In hypereutrophic Lake Taihu (China), Ding et al. (2021) showed that bloom-forming phytoplankton act as a 'biological pump' for hydrophobic organic contaminants (HOCs): algal uptake removes dissolved organochlorine pesticides and polycyclic aromatic hydrocarbons from the water, settling biomass carries them to the sediment, and phytoplankton-derived sediment organic matter (SOM) promotes their accumulation in benthos (the Asian clam Corbicula fluminea). The mechanistic control behind these correlations is, however, unquantified, and the authors explicitly call for further research to elucidate the phytoplankton biological pump effects on the fate of HOCs in the benthic food chain, especially for hypereutrophic waters. The open problem is to elucidate, with mechanistic and seasonal evidence, how the biological pump controls the fate and trophic transfer of HOCs within benthic food chains of hypereutrophic waters. Candidate mechanistic pathways suggested by the source study's findings include: (i) species-specific phytoplankton uptake and particle-size-dependent export to the sediment, (ii) in-situ transformation and metabolism of HOCs within phytoplankton and benthos, (iii) seasonal changes in benthic organism behavior (winter dormancy versus active feeding), and (iv) alteration of desorption kinetics and bioavailability of sediment-bound HOCs by phytoplankton-derived SOM.

Background

Hydrophobic organic contaminants (HOCs) — a class of persistent organic pollutants including organochlorine pesticides (OCPs) and polycyclic aromatic hydrocarbons (PAHs) — bind strongly to organic matter rather than staying dissolved, so their environmental fate is governed by particle cycling. In oceanography, the 'biological pump' denotes the process by which photosynthetic plankton fix carbon in the surface ocean and sinking particles export it to depth; the same physical logic applies to pollutants: bloom phytoplankton take up dissolved HOCs (by surface sorption and passive uptake), and settling biomass, aggregates, and fecal pellets transport the contaminants to the sediment, a mechanism sometimes called the biological pollutant pump. In a eutrophic (nutrient-overloaded) lake this coupling is amplified: hypereutrophic lakes such as Lake Taihu, China's third-largest freshwater lake, sustain recurrent cyanobacterial blooms dominated by Microcystis, producing large seasonal pulses of settling algal biomass and labile organic matter. Sediment organic matter (SOM) derived from this biomass can both sequester HOCs and, because fresh algal organic matter is easily degraded, change how strongly HOCs are bound — their bioavailability, i.e., the fraction of total sediment contaminant that organisms can actually take up, which depends on desorption kinetics (the rate at which contaminants leave sediment particles into pore water or organism guts). Bioaccumulation in benthic organisms (bottom-dwelling animals such as the Asian clam Corbicula fluminea, a common bioindicator that buries in sediments and filters phytoplankton, with a winter dormancy phase during which it stops feeding) is commonly quantified by the biota–sediment accumulation factor, the ratio of contaminant concentration in organism lipid to that in sediment organic carbon, and trophic transfer through food webs by trophic magnification factors, the change in lipid-normalized concentration per trophic level. A countervailing process is biodilution: contaminant concentrations per unit biomass fall when bloom biomass dilutes the contaminant pool. The prior results framing this problem are: (1) Zhao et al. (2015) showed in Lake Taihu that phytoplankton biomass strongly modulates dissolved-phase OCP and PAH occurrence in the water column — the water-column half of the pump. (2) Ding et al. (2021), the source study, supplied the benthic half: across Taihu sites and seasons, sediment HOC concentrations rose with phytoplankton biomass together with SOM content; sediment and benthos (Corbicula) HOC concentrations were significantly positively related during winter benthos dormancy; and phytoplankton appeared to strengthen benthic bioaccumulation through their contribution to SOM and the resulting increased bioavailability of sediment-bound HOCs. (3) Mechanistic strands have since advanced in adjacent settings: Wei et al. (2021) showed that the hydrophobicity of Microcystis-derived algal organic matter, and hence PAH and chlorobenzene partitioning to it, changes non-monotonically over the growth cycle; Xia et al. (2022) found phytoplankton PAH accumulation to be species-specific across eutrophication gradients; Tang et al. (2023, 2026) quantified biological-pump control of antibiotic bioaccumulation and trophic transfer in planktonic food webs of the Pearl River and its estuary, including controlled-biomass microcosms; Liu and Tao (2024) documented biodilution of phthalates in Taihu plankton, and Liu and Tao (2026) showed in controlled microcosms that an algal shift from Chlorella vulgaris to Microcystis aeruginosa non-monotonically alters PAH biomagnification in an algae–Daphnia food chain; Pei et al. (2022) used in-situ fish and Corbicula bioassays in the Tai Lake Basin and identified organism metabolism as an influential factor in sediment–clam contaminant distribution. What no study has done is close the loop for the benthic pathway in hypereutrophic lakes: quantify, seasonally and mechanistically, how the pump's delivery of algal biomass and organic matter controls the fate and food-chain transfer of HOCs in the benthos.

Problem Statement

Elucidate, with mechanistic and seasonal evidence, how the phytoplankton biological pump controls the fate and trophic transfer of hydrophobic organic contaminants (HOCs) within benthic food chains of hypereutrophic waters — the question the source study explicitly left open. The scientific objective is a mechanistic, quantitative account of how the pump's delivery of algal biomass and organic matter determines net HOC exposure of benthic consumers, addressing the candidate mechanistic pathways suggested by the source study's findings: (i) species-specific uptake of HOCs by phytoplankton and particle-size-dependent export of contaminant-bearing algal biomass to the sediment; (ii) transformation and metabolism of HOCs within phytoplankton and within benthic organisms; (iii) seasonal behavioral changes of benthic organisms, i.e., the contrast between winter dormancy and active feeding, as a driver of net exposure; and (iv) alteration of desorption kinetics and bioavailability of sediment-bound HOCs to benthic consumers by phytoplankton-derived sediment organic matter. Explaining the observed coupling between bloom dynamics, sediment contamination, and benthic bioaccumulation therefore requires resolving how these four processes jointly produce it; an account that resolves only a subset of the candidate pathways leaves the coupling unexplained. The sediment–benthos concentration relationships observed during dormancy in Lake Taihu are an instance that such an account is expected to reproduce, not the completion target.

References

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