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Coral recruitment onto an experimental pulverised fuel ash–concrete artificial reef
Marine Pollution Bulletin 46 (2003) 642–653 www.elsevier.com/locate/marpolbul
Coral recruitment onto an experimental pulverised fuel ash–concrete artificial reef Katherine K.Y. Lam
*
Swire Institute of Marine Science and Department of Ecology and Biodiversity, The University of Hong Kong, Cape DÕAguilar Road, Shek O, Hong Kong
Abstract An experimental artificial reef was deployed in December 1993 at Hoi Ha Wan Marine Park, Hong Kong. This is the first study documenting natural scleractinian coral recruitment onto a stabilised pulverised fuel ash (PFA)–concrete artificial reef. Visible recruits were first recorded 9–10 months after the placement of reef blocks, i.e., in the autumn of 1994. Two scleractinians, Oulastrea crispata and Culicia japonica, were recruited. The recruit density of the former was much greater than the latter. The spatial recruitment pattern of the corals was observed to be affected by the orientation of the attaching surface. O. crispata settled predominantly on the undersides of the reef blocks. There was an edge effect on O. crispata recruitment. C. japonica, however, had a preference for exposed surfaces. O. crispata did not show a preference for block composition whereas C. japonica favoured blocks with high (75% by volume) PFA levels. This shows that PFA–concrete is a potential substratum for artificial reef construction, especially when such reefs aim at rehabilitating corals. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Coral recruitment; Pulverised fuel ash; Artificial reef; Oulastrea crispata
1. Introduction The use of coal waste concrete blocks as artificial reef units has a history of almost two decades (Strobel et al., 1988; Waldichuk, 1988). Whether or not such material is a suitable substratum for coral and, hence, reef restoration, through natural coral recruitment or manual transplantation, is still unstudied. A possible reason for this is that most of the pulverised fuel ash (PFA)–concrete artificial reef projects so far have taken place in temperate waters, where coral reefs are not naturally found, and the major concern of such ash reef installations has focused on fishing (Lam, 1998). The degradation of coral communities in Tolo Channel and Hoi Ha Wan, Hong Kong, has been reported upon by numerous researchers over the past two decades (Scott and Cope, 1982, 1990; Morton, 1992, 1994, 1995; Cope, 1984; Zou et al., 1992; Collinson, 1997). The causes for this include man-made disturbances, such as increases in nitrates, phosphates and *
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sedimentation from the input of raw sewage (Morton, 1989; Wu, 1988) from both agricultural and domestic sources, as well as natural stresses such as the frequency of typhoons, and temperature and salinity changes (Collinson, 1997). Many other man-induced disturbances threaten local corals, such as dynamite fishing, coral collection by recreational and commercial divers, and marine sand dredging for major reclamation projects such as the new airport at Chek Lap Kok and Western Kowloon (Hodgson, 1994). On the other hand, coral communities in Hong Kong are expanding onto either artificial substrata or natural boulders. For example, a new coral community is developing on the submerged concrete dollos of the east High Island Reservoir dam which has been in existence for >20 years (Scott, 1984; personal observations) and small (<5 cm) corals were found settling on natural barren boulders at Ping Chau (personal observations). The artificial reef site of this study used to have a coral community. The removal of top soil from the Tai Leng Tung Peninsula, for land reclamation, caused the smothering and destruction of the coral community on the adjacent shore between 1981 and 1989 (Zou et al., 1992).
0025-326X/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0025-326X(02)00482-4
K.K.Y. Lam / Marine Pollution Bulletin 46 (2003) 642–653
Coral recruitment and growth on concrete constructions such as harbour moles, concrete pillars and concrete outflow pipes, or artificial reef structures, in tropical coastal waters has been reported (Schuhmacher, 1974, 1977, 1988; Chou and Lim, 1986; Clark and Edwards, 1994). Several studies have also reported upon the successful colonization of corals onto tyre reefs (Gomez et al., 1982; Fitzhardinge and Bailey-Brock, 1989), although there has been opposition to the idea since Fitzhardinge and Bailey-Brock (1989) pointed out a significantly smaller settlement of coral recruits onto scrap tyres than on concrete and metal surfaces. This study, therefore, was to test the feasibility of PFA–concrete blocks for coral reef restoration by natural coral larvae settlement. In the long term, this may yield coral colonies releasing larvae which further colonize the same reef. The first objective of this study was, therefore, to examine the spatial pattern of coral recruitment, i.e., to determine the choice of block type, orientation and edge effects. Recruitment patterns between species were compared. Second, the temporal pattern, in terms of recruitment season, was also identified. Third, the use of artificial reefs, especially those made of concrete or PFA–concrete, for reef restoration, is reviewed and discussed.
2. Materials and methods 2.1. Study site The artificial reef was deployed on an area of sea bed at )7 m C.D. within Hoi Ha Wan (Fig. 1). The sea bed of the study area had been smothered by the runoff of silt from surrounding hills which were used as a land borrow area between 1981 and 1989. Despite reforestation of the hills, the sea bed of the study area was still covered by at least 0.5 m thick of loose mud on top of a hard substratum, where there had been a coral community (Zou et al., 1992). The artificial reef comprised 1000 cuboidal structural blocks 500 mm in length, with a hollow central cylinder of 0.2 m diameter, to increase their surface areas as well as provide shelter for resident fish and invertebrates. The blocks were deployed in a random fashion from a barge within the gazetted site from 1–3 December 1993. The shape of the artificial reef was somewhat like a pyramid with a base area of approximately 10 m 10 m and a height of 2.5 m (Fig. 2). Additionally, 176 test blocks of 0:15 0:15 0:15 m dimensions made from four cement/PFA (pulverised fuel ash) mixes were placed to determine seasonal patterns of coral recruitment. All blocks had smooth surfaces. The test blocks were placed on top of the artificial reef. The structural and test blocks were made of different concrete to PFA compositions, as shown in Table 1. The formulation of
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these blocks was based on a cement replacement by PFA concept (Leung et al., 1997). 2.2. Sampling of test blocks During the deployment of the artificial reef in December 1993, an initial set of test blocks, including 90 each of the four PFA–concrete mixes, were placed on top of the structural reef blocks. These blocks (4 replicates for each of the 4 block types, i.e., 16 in total) were retrieved after 3, 6, 9, 12, 14, 18, 21 and 24 months post artificial reef deployment. Cumulative coral recruitment onto the artificial reef was thus studied over a period of two years. A second set of test blocks were retrieved and replaced trimonthly at the same time for the same period. No coral recruitment was recorded on these blocks. Only four faces of each test block were examined and each face was divided into categories as follows: (1) sheltered surface, i.e., the side of the test block most in contact with the main reef modules; (2) the vertical, or inclined, surface exposed to the open sea; (3) the top surface, which was more or less horizontal to the water surface and (4) the bottom, i.e., the base of the test block (Leung et al., 1997). The gap between the bottom surface of the test blocks and the reef modules ranged from 10 to 30 mm. Since the four side faces of each test block could be either exposed or sheltered, as they were resting on the main reef, various blocks had different numbers of exposed and sheltered surfaces. For standardisation and statistical analyses, therefore, only four instead of all the surfaces were examined. The total surface area studied in each season was, therefore, 0:15 0:15 m2 (surface area of each block) multiplied by 16 (number of test blocks) and 4 (number of each test block surfaces studied), making a total of 1.44 m2 . The retrieved test blocks were placed in a Perspex tank filled with seawater and scanned under the dissecting microscope. Larger pieces of algae and other macroinvertebrates were removed with forceps to facilitate observations, whenever necessary. Each test block surface was mapped using a transparent plastic sheet to determine the position and size of any recruits on the surfaces and their distances from the nearest edge of the block. Coral recruits were also assessed for size in terms of geometric diameter in mm (which equals the square root of the longest diagonal multiplied by the shortest one), and the number of polyps present. Each coral recruit was categorised under block type (types A, B, C and D as in Table 1) and face (sheltered, exposed, top and bottom) of settlement. Living recruits were identified by the presence of visible, intact and responsive, polyps whereas dead recruits were polyps without tissue, broken calcite or the presence of algal fouling. Recruitment here is referred to settlement followed by deposition of a recognisable skeleton and was alive at the time of retrieval of the plates.
