Insights into the ecological effects of deep ocean CO2 enrichment: The impacts of natural CO2 venting at Loihi seamount on deep sea scavengers

[1] Hydrothermal vents on Loihi seamount near Hawaii emit 5000–100,000 t/yr of CO2 at water depths of 1200–1300 m, yielding CO2 concentrations as high as 418 mol/m3. Because CO2 is the dominant toxic component of these vent fluids, Loihi provides an extraordinary setting in which to explore the effects of sustained CO2 enrichment on deep sea organisms and ecological processes. We used the Pisces IV submersible to manipulate baited traps and bait parcels in Loihi's plume to explore the effects of elevated CO2 on the feeding and behavior of typical deep sea scavengers. Necrophagous lysianassid amphipods (Eurythenes cf. obesus) and synaphobranchid eels were unusually abundant in the vicinity of Loihi's vents, suggesting that the CO2-rich plumes rising above Loihi may enhance the flux of carrion to the deep sea floor. Amphipods exposed to diluted vent waters with a mean pH of 6.3 (minimum 5.7) and temperature of 5.0°C became very active within seconds and then became narcotized over timescales of 2–15 min. Following 60 min of plume exposure, all amphipods revived within 30 min of removal from the plume and remained active until warmed to 10°C during submersible ascent into ocean surface waters. Open bait parcels placed in similar CO2 plumes were avoided by amphipods and fish, suggesting that they could detect either the elevated CO2 concentrations or other environmental parameters (e.g., temperature). Our results suggest that at least some scavenging deep sea amphipods may be able to escape relatively concentrated CO2 plumes (i.e., they do not become narcotized for several minutes) and that they may revive following 1 hour exposure to an oscillating CO2 plume (e.g., one meandering in a tidal current). In addition, our results suggest that scavenging amphipods and synaphobranchid eels may be able to detect and avoid intoxicating levels of CO2 resulting from deep ocean injection of CO2 and that specialized components of the deep sea fauna (e.g., some hydrothermal vent species) may be well adapted to exploit carrion accumulating on the periphery of injection sites.