Variation in the transfer of energy in marine plankton along a productivity gradient in the Atlantic Ocean

Plankton play a key role in oceanic carbon flux as the primary biological mechanism for the sequestration of carbon out of the atmosphere into surface waters. The transfer of energy between phytoplankton and zooplankton can be inferred from regular patterns in population size structure, where plots of abundance within size classes typically show a power-law dependence on size. Here we analyze such picoplankton to mesozooplankton size spectra along a 50uN to 50uS transect in the Atlantic. Contrary to common perception, the transfer efficiency in the oceans is not related to ecosystem productivity. Our results challenge the view that biomass transfer efficiency is lower in oligotrophic oceanic ecosystems. This suggests that global carbon flux models should reconsider the trophic transfer efficiency of productive and oligotrophic areas. The power decay of abundance with increasing body size in marine and freshwater plankton ecosystems is an indicator of the efficiency of biomass transfer between trophic levels (Gaedke 1993; Jennings and Mackinson 2003). Double log plots of normalized biomass versus body size can be interpreted as a continuous flow of biomass up the food web (Platt and Denman 1978). Although aquatic trophic chains can be complex, predators are larger than their prey and trophic position tends to increase with increasing size, i.e., the large eats the small (Jennings and Warr 2003). There may be some exceptions to this rule, such as parasites, some dinoflagellates, etc., but the majority of organic matter in pelagic ecosystems is transferred from small to large organisms, regardless of whether the individuals are autotrophic or heterotrophic. Hence, the steeper the slope of the log–log plot the larger the trophic step and the more energy lost in the transfer from the small and abundant primary producers to the larger and less dominant secondary consumers (Brown and Gillooly 2003). However, given that pelagic food webs are a non– steady state system, the slope cannot be considered an instantaneous measure of trophic transfer efficiency. The slope of the normalized biomass-size (NB-S) spectrum is, thus, an integration of the efficiency of biomass transfer to larger organisms in recent times and indicates the number of large individuals that are maintained by small ones (Jennings and Mackinson 2003; Jennings and Warr 2003). Primary production in oceanic oligotrophic areas is dominated by picoplankton (Agawin et al. 2000), which are not efficiently captured by most mesozooplankton. Therefore, the conventional wisdom is that in oligotrophic areas the ‘‘microbial loop’’ dominates the trophic web and that the energy transfer efficiency to mesozooplankton is lower 1 Corresponding author ([email protected]). Current address: ABP Marine Environmental Research Ltd, Suite B, Waterside House, Town Quay, Southampton SO14 2AQ, United Kingdom.