This paper extends previous modeling work by Ward and Follows, in order to examine whether effects of mixotrophy on ocean biogeochemistry are robust over long timescales. Overall the work is a useful contribution to biogeochemical modeling and mixotrophy research, and the manuscript is clearly written. I have a few minor suggestions that may help clarify the results and their interpretation. 

The details of the ecological model are summarized very briefly, because the model was described in a previous publication. However, there are some details that are important when trying to understand the model outcomes, which I think should be included in this manuscript. Specifically: 

-How does export flux work? Is there a relationship between organism size and sinking rate? Is there sinking of detrital particles? Is there any particle aggregation?

-How does cellular stoichiometry work? Is stoichiometry fixed or flexible?

-How many potentially limiting nutrients are tracked and included when modeling growth rates (e.g., N, P, Fe)?

The authors focus on gradients of DIC and phosphate, and never discuss nitrate. Is there a reason for this? Do the phosphate and nitrate patterns tell the same story? Nitrogen limits phytoplankton growth more commonly than phosphorus, so it seems important to have some discussion of nitrate.

The authors interpret the effects of mixotrophy on phosphate in high latitude regions in terms of iron limitation. Specifically, their hypothesis is that mixotrophy alleviates iron limitation, leading to greater phosphate drawdown. This is an interesting idea that would benefit from some more exploration. Which regions of the ocean are iron-limited in this model? And is there more evidence from the model output, such as the growth limitation terms, that can shed light on whether mixotrophy alters iron limitation?

The Sensitivity to Tradeoffs section belongs in the results section, not the discussion.