Nanoplastics rewire freshwater food webs

The study addresses the emerging environmental threat of nanoplastics in freshwater ecosystems. Plastic production and environmental loading are rapidly increasing, and plastics fragment into micro- and ultimately nano-sized particles with properties distinct from bulk materials. Laboratory studies indicate nanoplastics can penetrate cells and be toxic to aquatic biota across trophic levels, with evidence of trophic transfer and impacts on fish metabolism and behavior. However, most prior work used small-scale laboratory settings with arbitrary exposure levels, limiting environmental relevance. The authors aimed to quantify ecosystem-level effects at semi-natural scale across a gradient of environmentally relevant and predicted future nanoplastic concentrations, identify toxicity thresholds (tipping points) for key organisms, and test the hypothesis that some taxa and functions are more vulnerable than others. Specifically, they sought to determine which organisms (e.g., zooplankton grazers and phytoplankton groups) and ecosystem processes (e.g., benthic decomposition) are most at risk, providing information to support risk assessment and regulation.

Background literature highlights: global plastic production and waste are rapidly increasing, with large inputs to aquatic systems. Plastics degrade via mechanical, photolytic, and oxidative processes into micro- and nanoplastics; at nano-scale, particles exhibit large surface area and different behaviors compared to bulk materials. Nanoplastics can cross cell membranes, accumulate in tissues, and transfer through food webs, with reported adverse effects on phyto- and zooplankton, benthic invertebrates, and fish. Concentrations of nanoplastics in the environment are difficult to measure, but detections in surface waters (e.g., ~51 µg L⁻¹ in remote Siberian lakes and up to ~563 µg L⁻¹ in rural Swedish lakes) suggest current levels may approach biological effect thresholds. Reviews call for environmentally relevant, larger-scale mesocosm experiments to determine toxicity thresholds and improve risk assessment beyond small-scale lab exposures.

Design and setting: A replicated semi-natural wetland mesocosm experiment was conducted in a greenhouse (Lund, Sweden). Twelve glass mesocosms (1.0 × 0.3 × 0.2 m) were constructed and divided into a small lake compartment (8.4 ± 0.5 L, mean ± SD) followed by a sediment section with established macrophytes. Continuous flow-through tap water was supplied at 2 mL min⁻¹ via peristaltic pumps. Sediment tufts with macrophytes (collected from a natural wetland near Hästveda, southern Sweden; 56.2843° N, 13.9353° E) were installed two months prior to the experiment to establish communities (20 macrophyte species identified). Biota inoculation: Prior to the start, 10 individuals each of Daphnia magna (planktonic filter feeder) and Asellus aquaticus (benthic feeder) were added per mesocosm (a second inoculation of 10 Daphnia per mesocosm on Day 25). Additional organisms (copepods, chironomids, small worms) arrived with sediment. The experiment ran 73 days (2 May–15 July 2022). Nanoplastic treatments: Four treatments (n=3 each): Control (0), Low (L = 21.41 µg L⁻¹), Medium (M = 214.1 µg L⁻¹), High (H = 2141 µg L⁻¹). Additions were made weekly to the lake compartment; controls received equal MilliQ volumes. Gentle mixing dispersed particles without disturbing sediment. Weekly supplemental algal food was added: 100 mL Tetradesmus obliquus culture to the lake compartment (stock ~1995 µg Chl-a L⁻¹, yielding ~24 µg Chl-a L⁻¹ in the lake section). Nanoparticle characterization: Spherical aminated polystyrene nanoparticles (PS–NH₂), 53 nm diameter (Bangs Laboratories). Particles were dialyzed (MWCO 3.5 kDa; 72 h at 4 °C in 10 L MilliQ, refreshed daily). DLS confirmed post-dialysis size 53.8 ± 1.1 nm. Zeta potential measured: +26.23 ± 0.36 mV (MilliQ) and +27.47 ± 0.74 mV (tap). Dispersions stored at 4 °C until use. Sampling and measurements: Weekly, prior to NP additions, 50 mL of 100 µm–pre-filtered lake water was analyzed via AlgaeLabAnalyser spectrophotometer to quantify total chlorophyll and phytoplankton groups (cyanobacteria, cryptophytes, diatoms). Background variables (pH, dissolved oxygen, water color, temperature) were monitored; no treatment differences at experiment end. Temperature (hourly) ranged 18–27 °C. At termination (Day 73), decomposition was assessed via mesh bags (3 mm) containing pre-dried and weighed European beech leaf discs; remaining mass after drying provided mass loss. Zooplankton and benthic organisms (>100 µm) were enumerated by filtering water and sieving lake-compartment sediment (100 µm), preserving in Lugol’s solution, and counting under a stereomicroscope. Initial abundances for copepods, chironomids, and worms were unknown as they arrived with sediment. Experimental design and statistics: A repeated regression (RR) approach combined ANOVA and regression to assess both qualitative and quantitative responses across the NP gradient and identify tipping points. Abundances of Daphnia, Asellus, copepods, chironomids, worms, and decomposition were analyzed by one-way ANOVA (GraphPad Prism 7e) using log(x+1) transformed data. Bivariate correlations (one-tailed) assessed relationships between phytoplankton biomasses and NP concentrations or Daphnia abundances. Three-dimensional response surfaces (R 4.3.1; interp package) modeled final phytoplankton biomass versus NP exposure and Daphnia abundance; phytoplankton and NP data were log(x+1) transformed; negative fitted values set to zero.

