The impact of ocean liming on phytoplankton size-structure and the balance of photosynthesis and respiration in two contrasting environments

Current efforts to reduce CO₂emissions are being insufficient to decrease its atmospheric concentration and to avoid exceeding the warming threshold in the Paris agreement. Although reducing emissions remains essential, additional tools to limit global warming are being actively searched. These include methods to reduce the concentration of atmospheric CO₂ by capturing it from the air (the so-called Negative Emissions Technologies, NET). Ocean Alkalinity Enhancement (OAE) is a potentially viable NET that consists on the addition of alkaline substances, including slaked lime (calcium hydroxide), to the ocean, which enhances the ocean’s capture of atmospheric CO₂ and raises the pH of the seawater, thus countering ocean acidification. Beyond technological challenges to cost-effective OAE methods, a rigorous assessment of potential ecological and geochemical impacts is necessary. Ocean liming on the wake of ships is proposed as one of the most efficient ways for OAE. The discharge of slaked lime as a side activity of maritime traffic avoids the need of dedicated boats thus increasing the efficiency of OAE by reducing the amount of CO₂ emitted to perform this technique. Nevertheless, this procedure can cause local pH peaks, which may have temporary and local effects on the pelagic ecosystem, e.g. by selecting less sensitive plankton species and promoting the growth of calcifiers, thus shifting the phytoplankton composition and the functioning of the whole plankton community. The impact of OAE on the structure and functioning of plankton communities is however poorly known.

Here we present results of the impact on phytoplankton biomass and plankton community metabolism (photosynthesis and respiration) of repeated additions of slaked lime (Ca(OH)₂) during two mesocosm experiments in two contrasting coastal environments: the highly productive upwelling system of the Ría de Vigo (NW Spain) and the ultraoligotrophic eastern Mediterranean in Crete (Greece). The same experimental design was conducted at the CIM-ECIMAT (University of Vigo) and CRETACOSMOS (Hellenic Centre for Marine Research) facilities. Nine mesocosms were filled with natural coastal seawater. Three served as control, and Ca(OH)₂ slurry additions were repeated on days 1,3,5 (Vigo) and 1,3,5,7,9,11 (Crete) to simulate the chronic disturbance expected from repeated discharges from ships. Two different concentrations of calcium hydroxide were used, with three replicates each. pH, O₂, salinity, and temperature were recorded with a ten-minutes frequency. Size-fractionated chlorophyll a (0.2-2, 2-20, >20 μm) results indicate a dose-dependent effect on the phytoplankton community, with a differential response depending on the phytoplankton size-fraction. Gross primary production (GPP), community respiration (CR) and net community production (NCP) were determined from in vitro changes in O₂ concentration after 24 h light and dark incubations. Preliminary results indicate that the trophic functioning of the plankton community was impacted only by the high slurry addition treatment (H), and more notably in the eutrophic ecosystem of the Ría de Vigo. The response, however, was similar in both experiments, with GPP decreasing to a greater extent than CR, which caused a reduction of NCP in the H with respect to the L and control mesocosms.