Impact of El Niño-Southern Oscillation on the Alkalinity and Salinity of
a Coral Reef Lagoon in the Equatorial Pacific - Observations and a Model
Abstract
The impacts of El Niño-Southern Oscillation (ENSO) on salinity and
alkalinity in an equatorial coral reef lagoon (Kanton) are investigated
using water samples collected in 1973, 2012, 2015, and 2018. A simple
advective-diffusive model is developed to aid in the interpretation of
the sparse observations and make estimates of net ecosystem
calcification (NEC) rates. Salinity and alkalinity variations in Kanton
lagoon are primarily driven by ENSO variations in precipitation. During
non-El Niño years (1973, 2012, 2018), salinity increases from the ocean
(35.5 psu) to the back of the lagoon (38 psu) because evaporation
exceeds precipitation and water resides in the back of the lagoon for
~180 days. At the onset of the 2015-16 El Niño the back of
the lagoon is only ~1 psu saltier than the ocean because
precipitation had begun to exceed evaporation. The model suggests that
during El Niños, when precipitation exceeds evaporation, the back of the
lagoon is less salty than the ocean (30 – 32 psu). Alkalinity
variations in the lagoon are primarily due to dilution or concentration
driven by the ENSO variations in precipitation and NEC that causes an
alkalinity deficit of ~250 micromoles/kg in the back of
the lagoon. NEC rates in the early stages of the 2015 – 2016 El Niño
were ~10% lower (4.7 mmol/day) than in the non-El Niño
years (5.2 – 5. 5 mmol/day). The NEC rates and coral cover indicate
that Kanton Lagoon has recovered from the complete loss of coral cover
during the 2002-03 El Niño.