The persistence of Sargassum communities on coral reefs: resilience and herbivory

Loffler, Zoe (2019) The persistence of Sargassum communities on coral reefs: resilience and herbivory. PhD thesis, James Cook University.

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Many of the world's ecosystems are in decline, with the combined effects of climate change and local anthropogenic stressors leading to shifts in the dominant habitatforming organisms across a range of terrestrial and aquatic ecosystems. Such shifts not only represent a change in the structure and functioning of these ecosystems, but also the goods and services they provide. Within coral reef ecosystems, shifts from coral- to macroalgae-dominance are becoming increasingly common and, once established, macroalgae-dominated states are difficult to reverse. As such, understanding the capacity of herbivores to consume macroalgae, thereby preventing its establishment and reversing shifts to macroalgae-dominance is vital. However, the vast majority of research in this area has investigated the capacity of herbivores to remove macroalgal biomass without considering the longer term, ecological implications of the removal for macroalgal population dynamics. In this thesis, I seek to first understand how the ecology of a prevalent macroalga on degraded coral reefs, Sargassum, contributes to its resilience to disturbance, and then investigate herbivory of the Sargassum components (specifically, holdfasts and propagules) that are likely to be instrumental in promoting its persistence and spread on degraded and inshore coral reefs.

First, I examined the contrasting response of coral and macroalgae (Sargassaceae) communities to frequent disturbance using benthic monitoring data (2013-2017) from the Turtle Group, a series of inshore reefs in the northern Great Barrier Reef (GBR). Initially, five of the six monitored sites were either Sargassaceae or coral-dominated, while the sixth had a mixture of coral and macroalgae. Following three major disturbances in successive years (severe cyclones in 2014 and 2015, and a pan-tropical coral bleaching event in 2016), the ecosystem was essentially 'reset', as coral and Sargassaceae cover at all sites decreased to between 0-5 % in 2014-2015. However, the Sargassaceae at these sites quickly recovered to over 70 % cover only 1-2 years after the disturbance. In contrast, coral communities did not show any evidence of recovery. Furthermore, at one previously coral-dominated site, the cover of Sargassaceae increased from 0 % to 28 % cover post-disturbance, indicating that Sargassaceae has the capacity to not only rapidly recover after disturbance, but also to rapidly colonise sites where it was previously absent. However, it is unknown if regrowth from holdfasts remaining on the benthos or growth from newly settled recruits enabled the rapid rate of recovery.

To investigate the potential contribution of holdfasts and propagules to the recovery of Sargassum populations post-disturbance, I experimentally simulated the physical removal of macroalgal biomass from Sargassum beds at Orpheus Island, an inshore coral reef on the GBR, and monitored recovery for 11 months. Trimming Sargassum biomass but leaving the holdfast intact had no detectable effect on the density, height or biomass of Sargassum compared to adjacent intact, or control, areas after five months. In contrast, in areas where holdfasts were also removed, holdfast density and thalli height only recovered to 78 and 66 % of control values, respectively, and Sargassum biomass only recovered to 50 % of control biomass after 11 months. Given the importance of holdfasts to the resilience of Sargassum beds, I also investigated the ability of herbivores to remove holdfasts. Exposing pieces of dead coral with attached Sargassum holdfasts to local herbivore assemblages resulted in a 70 % decline in the number of holdfasts over four months compared to those protected from herbivores. The resilience of Sargassum to disturbance through regrowth from holdfasts indicates that storms and cyclones are unlikely to result in sustained reductions in Sargassum biomass on coral reefs, however, the removal of Sargassum holdfasts resulted in a significant decrease in the height and density of Sargassum, indicating that holdfasts must be removed in order to reduce Sargassum biomass.

In Chapter 4, I further explore the potential for herbivores to damage and/or remove holdfasts and investigate the capacity for Sargassum to regrow from damaged holdfasts. Underwater video cameras were used to investigate the susceptibility of Sargassum components (blades, stipes and holdfasts) to herbivory. Entire thalli of Sargassum swartzii were placed on the reef crest at Lizard Island in the northern GBR and monitored using photographs and video recordings for 24 days. The blades of the S. swartzii were rapidly removed (100 % in 2 days) by herbivores, whereas the stipes and holdfasts were less susceptible to herbivores. After 24 days, 72 % of experimental thalli had partial stipes remaining, and only one holdfast (out of 53) had been removed; all of the remaining holdfasts were largely undamaged. When S. swartzii holdfasts within natural stands were experimentally damaged (0, 25, 50, or 75 % removed), there was no detectable effect on thallus height or holdfast size among regrown thalli after one year. There was, however, a 50 % reduction in survival for S. swartzii individuals when 75 % of the holdfast was removed. This study demonstrated that holdfasts of S. swartzii are extremely resistant to herbivory, and that incidental bites on S. swartzii holdfasts are unlikely to affect their growth or survival unless three-quarters of the holdfast is removed.

