The importance of zooplankton life-cycle for the ocean carbon cycle

Posted by Design Studio

26 June 2024

Challenge 3: Marine Biota

Introduction Marine mesozooplankton play an important role for marine ecosystem functioning and for the ocean carbon cycle. Their size structure, varying spatially and temporally, heavily impacts biogeochemical processes and ecosystem services. But mesozooplankton exhibit size changes throughout their life cycle, affecting metabolic rates and functional traits. Despite this variability, many models oversimplify mesozooplankton as a single, unchanging size class, potentially biasing carbon flux estimates.

Method In this study (Clerc et al. in revision in Global Biogeochemical Cycles), we include a simple representation of mesozooplankton life-cycle, with explicit ontogenetic growth and reproduction, into the 3-dimensional global ocean biogeochemical model PISCES. We then investigate the subsequent effects on simulated mesozooplankton phenology, plankton distribution, and organic carbon export.

Architecture of the modified PISCES version including a simple representation of mesozooplankton life cycle.

Results Utilizing an ensemble of statistical predictive models calibrated with a global set of observations, we generated monthly climatologies of mesozooplankton biomass to evaluate the simulations. Our analyses reveal that the model and observation-based biomass distributions are consistent (rpearson=0.40, total epipelagic biomass: 137 TgC from observations vs. 232 TgC in the model), with similar seasonality (later bloom as latitude increases poleward).

Including ontogenetic growth in the model induced cohort dynamics and variable seasonal dynamics across mesozooplankton size classes and altered the relative contribution of carbon cycling pathways. Younger and smaller mesozooplankon transitioned to microzooplankton in this version of PISCES, resulting in a change in particle size distribution, characterized by a decrease in large particulate organic carbon (POC) and an increase in small POC generation. Consequently, carbon export from the surface was reduced by 10%.

Conclusion This study underscores the importance of accounting for ontogenetic growth and reproduction in models, highlighting the interconnectedness between mesozooplankton size, phenology, and their effects on marine carbon cycling.


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