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

• Published in: Limnology and Oceanography
• Main Authors: Boyd, PW, Doney, SC, Eggins, S, Ellwood, MJ, Fourquez, M, Nunn, BL, Strzepek, R, Timmins-Schiffman, E
• Format: Article in Journal/Newspaper
• Language: English
• Published: Amer Soc Limnology Oceanography 2022
• Subjects: global change; Southern Ocean; phytoplankton productivity; omics; physiology; Mesozooplankton
• Online Access: https://eprints.utas.edu.au/46763/; https://eprints.utas.edu.au/46763/1/Limnology%20%20%20Oceanography%20-%202022%20-%20Boyd%20-%20Transitioning%20global%20change%20experiments%20on%20Southern%20Ocean%20phytoplankton%20from%20lab.pdf

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/CO2/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.