Carbonate chemistry and coral reefs in a seasonal upwelling system: insights into ocean acidification scenarios

Chair: Ulf Riebesell

Celeste Sánchez-Noguera1,2,3, Ines Stuhldreier3,4, Carlos Jiménez1,5, Jorge Cortés1, Álvaro Morales1, Christian Wild3,4, Tim Rixen2,3

 

1 Centro de Investigación en Ciencias del Mar y Limnología (CIMAR), San Pedro,

11501-2060, San José, Costa Rica

2 Institute of Geology, University Hamburg, Hamburg, 20146, Germany

3 Leibniz Center for Tropical Marine Ecology (ZMT), Bremen, 28359, Germany

4 Faculty of Biology and Chemistry (FB2), University of Bremen, Bremen, 28359, Germany

5 Energy, Environment and Water Research Center (EEWRC) of The Cyprus

Institute (Cyl), Nicosia, CY-1645, Cyprus

 

Background:

Ocean acidification (OA) threatens coral growth and the future of coral reefs is still difficult to predict. Since reefs in the Eastern Tropical Pacific (ETP) are naturally exposed to acidic conditions during upwelling events and thus adapted to low pH, they are excellent locations to investigate thresholds of reef accretion under OA scenarios

Methods:

Culebra Bay, northern Pacific coast of Costa Rica, is a seasonal upwelling area. We characterised the carbonate chemistry in the bay during non-upwelling and upwelling season by deploying SAMI-sensors (pH and pCO2) and an underway pCO2 system (SUNDANS).

Findings:

As in other coral reefs, photosynthesis/respiration and calcification/dissolution produced a pronounced pCO2 diurnal cycle. During non-upwelling season, inputs of corrosive offshore waters superimposed this diurnal cycle and raised the pCO2 (658

µatm) to similar levels  as  those  occurring  during  upwelling  season  (645  µatm).

Associated changes in diurnal cycle of pH and pCO2 indicated that dissolution might exceed calcification during daytime, if corrosive waters enter the bay. Linear extension  rates  of  main  reef  building  corals  at  upwelling  locations  in  the  ETP correlate to Ωa, and growth in Culebra Bay is near zero when Ωa falls below 2.5.

Conclusions:

Mean  Ωa    occurring  during  upwelling  (2.68)  and  non-upwelling  season  (3.41) suggests that coral growth is mainly favoured during non-upwelling. The overall low reef accretion in the region reveals a sensitive balance between carbonate accumulation and dissolution. Inputs of corrosive waters during cold upwelling but

also during the comparable warm non-upwelling season could shift the system towards net dissolution. This supports studies linking reef gaps in the geological record of the ETP to periods of enhanced upwelling. Our results furthermore suggest that in addition to seasonal upwelling, advection of corrosive offshore waters during non-upwelling season may play an important role by hampering coral growth during the main growing season.