Zooplankton are crucial to marine food webs, acting as the primary energy conduit from phytoplankton to fish. Their diversity, encompassing 15 phyla and ~40% of global marine biomass, highlights their ecological significance. However, most ecosystem models simplify zooplankton representation, potentially overlooking crucial climate-driven changes in marine food webs. These changes have broad implications for ecosystem services, including biogeochemical cycling and fisheries. Zooplankton community structure is determined by the interplay of their functional traits and environmental interactions. These traits influence individual zooplankton fitness and energy transfer to fish. For instance, predator-prey mass ratios (PPMRs) affect the number of trophic steps between phytoplankton and fish, with higher PPMRs indicating fewer steps and less energy loss. Zooplankton carbon content, varying significantly across taxa, also impacts food quality for fish. This study aims to assess climate change's effects on global zooplankton community composition and its consequences for small pelagic fish (SPF), the primary zooplankton predators beyond other zooplankton.

Existing literature highlights the importance of zooplankton in marine food webs and their role in energy transfer from phytoplankton to fish. Studies emphasize the need for improved zooplankton representation in ecosystem models to accurately project climate change impacts. Research indicates that zooplankton functional traits, including PPMR and carbon content, significantly influence energy flow and food quality. Previous modelling and observational studies have demonstrated the strong relationship between zooplankton and phytoplankton biomass across large spatial scales, suggesting that declines in phytoplankton can lead to declines in zooplankton and fish biomass. However, a comprehensive global assessment of climate-driven changes in zooplankton community composition and its effects on fish is lacking, prompting this research.

This research employed the Zooplankton Model of Size Spectra (ZooMSS), a global marine ecosystem model resolving phytoplankton, microzooplankton, seven mesozooplankton and macrozooplankton groups, and three size-based fish groups. ZooMSS incorporates zooplankton traits such as size range, feeding characteristics (PPMR and feeding kernel width), and carbon content. The model was forced by sea-surface temperature and phytoplankton variables from five Coupled Model Intercomparison Project Phase 6 (CMIP6) Earth-system models under three Shared Socioeconomic Pathways (SSPs) (SSP 1-2.6, SSP 3-7.0, and SSP 5-8.5), using historical (1980-2014) conditions as a baseline. The analysis focused on mesozooplankton and macrozooplankton, categorized into three feeding groups: carnivores, omnivores, and filter feeders. The model's outputs were analyzed to assess changes in zooplankton biomass, community composition, and the impacts on SPF diet, trophic level, and biomass. Statistical methods, including Mann-Whitney U-tests, were used to determine the significance of changes observed.

Global zooplankton biomass declined by 7-16% from 1980 to 2100, primarily due to projected phytoplankton biomass reductions. Omnivorous zooplankton experienced the most significant decline (8-18%), while filter feeders showed a more modest decline (up to 6%), and carnivores the least (1-2%). These declines varied across ocean biomes, with the most pronounced effects in tropical regions, where omnivore biomass declined by >25% under SSP 3-7.0 and SSP 5-8.5. Climate change led to significant shifts in zooplankton community composition, particularly in expanding oligotrophic gyres. Carnivorous and gelatinous filter-feeding zooplankton increased in dominance, while omnivorous zooplankton declined. These shifts were driven primarily by changes in phytoplankton size structure, with picophytoplankton projected to increase, leading to a competitive advantage for filter feeders. These changes had profound implications for SPF. Globally, the proportion of SPF diet from omnivorous zooplankton decreased, with increases in filter feeders and carnivores. Although increases in carnivores should lengthen food chains, the rise of filter feeders partially offset this effect, resulting in only a modest increase in SPF trophic level. However, the increase in gelatinous zooplankton led to a decrease in the carbon content of SPF diets, with greater reductions in tropical regions. Global SPF biomass was projected to decline by 20% under SSP 5-8.5, with declines of –10–35% within biomes. These declines were slightly greater than phytoplankton declines in many regions, a phenomenon known as trophic amplification. A model simulation with constant zooplankton carbon content indicated that the decrease in SPF biomass in tropical waters would have been smaller without the reduced carbon quality of food.

The findings challenge the expectation that future food webs will be significantly longer due to climate change. While reduced phytoplankton biomass leads to lower fish biomass, as expected, the rise of gelatinous filter feeders partially mitigates the increase in trophic steps, as they provide a direct pathway from small phytoplankton to fish. The shift towards gelatinous zooplankton results in lower-carbon diets for SPF, particularly in tropical regions. These results are consistent with observations from recent marine heatwaves, which have led to similar shifts in zooplankton communities and fish diets. The model’s limitations, including simplifications of reproductive processes and the absence of direct human impacts, could potentially underestimate the extent of climate-driven changes in marine food webs. The model’s strong and weak points are elaborated in the text, with uncertainty quantification, sensitivity analysis, and specific caveats presented and explained.

This study demonstrates that climate change will lead to substantial shifts in global zooplankton community composition, with a dominance of gelatinous zooplankton at the expense of omnivores. These changes will result in reduced carrying capacity and lower nutritional quality of food for SPF, leading to further declines in fish biomass. The study highlights the importance of considering zooplankton functional traits and their role in mediating climate change effects on marine food webs. Further research should investigate the complex interactions between zooplankton and fish in greater detail, considering factors such as species-specific responses, seasonal variations, and the synergistic effects of climate change and fishing pressure.

The model simplifies several key biological processes, such as zooplankton reproduction and seasonal cycles, and lacks explicit representation of direct human impacts on ecosystems like fishing. The assumption of a uniform Q10 value across all zooplankton groups may also affect the accuracy of projections. Spatial resolution limitations of the model might affect the accuracy of results in coastal areas. The model’s reliance on near-surface chlorophyll a for certain simulations could lead to underestimation of primary producer biomass in stratified waters. The study also doesn't explicitly resolve the possibility of zooplankton adaptations to changing environmental conditions.