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Fig. 1. A map of Hoi Ha Wan, Hong Kong, showing the location of the artificial reef.
Fig. 2. An impression of Hong KongÕs first experimental artificial reef at Hoi Ha Wan, with test blocks for coral recruitment study resting on it.
K.K.Y. Lam / Marine Pollution Bulletin 46 (2003) 642–653 Table 1 The composition in terms of dry weight of the different types of test blocks deployed on the artificial reef Block type
Test block A
B
C
D
PFA/cement by volume PFA (%) Portland cement (%) 20 mm aggregate (%) 10 mm aggregate (%) Fine stone (%) Water (%)
0:1 0 13.1 41.8 11.3 33.8 6.9
1:3 3.2 9.7 41.9 11.3 33.9 6.4
1:1 6.6 6.6 41.6 11.2 33.8 9.0
3:1 9.7 3.2 41.8 11.3 33.9 7.6
Structural block composition was the same as type C.
2.3. Statistical analyses To assess differences between the number of coral recruits between test blocks with various submergence periods, block type and orientation of settlement, oneway ANOVA and Student–Newman–Keuls (SNK) test, at the 0.05 level of significance, were used (Sokal and Rohlf, 1995). Between-species differences were analysed by t-test from pooled data of the total number of living coral recruits onto all the test blocks retrieved throughout the two-year study period. To determine edge effects on coral recruitment onto block surfaces with different orientations, the nearest distance of each recruit to a block edge was measured. The frequency of the number of coral recruits with respect to every 5 mm (up to 75 mm, i.e., the centre of the 150 mm 150 mm surface) from the block edge was also obtained. Pearson correlation coefficients between the frequency of the coral recruits versus the distance of the living coral recruits from the block edge and coral recruit and the distance of the coral recruits from the block edge, respectively, were calculated for the four block surface orientations, so as to test the null hy-
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pothesis that there was no correlation between the two. On surface(s) where edge effect occurred, regression models were tested to describe the relationship between living coral recruits and distance from the block edge.
3. Results 3.1. Living coral recruitment Only two species recruited onto the test blocks, Oulastrea crispata (n ¼ 387, range of density ¼ 0–64.58 m2 and 92.81% of the total number of coral recruits) was much more abundant (paired t-test, t ¼ 8:37, df ¼ 509, p < 0:01) (Fig. 3) than Culicia japonica (n ¼ 30, range of density ¼ 0–6.25 m2 , 7.19% of the total number of coral recruits). 3.2. Oulastrea crispata recruits O. crispata recruits were first identified on the test blocks retrieved in September 1994, nine months after artificial reef installation. A total of 387 recruits were recorded. Fig. 4 shows the population dynamics of the recruits from nine to 24 months post-deployment of test blocks. Results of a one-way general factorial ANOVA (Table 2) showed that the numbers of O. crispata recruits were significantly different between the period of block immersion and settlement orientation (p-values < 0:05) but not between block type (p ¼ 0:45). The SNK test grouped the subsets in terms of mean number of coral recruits according to the period in water in ascending order as (1) 3, 6, and 9 months and (2) 24, 12, 14, 18, and 21 months. The subsets for surface orientation were (1) sheltered and top; (2) exposed and (3) bottom.
Fig. 3. Density of coral recruit (recruit m2 ) on the test blocks retrieved from the experimental artificial reef, at Hoi Ha Wan. Density was calculated as pooled number of recruit over total studied surface, i.e. (15 15 64) cm2 .
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two variables for coral recruits on the bottom (df ¼ 282, r ¼ 0:0153, p > 0:05), exposed (df ¼ 77, r ¼ 0:0737, p > 0:05) and top surfaces (df ¼ 21, r ¼ 0:161, p > 0:05). Fig. 5 showed the data for the distances of the coral recruits from the block edge on the top, exposed and bottom surfaces. A Pearson correlation coefficient showed that the frequency of coral recruits and their distance from the block edge was not significantly correlated with the exposed (df ¼ 77, r ¼ 0:081, p > 0:05) and sheltered surfaces (df ¼ 21, r ¼ 0:15, p > 0:05). Distance from the block edge was, however, highly negatively correlated with both the frequency of the coral recruits on the bottom surface (df ¼ 284, r ¼ 0:65, p < 0:01) and the log transformed frequency (df ¼ 74, r ¼ 0:8, p < 0:01). Coral recruitment onto the sheltered surfaces was excluded from the analysis of edge effect because only one was recorded, at a distance of 1 mm. Analysis of the linear-regression residuals showed that the relationship between the two variables, log transformed frequency of coral recruits and distance from the bottom surface of the block edge, was nonlinear (Zar, 1984). Results of a curve estimation by SPSS (version 7.0) software indicated that the relationship was best described by an exponential decay curve model, as follows: X
Y ¼ a ec : In this model, the parameter a describes an hypothetical maximum recruitment frequency, and c describes the rate at which distance from the block edge affects the frequency of coral recruits, i.e., the edge effect. Non-linear regression analysis using this model explained the pattern of coral distribution on the bottom surface as a function of edge effect, i.e., Y ¼ 1:317 e0:0217X ðr2 ¼ 0:71; p < 0:01Þ (Fig. 6). 3.3. Culicia japonica recruits
Fig. 4. Size frequency distribution of O. crispata recruits on 1.44 m2 of test block surface for different periods of block immersion.
