Should we account for mesozooplankton reproduction and ontogenetic growth in biogeochemical modeling?

Abstract

Mesozooplankton play a key role in marine ecosystems as they modulate the transfer of energy from phytoplankton to large marine organisms. In addition, they directly influence the oceanic cycles of carbon and nutrients through vertical migrations, fecal pellet production, respiration, and excretion. Mesozooplankton are mainly made up of metazoans, which undergo important size changes during their life cycle, resulting in significant variations in metabolic rates. However, most marine biogeochemical models represent mesozooplankton as protists-like organisms. Here, we study the potential caveats of this simplistic representation by using a chemostat-like zero-dimensional model with four different Nutrient-Phytoplankton-Zooplankton configurations in which the description of mesozooplankton ranges from protist-type organisms to using a size-based formulation including explicit reproduction and ontogenetic growth. We show that the size-based formulation strongly impacts mesozooplankton. First, it generates a delay of a few months in the response to an increase in food availability. Second, the increase in mesozooplankton biomass displays much larger temporal variations, in the form of successive cohorts, because of the dependency of the ingestion rate to body size. However, the size-based formulation does not affect smaller plankton or nutrient concentrations. A proper assessment of these top-down effects would require implementing our size-resolved approach in a 3-dimensional biogeochemical model. Furthermore, the bottom-up effects on higher trophic levels resulting from the significant changes in the temporal dynamics of mesozooplankton could be estimated in an end-to-end model coupling low and high trophic levels.

Including filter-feeding gelatinous macrozooplankton in a global marine biogeochemical model: model–data comparison and impact on the ocean carbon cycle

“…Also, life cycles are currently not represented in the model, though it could significantly affect the temporal dynamics of a biogeochemical model (Clerc et al, 2021). Most FFGM have a complex life cycle, with an alternation between a sexual and asexual phase that could be a major driver of their population dynamics (Henschke et al, 2016).…”

Recommendations for the formulation of grazing in marine biogeochemical and ecosystem models

“…Ecologically, we do not have a firm understanding of how a myriad of complex interactions combine across innumerable zooplankton species and evolve over time to yield a reasonable approximation of the mean state. For instance, juvenile zooplankton have faster metabolic rates (Clerc et al, 2021) and graze with 𝐾 1∕2 an order of magnitude smaller than adults (Hirst and Bunker, 2003;Richardson and Verheye, 1998), suggesting the apparent 𝐾 1∕2 of the community could be substantially lower during spawning. On the other hand, most applications of the functional response assume an instantaneous response between increasing prey and faster grazing rates, while in reality there is likely a longer acclimation time as predators adapt to new conditions (Mayzaud and Poulet, 1978).…”

Including filter-feeding gelatinous macrozooplankton in a global marine biogeochemical model: model-data comparison and impact on the ocean carbon cycle

“…To further investigate the effect of high growth rates and clearance rates of FFGM, a better understanding of the physiological and environmental drivers of the FFGM mortality processes triggering the end of their swarms seems essential, as their causes are multiple and too poorly documented to be currently modeled (Pitt et al, 2014). Also, life cycles are currently not represented in the model though it could significantly affect the temporal dynamics of a biogeochemical-model (Clerc et al, 2021). Most FFGM have a complex life-cycle, with an alternation between a sexual and asexual phase that could be a major driver of their population dynamics (Henschke et al, 2016).…”