Zooplankton grazing of microplastic can accelerate global loss of ocean oxygen

The global ocean is experiencing a decline in dissolved oxygen, primarily attributed to climate change and its effects on warming, circulation, and biogeochemistry. However, plastic pollution, another significant anthropogenic stressor, is increasingly recognized as a potential threat to marine ecosystems and biogeochemical cycles. A growing body of research indicates that zooplankton consume microplastics, which are small plastic particles (0.1 µm to 5 mm). This consumption leads to a reduction in zooplankton grazing on primary producers (phytoplankton), thereby potentially altering the ocean's carbon cycle and oxygen balance. This study investigates this hypothesis by exploring the impact of zooplankton microplastic ingestion on ocean oxygen levels, offering a novel perspective on the combined effects of climate change and plastic pollution on marine biogeochemistry. The paper's purpose is to quantify the potential contribution of microplastic ingestion by zooplankton to global ocean deoxygenation, highlighting a previously unconsidered feedback mechanism in climate models. The importance of this study lies in its potential to improve the accuracy of future climate change projections by incorporating the biogeochemical impacts of plastic pollution and provide a more comprehensive understanding of the complex interactions within the marine ecosystem.

Previous research has established the decline in global oceanic oxygen content over recent decades, attributing this primarily to climate change. Studies have also documented the increasing presence of plastic pollution in the oceans and its ingestion by zooplankton. Early work highlighted the impact of microplastic ingestion on zooplankton feeding and fecundity. More recent research implemented microplastic consumption and aggregation in marine snow into Earth system models, demonstrating a significant influence on microplastic distribution. However, the biogeochemical consequences of this microplastic consumption, particularly its impact on oxygen levels, remained largely unexplored until this study.

This research employs the University of Victoria Earth System Climate Model (UVic ESCM) version 2, an intermediate-complexity model with a resolution of 1.8° latitude by 3.6° longitude and 19 vertical depth levels in the ocean component. The model incorporates a complex nutrient-phytoplankton-zooplankton-detritus model with three phytoplankton functional types and one zooplankton type. A microplastics component was added, tracking three microplastic tracers: 'free' microplastics (unattached), microplastics bound in marine snow, and microplastics in zooplankton fecal pellets. The model doesn't distinguish between polymer types or particle sizes but represents biologically interactive particles affecting the food web. Microplastic emissions start from 2 million metric tonnes in 1950, increasing at 8.4% annually, spatially weighted by coastal and shipping lane emissions. The model simulates zooplankton grazing on 'free' microplastics, with a Holling II grazing formulation expanded to include microplastic grazing. Relative grazing selectivities for different food types are tested, ranging from aversion to high preference for microplastics. The model assumes 100% plastic egestion efficiency and considers the effects of marine snow aggregation and sinking on microplastic distribution and bioavailability. The model was integrated with year 1765 boundary conditions until equilibrium, then forced with historical CO2 forcing (to 2000) and an RCP8.5 projection to 2100. Four simulations (No Bio, Low Concentration, High Concentration, Moderate Concentration) representing different microplastic parameters were selected from a 300-member Latin Hypercube ensemble to explore the solution space, focusing on plausible global microplastic inventories and subsurface particle maxima.

The study's simulations reveal that zooplankton ingestion of microplastics significantly alters marine ecosystems and oxygen levels. Reduced zooplankton grazing pressure on primary producers leads to different outcomes depending on nutrient availability. In macronutrient-replete regions (e.g., western North Pacific), reduced grazing enhances export production and subsequent oxygen consumption via remineralisation at depth. The Low Concentration simulation shows the largest increase in export production and oxygen loss, despite the lowest surface microplastic concentrations, highlighting the importance of zooplankton's relative grazing selectivity. In contrast, in nutrient-limited regions, reduced grazing pressure leads to a decline in export production, potentially increasing oxygen levels. The North Pacific demonstrates the largest impact, showing up to a 10% decrease in oxygen inventory by 2020 in the Low Concentration simulation. Similar patterns are observed in the North Atlantic, but with smaller magnitudes. In the tropical Pacific, increased export production due to reduced grazing doesn't lead to significant oxygen loss as remineralisation occurs in suboxic zones. Conversely, reduced export production in the western tropical Pacific and tropical Atlantic increases oxygen inventory. The Southern Ocean, although having low microplastic concentrations, shows a significant oxygen loss due to increased export production. Globally, the additional oxygen loss from zooplankton microplastic ingestion is estimated to be between 0.2% and 0.5% relative to 1960 values by 2020 and 0.2-0.7% by 2100, exacerbating the climate-induced oxygen loss. The study's sensitivity analysis shows that the biogeochemical response is more sensitive to zooplankton's relative grazing selectivity for microplastics than to microplastic concentrations themselves.

The findings highlight a critical, previously unconsidered feedback mechanism between plastic pollution and ocean deoxygenation. The significant regional and global impacts of zooplankton microplastic ingestion demonstrate the potential for even low microplastic concentrations to influence ocean biogeochemistry. The model's sensitivity to zooplankton grazing selectivity underscores the need for further research to better constrain this parameter. The results suggest that the combined effects of climate change and plastic pollution are likely to exacerbate ocean deoxygenation. While the model's limitations (e.g., simplified representation of microplastic properties, lack of other potential feedbacks) warrant caution, the study’s qualitative findings strongly suggest the need for incorporating plastic pollution's biogeochemical effects into climate change projections. The study significantly contributes to understanding the complex interplay between physical pollution and marine ecosystem functioning.

This research demonstrates the potential for zooplankton grazing on microplastics to significantly impact regional and global ocean oxygen levels. The model simulations show that even relatively low microplastic concentrations can exacerbate ocean deoxygenation, highlighting a crucial feedback mechanism not previously accounted for in climate models. The study emphasizes the urgent need to better quantify zooplankton's relative grazing selectivity for microplastics and to incorporate the biogeochemical impacts of plastic pollution into future climate projections. Future research should focus on refining the model to incorporate a wider range of factors affecting microplastic bioavailability, such as particle size and species-specific interactions, and further explore the long-term implications of plastic pollution on ocean biogeochemistry and the carbon cycle.

The study utilizes a simplified representation of microplastics, not differentiating between polymer types or particle sizes. The model also doesn't explicitly consider all potential biogeochemical feedbacks associated with microplastic contamination, such as altered particle sinking rates or zooplankton life cycle effects. The assumed 100% plastic egestion efficiency may overestimate microplastic's impact. The study's reliance on an intermediate-complexity model introduces uncertainties, particularly regarding the interaction of iron limitation and microplastic effects in macronutrient-limited regions. Despite these limitations, the study's findings provide valuable insights into a crucial feedback mechanism and underscore the need for further research.