Zooplankton grazing of microplastic can accelerate global loss of ocean oxygen

The global ocean is losing oxygen, with drivers attributed to climate change–induced warming, circulation changes, and biogeochemical impacts. Concurrently, plastic pollution is increasing, and numerous studies show zooplankton ingest microplastics (0.1 μm to 5 mm), which displace nutritional food and reduce grazing on primary producers. Reduced grazing can alter export production and remineralisation, impacting dissolved oxygen. Building on prior modeling that implemented explicit zooplankton ingestion of microplastics and aggregation in marine snow, this study investigates whether such biological interactions can already be producing a detectable, globally significant deoxygenation signal, potentially comparable to a substantial fraction of climate-warming effects, and identifies a mechanism not yet represented in most Earth system models.

Prior work has documented a multi-decadal decline in global ocean oxygen and identified key physical and biogeochemical drivers. Observations and experiments show microplastic ingestion by diverse zooplankton taxa, with potential impacts on feeding, function, and fecundity. Modeling has indicated that marine snow and zooplankton fecal pellets can strongly influence the vertical and horizontal distribution of microplastics, creating subsurface maxima consistent with observations. Observational syntheses have found near-surface microplastic datasets with large regional variability, including evidence for substantial plastic beneath the surface and uncertain gyre maxima. Literature has also hypothesized microplastics could disrupt the biological carbon pump via altered pellet properties, microbial activity, and gelatinous zooplankton interactions, though mechanisms and large-scale implications remain uncertain.

The study uses the University of Victoria Earth System Climate Model (UVic ESCM) v2, an intermediate-complexity model with 1.8° × 3.6° ocean resolution and 19 vertical levels (surface layer 50 m). The atmosphere is a 2-D energy–moisture balance model; winds are prescribed (NCEP/NCAR) and geostrophically adjusted. Ocean biogeochemistry employs a nutrients–phytoplankton–zooplankton–detritus (NPZD) framework with three phytoplankton functional types (diazotrophs, calcifiers, general phytoplankton) and one zooplankton type. Fecal pellets are separated from general detritus (identical sinking rates); marine snow production is a fraction of detritus production. A microplastics module introduces three tracers: (1) free (unattached) microplastic (particles m^-3), passively advected/diffused; a fraction is assigned a fast rise rate to represent buoyant particles; no abiotic degradation; (2) microplastic bound in marine snow, formed by aggregation with a fixed fraction of detritus, sinking with detritus; a fraction reaching the seafloor is buried, the rest returned to free particles in the bottom cell via remineralisation; biofouling is represented implicitly via microbial loop breakdown; (3) microplastic in zooplankton fecal pellets, formed via ingestion and 100% egestion efficiency, sinking like detritus, with a seafloor burial fraction and remainder returned to free particles via bacterial remineralisation. Zooplankton graze free microplastic alongside organic prey via an extended Holling Type II formulation with prescribed relative grazing selectivities (summing to 1) for each prey class (microplastic, detritus, phytoplankton groups, zooplankton). Microplastic confers no nutritional benefit; a particle-to-mass conversion and a 1:1 food substitution assumption (at Redfield) convert particles to mmol N equivalents for the grazing term. Relative selectivity values were varied to represent community-level preferences/avoidance and implicitly account for size, biofouling, and prey rejection effects. Diazotroph selectivity was fixed low (p_Z=0.1) to limit nitrogen cycle disruption. Experimental design: A 300-member Latin Hypercube parameter ensemble explored microplastic input fractions, buoyant fraction, marine snow aggregation fraction, seafloor burial return fraction, aggregate uptake constants, relative grazing selectivity for microplastic, and food-to-microplastic conversion ratios. Fourteen simulations matched observational constraints (global microplastic inventories around 2010 and plausible vertical profiles); from these, three biologically interactive cases spanning the plausible range and one control (No Bio) were highlighted: Low Concentration (LC), Moderate Concentration (MC), High Concentration (HC), and No Bio. Key differences include zooplankton relative grazing selectivity for microplastic: LC high selectivity (v_MP=0.260) with enhanced marine snow aggregation; MC moderate selectivity (v_MP=0.193) and aggregation; HC low selectivity (v_MP=0.132) with reduced aggregation. Greater biological interaction (higher selectivity and/or aggregation) lowers near-surface microplastic inventories. Forcing and spin-up: The model was equilibrated >10,000 years at 1765 boundary conditions. 1765–1950 used historical CO2 and wind anomalies; 1950–2100 used historical CO2 to 2000 and RCP8.5 thereafter. Physical fields are identical across simulations; only biological interactions with microplastic differ. Microplastic emissions begin at 2 million metric tonnes in 1950, increasing at 8.4% yr^-1, spatially weighted by coastal CO2 emissions and shipping lanes. An estimated 4% of total plastic waste enters the ocean; ranges of input fractions were applied. Analyses focus on regional and global export production, nutrient distributions, and water column oxygen inventories to 2020 and 2100.

