Cruise report RV Pelagia 64PE414 Saba Bank

This research expedition is part of the project “Caribbean Coral Reef Ecosystems”, funded within NWO’s Caribbean Research Program. This project consist of three parts: one is devoted to the physiological basis of aragonite dissolution by excavating sponges, one focusses on the field-validation of the impact of environmental conditions on the net accumulation or dissolution of coral reefs. The research expedition completes these efforts by aiming to quantify the large-scale interaction between coral reefs and seawater chemistry. The Saba Bank (figure 1) is an excellent study site, since it is a large shallow carbonate platform (approximately 1800 km2), harboring coral reefs, various benthic algal communities and large amounts of sponges and gorgonians. Besides a series of previous surveys on the bank, most of the bank area is still a terra incognita as are the characteristics of the water overlying the bank. The various benthic communities on the bank as well as the various water masses that envelop the bank are expected to develop a typical bank signature in the overlying water, while flowing over the bank. Horizontal and vertical gradients of dissolved and suspended organic matter (phytopigments and particulate organic matter, POM) concentrations and pico-and nanoplankton are assessed during this cruise to shed light upon this benthic-pelagic coupling of the bank. Considering the large abundance of sponges and gorgonians on the bank that retain and/or filter huge amounts of water we expect depletion of pelagic plankton by zones in which such organisms dominate the benthic community of the bank. Depletion of pelagic plankton has been found previously on reefs elsewhere and may explain that certain reef zones might be net heterotrophic and may enhance acidification of the overlying water. 

Depth is undoubtedly the most important variable as it regulates the amount of light that penetrates the water column and is used for photosynthesis of corals and algae. Depth also is related to the shear stress that is exerted on the organisms living attached to the bottom. Wave energy determines if organisms can survive (stay attached) under extreme conditions such as storms and hurricanes. Furthermore, depth will have an influence on the composition and fate of the sediment that is generated on a coral reef. The sediment will be very fine in less exposed environments and virtually absent in exposed forereef conditions. Grain size of the sediment determines to a large degree the animals that can live in, on, and near it. Sediment transport on the bank may also influence settlement success of larvae of benthic species. Next to depth the exact position on the Saba Bank in relation to both incoming waves and currents and the edge of the bank will also influence the availability and concentration of food and nutrients. Closer to the edge of the bank an organism is closer to the incoming water with fresh nutrients and plankton. 

To understand the interaction between the environment and coral reef functioning, this research expedition will integrate ecological mapping, (carbonate) chemistry and physical oceanography. Spatial coverage of corals and other calcifying organisms will be mapped and linked to transects along which seawater will be analyzed for chemical constitution. Changes in total alkalinity, pH, DIC and nutrients will indicate what the net calcium carbonate production and dissolution, as well as release and uptake of nutrients over the Saba Bank are. Additional near-bottom seawater sampling by a custom-designed ‘gradient sampler’ may provide a local, instantaneous estimate of these processes. In combination with (micro-) turbulence estimates, the fluxes of CaCO3 accretion (or loss), nutrient dynamics et cetera may be inferred. 

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