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

Published in Latin American Journal of Aquatic Research, v. 45(1):41-54

Valdés, V., Escribano, R., Vergara , O.

Publication year 2017
IAI Program


IAI Project CRN3094


Crustacean zooplankton, often dominated by copepods and euphausiids, are the major phytoplankton grazers in coastal upwelling systems. It has been argued that zooplankton grazing 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 0.14-353.97 (ng C µg C-¹ h⁻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 ind⁻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 ind⁻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 the phytoplankton C flux through the pelagic food web, thus controlling production of higher trophic levels.