Effects of plankton temperature dependence in ocean biogeochemical models

Friederike Prowe, GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany and Iris Kriest, GEOMAR, Kiel, Germany
Global ocean biogeochemical models (OBGCMs) are used to predict plankton ecosystem and biogeochemistry changes in a future climate. Yet, simulated changes in ecosystem functions such as net primary production (NPP) and export production (EP) of different models do not even agree in sign. In these models, one important driver of future plankton dynamics is the response to temperature. A potential link between plankton temperature dependencies and variability in NPP predictions, however, cannot be identified as models differ in their representation of ecological, biogeochemical and physical processes, and the simulation setups.
In this study, we isolate the effect of temperature dependent plankton rates on simulated NPP and EP by comparing different temperature formulations in a common circulation using a simple OBGCM. Optimising the different configurations against a reference simulation yields similar representations of preindustrial steady state. We then project only temperature effects of different food web fluxes individually and in combination under prescribed rising temperatures, thereby excluding physical changes of circulation, stratification and nutrient supply.
In the absence of circulation changes, different assumptions of the grazing temperature dependence might predict increasing or decreasing EP under warming. Moreover, predictions diverge for different biogeochemical provinces and may explain a large part of the variability seen in future scenarios of coupled model ensembles.
Based on thorough methodology, we objectively highlight the uncertainty introduced by divergent temperature formulations to model predictions of global change, and stress the need for a better understanding of temperature-dependent processes in the plankton.