Tropical coral reefs shift to greater algal abundance in response to multiple anthropogenic stresses but show resilience when environmental conditions recover

Background/Question/Methods

The role of anthropogenic influences on the structure of tropical reef communities has received substantial interest in light of catastrophic shifts from communities dominated by coral to reefs dominated by less desirable community members such as macroalgae or algal turfs. In addition, understanding the resistance and resilience of reefs to these transitions is valuable to predicting the future responses of reefs and to developing effective management strategies for their conservation.   A large-scale field experiment manipulating sediment loading, nutrient levels and herbivore abundance was conducted using a fully crossed design, and benthic community composition was monitored to investigate the effects and potential interactions of these three common anthropogenic stresses on community structure.  Experiments were conducted on 0.84m² plots of live coral on two reefs (Suecas and Larga) at Isla Contadora in the Eastern Tropical Pacific.  Manipulations were maintained and responses monitored for ~6 months, which included both a non-upwelling and upwelling period.  Recovery was then monitored for 1.5 years after manipulations were removed.

Results/Conclusions

All stressors produced increases in algal abundance but the strengths and relative importance of different stressors varied between reefs and particular stressor combinations.  In general, the pattern of response to treatments was consistent between upwelling conditions.  However, ambient levels of algal growth were marginally higher during the upwelling, which affected the absolute magnitude of algal growth in any particular treatment.  The overall strongest responses were seen from herbivore reduction, which nearly doubled the rate of algal accumulation, but this effect was only seen at Suecas reef.  Nutrient additions also had stronger effects at Suecas reef but had an impact at both sites. The effect of sediment was the least consistent, varying with respect, location and combination with other stressors. After manipulations were ended, at least partial recovery was seen in all treatments.  This suggests that these reefs are resilient to moderate shifts in community structure produced by changes in environmental conditions, provided the environmental shift is only temporary, not persistent.