Abstract
The performance of four contemporary formulations describing trophic transfer, which have strongly contrasting assumptions as regards the way that consumer growth is calculated as a function of food C:N ratio and in the fate of non-limiting substrates, was compared in two settings: a simple steady-state ecosystem model and a 3D biogeochemical general circulation model. Considerable variation was seen in predictions for primary production, transfer to higher trophic levels and export to the ocean interior. The physiological basis of the various assumptions underpinning the chosen formulations is open to question. Assumptions include Liebig-style limitation of growth, strict homeostasis in zooplankton biomass, and whether excess C and N are released by voiding in faecal pellets or via respiration/excretion post-absorption by the gut. Deciding upon the most appropriate means of formulating trophic transfer is not straightforward because, despite advances in ecological stoichiometry, the physiological mechanisms underlying these phenomena remain incompletely understood. Nevertheless, worrying inconsistencies are evident in the way in which fundamental transfer processes are justified and parameterised in the current generation of marine ecosystem models, manifested in the resulting simulations of ocean biogeochemistry. Our work highlights the need for modellers to revisit and appraise the equations and parameter values used to describe trophic transfer in marine ecosystem models.
Highlights
Zooplankton are key players in the biogeochemical cycling of carbon and nutrients in marine ecosystems, especially in their roles in linking primary producers to higher trophic levels including fish (Beaugrand and Kirby, 2010; Beaugrand et al, 2010) and in the export of organic matter to the deep ocean (e.g., González et al, 2009; Juul-Pedersen et al, 2010).Parameterising zooplankton in models is far from straightforward (Carlotti and Poggiale, 2010)
Size structure may be expected to affect the relationship between primary production, secondary production and export, with greater transfer of carbon to higher trophic levels in systems dominated by large organisms ordered in short food chains (Michaels and Silver, 1988)
Zooplankton production as a fraction of primary production was lower than in Yool et al (2011) because of lower gross growth efficiency (GGE) and, because grazing on detritus was removed, predicted export was higher in the new simulation (Table 2)
Summary
Zooplankton are key players in the biogeochemical cycling of carbon and nutrients in marine ecosystems, especially in their roles in linking primary producers to higher trophic levels including fish (Beaugrand and Kirby, 2010; Beaugrand et al, 2010) and in the export of organic matter to the deep ocean (e.g., González et al, 2009; Juul-Pedersen et al, 2010).Parameterising zooplankton in models is far from straightforward (Carlotti and Poggiale, 2010). Food items are used for growth, with associated losses via faecal material and respiration/excretion. Food quality may interact with food quantity (in terms of carbon), yet C may often be in stoichiometric excess when present in the food of herbivorous zooplankton to the extent that "leftover C" must be disposed of via faecal material or increased metabolic activity and respiration (Hessen and Anderson, 2008). These pathways for disposal have important implications for C cycling and
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