When the size of the coral recruits and the distance of the recruit from the edge of the block were tested using the Pearson correlation coefficient (2-tailed, at p ¼ 0:05 significance), no correlation was obtained between these
Only 30 C. japonica recruits were recorded throughout the two-year study period. The results of the ANOVA showed that the number of C. japonica recruits was significantly different between periods of immersion of the test blocks (p < 0:05), block type (p < 0:05) and their surface orientation (p < 0:05). The periods were grouped by the SNK test into two subsets with respect to a similar mean number of C. japonica recruits, in an ascending order as follows: (1) 3, 6, 9, 21, 14, and 18 months and (2) 21, 14, 18, 12, and 24 months. In general, more C. japonica recruits were recorded on test blocks with an increased period of immersion. The SNK test grouped the block type factor into two subsets, i.e.: (1) block types A, B and C and (2) block types C and D. The subsets for surface orientation were (1) sheltered, top and bottom and (2) exposed.
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Table 2 One-way ANOVA analysis of number of coral recruits between period of immersion, block type and surface orientations, on test blocks retrieved from the experimental artificial reef at Hoi Ha Wan Source of variation O. crispata Period Between groups Within groups Total SNK test groupings in Block Between groups Within groups Total
df
Mean square
F
Significance
Remarks
7 25.06 7.89 0 * 504 3.18 511 an ascending order of means: 3-month; 6-month; 9-month < 24-month; 12-month; 14-month; 18-month; 21-month 3 508 511
3.11 3.48
0.89
0.45
Ns
0
*
Orientation Between groups Within groups Total SNK test groupings in
3 130.17 47.71 508 2.73 511 an ascending order of means: sheltered; top < exposed < bottom
C. japonica Period Between groups Within groups Total SNK test groupings in
7 0.2 3.16 0.003 * 504 0.06 511 an ascending order of means: 12-month; 14-month; 18-month; 6-month; 24-month; 3-month; 21-month; 9-month
Block Between groups Within groups Total SNK test groupings in
3 0.18 2.78 508 0.07 511 an ascending order of means: Type A; Type B; Type C; Type D
0.04
*
Orientation Between groups Within groups Total SNK test groupings in
3 0.55 8.8 508 0.06 511 an ascending order of means: top; bottom < exposed; sheltered
0
*
Computed using a ¼ 0:05. Period ¼ time since deployment, block ¼ test block type, orientation ¼ block surface orientation, ¼ significant at p < 0:05, Ns ¼ not significant.
Only the exposed surfaces were tested for an edge effect as C. japonica settled on this side only. The distance of C. japonica recruits from the block edge ranged from 3 to 73 mm, with a mean of 38:97 19:86 mm (Fig. 7). A Pearson correlation coefficient between frequency of C. japonica recruits and distance from the block edge was )0.052 (df ¼ 28; p > 0:01), indicating that the two were not correlated.
4. Discussion The absence of coral recruits on the test blocks collected and replaced trimonthly for two years, and the appearance on the test blocks deployed nine months after reef immersion, have shown that ÔconditioningÕ is required for the substratum surface prior to coral larval settlement and subsequent metamorphosis. Colonization by bacteria, diatoms and algae are necessary for the development of settlement cues (Morse et al., 1996).
This is consistent with past studies which have shown that the conditioning period for coral recruitment varies from six months to more than one year (Birkeland et al., 1982; Wallace, 1985; Benayahu and Loya, 1987; Harriott and Fisk, 1987; Bailey-Brock, 1989; Edwards and Clark, 1992; Wittenberg and Hunte, 1992). Also, sexual reproduction of O. crispata in Hong Kong occurs between July and September (Lam, 2000a). The combination of conditioning and availability of larvae, which is directly associated with the timing of spawning, explain why corals were recruited in this study only onto test blocks which had been immersed for a period exceeding nine months. The number of recruits on the 12 to 24th-month samples were the same, implied that recruitment onto the PFA–concrete blocks had reached an equilibrium after the 12th month of immersion, i.e., the rate of settlement more or less equalled the rate of recruit mortality. A decrease in settlement and increase in mortality with time may be attributed to diminished surface
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Fig. 5. The frequency of O. crispata recruits as a function of distance from the edges of the A, top, B, exposed and C, bottom surfaces of the test blocks on the experimental artificial reef at Hoi Ha Wan.
through occupation by other benthic macroinvertebrates and grazing activity (Lam, in press a). The recruitment density of O. crispata on the experimental artificial reef is due to both sexual and asexual reproduction. O. crispata spawns between July and September (Nakano and Yamazato, 1992; Lam, 2000a). It also releases asexual planulae in Hong Kong during the resting phase of its annual gametogenic cycle as well as in Japan (Nakano and Yamazato, 1992). This can partly account for the first appearance of recruits in September 1994. There was, however, no distinct increase in accumulated recruitment from June to
September 1995. Such asexual planulae were most likely released by the O. crispata individuals that had settled on the artificial reef in the previous year, i.e., 1994. Only one coral recruit was recorded from the sheltered surfaces. Coral recruitment onto this surface was much lower as compared with the exposed and bottom ones that had totals of 79 and 284 spat, respectively. Contrasting results of coral recruitment onto cryptic habitats created by artificial substrata have been described previously. Harriott and Fisk (1987, 1988) showed that Great Barrier Reef coral planulae prefer a
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Fig. 6. Regression analysis using an exponential decay model to explain the pattern of coral distribution as a function of distance from the edge of the test blocks on the experimental artificial reef at Hoi Ha Wan.
