Biogenic acidification reduces sea urchin gonad growth and increases susceptibility of aquaculture to ocean acidification

• Sea urchin growth and gonads are reduced at high densities/low seawater exchanges.
• CO2 is increased and total alkalinity decreased in sea urchin culture water.
• These changes are caused by respiration and calcification = biogenic acidification.
• Reduced growth and gonad production was attributed to biogenic acidification.
• Low total alkalinity may leave aquaculture vulnerable to ocean acidification.

Decreasing oceanic pH (ocean acidification) has emphasised the influence of carbonate chemistry on growth of calcifying marine organisms. However, calcifiers can also change carbonate chemistry of surrounding seawater through respiration and calcification, a potential limitation for aquaculture. This study examined how seawater exchange rate and stocking density of the sea urchin Tripneustes gratilla that were reproductively mature affected carbonate system parameters of their culture water, which in turn influenced growth, gonad production and gonad condition. Growth, relative spine length, gonad production and consumption rates were reduced by up to 67% by increased density (9–43 individuals.m−2) and reduced exchange rates (3.0–0.3 exchanges.hr−1), but survival and food conversion efficiency were unaffected. Analysis of the influence of seawater parameters indicated that reduced pH and calcite saturation state (ΩCa) were the primary factors limiting gonad production and growth. Uptake of bicarbonate and release of respiratory CO2 by T. gratilla changed the carbonate chemistry of surrounding water. Importantly total alkalinity (AT) was reduced, likely due to calcification by the urchins. Low AT limits the capacity of culture water to buffer against acidification. Direct management to counter biogenic acidification will be required to maintain productivity and reproductive output of marine calcifiers, especially as the ocean carbonate system is altered by climate driven ocean acidification.

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