Ocean Alkalinity Enhancement (OAE) is one of the Negative Emissions Technologies (NETs) with the highest climate mitigation potential. Yet, despite its potential to enhance long-term carbon storage in ocean water as bicarbonate ions, and therefore to mitigate ocean acidification, little is known about its possible side effects and/or co-benefits on natural planktonic communities. To address this knowledge gap, a mesocosm experiment was carried out in the Taliarte Harbor, on Gran Canaria, Spain. Nine mesocosms were deployed and a CO₂-equilibrated alkalinity gradient was applied in increments of 300 &microEq &#183 L^-1, from 2400 to 4800 &microEq &#183 L^-1. This mesocosm study was the first attempt at evaluating OAE’s potential impacts <em>in situ</em>. The metabolic response of the microbial plankton community was monitored from mid-September to mid-October 2021. The results show that Net Community Production (NCP) and Gross Community Production (GCP) rates, as well as Community Respiration (CR) and metabolic balance (GP: R), did not present a linear response to the applied TA gradient. Yet, a non-significant optimum curve was observed in the form of a slight increase in all rates up to &Delta1800 &microEq &#183 L^-1, followed by a drop. In fact, phytoplankton blooms were observed in the &Delta1500 and &Delta1800 &microEq &#183 L^-1 treatments where, from a heterotrophic balance state, NCP rates increased to 4 and 8 &micromol O₂ &#183 kg^-1 &#183 d^-1, respectively. These blooms and the optimum curve were also reflected in the size fractionated chlorophyll a and 14C uptake data. More specifically in the total chlorophyll a concentration, Particulate Organic Carbon (POC in &microg of 14C &#183 m^-3 &#183 d^-1) and total organic carbon (POC+DOC) production through 14C uptake, in both parameter’s 2-20 &microm size-fraction, but also in the nanoeukaryotic abundance data obtained through flow cytometry. Lastly, abiotic precipitation occurred in the highest TA treatment (&DeltaTA 2400 &microEq &#183 L^-1) after day 18, but no effect on the metabolic rates measured was detected. In summary, a damaging effect of CO₂-equilibrated alkalinity enhancement, in the range applied here, on the microbial plankton community, in terms of its production and respiration rates, cannot be inferred from this study. Nonetheless, additional experimental work is necessary to reject a non-linear response and to further understand the effects of OAE, both short and long term, on the planktonic community.

Primary Presenter: Laura Marin-Samper, Universidad de Las Palmas de Gran Canaria (laura.marin@ulpgc.es)

Authors:

Laura Marín-Samper, Universidad de Las Palmas de Gran Canaria  (laura.marin@ulpgc.es)

Javier Arístegui, Instituto de Oceanografía y Cambio Global, Universidad de Las Palmas de Gran Canaria  (javier.aristegui@ulpgc.es)

Nauzet Hernández-Hernández, Insituto de Oceanografía y Cambio Global, Universidad de Las Palmas de Gran Canaria  (nauzet.hernandez@ulpgc.es)

Joaquin Ortiz, GEOMAR Helmholtz centre for ocean research kiel  (jortiz@geomar.de)

Andrea Ludwig, GEOMAR Helmholtz centre for ocean research kiel  (aludwig@geomar.de)

Steve Archer, Bigelow Laboratory for Ocean Sciences  (sarcher@bigelow.org)

Jens Hartmann,  Institute for Geology, Universität Hamburg  (jens.hartmann@uni-hamburg.de)

Ulf Riebesell, GEOMAR Helmholtz Centre for Ocean Research Kiel  (uriebesell@geomar.de)