Quantifying Local Currents and their effects on Coral Reefs in French Polynesia

Abstract

Coral reefs around the world are being significantly affected by global changes in sea surface temperature as well as ocean acidification. However, scientists have a limited understanding of how local currents affect the chemical and thermal relationships between the reef building corals and the surrounding saltwater environment. Coral reefs are known to cause a diel-cycle with dramatic changes in the carbonate system along with oxygen. As calcifying reefs grow, they remove calcium and carbonate ions, reducing dissolved inorganic carbon (DIC) and total alkalinity (TAlk) and decreasing pH levels. The carbonate ions must be replaced in order for the coral to continue its calcification at a consistent rate. This residence time of surrounding seawater is very difficult to locally quantify in a coral reef as the bathymetry is complex, creating variable currents in all 3 dimensions. Ocean acidification and its negative impact on coral reef health is expected to increase, and fine-scale observations of the link between residence time of surrounding seawater and the dynamic change in water properties remain unknown. A prototype sensor was built with the aim to provide direct measurements of the fine-scale mixing dynamic to collect data from which estimates of mass transfer can be calculated within the reef environment. The neutrally buoyant passive sensor tracks local water parcels in 3 dimensions and records water parameters in a small area on a coral reef. The design and prototyping of this type of sensor will lead to an improved observational tool in coral reef research.