Observations and a Model of Net Calcification Declines in Palau's Largest Coral Reef Lagoon Between 1992 and 2015

Net ecosystem calcification (NEC) rates of Palau's largest lagoon and barrier reef system between 1992 and 2015 are estimated from sparse total alkalinity (TA) and salinity measurements and a tidal exchange model in which surface lagoon water transported offshore on the ebb tide is replaced by saltier (denser) ocean water that sinks to the bottom after entering the lagoon on the flood tide. Observed lagoon salinities are accurately reproduced by the model with no adjustable parameters. To accurately reproduce observed lagoon TA, NEC for the lagoon-barrier reef system was 70 mmols m(-2) day(-1) from 1992 to 1998, 35 mmols m(-2) day(-1) from 1999 to 2012, and 25 mmols m(-2) day(-1) from 2013 to 2015. This indicates that Palau's largest lagoon and barrier reef system has not recovered, as of 2015, from the 50% decline in NEC in 1998 caused by the loss of coral cover following a severe bleaching event. The cause of the further decline in NEC in 2012-2013 is unclear. Lagoon residence times vary from 8 days during spring tides to 14 days during neap tides and drive substantial spring-neap variations in lagoon TA (similar to 25% of the mean salinity-normalized ocean-lagoon TA difference). Sparse measurements that do not resolve these spring-neap variations can exhibit apparent long-term variations in alkalinity that are not due to changes in NEC. Plain Language Summary The primary objective of this study is to determine long-term variations in the rate at which corals and other organisms are building calcium carbonate skeletons that form the reefs in Palau's largest lagoon and barrier reef system in the western Pacific. Determining long-term variations in the rate at which coral reef structures are either growing or shrinking is critical for assessing the impact of climate change, including global warming and ocean acidification, on coral reefs. Measurements spanning 1992 to 2015, combined with a model of the system, indicate that the rate corals and other organisms were building skeletons declined by 50% in 1998 because of a loss of coral cover following a severe coral bleaching event. As of 2015, this coral reef system has not recovered from a devastating bleaching event; in fact, there has been a further decline in the rate corals are building reef structure.