Bivalve Farming Is Not a CO2 Sink: From Myth to Legend, Where Is Science?

Abstract

We recently published a paper entitled “Cracking the Myth: Bivalve Farming Is Not a CO₂ Sink” [1], which challenges the growing narrative that bivalve aquaculture sequesters atmospheric CO₂. We demonstrated that, when considering the full biogeochemical picture, bivalve farming in fact contributes to CO₂ emissions. By clarifying these dynamics, our intention was to correct a misconception and prevent the misclassification of shellfish aquaculture as a carbon sink.

A response entitled “The Legend Continues: The Critical Evidence Showing That Bivalve Farming Is a Carbon Sink with a Novel Budget Framework” [2] was published in the same journal. In this letter, the authors rely on estimated air–sea CO₂ fluxes in a mussel aquaculture area, published in He et al. [3], and state “Our observations of air–sea CO₂ flux provide definitive evidence that mussel farming can be characterized as a weak carbon sink, although its effectiveness is constrained by seasonal variations.”

A careful examination of the data and methods presented in the article on which their response is founded reveals critical methodological and interpretative biases. He et al. [3] report that the bivalve farming area investigated is actually a strong source of CO₂ to the atmosphere in summer and fall (between 10 and 40 mmol CO₂ m⁻² d⁻¹) while being a very weak sink in spring (~ −0.15 mmol CO₂ m⁻² d⁻¹). In addition to this significant misinterpretation of their data, the conclusion is fundamentally flawed for two key reasons.

First, it seems that the authors did not estimate air–sea CO₂ fluxes outside the aquaculture area. This omission makes it impossible to isolate the effect of aquaculture on the CO₂ dynamics. In fact, the few studies comparing CO₂ fluxes within and outside bivalve farms suggest that the surrounding environment—unaffected by farming activity—exhibits a stronger CO₂ sink than the aquaculture zones themselves [4, 5]. In their response letter, the authors present a schematic of the carbon budget both inside and outside the mussel farm that includes air–sea CO₂ fluxes, despite the absence of data for the external area [2]. Without such data, the external component of their framework is purely speculative. Although a Letter to the Editor can serve as a forum for proposing speculative ideas or hypotheses, it is essential that such speculation be clearly distinguished from evidence-based conclusions.

Second, although the manuscript does not state it explicitly, it appears that CO₂ fluxes were only estimated during daylight hours—and notably, no sample was taken in winter. Consequently, the dataset overrepresents periods of high photosynthetic activity, overestimating CO₂ uptake and/or underestimating CO₂ outgassing.

In conclusion, although the response letter of He et al. [2] represents an effort to integrate theoretical, observational, and experimental approaches, it suffers critical limitations. The interpretation that bivalve farming can be definitively characterized as a CO₂ sink is not supported by the cited paper. Assessing the impact of shellfish farms on air–sea CO₂ fluxes requires robust, year-round measurements. These must include well-chosen reference sites located outside the farming area, sufficient replication to ensure statistical reliability, and a sampling frequency capable of capturing diel, seasonal, and interannual variability. Equally important is a comprehensive evaluation of carbon stocks and fluxes across key ecosystem compartments, such as bivalves, phytoplankton, and sediments, as well as exchanges with adjacent systems.

The authors have nothing to report.