the mechanisms that produce high Hg concentrations in fish. Controlled studies in natural surface waters that quantify the uptake and retention of Hg in fish tissues following exposures from the aqueous phase and from invertebrate prey diets are rare. Using 203Hg, we contrasted the accumulation of inorganic Hg (HgI) and methylmercury (MeHg) from the dissolved phase and from invertebrate food in mosquitofish (Gambusia affinis) feeding on Daphnia pulex (representing a pelagic food chain) and in redear sunfish (Lepomis microlophus) feeding on amphipods (Hyallela sp., representing a benthic/macrophyte-based chain). Experiments were conducted with environmentally realistic Hg concentrations in two freshwaters from the San Francisco Bay Delta (CA, USA) with significantly different dissolved organic carbon (DOC) concentrations. Mercury uptake rates following aqueous exposures were consistently higher for fish in the water with lower DOC, whereas efflux rates were similar for both water types. Approximately 50% of the ingested Hg, associated with invertebrate prey was lost from mosquitofish, and 90% or more from sunfish, within 48 h. Assimilation efficiencies for ingested MeHg for both fish were 86 to 94%, substantially higher than those for HgI regardless of water type. Biokinetic modeling using the parameters determined in these experiments accurately predicted Hg burdens for fish in the San Francisco Bay Delta system. Despite considerable accumulation of HgI from both aqueous and dietary exposure routes, the high assimilation efficiencies and slow loss of MeHg from dietary sources are the principal determinants of predicted Hg burdens in both fish species.

Impacts of zooplankton composition and algal enrichment on the accumulation of mercury in an experimental freshwater food web.

There is a well documented accumulation of mercury in fish to concentrations of concern for human consumption. Variation in fish Hg burden between lakes is often high and may result from differences in Hg transfer through lower levels of the food web where mercury is bioconcentrated to phytoplankton and transferred to herbivorous zooplankton. Prior research derived patterns of mercury accumulation in freshwater invertebrates from field collected animals. This study provides results from controlled mesocosm experiments comparing the effects of zooplankton composition, algal abundance, and the chemical speciation of mercury on the ability of zooplankton to accumulate mercury from phytoplankton and transfer that mercury to planktivores. Experiments were conducted in 550-L mesocosms across a gradient of algal densities manipulated by inorganic nutrient additions. Enriched, stable isotopes of organic (CH3(200HgCl)) and inorganic (201HgCl2) mercury were added to mesocosms and their concentrations measured in water, seston, and three common zooplankton species. After 2 weeks, monomethylmercury (MMHg) concentrations were two to three times lower in the two copepod species, Leptodiaptomus minutus and Mesocyclops edax than in the cladoceran, Daphnia mendotae. All three zooplankton species had higher MMHg concentrations in mesocosms with low versus high initial algal abundance. However, despite higher concentrations of inorganic mercury (HgI) in seston from low nutrient mesocosms, there were no significant differences in the HgI accumulated by zooplankton across nutrient treatments. Bioaccumulation factors for MMHg in the plankton were similar to those calculated for plankton in natural lakes and a four-compartment (aqueous, seston, macrozooplankton, and periphyton/sediments) mass balance model after 21 days accounted for approximately 18% of the CH3(200Hg) and approximately 33% of the 201Hg added. Results from our experiments corroborate results from field studies and suggest the importance of particular zooplankton herbivores (e.g., Daphnia) in the transfer of Hg to higher trophic levels in aquatic food webs.

Algal blooms reduce the uptake of toxic methylmercury in freshwater food webs.

Mercury accumulation in fish is a global public health concern, because fish are the primary source of toxic methylmercury to humans. Fish from all lakes do not pose the same level of risk to consumers. One of the most intriguing patterns is that potentially dangerous mercury concentrations can be found in fish from clear, oligotrophic lakes whereas fish from greener, eutrophic lakes often carry less mercury. In this study, we experimentally tested the hypothesis that increasing algal biomass reduces mercury accumulation at higher trophic levels through the dilution of mercury in consumed algal cells. Under bloom dilution, as algal biomass increases, the concentration of mercury per cell decreases, resulting in a lower dietary input to grazers and reduced bioaccumulation in algal-rich eutrophic systems. To test this hypothesis, we added enriched stable isotopes of Hg to experimental mesocosms and measured the uptake of toxic methylmercury (CH3 200Hg+) and inorganic 201Hg2+ by biota at several algal concentrations. We reduced absolute spike detection limits by 50-100 times compared with previous techniques, which allowed us to conduct experiments at the extremely low aqueous Hg concentrations that are typical of natural systems. We found that increasing algae reduced CH3Hg+ concentrations in zooplankton 2-3-fold. Bloom dilution may provide a mechanistic explanation for lower CH3Hg+ accumulation by zooplankton and fish in algal-rich relative to algal-poor systems.