NIOZ-Saba Bank/Saba Cruise report: Net coral community calcification and hydrodynamics (Saba Bank/Saba)

Cruise Background

Many islands in the Caribbean sea are fringed by coral reefs and/or coral veneers, which produce CaCO3 for their skeletons. Not only stony corals but also coralline algae and other organisms form CaCO3 skeletons and contribute
to the CaCO3 produc on of these systems. Synchronously forces are at work to mechanis cally and chemically de- struct the CaCO3 structure. Erosion of CaCO3 by organisms such as grazing sea urchins, fishes and by scouring of coral skeleton-derived debris (e.g. rubble, sand, silt) is mainly mechanical. Boring worms and sponges combine mechani- cal and chemical removal of CaCO3. Chemical dissolu on by sponges can be substan al. Up to 75% of the bioerosion (CaCO3 mass loss) by coral excava ng sponges has been reported to be due to dissolu on, but more frequently values between 10%-30% have been found. Endolithic microorganisms bore purely chemically and infest bare CaCO3 sub- strates on reefs.

CaCO3 produc on and dissolu on is influenced by the seawater chemistry of CO2 (Andersson & Gledhill, 2013). The average pCO2 in water is enhanced by ocean acidifica on and by increased benthic mineraliza on of organic ma er due to eutrophica on. There are indica ons that seawater chemistry of CO2 and carbonate effects calcifying as well as decalcifying organisms differently and as such influence the balance between construc on and destruc on. How the balance is influenced in rela on to varia on in the carbonate system over different coral reef communi es and whole reef systems of different trophic status is s ll poorly understood.

Focus of this short NIOZ cruise was on benthic community carbon metabolism and the seawater carbonate system. With different methods we explored the balance between calcifica on and dissolu on of CaCO3. Bo om-water fluxes of the carbonate system were measured in rela on to the cover and composi on of the coral reef bo om commu- nity. This was done by (1) placing a dome tent over 4m2 of a coral reef community (and a small triangle tent as con- trol) and follow the diurnal changes of the chemistry in the tents and (2) by measuring diurnal changes in carbonate chemistry (e.g. pH and DIC in rela on to light) at the reef bo om outside of the tent in conjunc on with physical and hydrodynamic measurements and (3) by determining the benthic community with special focus on cover of calcifying and bioeroding organisms. In addi on depth profiles of conduc vity, temperature, pressure and oxygen were taken, to characterize the water masses over me in the research area.

The Saba Bank site was selected on basis of the purported oligotrophic status of the Bank. The selected site on the bank was at least 20 nau cal miles removed from coastal run-off and/or sewage pollu on of closest islands, the small islands of Saba an St Eusta us. Moreover the Saba Bank is separated from these islands by >800 m deep wa- ters. The selected loca on at Saba was more eutrophic than the Saba Bank site due to run-off from land. Physics and hydrodynamics (see CHAP II) were addressed with the aim to describe characteris cs of water masses flowing over the reef, and water movement profiles from the bo om to the surface. In CHAP III the experiments addressing the C metabolism and the carbonate system in the benthic compartment in and outside incuba on tents (placed in situ over benthic communi es) were elucidated. In Chap IV methods used to survey the benthic community were explained and preliminary results are presented. Chap V comprises a report of the coral species surveys of a site on the Saba Bank and a site in Ladder Bay, Saba. 

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