Effects of mesozooplankton growth and reproduction on plankton and
organic carbon dynamics in a marine biogeochemical model
Abstract
Marine mesozooplankton play an important role for marine ecosystem
functioning and global biogeochemical cycles. Their size structure,
varying spatially and temporally, heavily impacts biogeochemical
processes and ecosystem services. 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. Here, we include mesozooplankton ontogenetic
growth and reproduction into a 3-dimensional global ocean biogeochemical
model, PISCES-MOG, and investigate the subsequent effects on simulated
mesozooplankton phenology, plankton distribution, and organic carbon
export. 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 of
PISCES-MOG. Our analyses reveal that the model and observation-based
biomass distributions are comparable (r$_{pearson}$=0.40, total
epipelagic biomass: 137TgC from observations vs. 232TgC in the model),
with similar seasonality (r$_{pearson}$=0.25 for the months of
maximal biomass). 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 mesozooplankton transitioned to
microzooplankton in PISCES-MOG, 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\%. 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.