Hurricane control on shelf‐edge‐reef architecture around Grand Cayman

Abstract

Rimming the outer shelf of Grand Cayman is a submerged, 87 km long shelf‐edge reef that rises to within 12 m of mean sea level. It consists of an array of coral‐armoured buttresses aligned perpendicular to shore and separated by steep‐sided sediment‐floored canyons. Individual buttresses have a diverse coral‐dominated biota and consist of three architectural elements: a shield‐like front wall colonized by platy corals, a dome‐shaped crown colonized by head corals, and a shoreward‐projecting spur covered by varying amounts of branching coral. Buttresses are commonly fronted by coral pinnacles that, in some areas, have amalgamated with buttress walls to produce pinnacle‐and‐arch structures. As margin orientation changes, shelf‐edge‐reef architecture shows systematic variations that are consistent with changes in fetch and height of hurricane waves. Along margins exposed to fully developed storm waves, shelf‐edge‐reef buttresses are deep, have large amplitudes, and are dominated by robust head corals. These characteristics are consistent with hurricane‐induced pruning of branching corals and the flushing of significant quantities of sand from buttress canyons by return flows. Along margins impacted by fetch‐limited storm waves, reef buttresses are shallower, have intermediate‐amplitudes, and have a significantly higher proportion of branching corals. These characteristics are consistent with less coral pruning and sand flushing by weaker hurricane waves. Along margins fully protected from storm waves, the buttresses‐canyon architecture of the shelf‐edge reef breaks down producing a series of shallow, undulating, branching‐coral‐dominated ridges that merge laterally into an unbroken belt of coral. These characteristics correspond with negligible amounts of pruning and flushing during hurricanes. In addition to differences between margins, local intra‐marginal changes in shelf‐edge reef architecture are consistent with changes in the angle of hurricane‐wave approach. Open sections of the shelf‐edge reef, which face directly into storm waves, are pruned of branching corals and the fragments swept back onto the shelf producing extensive spurs. By contrast, on more sheltered, obliquely orientated sections, storm‐waves sweep debris along and off shelf producing little or no spur development. Instead, the debris shed seawards accumulates in front of the buttress walls and initiates the development of coral pinnacles. Over time, repeated buttress pruning and canyon flushing during hurricanes not only controls reef architecture but may also influence accretion patterns. Vertical accretion is limited by the effective depth of storm‐wave fragmentation. Once this hurricane‐accretion threshold is reached the reef moves into a shedding phase and accretes laterally via pinnacle growth, amalgamation, and infilling. Consequently, the reef steps out over its own debris in a kind of balancing act between lateral growth and slope failure — a pattern widely recognized in ancient reefs.

Field observations of wave‐driven circulation over spur and groove formations on a coral reef

“…Although the geometric properties of SAG formations are well documented [Munk and Sargent, 1954;Roberts, 1974;Blanchon and Jones, 1997;Storlazzi et al, 2003], analysis of their hydrodynamic function has been limited [Rogers et al, 2013]. Using a depth-averaged, phase-resolving model (funwaveC), Rogers et al [2013] showed that SAG formations together with shoaling waves induce a nearshore Lagrangian circulation pattern of counter-rotating circulation cells, confirming suggestions from the geologic literature [Munk and Sargent, 1954;Roberts et al, 1977;Storlazzi et al, 2003].…”

A morphometric assessment and classification of coral reef spur and groove morphology

“…distance between consecutive spur crests) and -amplitude‖ (i.e. the distance from spur crest to groove trough) while Roberts (1974) and Blanchon and Jones (1997) report -spur frequency‖ (i.e.…”

“…In addition, descriptions are frequently qualitative and adjective-based rather than quantitative (Duce and Janowicz, 2010). For example, adjectives such as -narrow‖ are used in place of quantitative values to describe groove width (Roberts 1974;Blanchon and Jones, 1997;Hamanaka et al, 2015).…”

“…Erosional processes include wave and current driven abrasion of grooves by rubble and/or sediment (Cloud, 1954); -pruning‖ of corals on spurs and spur walls by occasional hurricanes (Blanchon and Jones, 1997); and limestone solution of antecedent topography by fresh water flows during sea level low-stands to form grooves (Newell, 1954;Purdy, 1974). Constructional processes include water motion promoting aligned coral growth and eventual colony coalescence to form spurs (Shinn, 1963;Shinn et al, 1981;Kan et al, 1997a); and coral debris being washed together and rapidly cemented by robust, wave-resistant coralline algae forming spurs…”

Cayman Islands. Status of Coral Reefs of Little Cayman, Grand Cayman and Cayman Brac, British West Indies, in 1999 and 2000 (Part 1: Stony Corals and Algae)

“…The shallow i2cropora yalmata zone, as described for the 1970's and 1980's (Rigby and Roberts, 1976;Logan, 1994), no longer exists. In 1988, Hurricane Gilbert heavily impacted the north side of GC and destroyed much of the A. palmata-dominated fringing reef (Blanchon et al, 1997; T. Austin, personal communication). The predominance of standing dead colonies of A. palmatca in many of the upper-terrace study sites on GC (CI 19,20,21,24,29, 3 1) is indicative of prior, major mass mortality, possibly from white-band disease or, in part, from one or more of the earlier mass bleaching events.…”