cryptic, low-light, microhabitat for settlement while Collinson (1997) believed the reverse is true locally. Suboptimal light regimes attributed to increased sedimentation would make gap habitats unsuitable for coral recruitment (Maida et al., 1994), as demonstrated in both studies at Hoi Ha Wan (Collinson, 1997; Lam, 1999). Edge effect shaped the recruitment pattern in the zooxanthellate O. crispata but not in the azoxanthellate C. japonica. O. crispata recruitment onto the bottom surface, as described by the exponential decay curve, is similar to a previous finding on the Great Barrier Reef
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(Maida et al., 1994) in which a sigmoid curve model described the recruitment of zooxanthellate, hermatypic, corals as a function of distance from the plate edge. The edge effect of zooxanthellate coral recruitment onto the lower surfaces was further shown to be significantly related to light intensity (Maida et al., 1994). O. crispata seems to have a stronger tolerance to low light intensity than other species as it can locally be found on shaded boulders and in areas with high turbidity where no other corals are present. Its zooxanthellate polyps usually expand fully, even during the daytime, in contrast to other hermatypic corals (Kawaguti and Sakumoto, 1952; personal observations). This behaviour probably enables it to increase its photosynthetic surface and thus inhabit areas with low light intensity. Green fluorescent pigment is present in the epidermis of this species and is thought to be essential for converting short-wavelength light into a suitable light for zooxanthellae photosynthesis (Kawaguti, 1973). The results suggested that the larvae are, however, still selective in choosing a more favourable light regime for settlement. O. crispata recruitment onto PFA–concrete blocks (types B, C and D) was similar to that on concrete blocks (type A). C. japonica preferred to settle on types D and C blocks, i.e., blocks with a higher PFA content. These results indicate that the surface texture and chemistry of the PFA–concrete blocks were suitable for coral larval settlement. Macroinvertebrate colonization on these block types were also the same (Lam, 2000b). Some studies have shown a higher settlement, in terms of both species richness and percentage cover, on ash blocks in comparison with concrete ones because the former had a more friable surface (Relini et al., 1995). The friable PFA–concrete surfaces are, probably, preferred by the larvae of some corals such as C. japonica. Chou and Lim (1986) have also shown that concrete pillars, in comparison with natural substrata on an
Fig. 7. The frequency of C. japonica recruits as a function of distance from the edges of the exposed surfaces of the test blocks on the experimental artificial reef at Hoi Ha Wan.
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adjacent reef slope, were preferred for coral settlement. Greater numbers of coral colonies and size and depth distribution ranges were also recorded on such concrete structures. While coral recruitment has been recorded on artificial reefs constructed from oil ash blocks (Savercool, 1988) and concrete modules (Raymond, 1975; Bailey-Brock, 1989; Clark and Edwards, 1994), there is little or no documented information on coral recruitment onto PFA blocks. This is due to the fact that most coal ash reefs have been deployed in either temperate waters or estuaries (Lam, 1998), where corals are absent naturally. There have been limited studies concerned with coral settlement and recruitment in Hong Kong. Only Platygyra sinensis has been recorded from the pilings of the pier at Ping Chau, Mirs Bay, after an unknown period of submergence (Huang et al., 1992). A recent study by Collinson (1997) showed that four coral species, i.e., O. crispata, C. japonica, Oulangia stokesiana and Porites lobata were recruited onto PVC plates after two years at Hoi Ha Wan. Corals growing within five years on dollos of the High Island Reservoir Dam, which were deployed between February and August 1974, included Montipora informis, Cyphastrea microphthalma, Acropora pruinosa, M. venosa, Cyphastrea serailia, P. lobata and trace additional species (Scott, 1984). Although a smaller number of species (O. crispata and C. japonica) were identified in this study, it is still the first successful attempt, locally, to identify in detail coral recruitment onto a bare artificial substratum. The above studies demonstrate that recruitment only occurs at sites where natural coral communities were present and is limited by a lack of viable larvae (Collinson, 1997), in contrast to the situation on other coral reefs, where recruitment is restricted by suitable substrata rather than larval supply (Fisk and Harriott, 1990). In comparison to other studies in different locations, coral recruitment on artificial substrata in Hong Kong is generally low, in terms of both the number of species and recruit density (Alcala et al., 1982; Birkeland et al., 1982; Wallace and Bull, 1982; Fisk and Harriott, 1990; Latypov, 1991; Harriott, 1992; Maida et al., 1994). The experiments undertaken in this study show that PFA–concrete blocks, like concrete, is a substratum potentially suitable for coral recruitment. The low species composition of scleractinian recruits onto the artificial reef, i.e., O. crispata and C. japonica only, may be due to either the unavailability of coral planula-larvae or environmental conditions at the site at the time of artificial reef deployment. For the same bay, Collinson (1997) showed that only four species of scleractinian corals recruited (all in low densities) onto his settlement plates over a period of approximately 24 months. In conclusion, therefore, coral recruitment onto artificial substrata in Hoi Ha Wan was low, despite the natural diversity of 26 coral species present (Thompson and
Cope, 1982; Cope and Morton, 1988; Collinson, 1997). Both findings represent a picture of early succession by a coral community, with O. crispata and C. japonica being the pioneer species. Juveniles of three other faviids, Favites abdita, Goniastrea aspera and Favia speciosa, with maximum diameter of 5 cm, were observed on the reef modules during a site revisit in 1999. These were estimated to have recruited after this study. With increasing substratum immersion time, a greater diversity of coral species can be expected. There is, however, no previous study on estimating generation time of a new coral community onto an artificial substratum. The time required for the artificial reef at Hoi Ha Wan to recreate an original coral community comparable to that at Coral Beach in Hoi Ha Wan (Cope and Morton, 1988; Collinson, 1997) will, as a result, not to be accomplished within a decade. It has been shown that artificial substrata, e.g., plastic, appear to mainly attract coral recruits of genera usually regarded as opportunists, such as Pocillopora, Seriatopora and Acropora, while a far wider range of genera is recruited onto natural substrata (Wallace and Bull, 1982). If the species composition of coral recruits on natural substrata and PFA–concrete could be further compared, the feasibility of such PFA–concrete artificial reef installations in rehabilitating coral reefs would be better determined. Certain planulators, such as Stylophora pistillata (Loya, 1976), Pocillopora damicornis (Harrigan, 1972) and O. crispata (this study) are classified as pioneer species, that is, they