Algal blooms reduce the uptake of toxic methylmercury in freshwater food webs

Abstract

Mercury accumulation in fish is a global public health concern, because fish are the primary source of toxic methylmercury to humans. Fish from all lakes do not pose the same level of risk to consumers. One of the most intriguing patterns is that potentially dangerous mercury concentrations can be found in fish from clear, oligotrophic lakes whereas fish from greener, eutrophic lakes often carry less mercury. In this study, we experimentally tested the hypothesis that increasing algal biomass reduces mercury accumulation at higher trophic levels through the dilution of mercury in consumed algal cells. Under bloom dilution, as algal biomass increases, the concentration of mercury per cell decreases, resulting in a lower dietary input to grazers and reduced bioaccumulation in algal-rich eutrophic systems. To test this hypothesis, we added enriched stable isotopes of Hg to experimental mesocosms and measured the uptake of toxic methylmercury (CH 3 200 Hg ؉ ) and inorganic 201 Hg 2؉ by biota at several algal concentrations. We reduced absolute spike detection limits by 50 -100 times compared with previous techniques, which allowed us to conduct experiments at the extremely low aqueous Hg concentrations that are typical of natural systems. We found that increasing algae reduced CH 3Hg ؉ concentrations in zooplankton 2-3-fold. Bloom dilution may provide a mechanistic explanation for lower CH 3Hg ؉ accumulation by zooplankton and fish in algal-rich relative to algal-poor systems. N utrient enrichment with subsequent eutrophication is one of the most important problems impacting lakes worldwide (1, 2). Increased nutrient concentrations produce algal blooms, which in turn alter concentrations of nutrients, gases, pH, and metal ions in the water (3). It is our hypothesis that by increasing algal abundance, nutrient enrichment also alters Hg inputs to lake food webs. Mercury concentrations in fish have been related to metal burdens in their zooplankton prey (4-8), but the connection between Hg accumulation by zooplankton and increasing algal density under nutrient enrichment has not been established. It is critical to discern this association because algae can concentrate Hg from the aqueous phase (e.g., by 100-10,000ϩ times) and thus provide the greatest inputs of Hg to the food chain (9, 10). Here we report how an induced algal bloom affects the accumulation of methyl and inorganic Hg in the cladoceran Daphnia after 2 and 3 weeks of grazing on algae labeled with stable isotopes of Hg. Daphnia is a common zooplankton herbivore and known to be a major food for planktivorous fish (11), therefore factors affecting Hg burdens in this ''keystone'' (12, 13) prey taxon may have important ramifications for predicting CH 3 Hg ϩ burdens in fish across lakes of varying trophic status.We experimentally tested the hypothesis that at equal initial concentrations of aqueous Hg, an increase in algae will result in a decrease in Hg uptake-by zooplankton grazers. Our rationale for this hypothesis was that the concentration of metal per cel...

Difference of mercury bioaccumulation in red mullets from the north-western Mediterranean and Black seas

“…However, the methylmercury concentration in phytoplankton is the result of both MeHg uptake rate and phytoplankton growth. Uptake, which is partially governed by MeHg concentration in water (Mason et al 1996;Pickhardt and Fisher, 2007), will result in various MeHg concentration in plankton depending the rate of building cellular material; indeed a high primary production induces a biomass dilution phenomenon (Pickhardt et al, 2002). Joiris et al (1995) observed a significant negative correlation between Hg concentration and particulate organic carbon concentration in the particulate matter of the European Arctic Seas.…”

Organochlorine contaminants in different tissues from Platichthys flesus (Pisces, Pleuronectidea)

“…This can also be associated to an increase in metabolic activity and growth dilution (Pastor et al, 1996). Growth dilution can be defined as biomass-specific concentration of pollutants diminishes by accumulation of new biomass (Pickhardt et al, 2002). Growth dilution was observed in other species, such as in the Northern pike (Esox lucius), for mercury contamination (Verta, 1990).…”

Heavy metal migration and risk transference associated with cyanobacterial blooms in eutrophic freshwater

“…There is strong empirical evidence that the density of phytoplankton blooms affects the bioaccumulation of metals by algae biomass and metals biomagnifications (Pickhardt et al, 2002). To further clarify the influence of phytoplankton driven processes on metal distribution, the annual dynamics of cellular bound metals were measured and compared to phytoplankton growth during September of 2009 to August of 2010 in Lake Taihu.…”