Interactions between macroalgae and scleractinian corals in the context of reef degradation

Jompa, Jamaluddin (2001) Interactions between macroalgae and scleractinian corals in the context of reef degradation. PhD thesis, James Cook University.

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Abstract

Competition between hard corals and macroalgae is important to the overall status of coral reefs, especially during reef degradation which often involves a "phase shift" from coral to algal dominated reefs. Declining levels of herbivory due to over-fishing (the "top-down" model), or increasing supply of nutrients (eutrophication, the "bottom-up" model) have both been suggested to cause increased algal abundance and consequent competitive overgrowth of corals. Despite the importance of coral-algal competition to either of these models, there is little direct evidence demonstrating competition with algae as the cause of coral declines, and in particular, very little evidence unambiguously demonstrating shifts in competitive balance due to either reduced herbivory or increased nutrient supply. This thesis aims to provide more experimental evidence on the processes, mechanisms and outcomes of the interactions between corals and algae for (i) a range of levels, from individual to community; (ii) a range of algal taxa and functional groups; and (iii) ranges of ecological factors such as water quality or nutrients and herbivory.

The first study compared the effects of a turfing, filamentous red alga, Anotrichium tenue, and general mixed, filamentous algal turfs, on massive Porites corals. Comparisons of plots with A. tenue present, A. tenue experimentally removed, and with mixed turfs only present, indicated that A. tenue was able to overgrow and kill healthy coral, whereas mixed algal turfs could not. These contrasting effects demonstrate the potential variability in coral-algal competitive effects and outcomes, even within a functional group.

I further explored this variability using three different algal species; the turfing, filamentous red alga Corallophila huysmansii, the non-turfing, large green filamentous alga Chlorodesmis spp., and the corticated red alga Hypnea pannosa. Experimental tests again indicate considerable variation in the effects on corals, with C. huysmansii causing considerable coral tissue mortality, whereas neither Chlorodesmis nor H. pannosa had major effects on the corals.

To explore how herbivory may affect coral-algal competitive outcomes, I examined the interaction between the creeping foliose brown alga, Lobophora variegata, and the branching coral, Porites cylindrica, under natural and experimentally reduced levels of herbivory. These experiments used exclusion cages to test for effects of herbivores, and removal of algae or coral tissue, at their interaction boundary, to test for competitive inhibition of each competitor by the other. The results showed that overgrowth of the alga caused significant coral tissue mortality, but that the coral also inhibited algal growth. Nonetheless, the algae were markedly superior competitors. Importantly, reduced herbivory resulted in faster algal (net) growth and consequent overgrowth and mortality of coral tissue, demonstrating the critical importance of herbivory to the outcome of the competitive interaction.

This approach was extended to apply simultaneous, factorial tests of the effects of herbivores, nutrients and algal competitor on the coral. Coral tissue mortality was strongly enhanced by the presence of the algal competitor and this effect was significantly higher when herbivores were excluded. Addition of nutrients had no significant effect on corals overall, but had a small effect on algal growth and consequent coral tissue mortality when herbivores were excluded. The factorial design of this experiment not only provides the first direct comparisons of the strength of all 3 main effects, but also explores the interactions between those effects, and hence the processes involved. Importantly, whilst algal abundance and hence competitive impact were affected by herbivore consumption at all levels of nutrient supply, nutrient supply did not influence algal abundance except when consumption by herbivores was reduced.

Inshore reefs of the Great Barrier Reef commonly have extensive beds of large brown macroalgae (seaweeds) such as Sargassum spp., that are widely assumed to have negative impacts on coral populations. To test this assumption, I compared coral performance in large (5 x 5 m) plots in which the macroalgal canopy was removed, with that in control plots with intact algal canopies. In one study, the macroalgae had a negative impact on corals, reducing their recruitment, growth and survival. However, a second study showed a surprising increase in coral bleaching in plots from which the Sargassum canopy had been experimentally removed. Combined bleaching and recovery results suggest that coral bleaching-related mortality was higher in the removal plots. Thus the protection from bleaching provided by the Sargassum canopy may benefit long-term coral populations, as well as competing with them.

Comparison of competitive outcomes involving different algal types and functional groups, and under different herbivory and nutrient levels suggests three general conclusions. Firstly, the outcomes of the interactions are variable depending on coral-algal taxa and/or group involved. Secondly, this variability is readily interpreted in terms of a limited number of mechanisms by which corals and algae can interact: direct overgrowth, shading, abrasion, chemical or allelopathic effects, pre-emption of space,and sloughing of epithelial or mucus layers. The potential importance of these mechanisms can in turn be interpreted in terms of a limited number of properties of the algal taxa involved. These properties, which include thallus size, structure, growth form, reproductive mechanisms, and allelochemical production, are largely, but not entirely, summarised by existing algal functional groups. Thirdly, the specific interactions between competition, herbivory and nutrient enhancement, in which nutrient effects depended on levels of herbivory, but not vice versa, suggest that "top-down" control appeared to over-ride "bottom-up" control of algal abundance and competitive impact. Given the central importance of coral-algal competition to the process of coral reef phase shifts, understanding the variability and complexity in that competition will have important implications for the prediction and consequences of such phase shifts.

Item ID: 27497
Item Type: Thesis (PhD)
Keywords: coral-algal interactions; competitive outcomes; herbivory influences; phase shifts
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Publications arising from this thesis are available from the Related URLs field. The publications are:

McCook, L.J., Jompa, J., and Diaz-Pulido, G. (2001) Competition between corals and algae on coral reefs: a review of evidence and mechanisms. Coral Reefs, 19 (4). pp. 400-417.

Jompa, Jamaluddin, and McCook, Laurence J. (2002) Effects of competition and herbivory on interactions between a hard coral and a brown alga. Journal of Experimental Marine Biology and Ecology, 271 (1). pp. 25-39.

Jompa, Jamaluddin, and McCook, Laurence J. (2002) The effects of nutrients and herbivory on competition between a hard coral (Porites cylindrica) and a brown alga (Lobophora variegata). Limnology and Oceanography, 47 (2). pp. 527-534.

Jompa, Jamaluddin, and McCook, Laurence J. (2003) Contrasting effects of turf algae on corals: massive Porites spp. are unaffected by mixed-species turfs, but killed by the red alga Anotrichium tenue. Marine Ecology Progress Series, 258 . pp. 79-86.

Jompa, Jamaluddin, and McCook, Laurence J. (2003) Coral-algal competition: macroalgae with different properties have different effects on corals. Marine Ecology Progress Series, 258 . pp. 87-95.

Date Deposited: 25 Jun 2013 05:03
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 60%
05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050199 Ecological Applications not elsewhere classified @ 20%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 20%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 51%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 49%
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