are the first to colonize an open area. Coral colonization probably also follows a successional pattern, in which different species appear in the community as it ages. The herein identified coral recruitment at Hoi Ha Wan, which obtained a single pioneer species, has probably only identified an initial stage, i.e., within the first two years, of a longer term pattern of coral community development. This could be confirmed, if coral recruitment status on the artificial reef were to be studied for a longer time. The choice of any substrata for use in artificial reef construction must satisfy four criteria: durability, safety, functionality and economy (Grove et al., 1991). For durability, the material used can be tested for its structural integrity pre-deployment. The Japanese standard for reef material durability is a minimum 30-year life span without deterioration (Grove et al., 1991). The last criterion can be predicted easily by calculations of costeffectiveness. The second criterion involves studies on leaching of potentially harmful materials, such as trace metals, from the substratum. Preliminary results from studies in the USA (Hockley and Van der Sloot, 1991) and the UK (Collins et al., 1990, 1994) have indicated that trace metal leaching from PFA–cement blocks is insignificant, even after long periods of immersion in seawater because of the formation of a surface salt barrier. The amount and rate of trace metal released
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from blocks used in the present study are not expected to influence adversely the existing flora and fauna of Hoi Ha Wan (Leung et al., 1997). The leaching of metals, i.e., chromium, copper, nickel and zinc, from the Hoi Ha Wan artificial reef structural blocks has been demonstrated to be insignificant (Leung et al., 1997). Any toxicity to the attached biota can be evaluated by studying the accumulation of leachates in it. The material used should also have satisfactory organism-aggregating abilities. This can be tested by comparing the population dynamics and behaviour of individual species between the artificial and natural substrata. The present study shows that PFA-containing concrete (25%, 50% and 75% of PFA by volume), does not differ from true concrete in terms of species diversity, recruitment and succession, coral recruitment, growth of coral transplants and O. crispata growth studies (Lam, 2000b,c,d, in press b, this study). Of the 28 recorded attaching species, only Acrochaetium robustum, Watersipora cucullata, Serpula vermicularis, Spirobranchus tricornis, Pomatoleios kraussii, Corophium crassicorne and Chirona tenuis, showed variations in recruitment between block types. Other studies comparing epifaunal recruitment onto substrata with different PFA contents have also shown that certain species associate better with coal ash concrete blocks than concrete ones, and vice versa (Hatcher, 1993; Sampaolo and Relini, 1994; Relini et al., 1995). In past studies, the higher settlement of biota (in terms of the number of species and individuals) on ash as compared with concrete blocks was suggested to be because of the more friable surface of the former (Relini et al., 1995). The chemical properties of the block surfaces may, therefore, be suitable for algal and invertebrate settlement. Coral recruit densities on such PFA materials, i.e., 48.6 recruit m2 , is fourfold that on PVC plates, i.e., 11.9 recruit m2 (Collinson, 1997). Concrete may be a better substratum for coral settlement than a natural reef slope (Chou and Lim, 1986). These suggest that PFA–concrete is an appropriate material for artificial reef construction, especially when such structures aim at restoring the coral community through natural recruitment. The feasibility of using PFA (up to 75% of PFA by volume) in artificial reef construction for this purpose is, thus, confirmed by this study. This experimental artificial reef deployment study evaluated the use of a concrete artificial reef in habitat restoration of similar marine habitats lacking a hard substratum, especially those previously destroyed by either terrestrial runoff or dredging. Past researches have produced supporting findings for the use of power station wastes in building artificial reefs (Pickering, 1996). The two Hong Kong coal-fired power plants produce approximately one million tonnes of coal waste each year. Land fills and its use in some civil engineering projects are the only available disposal options at pre-
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sent. Such cement-stabilised coal ash could, however, be also used to construct artificial reefs and provide an alternative channel for disposal.
Acknowledgements I am grateful to Prof. B. Morton, Director of the Swire Institute of Marine Science (SWIMS), for his supervision and reading the first drafts of the manuscript. Special thanks are due to Dr. C. Sheppard who came to SWIMS in person and have the recruited corals identified. I also thank Prof. Y. Benayahu, Prof. C. Birkeland, Prof. L.M. Chou and other anonymous reviewers for their comments on this manuscript. This research was funded by the China Light and Power Co. Ltd. and Hong Kong Electric Co. Ltd. This paper is part of a thesis submitted to The University of Hong Kong in partial fulfilment of the requirements for a Ph.D. degree. References Alcala, A.C., Alcala, L.C., Gomez, E.D., Cowan, M.E., Yap, H.T., 1982. Growth of certain corals, molluscs and fish in artificial reefs in the Philippines. Proceedings of the Fourth International Coral Reef Symposium 2, 215–220. Bailey-Brock, J.H., 1989. Fouling community development on an artificial reef in Hawaiian waters. Bulletin of Marine Science 44, 580–591. Benayahu, Y., Loya, Y., 1987. Long-term recruitment of soft-corals (Octocorallia: Alcyonacea) on artificial substrata at Eilat (Red Sea). Marine Ecology Progress Series 38, 161–167. Birkeland, C., Rowley, D., Randall, R.H., 1982. Coral recruitment patterns at Guam. Proceedings of the Fourth International Coral Reef Symposium 2, 339–344. Chou, L.M., Lim, T.M., 1986. A preliminary study of the coral community on artificial and natural substrates. Malayan Naturalist Journal 39, 225–229. Clark, S., Edwards, A.J., 1994. Use of artificial reef structures to rehabilitate reef flats degraded by coral mining in the Maldives. Bulletin of Marine Science 55, 724–744. Collins, K.J., Jensen, A.J., Lockwood, A.P.M., 1990. Fishery enhancement reef building exercise. Chemistry and Ecology 4, 179– 187. Collins, K.J., Jensen, A.J., Lockwood, A.P.M., Lockwood, S.J., 1994. Coastal structures, waste materials and fishery enhancement. Bulletin of Marine Science 55, 1253–1264. Collinson, P.R.J., 1997. The ecology of a peripheral, subtropical coral community in Hong Kong. Ph.D. Thesis. The University of Hong Kong. Cope, M.A., 1984. An ecological survey of the scleractinian coral community in Hoi Ha Wan, Hong Kong. M.Phil. Thesis. The University of Hong Kong. Cope, M.A., Morton, B., 1988. The scleractinian coral community at Hoi Ha Wan, Hong Kong. Asian Marine Biology 5, 41–52. Edwards, A., Clark, S., 1992. Re-establishment of reef fish populations on a reef flat degraded by coral quarrying in the Maldives. Proceedings of the Seventh International Coral Reef Symposium 1, 593–600. Fisk, D.A., Harriott, V.J., 1990. Spatial and temporal variation in coral recruitment on the Great Barrier Reef: implications for the dispersal hypothesis. Marine Biology 107, 485–490.
