The study was carried out in reefs fringing the island of Phuket as well as the Phi Phi Islands in Phang Nga Bay, which were affected to various degrees by the 2004 tsunami. Two depths were chosen in the context of higher tsunami impact in the shallow reef area (<4 m). The Thai region features a monsoonal climate, where the wet and stormy SW monsoon season is from May to November, and the dry season with the calm NE monsoon is from December to April. In 2005, the SW monsoon was exceptionally dry in the beginning (except May) and above-average rainfall occurred during the end of the season (September–November) (Southern Meteorological Center, Thailand). The Andaman Sea is comparatively nutrient rich, due to upwelling and land run-off (Janekarn and Hylleberg 1989; Brown et al. 1999). In spite of growing anthropogenic pressures, the coral reefs are still in a fairly good condition (Brown 2007).
Phi Phi islands (Krabi Province)
The Phi Phi Islands consist of 6 limestone islands located 40 km southeast of Phuket and about 30 km west of Krabi in Phang Nga Bay, a large shallow bay not deeper than 30 m (Fig. 1). Well-developed fringing coral reefs are found on the eastern sides of the islands or in areas protected from SW storms. At most sites, coral grow down to about 8–10 m depth, but to about 15–20 m at Ko Phi Phi Lae. The visibility ranges from 5 to 25 m. Massive P. lutea, branching Acropora spp., such as A. formosa, A. grandis, A. subulata and A. austera and the tabular A. hyacinthus and A. subulata are the dominant species.
Although the Phi Phi Islands were declared a marine national park in 1983, unrestricted access by the tourism industry has led to the degradation of the surrounding reefs (Chou et al. 2002). There is no sewage treatment plant, only some collecting ponds and it remains unclear, where the wastewater enters the sea. Some of the coral reefs were strongly hit by the tsunami in December 2004 and suffered severe damage, while other areas remained untouched, providing an ideal setting for testing small-scale differences (<10 km) in coral recruitment as a function of reef damage.
Seven study sites were chosen, three damaged (marked with a ‘D’ superscript in the following) and four undamaged sites (Fig. 1). Highest tsunami damage occurred at Ko Pai and at Lolana Bay (up to 50% damage) and at the northern end of Ko Phi Phi Lae (30–50% damage) (DMCR 2005).
The remaining study sites Ko Yoong, Leam Tong, Hin Phae and Ko Phi Phi Lae SE were only slightly damaged or completely untouched (DMCR 2005).
Coral reefs are well developed on the west coast in protected bays and on some areas along the southern coast. The study site South Patong in the Southwest of Phuket (Fig. 1) is a tourist hotspot with over 30,000 hotel bedrooms. Corals grow to about 7 m depth and the dominant species are P. lutea, D. heliopora, Millepora sp., H. coerulea, Lobophyllia sp., few branching corals such as A. formosa, few table corals and encrusting corals. About 30% of waste waters of the city of Patong are untreated and being discharged into the bay only a few hundred meters away from the investigated reef smothering the corals. The visibility is 5–10 m.
South Patong is the only reef area on Phuket, which was severely impacted by the tsunami (DMCR 2005).
A settlement experiment was carried out 1 year after the tsunami to evaluate coral larvae distribution between damaged and undamaged sites and to quantify space-competing algae and fouling organisms. From acrylic board (2.5 mm thick), 12 cm × 12 cm plates were cut, roughened with a metal brush and fixed in triplicates in a vertical position on 1-m iron rods. Six rods were fixed in an upright position for every location and depth (shallow: reef edge ~3 m and deep: lower reef slope ~7–10 m) resulting in 2 depths × 6 rods × 3 plates = 36 plates per study site. Although acrylic shares the disadvantages of all artificial settlement plate material compared so far in being highly selective for certain taxa (Harriott and Fisk 1987; Dunstan and Johnson 1998; Heyward and Negri 1999; Petersen et al. 2005; Mangubhai et al. 2007), as opposed to natural reef substrate (e.g.Morse and Morse 1996), they are nevertheless useful in detecting differences in coral recruitment in both, space and time.
