Effects of nutrient enhancement on the fecundity of a coral reef macroalga

Diaz-Pulido, Guillermo, and McCook, Laurence J. (2005) Effects of nutrient enhancement on the fecundity of a coral reef macroalga. Journal of Experimental Marine Biology and Ecology, 317 (1). pp. 13-24.

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Abstract

On coral reefs, there is concern that increased nutrient supply (e.g. due to eutrophication) causes increased algal growth and hence increased algal abundance, in turn causing colonisation and invasions of coral populations, resulting in reef degradation, or a coral–algal phase shift. For example, species of Sargassum, a highly seasonal, large, brown seaweed, are suggested to be colonising corals on inshore coral reefs of the Great Barrier Reef, as a result of anthropogenic increases in terrestrial runoff of sediments and nutrients. However, implicit in this argument is the assumption that nutrient-related increases in growth will lead to increased fecundity (and/or propagule success), since without such changes, increased abundance can only occur by vegetative means. Whilst plausible, there is no experimental evidence for this assumption in coral reef algae. We here present an initial study in which experimental increases in nutrient supply apparently did not lead to increased fecundity of Sargassum siliquosum; rather, density and biomass of receptacles were reduced in nutrient-enhanced algae. There was little effect of nutrient treatments on the proportional allocation of biomass to reproductive and vegetative structures: nutrient enhancement led to similar decreases in biomass of reproductive and vegetative tissue. Tissue nutrient levels indicated effective enhancement of nitrogen supply, although phosphorus levels were not different across nutrient treatments. The reduced fecundity with increased nutrients may reflect either a genuine inhibition by higher nutrient levels, as found in previous studies, or accelerated maturation, causing increased tissue losses due to more advanced seasonal senescence. Either way, this exploratory experiment provides initial evidence that nutrient effects on tropical coral reef macroalgae may be complex, and does not support the assumption that increased nutrient supply will result in a numeric increase in populations of Sargassum spp. Our results should not be taken as an unequivocal demonstration that nutrients inhibit fecundity overall, but illustrate the need to distinguish between effects on different life-history processes (e.g. growth and reproduction). For increased growth of pre-existing individuals to contribute to algal invasions and phase shifts, that growth must result in either increased fecundity, or increased propagule success.

Item ID: 1478
Item Type: Article (Research - C1)
ISSN: 0022-0981
Keywords: algae, coral reef phase shifts, fecundity, nutrient effects, reproduction, Sargassum
Date Deposited: 01 Jun 2007
FoR Codes: 06 BIOLOGICAL SCIENCES > 0607 Plant Biology > 060701 Phycology (incl Marine Grasses) @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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