Nutrient enrichment strengthens feedbacks that promote and stabilize coral‐to‐macroalgae regime shifts on tropical reefs

Abstract Ecosystems around the world have experienced shifts in community state as a result of anthropogenic activities that alter important ecological feedbacks. On coral reefs, transitions to a state where fleshy macroalgae are a major space holder have often been associated with the loss of top‐down control, which frequently has been attributed to overharvesting of herbivorous fishes. However, nutrients could also facilitate the proliferation of macroalgae, but evidence for the role of this mechanism in state shifts on coral reefs is inconsistent. Here, we combine a common garden nutrient enrichment experiment with surveys of algal traits to assess potential mechanisms by which nutrient subsidies could trigger and/or maintain a regime shift to abundant macroalgae on reefs. We explored how nutrients influenced individual performance at three life stages of the frondose brown alga Turbinaria ornata , which is often a dominant fleshy macroalga on coral reefs throughout the Indo‐Pacific. Our enrichment experiment revealed that nutrients greatly increased growth and survival rates of young Turbinaria recruits during this brief life stage when they are most vulnerable to herbivory. For the juvenile stage, we found strong correlations between nutrient availability and the expression of physical defenses that deter herbivory. Additionally, reproductive investment by adults in natural populations was positively correlated with nutrient availability. Taken together, our findings provide several mechanisms by which nutrients could facilitate a persistent shift from coral to fleshy macroalgae by allowing more vulnerable recruit and juvenile life stages to escape herbivore control and by enhancing the reproductive output of adults that produce germlings that recruit locally. These results lend novel insights into bottom‐up mechanisms by which increases in nutrients can bolster the establishment and subsequent replenishment of populations of fleshy macroalgae, which can inform management strategies for conserving or restoring coral reefs.