Zooplankton regulation of surface ocean POC:PON ratios

The elemental composition of particulate organic matter in the surface ocean significantly affects the efficiency of ocean carbon storage. Though the elemental composition of primary producers is an important factor, recent observations from the western North Atlantic Ocean revealed that carbon-to-nitrogen ratios (C:N) of phytoplankton were significantly higher than the relatively homeostatic ratio of the total particulate pool (Particulate Organic Carbon: Particulate Organic Nitrogen; POC:PON). We suggest interactions between primary and secondary producers can regulate the mean composition of surface particulate, and the difference between primary producers and bulk material. We present simplified models of phytoplankton and zooplankton metabolism, developed and tested against laboratory data, which reveal contrasting autotrophic and heterotrophic responses to nitrogen limitation: phytoplankton accumulate carbon in carbohydrates and lipids while zooplankton deplete internal C reserves to fuel respiration. We couple the physiological models with an ocean general circulation model and scale up to predict global distributions of the elemental composition of living and detrital particles. We predict elevated phytoplankton C:N ratios in the high light, low nutrient regions of the ocean despite a lower, homeostatic POC:PON ratio maintained via respiration of excess carbon by zooplankton, providing an interpretation for a recent, global compilation of surface ocean POC:PON data.