Predator: prey size relationships of marine zooplankton: Testing the 10:1 hypothesis with stable isotopes
Size-based theories of energy flow in marine plankton ecosystems, as well as numerous conceptual models, assume that predators and prey are linked, on average, by a body length ratio of 10:1. We tested this hypothesis using bulk N isotopes and Compound-Specific Isotope Analyses of individual Amino Acids (CSIA-AA) on size-fractioned zooplankton representing a 10-fold increase in body size. We also compared collections from four regions which vary substantially in biogeochemistry of source N: the California Current (coastal upwelling), the Costa Rica Dome (denitrification), the central equatorial Pacific (high nitrate, iron-limited) and the subtropical North Pacific (nitrogen fixation). Upon analysis, size relationships were significantly greater (>20:1) than predicted by the 10:1 hypothesis. For contrasting ecosystems, we found similar variations of δ15N trophic enrichment with zooplankton size. Underlying predator-prey size relationships therefore appear to be similar for these systems. CSIA-AA confirmed that substantial 6-8 ‰ offsets in δ15N among the regions reflected differences in source N to the food-web base, rather than differences in mean trophic enrichment for animals of a given size.