Ecological drivers of the brown macroalgal genus Lobophora on Pacific coral reefs

Abstract

Coral reefs are threatened by increasingly frequent large-scale coral bleaching events that often lead to mass coral mortality. In the wake of coral mortality, macroalgae cover can increase drastically. A common alga contributing to these blooms is the brown macroalgal genus Lobophora. Lobophora has been shown to have many negative effects on corals, including severely reduced coral larval settlement, reduced coral fecundity and growth, and direct competition with adult corals. However, it is still unclear what drives the establishment of Lobophora on coral reefs. Therefore, the overall aim of this thesis is to study drivers of Lobophora assemblages and the control of the alga by herbivorous fishes.In Chapter 2 I examine the local cryptic diversity of Lobophora in Palau and study how wave exposure, herbivorous fish biomass and depth influence Lobophora diversity and species assemblages. I collected Lobophora specimens from 12 reefs around Palau along a wave exposure and herbivorous fish biomass gradient at 3 m and 10 m depth. The specimens were identified to species level using two chloroplast and one mitochondrial marker. I found 15 species of which ten are currently undescribed. The results further suggest that neither of the studied environmental factors influences the diversity of Lobophora. Species assemblages are shaped only by wave exposure on an island-wide geographic scale but by wave exposure as well as herbivore biomass and depth on a more local geographic scale. I also identify generalist and specialist species. These findings have important implications as Lobophora species may have different competitive strengths and the coral-algal interactions may, therefore, vary between Lobophora species. Understanding the drivers of Lobophora assemblages and which species may have a particularly large potential to spread to other reefs (i.e., generalists) may guide future studies of Lobophora-coral interactions.In Chapter 3 I study the life-stage specific control of Lobophora by herbivorous fishes. Using field observations and caging experiments, I found that Lobophora is more readily controlled at the recruit stage than the adult stage. I also observed a temporal trend in Lobophora growth and recruitment, with growth overpowering herbivory over the course of a few weeks. In a controlled tank experiment and camera-based field observations, I identify certain fish species as particularly important for the control of Lobophora at the recruit stage. Out of seven observed fish species, only three (the parrotfish Scarus niger, Chlorurus spilurus and the surgeonfish Acanthurus nigrofuscus) removed Lobophora recruits. These findings are important as they show that fish identity plays a role in the control of Lobophora – even at the recruit stage. Further, my results suggest that if a reduction in herbivory was to occur at a time of high Lobophora growth and recruitment, the alga may be able to overpower herbivory and increase its cover on a reef.In Chapter 4 I study the role refuges play for the establishment of Lobophora on coral reefs. Using a combination of field surveys and manipulative field experiments, I show that concealed microhabitats (i.e. crevices) provide a refuge for Lobophora recruits and that Lobophora abundance in the field is negatively associated with crevice size. My results also suggest that parrotfish are the main control of Lobophora abundance and that fish size is a good predictor of grazing rates in microhabitats. These findings show that Lobophora gains protection from herbivory in crevices and may be able to spread from these microhabitats when herbivory is decreased due to fishing or mass coral mortality to establish dominance on a coral reef.In Chapter 5 I developed an individual-based model using Lobophora recruitment rates, recruit and adult mortality associated with specific fish species and microhabitat types collected in Chapters 3 and 4 to simulate Lobophora dynamics. Some additional information from existing literature was incorporated and some minor adjustments allowed the model to successfully simulate Lobophora cover within different microhabitat types. The model predictions show the impact of fish species-specific overfishing on Lobophora cover and highlight the importance of concealed microhabitats in facilitating Lobophora.Overall, this thesis increases our knowledge of Lobophora diversity and ecology and its control by herbivorous fishes. I have shown that a strong shift in grazing susceptibility exists between early life-stage and adult Lobophora and that microhabitats are important in facilitating Lobophora. Further, this thesis has revealed fish species-specific abilities to control Lobophora and has identified parrotfish and Acanthurus nigrofuscus as the herbivorous fish species to have the strongest control over the alga’s recruits. However, a diverse suite of herbivorous fish species may be more effective at controlling Lobophora in a variety of microhabitats. These findings have implications for fisheries management and managers should consider protecting herbivorous fish species diversity, possibly with particular attention to Acanthurus spp. and parrotfish. The strong ontogenetic shift in grazing susceptibility further indicates that fisheries restrictions will likely be more effective if implemented before a shift to Lobophora dominance occurs.