Algal recruitment and reproduction in the context of reef degradation: interactions with corals, substrates, herbivores and nutrients

Diaz-Pulido, Guillermo (2002) Algal recruitment and reproduction in the context of reef degradation: interactions with corals, substrates, herbivores and nutrients. PhD thesis, James Cook University.

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Recruitment and reproduction are important processes in the dynamics of ecological populations and communities. Recruitment dynamics have been shown to influence the structure of tropical fish and coral, and of temperate algal populations, but there is a significant lack of information about the recruitment of tropical algae. Coral reef degradation commonly involves overgrowth and invasion of abundant corals by abundant algae. Reduction in herbivore populations, increases in nutrient supply and coral disturbances are considered as causes of algal invasions and coral-algal phase shifts, but conceptually, for algal invasions to occur they generally require dispersal, recruitment and colonisation processes. Despite the importance of algal recruitment to such invasions, there is little information about the recruitment dynamics and reproduction of coral reef algae. The aim of this thesis was to investigate the dynamics of, and factors that affect, the recruitment and reproduction of coral reef algae and their interactions with corals, particularly in the context of reef degradation. I addressed this aim by exploring i. The variability in algal recruitment in response to substrate availability, coral disturbances, reductions in herbivory, and addition of nutrients; ii. Variability in algal fecundity in response to nutrient enhancement. I compared the outcomes in terms of both numeric responses and growth responses, and included comparisons between two taxa of brown algae with contrasting morphologies (Sargassum spp. and Lobophora variegata). The study took place in a range of inshore and mid-shelf reefs on the Great Barrier Reef (GBR), Australia.

The first study tested the ability of Sargassum spp. and L. variegata to settle and grow on healthy coral tissue, and explored the effects of interactions with prior occupants, and of abiotic substrate properties. Healthy corals prevented attachment or survival of recruits of these macroalgae. This is a significant point, since it suggests that the replacement of corals by algae may often require prior stress or death of the coral tissue. Pre-conditioning of ceramic settlement plates at different sites had some effects on growth and survival, but these were relatively minor, whereas there was considerable variation within sites. Some of this variation appeared related to the amount of turf algae or crustose calcareous algae on the plates. Recruitment was generally but not always higher on plates with rougher texture. These results indicate considerable potential for variability in outcomes of algal colonisation, with implications for the dynamics of algal invasions. Furthermore, growth and survival responded differently to substrate manipulations, emphasising the importance of using a variable appropriate to the process being questioned. The results do not support suggestions that planktonic algal propagules can directly settle on and colonise healthy coral tissue.

The massive bleaching of corals that occurred on the GBR, in early 1998 provided an opportunity to explore the roles of coral disturbances in the recruitment of algae and shifts in dominance from corals to algae. In this study I described the composition and time course of algal recruitment on bleached corals, and the possible roles of that recruitment on the fate of the corals. Massive Porites corals were selected with different degrees of bleaching, and the cover of live coral tissue, and relative abundance and composition of algal recruitment, were followed in small plots over 2.5 years. The bleaching disturbance caused a major shift in abundance of corals and algae. All dead corals were colonised by a diverse assemblage of epilithic and endolithic algae, the nature and composition of which was variable and related to the stage of the succession, the severity of bleaching and reef location. Quantitative data on species composition of colonising algae are given, apparently the first such data. The epilithic algal assemblage was initially dominated by diatoms and blue-green algae, but rapidly shifted to an assemblage dominated by upright and branched filamentous algae and, on one reef, fleshy macroalgae. Algal colonisation was not the initial cause of coral tissue mortality, although it may have contributed to the failure of corals to recover after bleaching. The results thus emphasise the role of coral disturbances and substratum availability in controlling the abundance of coral reef benthic algae, in contrast to "bottom-up" and "top-down" views that assume changes in algal abundance are the major cause of changes in coral abundance. The considerable variability in outcome of bleaching damage and algal colonisation demonstrates the potential for major and variable effects on the recovery of coral populations, with implications for future reef status.

To explore the relative and interactive effects of bottom-up regulation (nutrient enhancement) and top-down control (herbivory reductions) as causes of algal invasions, I simultaneously manipulated nutrient supply to and herbivory on recruits of Sargassum fissifolium and L. variegata on a coral reef in the GBR. Herbivory strongly reduced both density and growth of recruits for both species, whereas nutrient supply had only minor effects on growth of L. variegata recruits and no detectable effects on S. fissifolium recruits. Notwithstanding the dominance of herbivory over nutrient treatments, herbivore effects were not uniform, but varied between taxa, response variables (growth and density), and were apparently stronger for nutrient-enriched plants. These results emphasise the importance of protecting herbivores for coral reef management, complement previous work with results for recruitment of tropical marine algae, and demonstrate that interpretations of the relative importance of bottom-up and top-down processes may depend on the species, circumstances, and life-history processes (survival and growth) under consideration.

It is often assumed that enhancement of nutrient levels and supply will lead to increases in area colonised by algae. In the case of algae with distinct individuals (e.g. Sargassum spp.), such increases must involve increases in propagule numbers, presumably by facilitating algal fecundity (or reproductive output). However, there is no experimental evidence, for that assumption in tropical coral reef algae. In the final study I experimentally explored the assumption that nutrient enhancement will facilitate algal fecundity. Although by no mean a comprehensive test, the results suggest that the additions of nutrients may not necessarily lead to increased algal fecundity, since the number and biomass of reproductive structures (receptacles) decreased with nutrient additions. This indicates that increased nutrient inputs alone cannot be assumed to promote invasions of Sargassum spp. without further experimental evidence.

Three main conclusions arise from this thesis. Firstly, there is considerable variability in the outcomes of the algal recruitment process and in the effects of factors on recruitment. Factors that accounted for variability in the recruitment dynamics included substrate availability, coral disturbances, location and successional stage, herbivory, taxa, and the life-history processes measured (growth and survival), and to a lesser extent nutrient enhancement. This variability in recruitment is very likely to have major consequences for changes in adult algal distributions and algal invasions on coral reefs. Secondly, the distinctions between growth and numeric responses are important to the understanding of algal dynamics during phase shifts. Growth is a property of an individual whereas density is a measure of the population. As algal invasions will generally require an increase in number of individuals, density seems to be a more relevant variable in the context of those invasions. Lastly, the replacement of corals by algae through the mechanism of direct settlement,of propagules from the plankton onto live coral is unlikely, but that mechanism appears important following coral disturbances. Given the fundamental importance of recruitment in the colonisation and invasion of algae on coral reefs, exploring the dynamics and variability of, and factors that affect, the recruitment and reproduction of algae is important to understanding the dynamics of coral-algal phase shifts and reef degradation.

Item ID: 27397
Item Type: Thesis (PhD)
Keywords: algal recruitment; algal reproduction; recruitment variability; algal colonisation variability; algal colonisation characteristics; algal dymanics
Date Deposited: 25 Jun 2013 02:42
FoR Codes: 06 BIOLOGICAL SCIENCES > 0607 Plant Biology > 060799 Plant Biology not elsewhere classified @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060399 Evolutionary Biology not elsewhere classified @ 33%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 51%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 49%
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