Biogenic acidification drives density-dependent growth of a calcifying invertebrate in culture

A major factor often overlooked in the production of calcifying marine invertebrates in aquaculture is the effect of changes in the carbonate chemistry of culture water caused by organism metabolism and calcification. This study examined the effects of temperature, stocking density and seawater exchange rate on the survival and growth of the juvenile sea urchin, Tripneustes gratilla with particular reference to effects of these factors on the carbonate chemistry of culture water. Growth and survival of T. gratilla were highest at 26-28 degrees C. Growth rates, relative spine length, consumption rates and food conversion efficiency were reduced by up to 50 % by an increase in stocking density from 43 to 129 individuals m(-2) and a reduction in seawater exchange rate from 3.0 to 0.3 exchanges h(-1), but survival rates were unaffected. Analysis of the influence of seawater parameters indicated that total alkalinity (A(T)), calcite saturation state (Omega Ca) and dissolved CO2 were the primary factors limiting growth at high densities and low seawater exchange rates. Uptake of bicarbonates for calcification and release of respiratory CO2 by T. gratilla appeared to be driving changes in carbonate chemistry. Our results show that reduced capacity of culture water to buffer acidification (low A(T)) can be a limiting factor in intensive production of calcifying marine invertebrates. This study demonstrates the importance of considering the influence of biogenic CO2 on seawater carbonate chemistry in aquaculture, particularly given the threat of reduced pH of source water due to ocean acidification, and also in experiments investigating the influence of anthropogenic CO2.