Continuous determination of dissolved inorganic carbon fluxes from pumping suspension feeders

Suspension-feeding organisms play a pivotal role in the cycling of carbon in the oceans. They filter large amounts of water, filter out organic matter, remineralize it, and release respiratory CO₂ back into the water column. Measuring emissions of respiratory CO₂ in situ from suspension feeders poses the challenge of detecting small changes in dissolved inorganic carbon (DIC). To address this issue, we propose a method for measuring CO₂ excretion rates directly and continuously for undisturbed pumping suspension-feeders in the lab and in situ. This technique involves using miniature optodes to measure the pH of the water inhaled and exhaled by suspension-feeders. Dissolved inorganic carbon concentrations are calculated using the pH data along with ancillary measurements of ambient total alkalinity, temperature, salinity, and pressure. The DIC mass flux can be determined by multiplying the difference in DIC concentrations between the inhaled and exhaled water by the pumping rate of the target organism. The method was tested for sponges in situ. pH-based, continuously measured DIC mass flux rates were found to be higher but comparable to those calculated from discrete DIC measurements and slightly lower than concurrently measured oxygen consumption rates. Assuming that total alkalinity remains constant throughout the brief (seconds) passage of the water through the filtration system, this technique provides a reliable and continuous assessment of DIC fluxes from suspension-feeders. Furthermore, pH-optodes can be coupled with O₂-optodes to provide measurements of the respiratory quotient (RQ) that is widely used in ecology but rarely measured continuously, let alone in the field.