Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

eactive pool of solid-phase sulfide. This phase is available to bind divalent metals that then
become unavailable for uptake by aquatic biota. The models proposed by Di Toro et al. (1992)
and Casas and Crecelius (1994) predict that when the molar ratio of simultaneously extractable
metals (SEM) to AVS in sediments is less than one, the metals will not be bioavailable due to
complexation with available sulfide.
Salinity is an important modifying factor to metal toxicity. Verslycke et al. (2003)
exposed the estuarine mysid Neomysis integer to individual metals, including Pb, and metal
mixtures under changing salinity. At a salinity of 5%, the reported LC50 for Pb was 1140 µg/L
(95% CL = 840, 1440 µg/L). At an increased salinity of 25l, the toxicity of Pb was substantially
reduced (LC50 = 4274 µg/L [95% CL = 3540, 5710 µg/L]) (Verslycke et al., 2003). The
reduction in toxicity was attributed to increased complexation of Pb 2+ with Cl ! ions.
Nutritional Factors
The relationship between nutrition and Pb toxicity has not been thoroughly investigated in
aquatic organisms. In fact, algae species are the only aquatic organisms to have been studied
fairly frequently. Although nutrients have been found to have an impact on Pb toxicity, the
mechanisms involved are poorly understood. It is unclear whether the relationship between
nutrients and toxicity comprises organismal nutrition (the process by which a living organism
assimilates food and uses it for growth and for replacement of tissues), or whether nutrients have
interacted directly with Pb, inhibiting its metabolic interaction in the organism. This section
reviews the little information that has been gathered from studies documenting apparent Pbnutrition
associations in aquatic organisms.
Jampani (1988) looked at the impact of various nutrients (i.e., sodium acetate, citric acid,
sodium carbonate, nitrogen, and phosphates) on reducing growth inhibition in blue-green algae
(Synechococcus aeruginosus) exposed to 200 mg Pb/L. Exposure to this Pb treatment
concentration caused 100% mortality in algae. Results indicated that additional nitrogen,
phosphates, and some carbon sources, including sodium acetate, citric acid and sodium
carbonate, all protected the algae from Pb toxicity. Algae that had been starved prior to the
experiment were found to be significantly more sensitive to Pb exposure. Glucose was the only
nutrient tested that did not have a significant impact on Pb toxicity in S. aeruginosus. In a
similar study by Rao and Reddy (1985) on Scenedesmus incrassatulus, nitrogen, phosphate and
AX7-169