• Seaweeds and diatoms on settlement plates created low pH and high pH conditions as they respired and photosynthesised.
• Low pH had adverse effects on growth and morphology of sea urchin post-larvae.
• High pH generally had little effect on growth and development, but reduced settlement rates.
• Controlling pH in invertebrate culture systems might improve settlement rates and post-settlement growth.
Respiration, photosynthesis, and calcification of cultured organisms and biological substrata can substantially alter the pH and other carbonate parameters of water in aquaculture systems. One such example is the diel cycle of photosynthesis and respiration by diatoms and seaweeds growing on ‘settlement plates’ used to induce metamorphosis of invertebrate larvae and as food for post-larvae. We documented low pH and high pH conditions in nursery raceways and simulated settlement tanks that were as much as 0.26 pH units lower and 0.52 pH units higher than the pH of the source seawater supplied to the systems. To better understand whether the low pH and high pH conditions commonly found in aquaculture culture systems affected the success of the settlement stage of the sea urchin Centrostephanus rodgersii, we induced larvae to settle at pH 7.6, 7.8 (created by injecting CO2), 8.1 (ambient), 8.2, and 8.3 (created by raising total alkalinity), and followed post-settlement growth, development, and survival for 16 d. At metamorphosis, low pH significantly increased the occurrence of abnormalities and reduced the number and length of the sea urchins’ spines and pedicellaria, but did not affect settlement rate or size compared to ambient pH. In contrast, high pH generally had little effect on morphological traits, but settlement was significantly reduced by 14–26% compared to ambient and low pH treatments. After 16 d, juveniles in the low pH treatments were as much as 7% smaller, had 2–4 fewer and 9–13% shorter spines, and had less-developed digestive systems compared to juveniles in ambient or high pH treatments, and there was a non-significant trend towards lower survival in low pH treatments. Our results highlight that the low pH and high pH conditions in invertebrate settlement and nursery culture systems have the potential to hamper production through reduced settlement or growth rates. We need to understand the impacts of fluctuating pH in culture systems, especially day-night oscillations. Treating seawater with alkali chemicals to stabilise pH and counter acidification should be done with caution. Due to the potential for deleterious effects on settlement, dosage regimens will need to be optimised.
Mos B., Byrne M. & Dworjanyn S. A., 2020. Effects of low and high pH on sea urchin settlement, implications for the use of alkali to counter the impacts of acidification. Aquaculture 528: 735618. doi: 10.1016/j.aquaculture.2020.735618. Article (subscription required).