Self-generated morphology in lagoon reefs

Emergent coral reef patterning via spatial self-organization

Regularly patterned reef ridges develop in the lagoons of at least one-third of Earth's coral reefs. The interactions between corals and their environment, occurring at scales from millimeters to meters, can lead to self-organized spatial patterns spanning hundreds of meters to kilometers. To understand the mechanism behind pattern formation, we first characterize these spatial patterns using satellite imagery from 63 sites across the Atlantic, Pacific, and Indian Oceans. Next, we develop a generalized Turing morphogenesis model. Corroborated by observed spatial patterns, results from our numerical model suggest that patterned ridges develop through a four-phase trajectory, dictated by changes in the lagoon's hydrodynamic regime. Initially, after an atoll lagoon forms, the first colonizing reefs establish as isolated pinnacles. These pinnacles then evolve into low-relief ridges and eventually form semi-enclosed inter-ridge ponds. In the terminal phase, a dense interconnected, branching, and rejoining ("anastomosing") pattern of reef ridges develop into a network, fully enclosing the ponds. Once enclosed, wind- and tide-induced currents are significantly reduced. Since corals rely on flow for feeding and shedding metabolites, ridge development stalls, and the pattern stabilizes. By combining empirical observations from around the world with a theoretical model, our study reveals the mechanism of reef pattern formation. Such a mechanistic understanding enables the use of emergent reef patterns to identify reef stress at the coral colony scale.

The lagoon geomorphology of pearl farming atolls in the Central Pacific Ocean revisited using detailed bathymetry data

The lagoons of seven French Polynesia and Cook Islands pearl farming atolls (Raroia, Takume, Mopelia, Takapoto, Ahe, Takaroa and Manihiki) were surveyed using multibeam and mono-beam sounders. From the detailed bathymetry, morphometric variables (average and maximum depth, frequency-area of depth, lagoon area and volume) are computed and compared. Remarkable geomorphological structures highlighted by bathymetric variations include deep reticulated structures and pinnacles. The seven atolls appear very different in abundance, size and density of these entities. Considering them as markers of the geological, sedimentological and eustatic processes that shape atoll lagoons, they are discussed in the context of the general theory of atoll lagoon formations involving karstic dissolution during Pleistocene or earlier low sea-level stands. In terms of pearl farming management, accurate bathymetric maps help pearl oyster wild stock assessment, development of circulation and biogeochemical models, better lagoon zoning and strategy to remove pearl farming derelict gears.