Transitioning global change experiments on Southern Ocean phytoplankton from lab to field settings: Insights and challenges

Abstract

The influence of global change on Southern Ocean productivity will have major ramifications for future management of polar life. A prior laboratory study investigated the response of a batch‐cultured subantarctic diatom to projected change simulating conditions for 2100 (increased temperature/CO 2 /irradiance/iron; decreased macronutrients), showed a twofold higher chlorophyll‐derived growth rate driven mainly by temperature and iron. We translated this design to the field to understand the phytoplankton community response, within a subantarctic foodweb, to 2100 conditions. A 7‐d shipboard study utilizing 250‐liter mesocosms was conducted in March 2016. The outcome mirrors lab‐culture experiments, yielding twofold higher chlorophyll in the 2100 treatment relative to the control. This trend was also evident for intrinsic metrics including nutrient depletion. Unlike the lab‐culture study, photosynthetic competence revealed a transient effect in the 2100 mesocosm, peaking on day 3 then declining. Metaproteomics revealed significant differences in protein profiles between treatments by day 7. The control proteome was enriched for photosynthetic processes (c.f. 2100) and exhibited iron‐limitation signatures; the 2100 proteome exposed a shift in cellular energy production. Our findings of enhanced phytoplankton growth are comparable to model simulations, but underlying mechanisms (temperature, iron, and/or light) differ between experiments and models. Batch‐culture approaches hinder cross‐comparison of mesocosm findings to model simulations (the latter are akin to “continuous‐culture chemostats”). However, chemostat techniques are problematic to use with mesocosms, as mesozooplankton will evade seawater flow‐through, thereby accumulating. Thus, laboratory, field, and modeling approaches reveal challenges to be addressed to better understand how global change will alter Southern Ocean productivity.

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