Size-structures of populations of the mushroom coral Fungia fungites: the role of disturbance.

Brief description

Changes in the size-structure of populations of the mushroom coral Fungia fungites were quantified at two reefs in the Dampier Archipelago during four annual surveys from April 1998 to April 2001. Exposure to disturbance was predicted to affect the size and frequency of life history stages of polyps at each reef and their variability through time.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: -Study sites-
Study sites are located approximately 5km apart in the dampier Archipelago on the north-west coast of Western Australia (see large thumbnail) and their coral communities are located in similar depths of water (3-5 m) on a narrow reef lat with a slight slope and a small drop off to sandy substrata. Apart from these similarities, they differ in a number of physical attributes that reflect their contrasting exposure to disturbances.

The Mainland Reef (ML) is adjacent to the mainland, has a low profile, and consists primarily of skeletons of fungiid and massive corals with mush of its free substrata covered by fine sediment. The Mainland Reef is exposed to the predominately westerly and north-westerly winds and waves produced by summer storms, in addition to swell generated by cyclones that travel through Mermaid Sound. The modified effective fetch for the Mainland Reef is 20km and the reef is classified as semi-exposed. Because wind speeds and directions are correlated to wave heights and sedimentation, levels of acute and chronic sedimentation are very high at the Mainland Reef.

In contrast, the East Lewis Reef (EL) has the opposite aspect and is located in a bay on the leeward side of East Lewis Island. Much of its coral growth occurs on top of thick (1m) stands of dead or partially dead Pavona decussata that elevate the corals approximately 1m above a sandy bottom (5-7 m depth). The East Lewis Reef is sheltered from the winds and waves resulting from storms and cyclones in summer and is buffered from the weaker winds that blow off the mainland in winter. There is little sediment accumulation at this reef because of its rates of sedimentation are comparatively low.

-Sampling regime-
In a random sample of over 50 free-living polyps from around the study area at each reef (see large thumbnail), ranging in diameter from 3cm to 21cm (mean = 10cm), all were identified taxonomically as F. fungites. Additionally, electrophoretic analysis of a random sample of 120 polyps from the same area at each reef, which excluded sexual recruits and asexual buds, indicated they were all F. fungites.

Along the narrow reef flat at each study site a permanent transect (150m) was established parallel to the shore in 3 - 5m depth, where F. fungites were distributed. A single large transect was used, rather than smaller replicate transects, because it sampled the majority of each population along a homogenous area within which there were no difference in the size-structure of polyps. A quadrat (50x50cm) was laid every 2m on alternate sides of the permanent transect (n = 75) at each reef. There was no evidence of migration by polyps off the reef flat at either study site, with most free-living polyps that had been tagged (>80%, n=67) moving < 30cm during this study, even following cyclones. Therefore, it can be assumed that the same population was censused through time. Within each quadrat, the maximum diameter and life history stage of F. fungites polyps > 0.3cm was recorded.

For parent polyps, the maximum diameter of the skeleton was recorded. Polyps were divided among the following life history stages: sexual recruits that were attached to the substrata via a stalk; free-living polyps that were not attached; parent polyps that had no live tissue other than one or more asexual buds; and asexual recruits that were attached to a parent polyp. Injury to polyps was also recorded, but it was not possible to accurately quantify the number of dead polyps because much of the reef matrix consisted of skeletons of polyps that had died over many years.

Notes

Credit
K. Smith: Australian Bureau of Meteorology (BOM)

Credit
A. Grieco: Department of Environmental Protection

Credit
Australian Institute of Marine Science (AIMS)

Credit
Hamersley Iron

Credit
Sigma Xi

Credit
The University of Western Australia (UWA)