The effects of a changing marine environment on the bioeroding sponge Cliona orientalis
Ramsby, Blake Donald (2018) The effects of a changing marine environment on the bioeroding sponge Cliona orientalis. PhD thesis, James Cook University.
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Abstract
Bioeroding sponges are a unique group of coral reef sponges. They transform dissolved nutrients into particulate nutrients via active filter feeding whilst also eroding the coral reef framework that they inhabit. Despite their ecological importance, we know little about their distribution or abundance, especially along the inshore Great Barrier Reef (GBR). In addition, bioeroding sponges are often considered to be thermally tolerant, even though their thermal thresholds are unknown. Bioeroding sponges also occur in high abundance in polluted or eutrophic habitats, but it is unclear whether these conditions directly benefit sponges through accelerated growth or improved condition or benefit bioeroding sponges indirectly via negative effects on corals. To address these knowledge gaps, this thesis investigated whether bioeroding sponges and their photosynthetic symbionts can tolerate changing environmental conditions on coral reefs. Research focused on Cliona orientalis as it is a conspicuous bioeroding sponge on the GBR. Field surveys were used to measure the abundance of C. orientalis on the inshore GBR and laboratory experiments were performed to investigate the response of C. orientalis to ocean warming and nutrient enrichment.
Decreasing coral cover on the GBR may provide opportunities for rapid growth and expansion of other taxa. The bioeroding sponges Cliona spp. may increase in abundance after coral bleaching, damage, and mortality as they withstand elevated temperatures without bleaching. In Chapter 2, I analysed benthic surveys of the inshore GBR (2005–2014) which revealed that the percent cover of C. orientalis has not increased in the past decade, as would be expected if the sponge benefited from coral bleaching or mortality. I found that the proportion of fine particles in benthic sediments was negatively associated with the presence-absence and the percent cover of this sponge, indicating that C. orientalis requires wave-exposed habitats where fine sediments are absent. The fastest increases in C. orientalis cover coincided with the lowest macroalgal cover and chlorophyll a concentration, highlighting the importance of macroalgal competition and local environmental conditions for this sponge. Given the observed distribution and habitat preferences of C. orientalis, bioeroding sponges likely represent site-specific rather than regional threats to corals and reef accretion.
Coral reefs face many stressors associated with global climate change, including increasing sea surface temperature and ocean acidification. In Chapters 3 and 4, I exposed C. orientalis to temperature increments increasing from 23 to 32 °C to define the thermal tolerance threshold of the sponge and its associated microbiome. At 32 °C, or 3 °C above the maximum monthly mean (MMM) temperature, sponges bleached and the photosynthetic capacity of Symbiodinium was compromised, consistent with sympatric corals. Cliona orientalis demonstrated little capacity to recover from thermal stress, remaining bleached with reduced Symbiodinium density and energy reserves after one month at reduced temperature. While C. orientalis can withstand current temperature extremes (<3 °C above MMM) under laboratory and natural conditions, this species would not survive ocean temperatures projected for 2100 without acclimatisation or adaptation (≥3 °C above MMM). In Chapter 4, I demonstrated that bleaching of C. orientalis is preceded by a change in its microbial community, which is not restored after the thermal stress is removed. In Chapter 5, I investigated the effects of dissolved inorganic nutrients and light intensity on the growth and condition of five common Great Barrier Reef sponges, including C. orientalis, to test whether C. orientalis responds differently than other sponge species. Dissolved nutrients up to 7 μM total DIN did not significantly affect the growth, condition, or chlorophyll content of any sponge species after 10 weeks of exposure. Light (80 vs 160 μmol quanta m⁻²-2 s⁻¹) did not affect four of the five sponge species, but higher irradiance resulted in higher organic content and chlorophyll levels in C. orientalis.
Hence, as ocean temperatures increase above local thermal thresholds, C. orientalis will have a negligible impact on reef erosion, and nutrient enrichment is unlikely to alter these effects.
| Item ID: | 59136 |
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| Item Type: | Thesis (PhD) |
| Keywords: | bleaching, climate change, Cliona orientalis, Rhodothalassium, symbiosis, ocean warming, bioeroding sponges, stressors, thermal tolerance |
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| Copyright Information: | Copyright © 2018 Blake Donald Ramsby. |
| Additional Information: | For this thesis, Blake Ramsby received the Dean's Award for Excellence 2019. Publications arising from this thesis are available from the Related URLs field. The publications are: Chapter 2: Ramsby, Blake D., Hoogenboom, Mia O., Whalan, Steve, Webster, Nicole S., and Thompson, Angus (2017) A decadal analysis of bioeroding sponge cover on the inshore Great Barrier Reef. Scientific Reports, 7. 2706. Chapter 3: Ramsby, Blake D., Hoogenboom, Mia O., Smith, Hillary A., Whalan, Steve, and Webster, Nicole S. (2018) The bioeroding sponge Cliona orientalis will not tolerate future projected ocean warming. Scientific Reports, 8. 8302. Chapter 4: Ramsby, Blake D., Hoogenboom, Mia O., Whalan, Steve, and Webster, Nicole S. (2018) Elevated seawater temperature disrupts the microbiome of an ecologically important bioeroding sponge. Molecular Ecology, 27 (8). pp. 2124-2137. Appendix B: Ramsby, Blake D., Hill, Malcolm S., Thornhill, Daniel J., Steenhuizen, Sieuwkje F., Achlatis, Michelle, Lewis, Allison M., and LaJeunesse, Todd C. (2017) Sibling species of mutualistic Symbiodinium clade G from bioeroding sponges in the western Pacific and western Atlantic oceans. Journal of Phycology, 53 (5). pp. 951-960. |
| Date Deposited: | 05 Aug 2019 02:18 |
| FoR Codes: | 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34% 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050206 Environmental Monitoring @ 33% 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060306 Evolutionary Impacts of Climate Change @ 33% |
| SEO Codes: | 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 35% 96 ENVIRONMENT > 9603 Climate and Climate Change > 960305 Ecosystem Adaptation to Climate Change @ 35% 96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961104 Physical and Chemical Conditions of Water in Marine Environments @ 30% |
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