All the settlement plates were deployed early November 2005. For a given site and depth, the triplicate plates of rod number one were replaced after 1 month, the plates of rod number two after 2 months, etc., resulting in a time series of settlement of varying exposure period (up to 4 months, early November 2005 to early March 2006; Table 1).
After removal, plates were allowed to dry in the sun for 2–3 days for examination with a magnifying glass and stereo microscope. Settlement plates were searched for coral spat and specimens identified to family level according to Babcock et al. (2003). The abundance of fouling organisms (filamentous algae, crustose coralline red algae and sessile fauna such as barnacles, bivalves, bryozoans and spirorbid worms) was recorded on an ordinal scale between 0 (absent) and 5 (replete). Category 5 applied to high densities of filamentous algae featuring long filaments; crustose coralline algae covering most of the plate in thick patches; bryozoans covering more than 40% of the plate; spirorbid worms in excess of 40 individuals per plate; bivalves and barnacles with more than 25 individuals per plate, respectively.
Statistical analyses were conducted with the software SAS. The data collected on one rod (3 plates) were treated as one replicate. A Poisson regression was carried out since the coral spat densities followed a Poisson distribution. The data set of the 4-month period was used to assess the effects of three indicator variables (1) ‘status’ (damaged, undamaged), (2) reef ‘sites’ (Ko Yoong, Leam Tong, Hin Phae, Phi Phi Lae SE, Phi Phi Lae N, Ko Pai, Lolana, Patong) and (3) ‘depth’ (shallow, deep) on the response variable ‘coral spat’ density, whereas ‘sites’ were nested within ‘status’. The effects of filamentous and crustose coralline red algae and of fouling organisms (additional indicator variables) on coral spat densities were analyzed in a separate run of Poisson regression.
To test for differences in coral spat abundances between the two seasons (November–January end of rainy season, January–March dry season) data of the 2-month period were used and Poisson regression was applied. The additional indicator variable was ‘season’, however, the variable ‘site’ was eliminated, because each site was represented by only two data points (one deep rod, one shallow rod). The effect of seasonality on algae and fouling organisms was tested with univariate ANOVA using season and depth as fixed factors; and the sites were treated as replicates. Levene′s test was applied to test homogeneity of residuals.
Succession over the 4-month period was analyzed graphically.
Visual census of coral recruits in situ
In this paper, we distinguish between newly settled corals (“coral spat”) on settlement plates and the young corals (“coral recruits”) in the field.
Visual census of coral recruits was done 1 year (January/February 2006) as well as 3 years (November 2007) after the tsunami in proximity to the settlement plates at all sites, along the reef edge and the lower reef slope in order to quantify the success of coral recruitment. Recruits were counted on natural substrate using a 0.5 × 0.5 m square (0.25 m−2) placed at random (n = 10) in areas dominated by dead coral substrate and summed up (recruits 2.5 m−2). Within the quadrates, 0.5–2.0-cm coral recruits (diameter) were recorded. Visual census with unaided eye allowed undoubted recruit identification only for the genus Pocillopora; all other scleractinians were recorded as “other”. Given linear extension rates of >1 cm year−1 even for slow-growing taxa in the area (Phongsuwan 1991; Scoffin et al. 1992), it was assumed that the bulk of the recruits had settled after the tsunami.
For statistical analyses, Levene’s test was applied to test homogeneity of residuals and univariate ANOVA was used to compare recruit abundances. Status and depth were fixed factors, and the sites were treated as replicates.
Line intercept transects
Line intercept transects (English et al. 1994) were carried out in January/February 2006 (i.e. 1 year after the tsunami) to determine live coral cover and the availability of suitable substrate for coral larvae settlement. With a measuring tape, 20-m transects (n = 3) were laid out along the reef edge and the lower reef slope in proximity to the settlement plates at all sites. The substrate directly under the tape was assigned to the following categories—live coral (LC), dead coral (DC), other organisms (including soft coral, sponges and giant clams), coral rubble (RB), sand (SA), macroalgae and categories recorded to the nearest cm. RB included small pieces of coral skeleton (<10 cm long), while DC was composed of coral skeleton pieces larger than RB, dead patches on coral colonies and entire dead colonies up to few meters in diameter. The respective mean percentage cover of each component was calculated for each location and depth.