Effect of eutrophication on mercury (Hg) dynamics in subtropical reservoirs from a high Hg deposition ecoregion

Abstract

Eutrophication can have opposite effects on mercury (Hg) bioavailability in aquatic systems, by increasing methylmercury (MeHg) production but reducing Hg biomagnification. Globally, the effect of eutrophication on Hg dynamics remains largely untested at lower latitudes such as eastern China, a productive subtropical ecoregion with Hg emission and deposition rates that are among the highest worldwide. Here, we quantify Hg (both MeHg and total Hg, THg) concentrations, Hg bioaccumulation, and Hg biomagnification rates in reservoir food webs across a gradient of eutrophication indicated by chlorophyll a (Chl a), zooplankton density, and total phosphorus (TP). We also assess the effect of hydrogeomorphic (HGM) features on Hg concentrations in water and biota. Water THg concentrations were strongly correlated with TP and were greater in reservoirs at higher elevations with short water retention times (WRT). Zooplankton and top predator THg concentrations were negatively correlated with Chl a, suggesting algal biodilution; evidence for zooplankton density dilution was also found when subtropical reservoirs were compared at a global scale with temperate lakes. Mercury bioaccumulation and biomagnification factors, respectively, did not correlate with increasing Chl a or zooplankton density suggesting no effect of plankton density on Hg trophic transfer. In subtropical reservoirs, eutrophication is associated with lower Hg concentrations in biota but does not explain Hg biomagnification; HGM features (i.e., elevation, WRT) and land use (i.e., % crop) appear to also influence Hg concentrations and bioaccumulation in reservoir food webs.

Key contributors to variations in fish mercury within and among freshwater reservoirs in Oklahoma, USA

“…For all species combined, [THg] was significantly (p=0.02) associated with TP (Figure 2a), indicating that Hg biomagnification occurred along the food chain in Grand Lake, with a trophic magnification factor (TMF) of 1.6 (95% CI = 1.1 – 2.1; calculated as the exponential of slope from the regression of log10-transformed [THg] with TP). This TMF value is much lower than the average TMF (3.4) found in freshwater ecosystems around the world 66 , but comparable to the low TMF (1.1–2.3) observed in fish from subtropical reservoirs in China 67, 68 . The lack of spatially and temporally resolved δ 15 N values for primary consumers in baseline correction may have increased the uncertainty in our TP calculations.…”

“…The negative correlation between chlorophyll-a and fish [THg] in our data provides evidence of algal biodilution. Low fish [THg] in eutrophic reservoirs can also result from growth dilution of the fish 67, 82 , since food is more readily available in nutrient-rich waters, allowing fish to accumulate relatively more biomass. Thus, because most freshwater systems are P-limited, the negative correlation between total P and fish [THg] suggests that growth dilution and biodilution may have limited Hg biomagnification, which is supported by the low TMF observed in Grand Lake.…”

“…This TMF value is much lower than the average TMF (3.4) found in freshwater ecosystems around the world, 66 but comparable to the low TMF (1.1-2.3) observed in sh from subtropical reservoirs in China. 67,68 The lack of spatially and temporally resolved d 15 N values for primary consumers in baseline correction may have increased the uncertainty in our TP calculations. Nevertheless, the signicant associations between sh [THg] and TP suggest that variations in d 15 N for zooplankton were smaller than variations among trophic levels.…”

Mercury and Amino Acid Content Relations in Northern Pike (Esox Lucius) in Subarctic Lakes Along a Climate-Productivity Gradient

“…Lake Kivi and Oiko, had high THg levels in pike in comparison to other oligotrophic lakes, where greater age can explain their high mercury content. Overall, THg biomagnification to top predators decreases towards more eutrophic lakes due to a biodilution process, however at the same time eutrophication and browning likely induce anoxia, that is known to enhance mercury methylation (Razavi et al, 2015;Kozak et al, 2021). Both biodilution and methylation processes contrast, thereafter high variation of THg content in top predators was observed.…”

Mercury Elevator in Lakes: A Novel Vector of Methylmercury Transfer to Fish via Migratory Invertebrates

“…Fish in alkaline (high pH) eutrophic lakes often exhibit lower tissue concentrations of MeHg than those in circumneutral unproductive basins. − Lower MeHg loads in fish in productive systems may result from complex interacting processes, including differences in algal size and edibility, selective feeding by fish on prey with low Hg content, zooplankton mass dilution of Hg transfer to fish, local physio-chemical controls of methylation, or uptake of MeHg by consumers as cited in ref . Notable exceptions to this pattern occur in eutrophic prairie lakes of North America and eutrophic geothermal lakes of New Zealand, as well as contaminated sites in China and the northeastern United States, where MeHg levels in fish are comparable to those observed in oligotrophic lakes.…”