Plankton food web structure and productivity under ocean alkalinity enhancement

Abstract

Ocean alkalinity enhancement (OAE) is a nature-based technology for CO₂ removal and storage, but little is known about its environmental safety. We tested a CO₂-equilibrated OAE deployment in a close-to-natural community using in situ mesocosms in the oligotrophic subtropical North Atlantic and assessed metazoan zooplankton to inform about food web stability, structure, and production. In addition, a literature review complemented experimental results by summarizing physiological responses of marine animals to decreasing proton concentrations, or increased pH. The food web studied proved resistant, and zooplankton physiologically tolerant, to the OAE tested. We observed short-term effects of OAE on zooplankton reproduction and productivity, which were likely trophically mediated. Yet, these did not affect zooplankton populations or their nutritional value as food for fish. Our study demonstrates an environmentally safe OAE application, but also stresses the risks of more intense OAE options, and the vulnerabilities of other marine ecosystems.

Fig. 1.. Metazoan zooplankton and lower trophic level responses to OAE.

We show a selection of key responses to represent each food web property (stability, structure and productivity) (A to D) and the potential for bottom-up mediation under OAE (E and F). The lower panels show the temporal development of each response, with the vertical dashed lines separating the experiment into three [(A) to (D)] and two [(E) and (F)] response phases. Response phases include an immediate (days 5 to 10), shorter- (11 to 22) and longer-term (23 to 33) phase for zooplankton [(A) to (D)], and a shorter- (days 6 to 20) and longer-term (21 to 33) phase for their potential food [(E) and (F)]. The upper panels show the phase averages, or the mean of all measurements taken within each phase, for each parameter and every treatment, on which linear regression models are tested.

Fig. 2.. Summary of metazoan zooplankton responses to OAE, informing about food web stability, structure, and productivity.

For every response, colored cells indicate the outcome of the linear regressions fitted to average values for each phase. Gray = no effect, orange = potentially adverse effect, and blank = no data. Arrows indicate the direction of change with OAE.

Fig. 3.. Metazoan zooplankton community and fatty acid composition under OAE.

Multidimensional analyses were performed on phase averages of zooplankton biomass (A) and relative fatty acid content (B). Ninety-five percent confidence ellipses (dashed circles) were computed for each response phase.

Fig. 4.. Effect sizes of significant responses to OAE across the food web.

For every impact, we report all metazoan zooplankton groups that were sampled to assess the consistency of the response. Effect sizes are linear regression slopes. Gray = no effect, orange = potentially adverse effect, and blank = no data.

Fig. 5.. Direct responses of marine animals to decreased [H⁺].

On the basis of the literature review, we show the normalized responses, across taxa and physiological functions, to decreased [H⁺] or increased pH. Non-significant impacts are assigned a value of zero, whereas significant effects are reported as the percent change with respect to the ambient (gradient-design experiments) or the control (replicated-design) values. Open circles highlight the effects of low [H⁺] on calcification, while closed circles represent all other physiological functions investigated. The blue bars indicate the range of proton perturbations that were tested in the mesocosm study. The right hand panel zooms in to the responses that ranged between −50 and 100%.