Context-dependent growth response of Halimeda opuntia to sediment and nutrients in a high light environment.

Coral reefs often experience simultaneous changes in multiple environmental drivers due to human impacts that can affect species' responses and ultimately alter community structure. Presently, the bulk of coral reef research is focused on the responses of coral, fish, and opportunistic algae to multiple stressors. Lacking are experiments investigating macroalgae typically associated with healthy reef systems. Here we explore how nutrients, sediment, and light affect a persistent macroalgal species using both field and mesocosm experiments. In the field, we quantified the response of Halimeda opuntia, a common calcifying alga on both less and more impacted reefs, to nutrients, sediment, and light. We found sediment and nutrient additions, conditions characteristic of more impacted reefs, interacted negatively to decrease H. opuntia growth. In a mesocosm experiment, we quantified the effects of sediment and light on H. opuntia growth and found in this extremely high light environment both sediment addition and light reduction positively affected H. opuntia. Our results demonstrate that the response of H. opuntia to these environmental drivers is context dependent. While the combination of nutrients and sediment may deter the growth of persistent macroalgal species, increased sediment alone may mediate the inhibitory effects of an extremely high light environment. These results suggest that macroalgal species that are typically associated with healthy coral reefs may suffer rather than benefit from shifts in environmental drivers impacted by anthropogenic factors.