Biodilution

Biodilution is the decrease in concentration of an element or pollutant with an increase in trophic level. This effect is primarily caused by the observed trend that an increase in algal biomass will reduce the overall concentration of a pollutant per cell, which ultimately contributes to a lower dietary input to grazers (and higher-level aquatic organisms).

The primary elements and pollutants of concern are heavy metals such as mercury, cadmium, and lead. These toxins have been shown to bioaccumulate up a food web. In some cases, metals, such as mercury, can biomagnify. This is a major concern since methylmercury, the most toxic mercury species, can be found in high concentrations in human-consumed fish and other aquatic organisms. Persistent organic pollutants, such as carcinogenic polycyclic aromatic hydrocarbons and alkylphenols, have also shown to biodilute in the marine environment.

Numerous studies have linked lower mercury concentrations in zooplankton found in eutrophic(nutrient-rich and highly productive) as compared to oligotrophic (low nutrient) aquatic environments. Nutrient enrichment (mainly phosphorus and nitrogen) reduce the input of mercury, and other heavy metals, into aquatic food webs through this biodilution effect. Primary producers, such as phytoplankton, uptake these heavy metals and accumulate them into their cells. The higher the population of phytoplankton, the less concentrated these pollutants will be in their cells. Once consumed by primary consumers, such as zooplankton, these phytoplankton-bound pollutants are incorporated into the consumer’s cells. Higher phytoplankton biomass means a lower concentration of pollutants accumulated by the zooplankton, and so on up the food web. This effect causes an overall dilution of the original concentration up the food web. That is, the concentration of a pollutant will be lower in the zooplankton than the phytoplankton in a high bloom condition.

...
Wikipedia