Low Ph Groundwater Discharge Drives Natural Acidification...

High pCO2, low pH groundwater discharge drives natural acidification of reefs Christina M. Richardson, Henrietta Dulai, Brian Popp, and Kathleen Ruttenberg Abstract Diel changes in the marine carbonate chemistry of reefs are thought to be primarily biologically-driven. The presence of groundwater discharge in nearshore reefs may complicate our understanding of carbon cycling in these systems, however. Here we show that coastal groundwater inputs shift and heighten biologically-induced changes in marine pH, driving natural acidification and CaCO3 dissolution along a salinity gradient in two Hawaiian coral reefs. We examined two contrasting fringing reefs with variable pCO2 (1800 - 4000 ?atm), low pHT (7.4 - 7.5), and low ?aragonite (0.3 ?†¦show more content†¦We observed changes in pHT from groundwater inputs ranging from 0.01 - 0.40 pHT units and mean daily changes of 0.04 ? 0.08 to 0.18 ? 0.16 pHT units at sites located closest to the groundwater springs. Further, our data revealed a shift in reef metabolism from net dissolution to net calcification across this groundwater-driven physicochemical gradient. At sites with high levels of g roundwater exposure, dissolution rates were elevated. Reef systems experiencing groundwater discharge may be placed under similar physicochemical constraints as expected with future ocean acidification conditions. As such, our results may shed light on how natural reef communities will respond to changes in carbonate chemistry due to ocean acidification. 1. Introduction Atmospheric CO2 levels are projected to reach 730 to 1090 ppm by 2100 due to rising anthropogenic CO2 inputs (IPCC 2007, Joos et al., 2001, Meehl et al., 2005, Wigley 2004, Wigley and Raper 2001). Oceanic uptake of CO2 is raising marine pCO2 and lowering pH as the dominant dissolved carbonate species in sea water shift from CO32- and HCO3- towards HCO3- and H2CO3, a process known as ocean acidification (OA) (Qu?r? et al., 2012). Declines in pH and ?aragonite associated with emerging changes in carbonate chemistry due to OA will impact diverse marine biota (Doney et al., 2009). Oceanic pH is predicted to decrease by 0.3 ? 0.4 pH units as a result of OA by the end of the century (Doney et al., 2009,