Futureproofing the green-lipped mussel aquaculture industry against ocean acidification

Ocean acidification is a potential threat to the growth, condition and survival of the green-lipped mussel (Perna canaliculus) and may have implications for the New Zealand shellfish aquaculture industry.

Two mitigation strategies – waste shell and aeration – were tested in field experiments to see how effective they are at mitigating acidification around mussel farms. This report outlines the results and recommendations from this research. 

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Primary results:

• The inner Firth of Thames currently experiences the lowest seasonal pH of the sites monitored, with a daily minimum of 7.84 (7.79–7.96) in autumn, with short-term (15-minute) pH minima as low as 7.2. Time-series data in the inner and outer Firth of Thames, and also on a mussel farm in the western Firth, show episodic declines in carbonate saturation to the critical carbonate saturation state ΩAR = 1.0 at which solid aragonite (the form of carbonate in mussel shells) will start to dissolve. Consequently, mussels in the Firth of Thames experience episodic corrosive conditions.
• The mean pH in the Marlborough Sounds region is projected to decrease by 0.15–0.4 by 2100 depending on future emission scenario. The corresponding decline of 0.5–1.25 in the saturation state of aragonite (ΩAR), results in the critical threshold of ΩAR =1 being reached by 2100 under the worst-case scenario. These projections are based only on future CO₂ emission scenarios and do not consider other coastal sources of acidity in coastal waters which may also alter in the future.
• At night pH within the boundary layer on a mussel dropper line may be up to 0.1 lower than the surrounding water due to respiration in the absence of photosynthesis. Consequently, mussels experience a more corrosive environment at night on dropper lines relative to the surrounding water.
• To raise pH from the present-day minimum to present-day mean in a one-hectare mussel farm would require instantaneous addition of 0.6 tonnes of dissolved carbonate. However, dilution by currents is high, and so to maintain this supply of dissolved carbonate would require 3.5–36 tonnes/day, depending on mussel farm location and current regime.
• Screening tests using mild acid to assess the factors influencing dissolution of waste shell identified shell particle size as a primary factor, with powdered shell dissolving faster than whole, fragmented, or crushed shell. Older shells dissolved more readily than fresh shell, and highest dissolution rates were obtained after waste shell had been submerged in coastal water or buried in sediment for one month.
• Laboratory tests using coastal seawater confirmed the threshold for mussel shell dissolution at a ΩAR = 1.1, slightly higher than the accepted ΩAR value of 1.0. This ΩAR value is already experienced episodically on mussel farms in the Firth of Thames.
• In dissolution tests, waste shell maintained on dropper lines showed an average mass loss of 0.012%/day (range 0.002–6%), which is lower than reported dissolution rates at the sediment surface. Maintenance of waste shell on dropper lines also had negligible measurable effect on pH.
• To raise pH from the present-day minima to present-day mean within the boundary layer (~10m3) of a dropper line would require dissolved carbonate addition of 8.5–85 kg/day at current speeds of 0.01–0.1 m/s. If this were delivered by waste shell the slow dissolution rate would necessitate loading of 0.9–3.6 tonnes waste shell/m dropper line at the lowest current speeds, which is not practical.
• The impact of waste shell addition on growth and survival of spat could not be tested on dropper lines due to low spat availability for seeding. Instead, juvenile mussels were tested in laboratory experiments under present-day pH and future pH projected for the year 2050. The presence of waste shell had no effect on juvenile mussel growth (shell length or width), or their overall physical or physiological condition over a 5-week experimental timescale.
• Although deposition of waste shell on the seafloor may be practical an excessive amount of waste shell (>11,000 tonnes) is required to significantly raise pH and carbonate availability on a one-hectare mussel farm. The waste shell would need to be at least 1.5 km upstream of the farm to ensure vertical mixing and availability of the resulting dissolved carbonate to mussels on the upper dropper lines.
• Aeration tests in a coastal seawater pond determined the efficiency of different tubing and flow rates, with the results extrapolated to a mussel farm. This technique has potential to raise pH and carbonate availability, but its effectiveness is also reduced by dilution. For example, aeration of 50% of the volume of a one-hectare mussel farm would raise dissolved carbonate levels by less than 5%. The associated air delivery requirement would be high and incur significant capital and power generation costs.
• Location of aeration needs to be assessed as a pilot test on a mussel farm identified that near-surface aeration (4 m depth) within the farm resulted in re-suspension of particulate matter and a reduction in pH and dissolved oxygen.

Recommendations

Within the scope of this study the application of waste shell has minimal potential for ameliorating ocean acidification within mussel farms. Alternative applications of waste shell, such as incorporation within the matrix of dropper lines and calcination to produce lime may represent more effective ways of using waste shell. CO₂ removal by aeration has limited potential for raising pH over a mussel farm, although further studies could investigate the potential of microscale aeration within dropper lines, and other potential “bio-buffering” options such as macroalgae beds upstream of mussel farms.