Coral recruitment dynamics and substrate mobility in a rubble-dominated back reef habitat

Some back reef habitats can be characterized by unconsolidated substrate with sand patches that, while potentially valuable as nursery habitat for reef fish species, are typically considered less suitable for settlement by sessile invertebrates. Long-term coral monitoring coupled with short-term rubble movement trials were conducted in a natural rubble field in Biscayne National Park, Florida, to quantify juvenile coral recruitment and species composition, as well as the direction and distance of coral rubble displacement. Mean juvenile density over the 5-yr study ranged from 2.59 to 4.09 juveniles m(2), while juvenile loss (emigration/mortality) was around 50% yr(-1). This loss, although higher than nearby reef sites, could be partially explained by rubble movement. During the four observational periods (each 4-7 wk), individual rubble pieces had mean displacement of 33 (SE 8.5) cm during the calmest period and 149 (SE 46) cm during the windiest period. Coral species composition at the natural rubble site consisted of 12-15 species of coral, predominantly brooding species. While coral density, recruitment, and overall coral cover may seem low at the natural rubble site, they are comparable to nearby consolidated reef sites and are significantly higher than those observed at a nearby restored rubble field (0.34-1.02 juveniles m(-2)). Despite the potential importance of rubble fields as source populations for reef species, population dynamics in unconsolidated environments are often overlooked. Thus, studies such as this may provide insight into restoration and management strategies for these understudied habitats.