Overview

Laboratory toxicity studies and a caged bivalve field study complemented by computer modeling were conducted to investigate the significance of effects on aquatic organisms due to dietary exposure to metals.  The lab studies were performed with saltwater and freshwater organisms. The saltwater studies provided partial support for previous research that showed dietary effects could occur when the food source, a mono-algal culture, was loaded with Ag, Cu, Ni or Zn at dissolved metal concentrations below than the current ambient water quality criteria (WQC). However, in contrast to the earlier studies, continuous 7-day exposures to Ag rather than a 4-hour pulse exposure were required to elicit a response by Acartia tonsa. The effects concentrations were also much higher than in the earlier studies and slightly higher than the proposed chronic water quality criteria (WQC) for Ag. The three other metals were only tested under a continuous-exposure regime and yielded similar test results to Ag, though the dissolved concentrations in which the algae were cultured were below the current chronic saltwater WQC.  In the freshwater studies performed with Ceriodaphnia dubia exposed to dietary Cu and Ag, reproductive impairment was not consistent among repeat tests. Variations in reproductive vitality of the C. dubia might have contributed to the variability in adverse responses by the organisms. Questions remain regarding the applicability of the saltwater and freshwater results to field conditions, where mitigating factors such as a more nutritional mixed-algal diet or increased metal complexation capacity could decrease the amount of the bioavailable form of the metals, thereby decreasing exposure and effects.  In a caged-bivalve field study in San Diego Bay, we suspended the mussel Mytilus galloprovincialis for 82 days in the water column, at four locations along a Cu gradient and at one location (SI) in Shelter Island Yacht Basin (SIYB), a quiescent area adjacent to the main Bay.  The range of Cu levels to which the bivalves were exposed bracketed the current and proposed saltwater acute and chronic WQC for Cu.  The mussels in the main Bay were able to regulate tissue Cu levels (<10 mg/gd) when dissolved Cu was below about 3 mg/L and dietary Cu was less than 226 mg/g.  In contrast, mussels in SIYB exposed to 7.5 mg/L dissolved Cu and 307 mg/g particulate Cu accumulated about 10-fold higher tissue Cu (108 mg/gd).  Survival was generally good at all stations (>79%). Effects on growth were observed for mussels from two main Bay stations as well as the SIYB station.  The main bay stations that exhibited the most significant growth effects (CAYS and EOD) accumulated relatively little Cu over the period of deployment. We believe the growth effects were most likely caused by the elevated temperatures at EOD and CAYS.  At SI, the station with the lowest mean concentration of suspended particulate matter, we attributed the decreased growth to a decreased food supply. The companion model was used to interpret the field study data and three additional data sets. It was not possible to evaluate the contributions of water and food to the body burden of Cu in the range in which regulation was achieved, but we conclude that the mussels could either regulate dietary Cu uptake within this range, or they did not accumulate much Cu from their food. At Cu levels above the threshold for regulation, the model indicated that most if not all of the excess Cu accumulation was from waterborne sources. Finally, the modeling indicated that the bivalves at SI were protected from elevated tissue Cu levels by intracellular detoxification processes that resulted in an accumulating pool of relatively slowly excreted Cu.

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