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While climate change and associated increases in sea surface temperature and ocean acidification, are among the most important global stressors to coral reefs, overfishing and nutrient pollution are among the most significant local threats. Here we examined the independent and interactive effects of reduced grazing pressure and nutrient enrichment using settlement tiles on a coral-dominated reef via long-term manipulative experimentation. We found that unique assemblages developed in each treatment combination confirming that both nutrients and herbivores are important drivers of reef community structure. When herbivores were removed, fleshy algae dominated, while crustose coralline algae (CCA) and coral were more abundant when herbivores were present. The effects of fertilization varied depending on herbivore treatment; without herbivores fleshy algae increased in abundance and with herbivores, CCA increased. Coral recruits only persisted in treatments exposed to grazers. Herbivore removal resulted in rapid changes in community structure while there was a lag in response to fertilization. Lastly, re-exposure of communities to natural herbivore populations caused reversals in benthic community trajectories but the effects of fertilization remained for at least 2 months. These results suggest that increasing herbivore populations on degraded reefs may be an effective strategy for restoring ecosystem structure and function and in reversing coral–algal phase-shifts but that this strategy may be most effective in the absence of other confounding disturbances such as nutrient pollution. [ABSTRACT FROM AUTHOR]

Reduced herbivory and enhanced nutrient concentrations have both been suggested as probable mechanisms driving phase shifts from coral toalgal dominance on tropical reefs. While there is considerable information regarding the effects of herbivores on reefs, there is little experimental evidence of nutrient effects or the interactive effects of both of these factors. This study experimentally tested the role of these factors on benthic community structure on a coral-dominated reef in Hawaii. A randomized factorial block design was used to simultaneously investigate the effects of nutrient enrichment and herbivoreexclusion on the development of benthic algal communities on artificial surfaces over a 6-month time period. Total algal biomass was greatest on settlement surfaces exposed to both nutrient enrichment and herbivore exclusion simultaneously. Fleshy algal biomass was greatest on surfaces removed from grazing whereas calcareous biomass was greatest on surfaces exposed to nutrient enrichment. Control surfaces exhibited consistently less total, fleshy and calcareous algal biomass than that on any of the experimental surfaces. Microinvertebrates were most abundant on surfaces within herbivore exclusion treatments but increased in number on all settlement surfaces over time. Sediment accumulation was positively correlated with fleshy algal biomass and wasmost abundant on surfaces within herbivore exclusion treatments; there was no pattern in sediment accumulation over time. This research demonstrates that on short time scales (less than 6 months), nutrient enrichment and herbivore exclusion can independently and interactively support shifts in benthic algal community structure on a Hawaiian reef. [ABSTRACT FROM AUTHOR]