Benthic micro‐ and macro‐community succession and coral recruitment under overfishing and nutrient enrichment

Abstract

Herbivory and nutrient availability are fundamental drivers of benthic community succession in shallow marine systems, including coral reefs. Despite the importance of early community succession for coral recruitment and recovery, studies characterizing the impact of top‐down and bottom‐up drivers on micro‐ and macrobenthic communities at scales relevant to coral recruitment are lacking. Here, a combination of tank and field experiments were used to assess the effects of herbivore exclusion and nutrient enrichment on micro‐ to macrobenthic community succession and subsequent coral recruitment success. Herbivore exclusion had the strongest effect on micro‐ and macrobenthic community succession, including a community shift toward copiotrophic and potentially opportunistic/pathogenic microorganisms, an increased cover of turf and macroalgae, and decreased cover of crustose coralline algae. Yet, when corals settled prior to the development of a macrobenthic community, rates of post‐settlement survival increased when herbivores were excluded, benefiting from the predation refugia provided by cages during their vulnerable early post‐settlement stage. Interestingly, survival on open tiles was negatively correlated with the relative abundance of the bacterial order Rhodobacterales, an opportunistic microbial group previously associated with stressed and diseased corals. Development of micro‐ and macrobenthic communities in the absence of herbivory, however, led to reduced coral settlement. In turn, there were no differences in post‐settlement survival between open and caged treatments for corals settled on tiles with established benthic communities. As a result, open tiles experienced marginally higher recruitment rates, driven primarily by the higher initial number of settlers on open tiles compared to caged tiles. Overall, we reveal that the primary interaction driving coral recruitment is the positive effect of herbivory in creating crustose coralline algae (CCA)‐dominated habitats, free of fleshy algae and associated opportunistic microbes, to enhance coral settlement. The negative direct and indirect impact of fish predation on newly settled corals was outweighed by the positive effect of herbivory on the initial rate of coral settlement. In turn, the addition of nutrients further altered benthic community succession in the absence of herbivory, reducing coral post‐settlement survival. However, the overall impact of nutrients on coral recruitment dynamics was minor relative to herbivory.

Coral recruitment in the Toliara region of southwest Madagascar: Spatio‐temporal variability and implications for reef conservation

“…In the context of resilience‐based management, actions should be taken to reduce the negative effects of nutrients and sediment loads from the Fiherenana and Onilahy rivers that have increased due to unmanaged mining and agricultural and deforestation activities (Bruggemann et al., 2012; Sheridan et al., 2014). These actions are critical as coral recruits are particularly sensitive to increasing levels of sedimentation (Brunner et al., 2021; Wakwella et al., 2020) and nutrients (D'Angelo & Wiedenmann, 2014; Evensen et al., 2021). Moreover, privileging better land and watershed management actions will also improve water quality and coral reef health to the benefit of other reef communities.…”

Larval precompetency and settlement behaviour in 25 Indo-Pacific coral species

“…Secondly, rubble fragments may have supported more complex and/or well-developed biofilms than the artificial discs, leading to the presence of stronger induction cues. Indeed, the presence of complex microhabitats likely created pockets of unique biofilm communities 83 – 85 that may have offered a wider diversity of potential inducers. It is also likely that biofilm discs supported early successional species (+4 weeks conditioned) 83 that were less mature and inductive than those on rubble fragments.…”

Priority effects in coral–macroalgae interactions can drive alternate community paths in the absence of top‐down control

“…In addition to preemption, early colonists often modify the environment in ways that favor their persistence, creating feedbacks that can inhibit new colonizers (Connell & Slatyer, 1977 ; Orrock et al, 2008 ; Peterson, 1984 ; Yelenik & D'Antonio, 2013 ). Large numbers of studies have shown that abundant algae can inhibit coral recruitment by preempting space and by creating an environment that is hostile to coral colonists (e.g., Bulleri et al, 2018 ; Evensen et al, 2021 ; Hughes et al, 2007 ; Mumby et al, 2016 ; Schmitt et al, 2021 ). In particular, the dominant alga that proliferated under reduced herbivory conditions in our study, the creeping foliose brown alga Lobophora , can inhibit coral recruitment by suppressing the settlement of coral larvae (Baird & Morse, 2004 ; Evensen, Doropoulos, Morrow, et al, 2019 ; Evensen, Doropoulos, Wong, et al, 2019 ; Kuffner et al, 2006 ; Morrow et al, 2017 ) and by killing newly settled corals via overgrowth and allelopathy (Evensen, Doropoulos, Wong, et al, 2019 ; Johns et al, 2018 ; Rasher & Hay, 2010 ).…”