Long-term impacts of mixotrophy on ocean carbon storage: insights from a 10 000 year global model simulation

Abstract. Mixotrophs – organisms that combine the use of light and inorganic resources with the ingestion of prey – have been shown in simulations to increase mean organism size and carbon export in the ocean. These simulations have, however, been limited to decade-long timescales that are insufficient to investigate the impacts of mixotrophy on the ocean's long-term capacity for carbon storage. Here we explore these long-term impacts using a low-resolution ocean model that resolves important feedbacks between surface ecology and the ocean interior over multi-millennial periods. The model was compared in two configurations: one with a strict distinction between phytoplankton and zooplankton populations and one in which all populations were assumed to be capable of mixotrophy. Consistent with earlier studies, we found that increased carbon and nutrient export associated with mixotrophy was rapidly established within the first few years of the simulation and was robust over long time scales. However, we also found that these increases were partially offset over longer time scales by a decline in “preformed” inorganic carbon and nutrients entering the deep ocean via the sinking of surface waters. Over the 10 000 year duration of the simulations, we found that ecologically-driven changes in C export increased the oceanic C inventory by up to 537 Pg, and that this was partially offset by decline of 150 Pg in the preformed C inventory, leaving a net increase of up to 387 Pg C (∼1 %).