Studies of nutrient variability and the consequences for benthic communities on the Coral Coast fringing reefs, Fiji

Author

Ulukalesi B. Tamata, University of WollongongFollow

Year

2007

Degree Name

Doctor of Philisophy

Department

School of Earth and Environmental Sciences - Faculty of Science

Recommended Citation

Tamata, Ulukalesi B, Studies of nutrient variability and the consequences for benthic communities on the Coral Coast fringing reefs, Fiji, PhD thesis, School of Earth and Environmental Sciences, University of Wollongong, 2007. http://ro.uow.edu.au/theses/83

Abstract

The Coral Coast region, in south-western Viti Levu, Fiji, is the ‘hub’ of tourism development in Fiji, and is one of the fastest developing areas in Fiji. Since the 1980s, development along the Coral Coast has occurred at a rate never previously seen in this area. Large resorts and smaller backpacker facilities emerged from the once coconut-tree lined coastal stretch, from Serua, to Natadola. Village populations have also increased. Anecdotal information and the few, sporadic studies conducted in this area have indicated that the water quality and the biological communities on the fringing reefs have deteriorated dramatically since the 1980s. Live corals which used to feature prominently on the fringing reefs (hence the name ‘Coral Coast), have been replaced by weedy macroalgae, the most common of which is the brown algae Sargassum. The Coral Coast fringing reefs were apparently undergoing phase shifts (like other reefs around the world), and anthropogenic factors have been blamed. The need for scientific information on the effects of anthropogenic activities on the Coral Coast water quality and fringing reef communities prompted this study.

The study had several objectives. Two major components addressed the need to establish baseline scientific information on the variability of dissolved nutrients in the water column, and the status of biological communities on the reef flats of the fringing reefs, for control sites (away from human impacts), and impacted sites (close to villages and resorts). Nutrients from anthropogenic sources on land have been identified as one the major bottom-up controlling factors for phase shifts on coral reefs. Control, village and resort sites were monitored for nutrient concentrations from 2003 – 2006, and sampling times covered seasonal as well as weather effects, particularly the effects of storm-associated rainfall. Benthic surveys were also carried out in control and impacted sites several times over the period 2004 – 2006. To complement the results from the longer term monitoring of water quality and reef benthic communities, a number of short-term, targeted experiments were conducted in the laboratory and in the field, to examine nutrient uptake by algae, herbivory impacts and nutrient sources.

A combination of the Line Transect and Quadrat Point Intercept methods were used for the assessment of the abundances and distribution of the main species on the fringing reefs, especially Sargassum sp. Nutrient enrichment experiments were conducted in the laboratory and also in the field to assess uptake of nutrients by Sargassum sp. Herbivore-exclusion caging experiments were conducted in the field to assess influence of herbivory in a ‘tabu’ or marine protected area (MPA) as well, as in non-‘tabu’ or open-fishing sites. Samples of Sargassum sp. and a few other dominant macroalgae species from the study sites were analysed for tissue nitrogen and phosphorus contents and for &#;15N content, to aid nutrient source identification.

Nutrient concentrations were highly variable, showed little association with season, but were strongly linked to rainfall. The results indicated the clear influence of pulse (storm runoff related) events on the nutrient concentrations in the water column. Control sites generally recorded lower nutrient concentrations than impacted sites, confirming the anthropogenic effects on water column nutrients. The biological communities on the fringing reefs reflected the status of nutrients in the water column, i.e., reefs close to human impacts recorded higher abundances of macroalgae, especially Sargassum sp., and lower abundances of live corals. The nutrient enrichment experiments showed the ability of Sargassum sp. to take up nutrients very quickly from the water column, but the rhizoids showed greater responses than the leafy shoots used in the experiments. The ‘low growth’ season for Sargassum sp. during the Cool season (May to October) may be the explanation for the differential responses between the rhizoids, and the leafy parts of the Sargassum plants used in the experiment. Caging experiments showed the significance of herbivory in the control of Sargassum sp., and the ‘tabu’ sites appeared to show a greater intensity of herbivory (lower suvivorship for uncaged shoots). Both tissue nutrient and &#;15N contents in macroalgae samples matched the findings from the water column nutrient studies, i.e., the human-impacted sites exhibited nutrient enrichment in the water column and in the macroalgae on the reef flats. Sargassum samples from impacted sites had higher tissue % N and higher &#;15N contents compared to the control sites confirming that human activities were enriching the coastal waters with nutrients.

On the basis of the results from this research, recommendations are proposed for better management of nutrient sources on land for the protection of the water quality, and therefore promote healthier coral reef systems. The significance of protecting herbivorous species is also important, and the setting aside of ‘tabu’ sites is encouraged. Areas of further research are also identified, for better understanding of the interrelationships among all the factors involved in the phase shift occurring on the fringing reefs along the Coral Coast of south-western Viti Levu, Fiji.