| 01 Jun 2026
Toward shellfish aquaculture circularity: stimulating mussel shell dissolution in marine sediments
Abstract. Ocean alkalinity enhancement (OAE) is receiving considerable attention as a carbon dioxide (CO2) removal strategy, and novel approaches to increase the total alkalinity (AT) of the surface ocean are being explored. In bivalve aquaculture, calcification during shell growth consumes AT, thus leading to CO2 emissions. After consumption, shells are typically landfilled or incinerated, which can generate additional CO2 emissions. Here, we investigate whether bivalve shells could be a potential resource for mineral-based OAE. The idea is to grind the calcium carbonate (CaCO3) shells to increase the reactive surface area and distribute them into permeable, oxygen-rich sediments, where their dissolution produces AT that could then compensate the CO2 emitted during calcification. To evaluate this concept, we conducted microcosm incubations of sediments amended with crushed mussel shells (~8 wt%), and monitored the sediment geochemistry and sediment-water exchange over 24 weeks. Control sediments exhibited low and constant CaCO3 dissolution rates (Rdiss = 0.9 ± 0.5 mmol m-2 d-1) and AT fluxes (FAT = 3.2 ± 1.1 mmol m-2 d-1). In contrast, shell-amended sediments showed markedly higher Rdiss and FAT values, which exhibited a transient response modulated by oxygen and organic matter availability. Initially, shell dissolution was restricted by oxygen availability due to the intense mineralization of shell-associated organic matter. Subsequently, following gradual sediment reoxygenation, dissolution rates increased, reaching a maximum Rdiss of 22.7 ± 2.6 mmol m-2 d-1 after 9 weeks, corresponding to a measured FAT of 43.0 ± 6.0 mmol m-2 d-1. After that, CaCO3 dissolution rates declined as organic matter availability decreased, thus reducing dissolution toward a constant steady-state Rdiss of 2.2 ± 1.1 mmol m-2 d-1. After 6 months, ~6 % of the initial shell mass had dissolved, and extrapolation of the new quasi-steady-state dissolution rate at the end of the experiment suggests that complete dissolution would take ~38 years. Our results suggest that organic matter availability limits CaCO3 dissolution in the permeable sediment investigated. This constraint, however, can be alleviated by targeting environments with high organic matter deposition for in-situ applications, such as sediments beneath mussel farms, thereby promoting mussel aquaculture circularity.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Biogeosciences.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
This paper explores the addition of crushed mussel shells to carbonate-poor, irrigated, permeable sediment as a technique for mitigating CO2 production during bivalve aquaculture. Rather than disposing of the shells into landfills or via incineration, the authors suggest crushing and spreading them along coastal areas. There, dissolution produces alkalinity that can drive atmospheric CO2 uptake that would offset CO2 produced during calcification. The addition of shells to the surface 2 cm of sediments incubated in chambers outfitted with a rotating disk used to induce advection resulted in enhanced alkalinity production via metabolic CaCO3 dissolution. The results indicate CaCO3 dissolution is limited by competing oxygen and organic matter availability over the course of the experiment. Therefore, the authors suggest maximum alkalinity production efficiency would be achieved by adding crushed shells to well irrigated areas that receive high organic matter loading to maintain metabolic CO2 production via aerobic respiration.
General Comments:
This paper is both well-written and timely and warrants publication with some minor revisions. I think the paper could benefit from further exploration of the rate of exchange between the OLW and pore water in terms of driving dissolution and the resultant enhanced alkalinity fluxes in their incubations. The authors address the importance of exchange rate and pore water residence time, so it would be interesting to see an estimate of their exchange rate, how it relates to measured exchange rates along the continental shelf, and how pore water chemistry evolves with depth in the sediment according to pore water residence time.
Additionally, when discussing optimal locations for shell amendment, some discussion of natural sediment deposition and burial rate could be useful. When these shells are buried below the oxygenated zone, they could result in enhanced CaCO3 precipitation rather than dissolution. However, overall I think this is a very good and interesting paper. See some specific comments below.
Specific Comments:
L24: Looking at Figure 5b, is it correct to say there is a new steady-state dissolution rate in the treatments? Seems like it’s still decreasing.
L78- The placement of “(i.e. the reverse of Eq. 1)” is a little confusing here. Should be moved to the end of the sentence, otherwise it sounds like calcification is the reverse of Eq. 1.
L108-109: Was the sediment sieved to remove macrofauna?
L145-146: Were you able to determine an OLW-pore water exchange rate?
L165: How long were At samples stored unpoisoned before analysis?
L248: Should the MCDE be 100*2*fcarb? Looks like it is reported as % in results rather than a fraction.
Table 1- Why don’t you report isotopic values for the organic and inorganic carbon from the mussel shells? If not measured, can you calculate values based on the 8% amendment and how much the isotopic composition of the top 2cm changed following amendment? Also, why is there no error on the mussel shell solid analysis results?
Figure 3: Why are you only showing results from one set of control and treatment incubations rather than all three of each? Can you show a plot of how OLW Ω changed in the control and treatment incubations? Also, the starting δ13C of the OLW seems pretty light. Do you know why that is the case?
L325: Is there any correlation between the depth of the grey/black transition depth in the sediment and the CaCO3 dissolution rate/alkalinity flux?
L401-405: Was there any attempt to determine pore water residence time with depth in the sediment column? Especially over the oxygenated zone?
L410-411: Could be helpful to readers if you show the reaction stoichiometry.
L492-495: The deposition of fresh organic matter is clearly important, however some note about burial would be useful as natural sediment deposition will eventually bury the added shells below the oxygenated zone and where precipitation of CaCO3 would be more likely.
Technical:
L23- Try to consistently use weeks or months rather than switch between the two.
L71- Consistently use CaCO3 or calcium carbonate.
L204- Redundant to say flux calculations were performed using the software that allows for the calculation of sediment-water fluxes
L393- “as such” seems out of place
Figure 1- somewhat difficult to see the green arrows on the blue background.