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Fitzhardinge, R.C., Bailey-Brock, J.H., 1989. Colonization of artificial reef materials by corals and other sessile organisms. Bulletin of Marine Science 44, 567–569. Gomez, E.D., Alcala, A.C., Alcala, L.C., 1982. Growth of some corals in an artificial reef off Dumaguete, central Philippines. Kalikasan, Philippines Journal of Biology 11, 148–157. Grove, R.S., Sonu, C.J., Nakamura, M., 1991. Design and engineering of manufactured habitats for fisheries enhancement. In: Seaman, W., Sprague, L.M. (Eds.), Artificial Habitats for Marine and Freshwater Fisheries. Academic Press, Inc., San Diego, pp. 109– 152. Harrigan, J., 1972. The planulae larva of Pocillopora damicornis; lunar periodicity of swarming and substratum selection behaviour. Ph.D. Thesis. University of Hawaii. Harriott, V.J., 1992. Recruitment patterns of scleractinian corals in an isolated sub-tropical reef system. Coral Reefs 11, 215–219. Harriott, V.J., Fisk, D.A., 1987. A comparison of settlement plate types for experiments on the recruitments of scleractinian corals. Marine Ecology Progress Series 37, 201–208. Harriott, V.J., Fisk, D.A., 1988. Recruitment patterns of scleractinian corals: a study of three reefs. Australian Journal of Marine and Freshwater Research 39, 409–416. Hatcher, A.M., 1993. Epifaunal development on an artificial reef. Ph.D. Thesis. University of Southhampton. Hockley, D.E., Van der Sloot, H.A., 1991. Long term processes in a stabilized coal-waste block exposed to seawater. Environmental Science Technology 25, 1414–1419. Hodgson, G., 1994. The environmental impact of marine dredging in Hong Kong. Coastal Management in Tropical Asia 2, 1–8. Huang, Z., Yan, S.K., Lin, S., Zheng, D.Q., 1992. Biofouling communities on pier pilings in Mirs Bay. In: Morton, B. (Ed.), The Marine Flora and Fauna of Hong Kong and Southern China III. Hong Kong University Press, Hong Kong, pp. 529–543. Kawaguti, S., Sakumoto, D., 1952. Pigment of skeleton of a reef coral, Oulastrea crispata. Biological Journal of Okayama University 1, 265–270. Kawaguti, S., 1973. Electron microscopy on symbiotic algae in reef corals. Publications of Seto Marine Biology Laboratory 20, 779– 783. Lam, K.K.Y., 1998. Colonization of an experimental artificial reef at Hoi Ha Wan Marine Park, Hong Kong. Ph.D. Thesis. The University of Hong Kong. Lam, K.K.Y., 1999. Hydrography, nutrients and phytoplankton, with special reference to an hypoxic event, at an experimental artificial reef at Hoi Ha Wan, Hong Kong. Asian Marine Biology 16, 35–64. Lam, K.K.Y., 2000a. Sexual and asexual reproduction of a lowtemperature tolerant coral, Oulastrea crispata (Scleractinia, Faviidae) in Hong Kong, China. Marine Ecology Progress Series 205, 101–111. Lam, K.K.Y., 2000b. Algal and sessile invertebrate recruitment onto an experimental PFA–concrete artificial reef in Hong Kong. Asian Marine Biology 17, 55–76. Lam, K.K.Y., 2000c. Early growth of a pioneer recruited coral Oulastrea crispata (Scleractinia, Faviidae) on PFA–concrete blocks in a marine park in Hong Kong, China. Marine Ecology Progress Series 205, 113–121. Lam, K.K.Y., 2000d. Coral transplantation onto a stabilised pulverised fuel ash substratum. Asian Marine Biology 17, 25–41. Lam, K.K.Y., in press a. Seasonally-varying competitive hierarchy of sessile invertebrates on an artificial reef. Asian Marine Biology 18. Lam, K.K.Y., in press b. Epibentic community development on an experimental pulverised fuel ash (PFA) artificial reef. Asian Marine Biology 18. Latypov, Y.Y., 1991. Pioneer settlements of reef-building corals on piles of oil platforms in the South China Sea. Biologica Moria 3, 93–99.
Leung, A.W.Y., Leung, K.F., Lam, K.K.Y., Morton, B., 1997. Final Report––The evaluation of an experimental artificial reef for the marine habitat rehabilitation and the feasibility of using stabilized coal combustion solid residues for reef construction. The Swire Institute of Marine Science, The University of Hong Kong, Hong Kong. Loya, Y., 1976. The Red Sea coral Stylophora pistillata is an r strategist. Nature 259, 478–480. Maida, M., Coll, J.C., Sammarco, P.W., 1994. Shedding new light on scleractinian coral recruitment. Journal of Experimental Marine Biology and Ecology 180, 189–202. Morse, D.E., Iwao, K., Baba, M., Shimoike, K., Hayashibara, T., Omori, M., 1996. An ancient chemosensory mechanism brings new life to coral reefs. Biological Bulletin 191, 149–154. Morton, B., 1989. Pollution of the coastal waters of Hong Kong. Marine Pollution Bulletin 20, 310–318. Morton, B., 1992. A case for marine conservation: Hong KongÕs scleractinian coral communities. In: Morton, B. (Ed.), The Marine Flora and Fauna of Hong Kong and Southern China III. Hong Kong University Press, Hong Kong, pp. 3–13. Morton, B., 1994. Hong KongÕs coral communities: status, threats and management plans. Marine Pollution Bulletin 29, 74–83. Morton, B., 1995. Editorial Hong KongÕs corals. Marine Pollution Bulletin 30, 230–231. Nakano, Y., Yamazato, K., 1992. Ecological study of reproduction of Oulastrea crispata in Okinawa. Zoological Science 9, 1292. Pickering, H., 1996. Artificial reefs of bulk waste materials: a scientific and legal review of the suitability of using the cement stabilised byproducts of coal-fired power stations. Marine Policy 20, 483–497. Raymond, W.F., 1975. Sand and coral monitoring at two artificial reefs in Florida. Sand and Beach 43, 3–10. Relini, G., Relini, M., Torchia, G., Tixi, F., Nigri, C., 1995. Coal ash tests in Loano artificial reef. In: Proceedings of the International Conference on Ecological System Enhancement Technology for Aquatic Environments, ECOSET Ô95. Japan International Marine Science and Technology Federation, Tokyo, pp. 107–113. Sampaolo, A., Relini, G., 1994. Coal ash for artificial habitats in Italy. Bulletin of Marine Science 55, 1277–1294. Savercool, D.M., 1988. A comparative study of fouling communities on oil ash block and concrete block reefs off Vero Beach, Florida. M.Sc. Thesis. Florida Institute of Technology, Melbourne, FL. Schuhmacher, H., 1974. On the condition accompanying the first settlement of corals on artificial reefs with special reference to the influence of grazing sea urchins (Eilat, Red Sea). Proceedings of the Second International Coral Reef Symposium 1, 257–267. Schuhmacher, H., 1977. Initial phases in reef development, studied at artificial reef types off Eilat (Red Sea). Helgolander wiss Meeresunters 30, 400–411. Schuhmacher, H., 1988. Development of coral communities on artificial reef types over 20 years (Eilat, Red Sea). Proceedings of the Sixth International Coral Reef Symposium 3, 379–384. Scott, P.J.B., 1984. The Corals of Hong Kong. Hong Kong University Press, Hong Kong. Scott, P.J.B., Cope, M., 1982. The distribution of scleractinian corals at six sites within Tolo Harbour and Channel. In: Morton, B., Tseng, C.K. (Eds.), Proceedings of the First International Marine Biological Workshop: The Marine Flora and Fauna of Hong Kong and Southern China, Hong Kong. Hong Kong University Press, Hong Kong, pp. 575–586. Scott, P.J.B., Cope, M., 1990. Tolo revisited: a survey of the corals in Tolo Harbour and Channel six years and half a million people later. In: Morton, B. (Ed.), Proceedings of the Second International Marine Biological Workshop: The Marine Flora and Fauna of Hong Kong and Southern China, Hong Kong. Hong Kong University Press, Hong Kong, pp. 1203–1220. Sokal, R.R., Rohlf, F.J., 1995. Biometry. W.H. Freeman, San Francisco.
