Deep zooplankton rely on small particles in response to low fluxes

Small particles are overlooked as a food source to deep ecosystems because of their often refractory nature at these depths. However, recent work has shown that small particles play a critical role in carbon export to deeper waters. Also, stable isotope analyses have revealed small particle utilization by zooplankton and micronekton in the mesopelagic waters of the oligotrophic North Pacific Subtropical Gyre. Nevertheless, the importance of small particles to deep ecosystems remains enigmatic; under what conditions are small particles important as a food source? Can this finding be generalized to other deep ecosystems? We used compound specific nitrogen stable isotope ratios of source amino acids in small (0.7-53 µm) and large (>53 µm) particle size classes and deep 1-2 mm zooplankton (700-1500 m) to determine the contribution of small particles to their diet. Using an end-member mixing model, we estimated that small particles constitute between 9 and 98% of mesopelagic zooplankton diets at four sites in the North Pacific. The differences can be explained by the quantity and quality of the export flux. Small particles are more important as a food source to deep zooplankton when the quantity of material transported passively or actively below the euphotic zone is lower, and when the nutritional quality, as measured by the C:N ratio, of large particles is poor relative to small particles. A small-particle based diet of lower mesopelagic zooplankton may enhance the efficiency of deep ocean carbon sequestration through the production of dense, quickly sinking fecal pellets. Furthermore, climate driven changes in primary production and export are expected to shift flux to smaller particles and thus their importance in midwater food webs should become widespread. The reliance of deep sea ecosystems on small particles may improve their resilience to the predicted changes in overlying surface waters.