 |
Indicators of
|
 |
|
|
Indicators of
|
|
These photographs may not be copied or down loaded without written permission. If your organisation would like to obtain these images in high quality slide or digital format for publishing, educational, commercial or other purposes then please contact us per e-mail.
Coral Bleaching
| The term coral bleaching refers to a serious stress condition in coral reefs where coral colonies appear white or bleached due to the expulsion of photosynthetic algae or zooxanthellae from their tissue.
The incidence of coral bleaching has been reported throughout the world and is linked to certain environmental conditions that cause stress in corals; predominantly sudden increases in Sea Surface Temperature (SST). Other causal factors include El Nino Southern Oscillations, low tides, and changes in salinity and sedimentation. In severe international bleaching events vast areas of reef may be affected. The corals may recover and regain their zooxanthellae and colour after weeks of the initial expulsion, or die and become covered with algae within days. Mass bleaching events are increasing in frequency with over six global events recorded since 1979. These events were not known to occur previously. This initial evidence coupled with long-term climate modelling studies suggest reefs are affected by global warming trends and could be seriously damaged by yearly bleaching events in 30-50 years time. |
 |
Coral breakages occur naturally on reefs as a result of wave action, strong current surges or marine life activity such as nest building fish species and boring invesrtebrates. It is estimated that up to 80% of the reef's annual growth may be lost due to physical and biological erosion. Some branching forms of coral (especially the Acropora staghorns) actually use breakages in their advantage as a form of asexual reproduction called fragmentation. Unnatural causes of coral breakages would be accidental fin contact with fragile corals, anchor damage, fishing line entanglement and destructive fishing practices such as reef bombing and cyanide fishing for the live fish trade.
Coral Malformations
One of the few long-term studies on the ecological significance of coral malformations was carried out by APEX Environmental in 1995-99. Coral malformations (warts and tumours) are irregular shaped skeletal abnormalities. Malformations are roughly round or oval in shape and normally white in colour and may grow over 5mm per month. This growth is thought to be non-productive and uncontrolled. Malformations are found predominantly within the reef lagoon at sites of high coral cover, and are especially abundant on the branching and plate corals of the Acropora family.
The malformations appear whitish as they lack photosynthetic algae (zooxanthellae) and affected polyps continuously extend their tentacles to feed. Coral malformations also have fewer protective mucous cells and a porous skeleton. These conditions expose the affected coral colony to increased predation by corallivores and bio-erosion from algae and boring organisms. These organisms can invade the porous, low quality coral skeleton and cause local coral mortality and may reduce coral colony fitness and fecundity.
The causes of coral malformation are unknown. Possible initiating factors range from biological pathogens transmitted by corralivore fishes, genetic mutations and external environmental conditions such as excessive UV radiation exposure. For a more detailed account on the ecological significance of coral malformations please contact our office for an abstract of the Acropora Coral Malformations Technical Report.
Filamentous or "turf algae" prefers habitats with good light conditions and relatively high nutrients. It can be found predominantly on the reef flat and in lagoons during the warmer months of the year. This may act as an efficient nutrient "sink" as turf algae converts nutrients into biomass. However, during high nutrient periods algal growth can outstrip algae consumption by herbivores and reef areas will become overgrown. This can result in localised eutrophication and permanent changes in the reef community.
Algal Blooms - Trichodesmium spp.| Phytoplankton species are not very abundant in the nutrient poor reef waters, with the exception of the blue green algae Trichodesmium (also called Oscillatoria - seen here with 50x magnification). Trichodesmium grows from small filaments that develop deep within the water column to aggregate and float to the surface as they age.
High concentrations of Trichodesmium may form large brownish slicks many kilometres long which float on the surface. Divers and snorkelers will recognise the milky surface layer up to 2-3m deep. Trichodesmium slicks may also develop during winter if calm and windy periods alternate. Increased phytoplankton growth decreases the availability of light for the symbiotic zooxanthellae of the corals and also increases the sedimentation load on reefs. This not only stresses corals directly but also favours various competitors of corals such as algae, boring sponges and filter feeders. Severe blooms can also kill marine life by depleting the oxygen contact of the water. |
 |
|
|
|
|
|
|
|
For more information please contact our office via this e-mail address.
Back to Home Page
No part of these Web pages may be reproduced without written permission
but you are welcome to create a link to
http://www.apex-environmental.com/