• Zooplankton ingestion of microplastics reduces grazing on primary producers, altering export production differently by nutrient regime.
• Macronutrient-replete regions: Reduced grazing increases export production and subsurface remineralisation, lowering dissolved oxygen. Macronutrient-limited regions: Reduced grazing enhances the microbial loop and decreases export, often increasing oxygen inventories.
• North Pacific (macronutrient-replete): Export production increases 10–30% by 2020; associated remineralisation reduces water column O2, with regional inventory losses up to roughly 10% (abstract) and strong declines in modeled O2 inventory. The LC case (highest microplastic selectivity) shows the largest biogeochemical impact despite lower surface microplastic concentrations.
• North Atlantic: Export increases 0–20% by 2020; oxygen inventory decreases are smaller (<10 mol O2 m^-2 by 2020), muted by warmer temperatures, stronger microbial processing, and air–sea gas exchange; high background ventilation reduces biogeochemical impact.
• Eastern tropical Pacific (upwelling, macronutrient-replete): Export rises 0–30% depending on region and configuration, but additional remineralisation occurs in suboxic zones dominated by nitrogen loss (denitrification), so little extra O2 loss. Enhanced local export reduces westward nutrient transport, increasing surface nitrate deficiency and suppressing export downstream in macronutrient-limited western tropical Pacific.
• Western tropical Pacific and tropical Atlantic (macronutrient-limited): Export decreases 10–40%, with increases in water column O2 inventory up to ~10 mol O2 m^-2 by 2020 relative to No Bio.
• Northern Indian Ocean: Net annually averaged export decreases up to ~30%, reflecting mixed seasonal transitions between nutrient regimes; signals in nitrate depletion relative to Redfield suggest potential seasonal increases in export in some configurations.
• Southern Ocean (macronutrient-replete): Even with low simulated microplastic concentrations, alleviated grazing elevates export and reduces water column O2 by as much as ~15 mol O2 m^-2 by 2020 across wide regions.
• Global trend: By 2020, microplastic-driven changes accelerate global oxygen inventory loss by an additional 0.2–0.5% relative to 1960, on top of ~1% climate-induced loss; by 2100, additional loss reaches 0.2–0.7%. Differences grow over time with increasing pollution and climate amplification.
• Sensitivity: Biogeochemical response is more sensitive to zooplankton relative grazing selectivity for microplastics than to surface microplastic concentrations, implying significant impacts are possible even without large surface accumulations.

The study identifies a previously underappreciated biogeochemical feedback: zooplankton ingestion of microplastics reduces grazing on primary producers, increasing export and remineralisation where macronutrients are abundant, thereby exacerbating deoxygenation; conversely, in nutrient-limited regions, export declines and oxygen may increase. Regionally, strong effects occur in the North Pacific and Southern Ocean, with muted or contrasting effects in tropical and subtropical regions depending on nutrient limitation and microbial loop dynamics. The modeled additional global oxygen loss by 2020 (0.2–0.5% of 1960 inventory) is potentially up to about half of the climate-warming effect simulated by the model by that date, suggesting microplastic pollution may already be measurably influencing deoxygenation trends. Sensitivity analyses highlight the dominant role of zooplankton relative grazing selectivity, underscoring the need for better constraints. While increased export in this framework does not enhance atmospheric CO2 drawdown in the model (anthropogenic carbon uptake remains effectively unchanged), the identified mechanism has implications for carbon cycling and potential climate mitigation strategies. The findings suggest Earth system models lacking microplastic–biology interactions may underestimate 21st-century deoxygenation.

This work proposes and quantifies a novel pathway by which microplastic pollution can affect ocean biogeochemistry: zooplankton ingestion of microplastics reduces grazing on phytoplankton, altering export production and oxygen consumption patterns. Modeling indicates significant regional deoxygenation, especially in the North Pacific and Southern Ocean, and an additional 0.2–0.5% global oxygen inventory loss by 2020 relative to 1960 (0.2–0.7% by 2100), exacerbating climate-driven deoxygenation. The response depends strongly on nutrient regime and zooplankton grazing selectivity for microplastics. Future research should (i) constrain zooplankton–microplastic grazing selectivity across taxa and particle characteristics; (ii) incorporate dynamic iron limitation and additional ecological complexity; (iii) assess other feedbacks (e.g., particle sinking rate changes, life-cycle and behavioral effects); (iv) refine microplastic distributions, sizes, and degradation pathways; and (v) evaluate consequences for carbon cycling and policy-relevant climate mitigation strategies.

• Parameter uncertainty is high, particularly zooplankton relative grazing selectivity for microplastics; biogeochemical responses are highly sensitive to this parameter.
• Sparse and uncertain observational constraints on global and vertical microplastic distributions; coarse model resolution underrepresents gyre maxima.
• Simplified microplastic representation: no polymer types or size spectra, no abiotic degradation; assumes 100% egestion efficiency; free microplastic only grazed (not marine snow–bound microplastic).
• Biological simplifications: one zooplankton type; no explicit species, size structure, life stages, or diel vertical migration; microbial and biofouling processes implicit.
• Does not include potential feedbacks such as altered particle sinking rates or explicit life-cycle impacts on zooplankton.
• Iron limitation is prescribed seasonally and is not the primary annual limiter; stronger or dynamic iron limitation could alter responses.
• Results are qualitative with respect to absolute magnitudes and depend on assumed emission scenarios (RCP8.5) and microplastic input fractions.