Current flow in low-energy coral forereef spurs and grooves

Coral forereef hydrodynamics, driven by waves and currents, play a critical role in controlling the distribution of nutrients, sediments and corals across platform reefs. These processes drive the formation of reef systems and are critical to determining the future of coral reef environments in a changing global climate. Despite this, limited in-situ research from the Great Barrier Reef (GBR) into the interactions between spurs and grooves (SaG) and forereefs currents is available. Here we analyse the first dataset of current measurements from a coral forereef SaG system in the GBR and assess forereef currents adjacent to a large reef lagoon that is isolated from the open ocean during low tides. We find that under prevailing weather conditions, waves are low (Hs = 0.25 m) and oblique to forereef isobaths. Currents in SaG under low wave exposure exhibit high velocity (2 m/s) with a dominant offshore component driven by surf zone processes and lagoonal outflow. This is consistent with postulated mechanisms of constructional development of SaG whereby offshore flow transports sediments seaward of the forereef, providing a substrate for coral larvae and living coral fragments to attach. These in-situ observations provide the first evidence from the GBR linking prevalent hydrodynamic conditions to the morphological evolution of coral forereef spur and groove systems. • First in situ measurements of water flow in reef spurs and grooves on the GBR. • Lagoon outflow creates steady offshore currents across the tidal cycle. • Strong outflows may contribute to spur and groove development. • Findings support a lagoon-driven pathway for reef growth and extension.