Abstract
Zooplankton occupy a key place in the ocean ecosystems as they constitute a link between primary producers and upper trophic levels, with many commercially important fisheries relying on the presence of zooplankton to sustain fish stocks. Moreover, zooplankton have an important role in supporting primary production as they can recycle large amounts of micronutrients such as iron, facilitating its retention in the surface ocean and alleviating iron limitation of phytoplankton. Intuitively, one may consider that a large quantity of prey should ensure a healthy zooplankton ecosystem, but the microbial oceanic food web is characterized by a great variability in both the composition and quality of preys. This variability may lead to mismatches between predator and prey stoichiometry, which can in turn affect the growth efficiency of zooplankton. Here we show that variations in food quality are the main drivers of changes in iron assimilation and recycling by zooplankton. Making use of a state-of-the-art biogeochemical model that explicitly accounts for the impact of multiple drivers on the iron assimilation efficiency, we quantify the relative drivers of iron recycling in different ocean regions and across seasons. Our results can be reconciled within a conceptual framework that links the assimilation efficiency of zooplankton to predator-prey stoichiometric mismatch and zooplankton physiological assumptions. If predator and prey stoichiometries are close, then the micronutrient assimilation by zooplankton is optimal and recycling is low. Any departure from this optimal stoichiometry leads to a decrease in assimilation efficiency and a subsequent increase in micronutrient recycling. This framework can be used to understand the impact of variability in prey food quality on iron recycling from previous experiments and generates clear hypotheses about the relative importance of recycling for other micronutrients such as copper, cobalt, manganese, and zinc. Finally, our findings highlight the importance of future changes in prey food quality in driving recycling rates of micronutrients that can amplify or attenuate any climate driven trends in upper ocean nutrient supply.
Highlights
Zooplankton are keystone species in the ocean ecosystem (Steinberg and Landry, 2017), and constitute the largest animal biomass on Earth (Richardson, 2008)
As over 80% of upper 100 m iron recycling is performed by microzooplankton, we will focus the rest of this study on microzooplankton
The detritivory proportion has a stronger impact on the variance in the iron assimilation efficiency in the Equatorial Atlantic, where it explains over 30% of iron assimilation efficiency variance, in the Southern Ocean where it explains about 20% of iron assimilation efficiency variance, and in the Arctic region where it explain over 50% of assimilation efficiency variance (Figure 5C)
Summary
Zooplankton are keystone species in the ocean ecosystem (Steinberg and Landry, 2017), and constitute the largest animal biomass on Earth (Richardson, 2008). Zooplankton act as phytoplankton grazers, but they can recycle nutritive elements such as iron very efficiently (Hutchins and Bruland, 1994; Barbeau et al, 2001; Boyd et al, 2012; Giering et al, 2012; Schmidt et al, 2016) This is important as net primary production is limited by iron over much of the ocean and in such areas, recycled iron is the major iron source for surface primary production for parts of the year (e.g., Hutchins et al, 1993; Boyd et al, 2015; Rafter et al, 2017; Tagliabue et al, 2017). The biotic and abiotic drivers of recycling are not well-characterized
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