Abstract
Individual hard coral colonies from four representative reef sites around Little Cayman were surveyed yearly between 2010 and 2015, a period of non-disturbance between two elevated seawater temperature anomalies. Photographic censuses produced 7069 annual transitions that were used to describe the demographics (size class frequencies, abundance, area cover) and population dynamics under non-disturbance environmental conditions. Agariciids, Porites asteroides, and Siderastrea radians have replaced acroporids as the predominant massive corals. Recruitment rates were generally low (2), except for a fourfold recruitment pulse of S. radians that occurred in 2011. On average, 42% of coral recruits survived their first year but only 10% lived longer than four years. Temporal comparisons allowed correction factors to be calculated for in-situ methods that overestimate recruitment of colonies ≤2 cm in diameter and overlook larger colonies. Size class transitions included growth (~33%), stasis (~33%), partial mortality (10% - 33%), and whole colony mortality, which decreased with increasing colony size (typically 30 cm2). Transition matrices indicated that Little Cayman assemblages have declining hard coral populations (λ 150 cm2 surface areas, live area cover may remain relatively stable. Projection models indicated that downward population trends would be exacerbated even by mild disturbance (5% - 10% mortality) scenarios. The fate of hard corals on Little Cayman’s reefs was determined to be heavily dependent on the health and transitions of agariciid colonies. Conservation strategies that currently focus on restoration of Caribbean acroporids should be expanded to include agariciids, which were previously considered “weeds”.
Highlights
Since the 1970s, Caribbean corals have been subjected to regional and local mass mortality events associated with disease outbreaks, temperature-induced bleaching, hurricanes, and anthropogenic stressors, which have resulted in a decline in average coral cover from 35% to 16% [1]
The fate of hard corals on Little Cayman’s reefs was determined to be heavily dependent on the health and transitions of agariciid colonies
Little Cayman is an ideal location within the Caribbean to study the population dynamics of hard coral populations because it lacks many of the anthropogenic stressors to which other reefs are exposed
Summary
Since the 1970s, Caribbean corals have been subjected to regional and local mass mortality events associated with disease outbreaks, temperature-induced bleaching, hurricanes, and anthropogenic stressors, which have resulted in a decline in average coral cover from 35% to 16% [1]. Coral dominance has shifted from these highly susceptible branching and tabular species to more resistant and resilient massive taxa. Little Cayman is an ideal location within the Caribbean to study the population dynamics of hard coral populations because it lacks many of the anthropogenic stressors to which other reefs are exposed. The remote island is approximately 16 km long by 1.6 km wide, with a maximum elevation of 12 m, and is surrounded by shallow and narrow reef shelves, some of which drop off to vertical walls (e.g. Bloody Bay Wall) that reach 300 m deep. More than half of the shallow reefs have been designated as protected areas (e.g. marine parks, replenishment zones) since 1986 [2]
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