K.K.Y. Lam / Marine Pollution Bulletin 46 (2003) 642–653 Strobel, C.J., Gentile, J.H., Schimmel, S.C., Carr, R.S., Williams, J.W., Redford, D., 1988. Proposed biological testing methods for the United States Incineration-at-Sea Research Program. Marine Pollution Bulletin 19, 605–610. Thompson, G.B., Cope, M., 1982. Estimation of coral abundance by underwater photography. In: Morton, B., Tseng, C.K. (Eds.), Proceedings of the First International Marine Biological Workshop: The Marine Flora and Fauna of Hong Kong and Southern China. Hong Kong University Press, Hong Kong, pp. 557– 573. Waldichuk, M., 1988. Incineration at sea and artificial reefs: options for marine waste disposal. Marine Pollution Bulletin 19, 589– 593. Wallace, C.C., 1985. Seasonal peaks and annual fluctuations in recruitment of juvenile scleractinian corals. Marine Ecology Progress Series 21, 289–298.
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Wallace, C.C., Bull, G.D., 1982. Patterns of juvenile coral recruitment on a reef front during a spring–summer spawning period. Proceedings of the Fourth International Coral Reef Symposium 2, 345–350. Wittenberg, M., Hunte, W., 1992. Effects of eutrophication and sedimentation on juvenile corals. I. Abundance, mortality and community structure. Marine Biology 116, 131–138. Wu, R.S.S., 1988. Marine pollution in Hong Kong: a review. Asian Marine Biology 5, 1–23. Zar, J.H., 1984. Biostatistical Analysis. Prentice-Hall, Engelwood Cliffs, NJ. Zou, R.L., Wang, Z.H., Cheung, C.P.S., 1992. A resurvey of the corals at Hoi Ha Wan after eight years and the Tai Long Tun Borrow Area. In: Morton, B. (Ed.), Proceedings of the Fourth International Marine Biological Workshop: The Marine Flora and Fauna of Hong Kong and Southern China. Hong Kong University Press, Hong Kong, pp. 11–29.
Coral recruitment onto an experimental pulverised fuel ash–concrete artificial reef Katherine K.Y. Lam
*
Swire Institute of Marine Science and Department of Ecology and Biodiversity, The University of Hong Kong, Cape DÕAguilar Road, Shek O, Hong Kong
Abstract An experimental artificial reef was deployed in December 1993 at Hoi Ha Wan Marine Park, Hong Kong. This is the first study documenting natural scleractinian coral recruitment onto a stabilised pulverised fuel ash (PFA)–concrete artificial reef. Visible recruits were first recorded 9–10 months after the placement of reef blocks, i.e., in the autumn of 1994. Two scleractinians, Oulastrea crispata and Culicia japonica, were recruited. The recruit density of the former was much greater than the latter. The spatial recruitment pattern of the corals was observed to be affected by the orientation of the attaching surface. O. crispata settled predominantly on the undersides of the reef blocks. There was an edge effect on O. crispata recruitment. C. japonica, however, had a preference for exposed surfaces. O. crispata did not show a preference for block composition whereas C. japonica favoured blocks with high (75% by volume) PFA levels. This shows that PFA–concrete is a potential substratum for artificial reef construction, especially when such reefs aim at rehabilitating corals. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Coral recruitment; Pulverised fuel ash; Artificial reef; Oulastrea crispata
1. Introduction The use of coal waste concrete blocks as artificial reef units has a history of almost two decades (Strobel et al., 1988; Waldichuk, 1988). Whether or not such material is a suitable substratum for coral and, hence, reef restoration, through natural coral recruitment or manual transplantation, is still unstudied. A possible reason for this is that most of the pulverised fuel ash (PFA)–concrete artificial reef projects so far have taken place in temperate waters, where coral reefs are not naturally found, and the major concern of such ash reef installations has focused on fishing (Lam, 1998). The degradation of coral communities in Tolo Channel and Hoi Ha Wan, Hong Kong, has been reported upon by numerous researchers over the past two decades (Scott and Cope, 1982, 1990; Morton, 1992, 1994, 1995; Cope, 1984; Zou et al., 1992; Collinson, 1997). The causes for this include man-made disturbances, such as increases in nitrates, phosphates and *
Tel.: +852-280-92179; fax: +852-280-92197. E-mail address: [email protected] (K.K.Y. Lam).
sedimentation from the input of raw sewage (Morton, 1989; Wu, 1988) from both agricultural and domestic sources, as well as natural stresses such as the frequency of typhoons, and temperature and salinity changes (Collinson, 1997). Many other man-induced disturbances threaten local corals, such as dynamite fishing, coral collection by recreational and commercial divers, and marine sand dredging for major reclamation projects such as the new airport at Chek Lap Kok and Western Kowloon (Hodgson, 1994). On the other hand, coral communities in Hong Kong are expanding onto either artificial substrata or natural boulders. For example, a new coral community is developing on the submerged concrete dollos of the east High Island Reservoir dam which has been in existence for >20 years (Scott, 1984; personal observations) and small (<5 cm) corals were found settling on natural barren boulders at Ping Chau (personal observations). The artificial reef site of this study used to have a coral community. The removal of top soil from the Tai Leng Tung Peninsula, for land reclamation, caused the smothering and destruction of the coral community on the adjacent shore between 1981 and 1989 (Zou et al., 1992).
0025-326X/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0025-326X(02)00482-4
K.K.Y. Lam / Marine Pollution Bulletin 46 (2003) 642–653
Coral recruitment and growth on concrete constructions such as harbour moles, concrete pillars and concrete outflow pipes, or artificial reef structures, in tropical coastal waters has been reported (Schuhmacher, 1974, 1977, 1988; Chou and Lim, 1986; Clark and Edwards, 1994). Several studies have also reported upon the successful colonization of corals onto tyre reefs (Gomez et al., 1982; Fitzhardinge and Bailey-Brock, 1989), although there has been opposition to the idea since Fitzhardinge and Bailey-Brock (1989) pointed out a significantly smaller settlement of coral recruits onto scrap tyres than on concrete and metal surfaces. This study, therefore, was to test the feasibility of PFA–concrete blocks for coral reef restoration by natural coral larvae settlement. In the long term, this may yield coral colonies releasing larvae which further colonize the same reef. The first objective of this study was, therefore, to examine the spatial pattern of coral recruitment, i.e., to determine the choice of block type, orientation and edge effects. Recruitment patterns between species were compared. Second, the temporal pattern, in terms of recruitment season, was also identified. Third, the use of artificial reefs, especially those made of concrete or PFA–concrete, for reef restoration, is reviewed and discussed.
