The effects of experimentally elevated nutrient concentrations on growth rate, skeletal architecture and soft tissue morphology of acroporid corals (Scleractinia: Acroporidae)

Daniel John Bucher

The symbiotic relationship between reef-building corals and their zooxanthellae allows for recycling of material between animal tissue and microalgae. The association is therefore regarded as adapted to tropical waters that are low in both plankton biomass and inorganic nutrients. Human activities around coral reef areas can increase concentrations of dissolved nitrogen and phosphorus, often accompanied by other particulate and dissolved pollutants. Large changes in community structure from coral-dominated to macroalgae-dominated communities, have coincided with nutrient pollution. Increasing urban and agricultural development within the coastal catchment of the Great Barrier Reef (GBR) has prompted concerns that nutrients from fertiliser and sewage inputs pose a potential threat to the coral communities within the GBR Marine Park. This study was part of an integrated long-term manipulative experiment ('ENCORE') to examine the response of intact reef systems to elevated nutrients at One Tree Island (southern GBR). During the 27 months of ENCORE, ammonium was added to three patch reefs, phosphate was added to another three reefs, three patch reefs received both nutrients, and three control reefs received no additional nutrients. For the present study, colonies of two species of the reef-building coral genus Acropora were transplanted into the ENCORE patch reefs. To assess changes in the growth rate of Acropora in response to elevated nutrient concentrations, three growth parameters (linear extension, buoyant weight and displacement volume) were measured. Changes in the tissue organisation and skeletal architecture of A. longicyathus and A. aspera were also investigated, to provide clues to the processes by which skeletal growth may be altered. The corals and their zooxanthellae within the ENCORE patch reefs were phosphorus limited in their annual growth. However, seasonal differences in limiting nutrients and growth responses occU1Ted, which highlight the need for long-term experiments such as ENCORE. Phosphate stimulated tissue growth, chlorophyll content and skeletal extension rates. Increased skeletal porosity in phosphate-treated corals resulted when skeletal extension increased more than the total calcification rate. Acropora corals with high zooxanthellae densities showed no evidence of inorganic carbon limitation of calcification. Faster growth of tissue, rather than reduced calcification, led to a more porous, and presumably less strong, skeletal structure in phosphate-treated corals. A reduction in the density of mucous bodies in tissues of phosphate-treated corals is potentially significant to the survival of corals in polluted water, because of the role of mucus in facilitating removal of sediment from the coral surface. Elevated ammonium increased skeletal bulk density and micro-density, reduced the ability of corals to repair lesions (unless phosphate was also elevated), and reduced the chlorophyll content of branch tips, relative to ambient ammonium concentrations. Buoyant weight growth was accelerated by elevated ammonium on an annual basis, but significant reductions occurred in some seasons. Inhibition by ammonium of lesion regeneration has implications for the recovery of polluted reefs following physical damage. When the results from this study are compared with other studies, it is evident that growth of Acropora corals may be less sensitive to elevated inorganic nutrient concentrations than growth of other coral taxa used in nutrient studies to date, providing that the nutrients are not accompanied by sediment and other pollutants. Growth of Acropora is also less sensitive to elevated nutrients than reproduction. The effects of elevated nutrients on many of the growth parameters measured in this study were subtle when compared with the differences between seasons, different species, different genotypes and different patch reef environments.

The effects of experimentally elevated nutrient concentrations on growth rate, skeletal architecture and soft tissue morphology of acroporid corals (Scleractinia: Acroporidae)

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The effects of experimentally elevated nutrient concentrations on growth rate, skeletal architecture and soft tissue morphology of acroporid corals (Scleractinia: Acroporidae)

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