Although the importance of herbivores in preventing shifts to macroalgae-dominance on coral reefs is well-established, the removal of macroalgal propagules within the Epilithic Algal Matrix (EAM: a conglomerate of short, productive turf algae, macroalgal propagules, detritus, microbes, and invertebrates) has largely been inferred rather than empirically demonstrated. To determine if the presence of Sargassum swartzii propagules in the EAM affected the feeding rate of grazing fishes, and if any grazing affected the survival of S. swartzii propagules, I compared feeding on settlement tiles with EAM only, to those with S. swartzii propagules settled within the EAM. Paired settlement tiles (EAM only and EAM with propagules) were deployed on the reef crest and flat at Lizard Island in the northern GBR with video cameras for six days, with caged pairs to act as controls. While survival of Sargassum propagules was 39 % lower on tiles exposed to local fish assemblages than on caged tiles, grazing rates were 36 % lower on tiles that had S. swartzii propagules within the EAM than on tiles with EAM only. Surprisingly, these patterns were largely driven by small-bodied fishes from the genus Ecsenius (F. Blenniidae), which took significantly more bites on the tiles than any other fishes, and likely contributed to the decrease in propagule density on exposed tiles. These results suggest that smaller-bodied grazers may play a greater role in propagule removal than previously assumed, and that grazing fishes are able to detect the presence of S. swartzii propagules growing within the EAM and may prefer to graze areas free of propagules.

Finally, I investigate the importance of structurally complex microhabitats to the recruitment, growth and survival of Sargassum swartzii propagules. Fertile thalli of S. swartzii were induced to release spores and the spores allowed to settle onto terracotta settlement tiles that had a series of 3 mm deep crevices evenly spaced on their top (exposed) surface. Recruitment of S. swartzii within crevices was 21% greater, but propagules were 18 % shorter, 18-days post-settlement than on adjacent exposed surfaces. Exposing tiles to local fish assemblages on the reef crest and flat at Lizard Island for five days showed that survival of propagules was 90 % greater in crevices than on exposed areas of the tiles on the reef crest, but not on the reef flat. Underwater video footage revealed that few fishes fed from within the crevices (18% of all bites) with the majority of feeding being concentrated on the exposed surface of tiles. Once again, small-bodied fishes from the family Blenniidae (predominantly Ecsenius spp.) accounted for the majority of the feeding activity on the tiles, and likely contributed to the mortality of propagules. Structurally complex microhabitats, such as crevices, may therefore be important for the proliferation of Sargassum on coral reefs through the provision of refugia from herbivory.

Overall, this thesis demonstrates the extraordinary resilience of Sargassum to disturbance. The capacity of Sargassum to recover after disturbance appears to be related to its ability to regenerate from holdfasts and may be further reinforced by the recruitment and growth of propagules. The persistence of Sargassum on coral reefs is likely enhanced by the reluctance of herbivores to consume holdfasts and the ability of Sargassum to regrow from holdfasts that have sustained significant damage. As a consequence, sustained browsing (preventing regrowth of the stipe and blades) may be more important in reversing macroalgae-dominance than physical removal of holdfasts by herbivorous fishes. Although small-bodied grazing fishes contributed to the mortality of Sargassum propagules, lower grazing rates on areas of the EAM containing propagules and within crevices may contribute to the survival of developing Sargassum, potentially enhancing the persistence of Sargassum beds on coral reefs. With predicted increases in the frequency and intensity of disturbances affecting coral reefs, the results of this thesis indicate that the ecology of Sargassum affords it a large capacity to withstand future disturbances, to the likely detriment of coral communities already under pressure from climate change.

Item ID: 60508
Item Type: Thesis (PhD)
Keywords: coral reef, cyclone, herbivory, holdfasts, hurricane, nitrogen phase-shift, plant population and community dynamics, plant-herbivore interactions, regime shift, resilience, Sargassum
Related URLs:
Copyright Information: Copyright © 2019 Zoe Loffler.
Additional Information:

Three publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 3: Loffler, Zoe, and Hoey, Andrew S. (2018) Canopy-forming macroalgal beds (Sargassum) on coral reefs are resilient to physical disturbance. Journal of Ecology, 106 (3). pp. 1156-1164.

Chapter 4: Loffler, Zoe, Graba-Landry, Alexia, Kidgell, Joel T., McClure, Eva C., Pratchett, Morgan S., and Hoey, Andrew S. (2018) Holdfasts of Sargassum swartzii are resistant to herbivory and resilient to damage. Coral Reefs, 37 (4). pp. 1075-1084.

Chapter 6: Loffler, Zoe, and Hoey, Andrew S. (2019) Microtopographic refuges enhance recruitment and survival, but inhibit growth of propagules of the tropical macroalga Sargassum swartzii. Marine Ecology Progress Series. pp. 61-70.

Date Deposited: 02 Oct 2019 21:58
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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