23. Sea cucumbers buffer seawater chemistry: prospects for a changing ocean

Kennedy Wolfe (1)*, Francisco Vidal Ramirez (2), Sophie G Dove (2), Maria Byrne (1,3)

(1) School of Medical Sciences, The University of Sydney, NSW 2006, Australia
(2) School of Biological Sciences, The University of Queensland, QLD 4067, Australia
(3) School of Biological Sciences, The University of Sydney, NSW 2006, Australia

Background
Deposit feeding sea cucumbers on tropical reefs have the potential to influence local carbonate chemistry through their digestive physiology and dissolution of CaCO3 sand. The influence of these species to local alkalinity levels may improve conditions for reef calcifiers, with implications for reef resilience in the face of global change.

Methods
We examined the impact of deposit feeding by the vulnerable Stichopus herrmanni, on seawater chemistry, CaCO3 dissolution, sediment pH, benthic productivity and infaunal communities in mesocosms maintained at present-day and near-future pCO2 (+570 µatm). After acclimation, the influence of S. herrmanni on the carbonate chemistry of mesocosm water was determined from day and night incubations.

Findings
In the presence of S. herrmanni, pH increased and total alkalinity (AT) decreased during the day, while the opposite occurred at night. The magnitude of these changes was significantly greater in mesocosms without sea cucumbers. The presence of S. herrmanni had a significant effect on buffering the extent of increased pH (low AT) during the day and decreased pH (high AT) at night, in both ambient and elevated pCO2 treatments. In control seawater, dissolution of CaCO3 by S. herrmanni at night (4.7 µmol day-1 g-1) was significantly greater than during the day (-0.5 µmol day-1 g-1), and this was greater at elevated pCO2 (night: 10.4 µmol day-1 g-1; day: 0.6 µmol day-1 g-1).

Conclusions
Stichopus herrmanni directly influenced the carbonate chemistry of surrounding water through dissolution of CaCO3, especially at night. In a climate change ocean, S. herrmanni and other deposit feeding sea cucumbers may play an increasingly important role in the CaCO3 budget of coral reefs. Albeit at a local scale, this is a critical aspect of their role in ecosystem function. This reveals an unanticipated negative effect of the commercial overharvest of sea cucumbers worldwide to coral reef resilience.