Metal Sediment Quality Guidelines and the Ecosystem Concept: Lessons from Field and Mesocosm Experiments

摘要

Background. Divalent metals, which can be chemically reactive in natural ecosystems,present a major challenge to the establishment of sediment quality guidelines (SQG).Ongoing efforts to measure bioavailable (i.e., potentially toxic) fractions of metal maylead to site-specific SQG that are a more appropriate estimate of hazard. Metal concentrationscorrected for acid volatile sulfide (AVS) and organic carbon are being promoted asa good alternative for setting site-specific SQG. We use results from a variety of mesocosmand field-based experiments to draw conclusions about conditions under whichAVS-based measures of bioavailability are accurate, conservative, or under-protective foraquatic invertebrates. Using the ecosystem concept, we explore how divalent metals canpotentially interact with abiotic and biotic compartments within contaminated ecosystemsto ultimately affect consumers.Approach. A series of field-based and laboratory mesocosm experiments were completedusing either metal-amended or naturally contaminated sediments. Geochemicallydiverse sediments were amended with Ni and placed in streams where we measuredcolonization by native invertebrates. In a second field experiment, primary producerswere exposed to Cu and Ni amended sediments and subsequently fed to herbivores. Inlaboratory mesocosms, we exposed snails and amphipods to sediment and dietary Ni inthe presence and absence of suspended sediment and dissolved organic carbon (DOC).Finally, we exposed amphipods and cladocerans to bedded, resuspended, and rebeddedfield-contaminated sediment similar to what we would expect during a dredgingoperation.Results. In general, AVS-based measures of bioavailability were protective of invertebrateconsumers and in some cases overly-conservative. Interactions between metals andabiotic binding fractions that are unaccounted for in the AVS-based measures (e.g., DOC,iron (hydr)oxides (FeOX)) tended to decrease the fraction of truly bioavailable metal. Forexample, in a field experiment with sediments that maintained stable AVS and total Niconcentrations, a reduction in invertebrate toxicity was related to an increase in FeOX,which presumably adsorbed Ni in excess of AVS. Sediments resuspended in oxic surfacewaters did release some bioavailable metals, yet the concentrations were below toxicitythresholds. In feeding experiments, biofilms growing on contaminated sediments incorporatedsediment-derived metals, which then altered consumer feeding rates. In labexperiments, snails and amphipods both demonstrated sensitivity to metals in periphytonfood. Photosynthetically active biofilms have the potential to oxidize surface sedimentand break metal sulfide bonds, which may lead to seasonal and diel variation in metal