Scaling copepod grazing in a coastal upwelling system: the importance of community size structure for phytoplankton C flux

• Valdés, Valentina ^[2] ; Escribano, Rubén ^[1] ; Vergara, Odette ^[1]
  1. [1] Universidad de Concepción

     Universidad de Concepción

     Comuna de Concepción, Chile

     
  2. [2] Instituto Milenio de Oceanografía
• Localización: Latin American Journal of Aquatic Research, ISSN-e 0718-560X, ISSN 0716-1069, Vol. 45, Nº. 1, 2017, págs. 41-54
• Idioma: inglés
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• Resumen

  • Crustacean zooplankton, often dominated by copepods and euphausiids, are the major phytoplankton grazers in coastal upwelling systems. It has been argued that grazing by zooplankton is a size-dependent process, such that models incorporating the size structure of zooplankton are appropriate for describing herbivore C-transfer. Here, based on the size-spectrum theory and on gut-fluorescence experiments, conducted with numerically dominant copepods from two upwelling sites off the Chilean coast, we show that C-specific ingestion rates of copepods are size-dependent. We further show that the size structure of the copepod community, synthesized by the slope of the normalized size spectrum, determines the impact of grazing on phytoplankton. C-specific ingestion rates, depending on species size, were in the range of 0.02-8.40 (µg C x µg C-1 d-1). A modelled normalized biomass-spectra of a copepod community in the size range of 0.5 to 74.0 µg C showed that C-specific grazing impact can increase by a factor of 4 when small-sized species (0.1-10 µg C indiv-1), such as Paracalanus cf. indicus, Acartia tonsa, Oncaea spp. and Corycaeus spp., dominate the community in terms of biomass. By contrast, when larger-sized copepods dominate (10-100 µg C indiv-1), such as Calanus chilensis, Calanoides patagoniensis and Rhyncalanus nasutus, total zooplankton biomass may increase, but with a sharp decrease in the efficiency of C transfer via herbivores. Our findings indicate that processes affecting the size structure of zooplankton communities can substantially impact on phytoplankton C flux through the pelagic food web, thus controlling production of higher trophic levels.