Linking plankton size spectra and community composition to carbon export and its efficiency

Abstract

The flux of detrital particles produced by plankton is an important component of the biological carbon pump. We investigate how food web structure and organisms' size regulate particulate carbon export efficiency (the fraction of primary production that is exported via detrital particles at a given depth). We use the Nutrient-Unicellular-Multicellular (NUM) mechanistic size-spectrum model of the planktonic community (unicellular protists and copepods), embedded within a 3D model representation of the global ocean circulation. The ecosystem model generates emergent food webs and size distributions of all organisms and detrital particles. Model outputs are compared to field data. We find that strong predation by copepods increases export efficiency, while protist predation reduces it. We find no clear relation between primary production and export efficiency. Temperature indirectly drives carbon export efficiency by affecting the biomass of copepods. High temperatures, combined with nutrient limitation, result in low growth efficiency, smaller trophic transfer to higher trophic levels, and decreased carbon export efficiency. Even though copepods consume a large fraction of the detritus produced, they do not markedly attenuate the particle flux. Our simulations illustrate the complex relation between the planktonic food web and export efficiency, and highlights the central role of zooplankton and their size structure.

Plankton energy flows using a global size-structured and trait-based model

“…8). These results are broadly consistent with other models (Barton et al, 2013; Serra-Pompei et al, 2022) and observations (Kiko et al, 2022). Additionally, we found a positive relationship between the slope of the size-abundance relationship and export efficiency (pe-ratio), similar to Serra-Pompei et al (2022).…”

“…These results are broadly consistent with other models (Barton et al, 2013; Serra-Pompei et al, 2022) and observations (Kiko et al, 2022). Additionally, we found a positive relationship between the slope of the size-abundance relationship and export efficiency (pe-ratio), similar to Serra-Pompei et al (2022). Model plankton communities with steeper (more negative) slopes were dominated by small phytoplankton with inefficient food webs, and thus had lower export efficiencies than plankton communities dominated by larger phytoplankton, consistent with observations (Henson et al, 2019).…”

Quantitative analysis of food web dynamics in a low export ecosystem

“…Pelagic marine food web processes establish a concentration gradient in organic matter from the sunlit surface to the ocean's depths that is driven by the downward flux of organic carbon from the surface to the ocean interior, known as the biological carbon pump (BCP) (Michaels and Silver, 1988;Ducklow et al, 2001;Boyd et al, 2019). The complexity of oceanic food webs directly influences both the amount and composition (e.g., phytoplankton cells, zooplankton fecal pellets, dissolved organic carbon (DOC), aggregates) of carbon that contributes to export (Carlson et al, 1994;Durkin et al, 2016;Guidi et al, 2016;McCave, 1975;Passow & Alldredge, 1995;Rynearson et al, 2013;Serra-Pompei et al, 2022;Steinberg & Landry, 2017;Turner, 2015). The links and losses within food webs influence the magnitude and strength of key export pathways within the BCP including sinking of individual phytoplankton cells, sinking aggregates, fecal pellets, and active vertical migration (Nowicki et al, 2022;Siegel et al, 2023).…”