Abstract
The biological carbon pump is a key controller of how much carbon is
stored within the global ocean. This pathway is influenced by food web
interactions between zooplankton and their prey. In global
biogeochemical models, Holling Type functional responses are frequently
used to represent grazing interactions. How these responses are
parameterised greatly influences biomass and subsequent carbon export
estimates. The half-saturation constant, or k value, is central to the
Holling functional response. Empirical studies show k can vary over
three orders of magnitude, however, this variation is poorly represented
in global models. This study derives zooplankton grazing dynamics from
remote sensing products of phytoplankton biomass, resulting in global
distribution maps of the grazing parameter k. The impact of these
spatially varying k values on model skill and carbon export flux
estimates is then considered. This study finds large spatial variation
in k values across the global ocean, with distinct distributions for
micro- and mesozooplankton. High half-saturation constants, which drive
slower grazing, are generally associated with areas of high
productivity. Grazing rate parameterisation is found to be critical in
reproducing satellite-derived distributions of nanophytoplankton
biomass, highlighting the importance of top-down drivers for this size
class. Spatially varying grazing dynamics decrease mean total carbon
export by >17% compared to globally homogeneous dynamics,
with increases in faecal pellet export and decreases in export from
algal aggregates. This study highlights the importance of grazing
dynamics to both community structure and carbon export, with
implications for modelling marine carbon sequestration under future
climate scenarios.