Pristine Reefs Recover Better: Examples from Chagos and the Red Sea
2010 Meeting of the Americas
Benthic Processes, Biodiversity, Ecosystems, Structure and Dynamics, Estuarine and Nearshore Processes
Consensus that coral reefs are at threat from large-scale and local environmental perturbations causes concern for their continued existence. A key question is whether the quality of a reef, pristine or impacted, plays a role for trajectories and resilience. Pristine reefs are usually remote and create challenges for sampling. We develop stage-based models for guilds of very fast, fast and slow-growing corals in the Chagos Archipelago and the Saudi Red Sea. Recruitment and size-classes were estimated from phototransects and direct counts, currents by ADCP, temperatures by in situ loggers. A transition matrix of four life-stages was inversely solved by quadratic programming from as many population vectors as phototransects. Space-for-time substitution assumed that transects with the most large colonies were the oldest. This model did not recreate observed size-frequencies. Next, we Monte-Carlo sampled model life-tables from the literature and hindcast population dynamics assuming that populations started at zero in 1998, as was indeed observed. We modelled the level of recruitment required (0 and 20.000 recruits per 10m2) to obatin size-frequency distribution comparable to 2006/8/9 field data. Models assuming connected populations and minimal to no local recruitment compared to field data. The models suggested in Chagos more recruitment inside lagoons than on outside reefs and a decline at 20m depth. In the Red Sea, we found more recruits on offshore, pristine reefs. Models, supported by field evidence, suggested that more large slow-growers than fast-growers had survived. Coral size-frequency distribution in Chagos deep lagoons was not compatible with the assumption of an overkill in 1998, suggesting a refuge. Our method and results demonstrate that even limited, snap-shot type, rapid ecological surveys can yield data that allow hind- and forecasting of population dynamics. Results were calibrated against current and temperature measurements. Lagoons had lower currents and outside reefs were subject to upwelling at >15m depth. Higher recruitment on lagoon reefs was due to more coral survival in deep lagoons and slower currents favoring larvae retention. Faster currents and colder temperatures on outside reefs correlate with lower recruitment and coral cover. Thus coral lagoons appear to be a key locus for reef resilience in the Chagos. In the Red Sea, we found equal impact on nearshore and offshore reefs, but better recruitment in the pristine settings. Thus, pristine reefs, while equally at threat of damage, recover better. Global change forecasts suggest a ~30% coral decline if recruitment declines by 1% per year over 50 years in the absence of mass mortality, and an about 50% decline in coral cover with decadal mass mortalities and an annual 1% decline in larval survival. This is a powerful argument for the conservation of lagoonal areas in atoll settings and for keeping reefs as pristine as possible.
Riegl, Bernhard; Purkis, Samuel J.; Sheppard, Charles R. C.; Harris, A.; Bruckner, Andrew; Rowlands, Gwilym; and Renaud, Philip, "Pristine Reefs Recover Better: Examples from Chagos and the Red Sea" (2010). Marine & Environmental Sciences Faculty Proceedings, Presentations, Speeches, Lectures. 73.
0000-0002-6003-9324; F-8807-2011; B-8552-2013