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Abstract. Eastern boundary upwelling systems (EBUS) are among the most productivemarine ecosystems on Earth. The production of organic material is fueled byupwelling of nutrient-rich deep waters and high incident light at the seasurface. However, biotic and abiotic factors can modify surface productionand related biogeochemical processes. Determining these factors is importantbecause EBUS are considered hotspots of climate change, and reliablepredictions of their future functioning requires understanding of themechanisms driving the biogeochemical cycles therein. In this fieldexperiment, we used in situ mesocosms as tools to improve our mechanisticunderstanding of processes controlling organic matter cycling in the coastalPeruvian upwelling system. Eight mesocosms, each with a volume of ∼55  m 3 , were deployed for 50 d  ∼6  kmoff Callao (12 ∘  S) during austral summer 2017, coinciding with acoastal El Nino phase. After mesocosm deployment, we collected subsurfacewaters at two different locations in the regional oxygen minimum zone (OMZ)and injected these into four mesocosms (mixing ratio ≈1.5  : 1 mesocosm: OMZ water). The focus of this paper is on temporaldevelopments of organic matter production, export, and stoichiometry in theindividual mesocosms. The mesocosm phytoplankton communities were initiallydominated by diatoms but shifted towards a pronounced dominance of themixotrophic dinoflagellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted insurface layers. The community shift coincided with a short-term increase inproduction during the A. sanguinea bloom, which left a pronounced imprint on organicmatter C : N : P stoichiometry. However, C, N, and P export fluxes did notincrease because A. sanguinea persisted in the water column and did not sink out duringthe experiment. Accordingly, export fluxes during the study were decoupledfrom surface production and sustained by the remaining plankton community.Overall, biogeochemical pools and fluxes were surprisingly constant for mostof the experiment. We explain this constancy by light limitation throughself-shading by phytoplankton and by inorganic nitrogen limitation whichconstrained phytoplankton growth. Thus, gain and loss processes remainedbalanced and there were few opportunities for blooms, which represents anevent where the system becomes unbalanced. Overall, our mesocosm studyrevealed some key links between ecological and biogeochemical processes forone of the most economically important regions in the oceans.

Factors controlling plankton community production, export flux, and particulate matter stoichiometry in the coastal upwelling system off Peru