Algal Density Controls the Spatial Variations in Hg Bioconcentration and Bioaccumulation at the Base of the Pelagic Food Web of Lake Taihu, China

Abstract

Algal density can significantly impact mercury (Hg) bioaccumulation and biomagnification in aquatic food webs, but the underlying mechanisms remain controversial especially in subtropical and tropical regions. We conducted a comprehensive field study on Hg bioconcentration in phytoplankton and bioaccumulation in size-fractionated zooplankton across 17 sampling sites in Lake Taihu, a large shallow lake in eastern China with large spatial differences in algal density. The higher algal density in the northern sites is highly associated with the lower THg bioconcentration factor (BCF) in phytoplankton and lower THg bioaccumulation factor (BAF) in zooplankton. The low Hg BCFs or BAFs at productive sites could not be explained by algal bloom dilution but attributed to the low Hg bioavailability, which is highly associated with the elevated pH levels at productive sites. The smaller body size of the dominant zooplankton species at higher algal density sites also contributed to their lower Hg bioaccumulation. Importantly, we provide evidence that high algal density is associated with a low proportion of methylmercury (MeHg) in total Hg (% MeHg) in phytoplankton, which is further transferred to zooplankton. Such a low THg BCF or BAF and low % MeHg in plankton at high algal density sites hamper the entry of Hg into the pelagic food webs, which are important but yet underestimated driving forces for the low Hg contents in pelagic fish that are commonly observed in anthropogenic-impacted eutrophic lakes in subtropical regions.

Bioconcentration of Inorganic and Methyl Mercury by Algae Revealed Using Dual-Mass Single-Cell ICP-MS with Double Isotope Tracers

“…With a cell density of 25-fold higher, IHg T and MeHg T in the cell population with a density of 5 × 10 6 cells mL –1 were approximately 2- and 5-fold lower than those with a density of 2 × 10 5 cells mL –1 , respectively. Accordingly, the proportion of cellular MeHg contents to the total Hg contents would be higher in low-density algae populations, which agrees with observations in natural water . Besides, another trend was that at lower algal densities, higher cellular Hg contents were always accompanied by a wider mass distribution, suggesting a higher degree of intraspecies heterogeneity.…”

“…Accordingly, in environmental scenarios, the cellular Hg contents would be lower in eutrophic water, further resulting in lower Hg levels in the food web. More importantly, both our laboratory results and previous field investigation found that the proportions of MeHg to total Hg on the algae negatively correlate with algal density, implying a lower transfer efficiency of MeHg in eutrophic water. Above all, validated by the experimental results, the proposed model equations can well describe the relations among m , c n , VCF, c Hg‑ini , and c Hg‑res .…”

“…Besides, it was reported that cell sizes are relatively smaller in the productive high-density algae populations, which may lead to lower cellular Hg contents, as we observed in the previous experiments. The above results can also partly explain the relatively lower IHg and MeHg contents of high trophic level species (e.g., zooplankton and fish) with increasing eutrophication. ,− Hg uptake from the food web would be largely reduced when consuming the same biomass of algae due to the lower Hg contents on single algae. It is noteworthy that even though both IHg and MeHg contents on single algae cells were negatively correlated with algal density, the total accumulated IHg and MeHg of the entire algae population were higher with increasing algal density (Figures B and S18B).…”

“…With the increase of algae population (Figure C), the bloom dilution effect has been considered an important factor affecting the accumulated Hg levels in biota. , We first exposed the algae with different densities (2 × 10 5 , 1 × 10 6 , and 5 × 10 6 cells mL –1 ) to the same concentrations of IHg and MeHg and then measured them using scICP-MS. Variations in cellular Hg masses driven by algal density were observed for both CH and CD (Figures B and S18B). Of note, cellular MeHg masses varied more with cell density compared to those of IHg.…”

“…Accordingly, the proportion of cellular MeHg contents to the total Hg contents would be higher in lowdensity algae populations, which agrees with observations in natural water. 52 Besides, another trend was that at lower algal densities, higher cellular Hg contents were always accompanied by a wider mass distribution, suggesting a higher degree of intraspecies heterogeneity. In short, density-driven variations in IHg and MeHg accumulation on single algae cells involved heterogeneity-related "competitive processes" rather than equal distribution among the cells.…”