Physical and biogeochemical characteristics of the aquatic environment that affect growth dynamics of phytoplankton and the zooplankton communities that depend on them may also affect uptake of methylmercury (MeHg) into the pelagic food web of oligotrophic reservoirs. We evaluated changes in the quality and quantity of suspended particulate material, zooplankton taxonomy, and MeHg concentrations coincident with seasonal changes in water storage of a mining-impacted reservoir in northern California, USA. MeHg concentrations in bulk zooplankton increased from 4 ngÁg -1 at low water to 77 ± 6.1 ngÁg -1 at high water and were positively correlated with cladoceran biomass (r = 0.66) and negatively correlated with rotifer biomass (r = -0.65). Stable isotope analysis revealed overall higher MeHg concentrations in the pelagic-based food web relative to the benthic-based food web. Statistically similar patterns of trophic enrichment of MeHg (slopes) for the pelagic and benthic food webs and slightly higher MeHg concentrations in zooplankton than in benthic invertebrates suggest that the difference in MeHg bioaccumulation among trophic pathways is set at the base of the food webs. These results suggest an important role for plankton dynamics in driving the MeHg content of zooplankton and ultimately MeHg bioaccumulation in top predators in pelagic-based food webs.Résumé : Les caractéristiques physiques et biogéochimiques du milieu aquatique qui affectent la dynamique de la croissance du phytoplancton et les communautés zooplanctoniques qui en dépendent peuvent aussi influencer l'introduction de méthylmercure (MeHg) dans le réseau alimentaire pélagique des réservoirs oligotrophes. Nous déterminons les changements dans la qualité et la quantité des matières particulaires en suspension, dans la taxonomie du zooplancton et dans les concentrations de MeHg qui coïncident avec les changements saisonniers d'emmagasinement d'eau dans un réservoir affecté par des activités minières dans le nord de la Californie, É .-U. Les concentrations de MeHg dans le zooplancton global augmentent de 4 ngÁg -1 aux basses eaux à 77 ± 6,1 ngÁg -1 aux hautes eaux et elles sont en corrélation positive avec la biomasse des cladocères (r = 0,66) et en corrélation négative avec la biomasse des rotifères (r = -0,65). Une analyse des isotopes stables indique des concentrations globalement plus élevées de MeHg dans le réseau alimentaire pélagique que dans le réseau alimentaire benthique. Les patrons statistiquement semblables d'enrichissement trophique de MeHg (pentes) dans les réseaux alimentaires pélagiques et benthiques et des concentrations légèrement supérieures de MeHg dans le zooplancton par rapport aux invertébrés benthiques font penser que la différence de bioconcentration de MeHg entre les voies trophiques s'établit dès la base des réseaux alimentaires. Ces résultats indiquent un rôle important joué par la dynamique du plancton dans le contenu en MeHg du zooplanction et, en fin de compte, dans la bioconcentration de MeHg chez les prédate...
We assessed methylmercury (MeHg) concentrations across multiple ecological scales in the Edisto (South Carolina) and Upper Hudson (New York) River basins. Out-of-channel wetland/floodplain environments were primary sources of filtered MeHg (F-MeHg) to the stream habitat in both systems. Shallow, open-water areas in both basins exhibited low F-MeHg concentrations and decreasing F-MeHg mass flux. Downstream increases in out-of-channel wetlands/floodplains and the absence of impoundments result in high MeHg throughout the Edisto. Despite substantial wetlands coverage and elevated F-MeHg concentrations at the headwater margins, numerous impoundments on primary stream channels favor spatial variability and lower F-MeHg concentrations in the Upper Hudson. The results indicated that, even in geographically, climatically, and ecologically diverse streams, production in wetland/floodplain areas, hydrologic transport to the stream aquatic environment, and conservative/nonconservative attenuation processes in open water areas are fundamental controls on dissolved MeHg concentrations and, by extension, MeHg availability for potential biotic uptake.
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