- Zooplankton responses: Daphnia magna showed a strong, dose-dependent negative response to nanoplastics (ANOVA F(3,8)=6.994, p=0.0126). Populations reached ~200–300 individuals in Control and Low (21 µg L⁻¹) treatments, showed higher variance and some declines at Medium (214 µg L⁻¹; not statistically significant vs control/low), and collapsed at High (2141 µg L⁻¹) in all replicates. Mortality onset occurred within ~2 weeks at concentrations between 214 and 2141 µg L⁻¹. In contrast, cyclopoid copepods were unaffected across the gradient (ANOVA F(3,8)=2.000, NS). - Benthic organisms and process: No significant effects of nanoplastics on Asellus aquaticus, chironomids, worms, or leaf-litter decomposition, even at the highest concentration (all ANOVAs NS). - Phytoplankton responses and grazing interactions: Nanoplastics reduced Daphnia populations, releasing phytoplankton from grazing. Total chlorophyll, cyanobacteria, and cryptophytes were not directly suppressed by nanoparticles and tended to have higher biomasses with increasing NP levels, likely via reduced Daphnia grazing. Diatoms were strongly and directly negatively affected by nanoparticles, showing low biomass at high NP concentrations and relatively high biomass at lower NP concentrations despite higher Daphnia grazing. Correlation statistics (one-tailed, n=12; log-transformed): • NP vs phytoplankton: Chlorophyll r=0.52 (p<0.05), Cyanobacteria r=0.47 (NS), Cryptophyta r=0.60 (p<0.025), Diatoms r=−0.53 (p<0.05). • Daphnia grazing vs phytoplankton: Chlorophyll r=−0.50 (p<0.05), Cyanobacteria r=−0.48 (NS), Cryptophyta r=−0.61 (p<0.025), Diatoms r=0.48 (p<0.05). - Environmental relevance: Reported environmental nanoplastic concentrations (~51 µg L⁻¹ in remote Siberian lakes; up to ~563 µg L⁻¹ in rural Swedish lakes) approach the observed threshold range for detrimental effects on Daphnia (between ~214 and 2141 µg L⁻¹).

Findings support the hypothesis that sensitivity to nanoplastics is taxon- and function-specific, leading to differential impacts that can rewire freshwater food webs. Severe Daphnia sensitivity at higher NP levels shifts herbivory from efficient filter-feeding cladocerans to less efficient omnivorous copepods, altering grazing pressure and primary producer dynamics. Phytoplankton communities shift away from diatoms (direct NP sensitivity) toward cyanobacteria and cryptophytes as NP concentrations increase, largely due to reduced cladoceran grazing. Such shifts may exacerbate nuisance cyanobacterial dominance already promoted by eutrophication and warming. Despite substantial NP loading, benthic invertebrates and decomposition were unaffected over the experimental timescale, suggesting pelagic compartments are more vulnerable than benthic ones, potentially due to particle aggregation or eco-corona formation reducing benthic toxicity. Consequences include a truncated pelagic food chain, reduced pelagic biodiversity and herbivory, and a bias of energy flow and interaction strengths toward benthic pathways. These ecosystem-level alterations align with the concept of Ecologically Disrupting Compounds (EcoDC), with nanoplastics predicted to disrupt pelagic processes and community structure while leaving benthic processes relatively intact at least in the short term.

This semi-natural mesocosm study quantifies tipping points and organism-specific responses to nanoplastics at environmentally relevant concentrations. Key contributions include: identification of a threshold range where Daphnia populations collapse (between ~214 and 2141 µg L⁻¹), demonstration of strong negative effects on diatoms but not on copepods, cyanobacteria, cryptophytes, or benthic biota and decomposition, and prediction of food-web rewiring from pelagic to benthic processes, from Daphnia- to copepod-dominated herbivory, and from diatom-based to cyanobacteria/cryptophyte-dominated primary production. These results bolster the classification of nanoplastics as Ecologically Disrupting Compounds and provide actionable insights for environmental risk assessment and regulation. Future research should elucidate mechanistic bases for pelagic versus benthic sensitivity (e.g., aggregation, eco-corona effects), assess longer timescales and seasonal dynamics, expand to diverse nanoplastic materials, sizes, and surface chemistries, improve in situ concentration measurements, and evaluate cascading impacts on higher trophic levels (e.g., fish) and ecosystem services.

- Semi-natural mesocosm setting may not capture full complexity of natural lakes/wetlands, though it improves environmental realism over lab tests. - Single nanoplastic type (53 nm aminated polystyrene) and weekly pulse additions; responses may differ for other polymers, sizes, shapes, or surface chemistries and exposure regimes. - Duration limited to 73 days; longer-term or seasonal effects, chronic accumulation, and delayed responses were not assessed. - Community resolution limited: organisms >100 µm were targeted; copepods, chironomids, and worms were not identified to genus/species; initial abundances for some taxa were unknown as they arrived with sediment. - Benthic mechanisms underlying the lack of observed toxicity were not investigated; particle fate and transformations in sediments were not the study’s focus.