Complex interactions among stressors evolve over time to drive shifts from short turfs to macroalgae on tropical reefs

Abstract

Citation: Fong, C. R., C. J., Gaynusand R. C., Carpenter. 2020. Complex interactions among stressors evolve over time to drive shifts from short turfs to macroalgae on tropical reefs. Ecosphere 11(5):Abstract. Multiple stressors acting simultaneously on ecological communities are the new normal state.Stressor number and strength will increase with rising anthropogenic activity, making it critical to understand both stressor effects and interactions. Stressor temporal regimes vary in intensity, frequency, and duration, ranging from press to pulse. While stressors with different temporal regimes likely have divergent effects, this remains mostly unexplored, though there is some evidence communities are more resistant to pulse than press stressors. Coral reefs are among the most impacted marine communities, and degradation from coral to algal dominance has been attributed to increases in both local and global stressors. Overfishing, nutrient pollution, and increased sedimentation are all local stressors that have been implicated in shift dynamics. Nutrients and sediments are anthropogenically derived stressors to reefs that can have press and pulse temporal regimes. We conducted a 6-month fully crossed factorial field experiment on algal turf communities in Moorea, French Polynesia, manipulating access by herbivores, enrichment regime, and sedimentation regime and tracked changes in benthic community composition. We found complex interactions among stressors and stressor regimes drove a series of transitions from healthy, short algal turf communities to degraded, long algal turfs, and ultimately to macroalgal-dominated communities. While herbivory controlled final community composition after 6 months, 2-and 3-way interactions among nutrient and sediment temporal stressor regimes over time drove transition dynamics, and matching of stressor temporal regimes accelerated shifts. Some stressors cryptically eroded the resilience of the community, which was only evident when the strong ecological processes that masked these effects were disrupted. Our research highlights the need to consider temporal stressor regime as well as stressor interactions, particularly in light of predicted increases in both local and global stressors and alterations to stressor temporal regimes. Our understanding of the impacts of local stressor temporal regimes is in its infancy. Here, we provide a novel demonstration that the effects of stressor temporal regime varied and multiple stressors interacted to exhibit complex, emergent interaction effects, demonstrating the need to explicitly contrast stressor temporal regimes under multiple conditions to understand how communities will respond to future challenges.

Storms may disrupt top-down control of algal turf on fringing reefs

“…1 ). While we did not estimate sediment loads, comparable rainfall events have pulsed sediment into this bay (Fong et al 2020a , b ), and the water was visibly discolored with red sediment from the agricultural watershed (Fig. 1 ).…”

“…To understand how rainfall events impact top-down control on short algal turf communities, we compared herbivore exclosure experiments from previously published experiments without rainfall (Fong et al 2018 ) and with light rainfall (Fong et al 2020b ) to our experiment conducted during an extreme rainfall that produced a visible sediment plume in January 2020. All three of these experiments were conducted on the same fringing reef along Cook’s Bay (− 17.489825, − 149.825907) in Moorea, French Polynesia within the same ~ 100 m × 20 m area.…”

“…Herbivory is a critical ecological force supporting coral reef resilience through strong top-down control on algal communities like turf (McClanahan et al 2003 ; Bellwood et al 2004 ; Hughes et al 2007 ). Short algal turfs are highly productive benthic space holders (Adey and Goertemiller 1987 ) that can transition to macroalgae or long turfs, depending on environmental conditions (Goatley et al 2016 ; Muthukrishnan et al 2016 ; Fong et al 2020b ). Generally, macroalgae is considered to be a degraded, undesirable state on coral reefs (e.g., Hughes et al 2007 ).…”

“…Storms may disrupt strong top-down control by herbivores on SPATs, and possibly mediate transitions to LSATs. One mechanism for this effect is increased fluxes of terrestrial sediment to nearshore environments, often via visible sediment plumes (Fong et al 2020a , b ). Sediment that settles on turf can disrupt strong top-down control by herbivores on SPATs, making it important to explore any potential relationship between storms and herbivory.…”

“…Research has demonstrated that many herbivores avoid sediment-laden algal turf (Goatley & Bellwood 2012 ; Clausing et al 2014 ; Gordon et al 2016 ; Tebbett et al 2017a , 2017b ). Since storms can drive plume events that enhance sediment transport (Fong et al 2020a , b ) and storms can increase sediment load (Goatley et al 2016 ), release from herbivory pressure associated with rainfall, possibly mediated by sediment, may drive transitions to either LSATs or macroalgae.…”