Food Chains Evaluated

Marine, freshwater, and terrestrial food chains determined to have the highest propensity for Ni bioaccumulation were identified. For example, the oystercatcher (Haematopus ostralegus) is the main predator of C. edule, which has a much higher propensity to bioaccumulate Ni than other marine organisms (Text Box 1).

Text Box 1

Overview of Nickel Bioaccumulation
and Biomagnification Potential


The bioaccumulation potential of Ni from water into an aquatic organism or from soil into a terrestrial organism is generally low. One notable exception is the marine bivalve Cerastoderma edule, which may have Ni concentrations in its soft tissue that are >25,000 times the Ni concentration in the seawater that it lives in. Interestingly, the bioaccumulation potential of Ni in other marine bivalves is much lower. Accordingly, marine food chains that include C. edule are of particular interest in the secondary poisoning assessment. In terrestrial systems, Ni concentrations in soil-dwelling organisms rarely exceed the Ni concentration in the soil. On average, Ni concentrations in earthworms, for example, are approximately 1/3 of the Ni concentration in soil.

There is no evidence that Ni biomagnifies in aquatic or terrestrial food chains (i.e., Ni concentrations do not increase with increasing trophic level). This contrasts with other chemicals, such as methyl mercury, which tends to biomagnify across multiple trophic levels, thereby resulting in higher trophic level organisms being potentially more susceptible to mercury poisoning. For Ni, the opposite is often the case, where “biodilution” of Ni may occur over increasing steps in the food chain (e.g., Campbell et al. 2005; Lapointe and Couture 2006).

 

Accordingly, an oystercatcher food chain was evaluated for marine birds. In terrestrial systems, earthworm-based food chains were identified because earthworms have a high exposure potential to chemicals in soil and are a common food item for a variety of bird and mammal species. The aquatic and terrestrial food chains identified for birds and mammals are provided in Table 1

System Consumer Organism Food Chain Conservative
Dietary Assumption1
More Realistic
Dietary Assumption2
Marine Bird (oystercatcher) seawater → bivalve mollusk → oystercatcher 100% C. edule 50% C. edule, 50% other mollusks
Mammal (harbor seal) seawater → fish/octopus/squid → harbor seal 100% mollusks or 100% fish not applicable
Freshwater Bird (mollusk-eating) freshwater → mollusk → bird 100% mollusks not applicable
Mammal (otter) freshwater → fish → otter 100% fish not applicable
Terrestrial Bird (worm-eating) soil → earthworm → worm-eating bird 100% earthworms 50% earthworms, 50% isopods
Mammal (shrew) soil → earthworm → shrew 100% earthworms 30% earthworms, 70% isopods
  1. In lower tiers it was assumed that the bird or mammals would feed exclusively on one prey organism.
  2. In the highest tier, where justified, a more realistic mixed diet was assumed.

Table 1: Food chains evaluated in the nickel secondary poisoning risk assessment