Thesis
The effects of elevated nitrogen and phosphorus on the reproduction of three species of acroporid reef corals
Southern Cross University, School of Resource Science and Management
Doctor of Philosophy (PhD), Southern Cross University
1997
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Abstract
Colonies of two scleractinian reef coral species, Acropora longicyathus and Acropora aspera were transplanted into patch reefs at One Tree Reef, Great Barrier Reef, Australia. These corals and colonies of A. aspera and Acropora bushyensis which were naturally present in the patch reefs, were exposed to four treatments over two years: controls with normal seawater, elevated levels of nitrogen only, phosphorus only, or nitrogen plus phosphorus. These corals were sampled and used for experiments to determine whether reproductive activity was affected by nutrient enrichment. Gametogenic cycles, fecundity, spawning behaviour, fertilization and settlement of corals were investigated. Colonies of Goniastrea aspera were also used for fertilization trials.
Acropora longicyathus had a single annual gametogenic cycle. Corals to elevated nitrogen produced significantly smaller and fewer eggs and contained less testes material compared with those which were not exposed to nitrogen. Exposure to elevated phosphorus resulted in corals producing more but smaller eggs, and more testes material. Timing and spawning behaviour were not obviously affected by elevated nutrients. Just prior to spawning the eggs of all the transplanted A. longicyathus exposed to phosphorus alone were very bright red and more highly pigmented than in the other treatments.
Fertilization rates were significantly reduced when gametes from A. longicyathus were exposed to nitrogen only, or phosphorus only, or nitrogen plus phosphorus. Of the eggs which did fertilize, there was a significantly greater percentage of irregular embryos and embryos which stopped their development at the first cleavage stage in nutrient treatments compared with controls. Fertilization trials using gametes from Goniastrea aspera resulted in higher incidence of formation of irregular embryos after exposure to all nutrient treatments compared with controls. Fertilization rates for this species were only reduced in the highest dose (50μm) of nitrogen plus phosphorus.
When corals were exposed to elevated nitrogen only during their gametogenic cycles and then taken from elevated nutrients for spawning, larval rearing and settlement, the larvae they produced had higher settlement rates than those from corals exposed to the other nutrient treatments or the controls. However, survivorship levels were low in these spat. Exposing larvae to elevated nutrients for the first five days of their development did not significantly affect their settlement rates. When larvae were exposed to nutrients for the settlement period in 1993, from 5 10 days after spawning, settlement was significantly reduced by the presence of both elevated nitrogen and phosphorus. When these tiles were rescored for the next year to investigate the survival of the experimental spat and new recruitment, settlement was reduced on those tiles exposed to elevated nitrogen compared to those without nitrogen and on those exposed to elevated phosphorus compared to those without phosphorus. At the end of 1994 settlement on tiles was mapped and these tiles were remapped until 1996. Larvae from spawning species had settled abundantly by the census dates in February 1995 and January 1996 but on the other sampling dates of May, August and November 1995, settlement was dominated by the spat of brooding species. Settlement was reduced in the nitrogen only treatments compared to the other treatments. Settlement of spat from brooding corals was enhanced in those tiles exposed to phosphorus compared to those without phosphorus, and there was no significant difference in the settlement of spat from spawning species for these phosphorus treatments.
The results of this study show that elevated nitrogen and phosphorus have some severe effects on reproductive activity in these species of scleractinian corals. Successful coral reproduction is essential for the continued health and growth of coral reefs so these results have important management implications for reef areas. Further research needs to be conducted to determine whether other species of corals are similarly affected by nutrient exposure.
Details
- Title
- The effects of elevated nitrogen and phosphorus on the reproduction of three species of acroporid reef corals
- Creators
- Selina Ward
- Contributors
- Peter L Harrison (Supervisor) - Southern Cross University
- Awarding Institution
- Southern Cross University; Doctor of Philosophy (PhD)
- Theses
- Doctor of Philosophy (PhD), Southern Cross University
- Publisher
- Southern Cross University, School of Resource Science and Management
- Number of pages
- [viii], 269
- Identifiers
- 991012958498302368
- Copyright
- © Selina Ward 1997
- Academic Unit
- Faculty of Science and Engineering
- Resource Type
- Thesis