2. Materials and methods 2.1. Study site The artificial reef was deployed on an area of sea bed at )7 m C.D. within Hoi Ha Wan (Fig. 1). The sea bed of the study area had been smothered by the runoff of silt from surrounding hills which were used as a land borrow area between 1981 and 1989. Despite reforestation of the hills, the sea bed of the study area was still covered by at least 0.5 m thick of loose mud on top of a hard substratum, where there had been a coral community (Zou et al., 1992). The artificial reef comprised 1000 cuboidal structural blocks 500 mm in length, with a hollow central cylinder of 0.2 m diameter, to increase their surface areas as well as provide shelter for resident fish and invertebrates. The blocks were deployed in a random fashion from a barge within the gazetted site from 1–3 December 1993. The shape of the artificial reef was somewhat like a pyramid with a base area of approximately 10 m 10 m and a height of 2.5 m (Fig. 2). Additionally, 176 test blocks of 0:15 0:15 0:15 m dimensions made from four cement/PFA (pulverised fuel ash) mixes were placed to determine seasonal patterns of coral recruitment. All blocks had smooth surfaces. The test blocks were placed on top of the artificial reef. The structural and test blocks were made of different concrete to PFA compositions, as shown in Table 1. The formulation of
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these blocks was based on a cement replacement by PFA concept (Leung et al., 1997). 2.2. Sampling of test blocks During the deployment of the artificial reef in December 1993, an initial set of test blocks, including 90 each of the four PFA–concrete mixes, were placed on top of the structural reef blocks. These blocks (4 replicates for each of the 4 block types, i.e., 16 in total) were retrieved after 3, 6, 9, 12, 14, 18, 21 and 24 months post artificial reef deployment. Cumulative coral recruitment onto the artificial reef was thus studied over a period of two years. A second set of test blocks were retrieved and replaced trimonthly at the same time for the same period. No coral recruitment was recorded on these blocks. Only four faces of each test block were examined and each face was divided into categories as follows: (1) sheltered surface, i.e., the side of the test block most in contact with the main reef modules; (2) the vertical, or inclined, surface exposed to the open sea; (3) the top surface, which was more or less horizontal to the water surface and (4) the bottom, i.e., the base of the test block (Leung et al., 1997). The gap between the bottom surface of the test blocks and the reef modules ranged from 10 to 30 mm. Since the four side faces of each test block could be either exposed or sheltered, as they were resting on the main reef, various blocks had different numbers of exposed and sheltered surfaces. For standardisation and statistical analyses, therefore, only four instead of all the surfaces were examined. The total surface area studied in each season was, therefore, 0:15 0:15 m2 (surface area of each block) multiplied by 16 (number of test blocks) and 4 (number of each test block surfaces studied), making a total of 1.44 m2 . The retrieved test blocks were placed in a Perspex tank filled with seawater and scanned under the dissecting microscope. Larger pieces of algae and other macroinvertebrates were removed with forceps to facilitate observations, whenever necessary. Each test block surface was mapped using a transparent plastic sheet to determine the position and size of any recruits on the surfaces and their distances from the nearest edge of the block. Coral recruits were also assessed for size in terms of geometric diameter in mm (which equals the square root of the longest diagonal multiplied by the shortest one), and the number of polyps present. Each coral recruit was categorised under block type (types A, B, C and D as in Table 1) and face (sheltered, exposed, top and bottom) of settlement. Living recruits were identified by the presence of visible, intact and responsive, polyps whereas dead recruits were polyps without tissue, broken calcite or the presence of algal fouling. Recruitment here is referred to settlement followed by deposition of a recognisable skeleton and was alive at the time of retrieval of the plates.
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Fig. 1. A map of Hoi Ha Wan, Hong Kong, showing the location of the artificial reef.
Fig. 2. An impression of Hong KongÕs first experimental artificial reef at Hoi Ha Wan, with test blocks for coral recruitment study resting on it.
K.K.Y. Lam / Marine Pollution Bulletin 46 (2003) 642–653 Table 1 The composition in terms of dry weight of the different types of test blocks deployed on the artificial reef Block type
Test block A
B
C
D
PFA/cement by volume PFA (%) Portland cement (%) 20 mm aggregate (%) 10 mm aggregate (%) Fine stone (%) Water (%)
0:1 0 13.1 41.8 11.3 33.8 6.9
1:3 3.2 9.7 41.9 11.3 33.9 6.4
1:1 6.6 6.6 41.6 11.2 33.8 9.0
3:1 9.7 3.2 41.8 11.3 33.9 7.6
Structural block composition was the same as type C.
2.3. Statistical analyses To assess differences between the number of coral recruits between test blocks with various submergence periods, block type and orientation of settlement, oneway ANOVA and Student–Newman–Keuls (SNK) test, at the 0.05 level of significance, were used (Sokal and Rohlf, 1995). Between-species differences were analysed by t-test from pooled data of the total number of living coral recruits onto all the test blocks retrieved throughout the two-year study period. To determine edge effects on coral recruitment onto block surfaces with different orientations, the nearest distance of each recruit to a block edge was measured. The frequency of the number of coral recruits with respect to every 5 mm (up to 75 mm, i.e., the centre of the 150 mm 150 mm surface) from the block edge was also obtained. Pearson correlation coefficients between the frequency of the coral recruits versus the distance of the living coral recruits from the block edge and coral recruit and the distance of the coral recruits from the block edge, respectively, were calculated for the four block surface orientations, so as to test the null hy-
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pothesis that there was no correlation between the two. On surface(s) where edge effect occurred, regression models were tested to describe the relationship between living coral recruits and distance from the block edge.
3. Results 3.1. Living coral recruitment Only two species recruited onto the test blocks, Oulastrea crispata (n ¼ 387, range of density ¼ 0–64.58 m2 and 92.81% of the total number of coral recruits) was much more abundant (paired t-test, t ¼ 8:37, df ¼ 509, p < 0:01) (Fig. 3) than Culicia japonica (n ¼ 30, range of density ¼ 0–6.25 m2 , 7.19% of the total number of coral recruits). 3.2. Oulastrea crispata recruits O. crispata recruits were first identified on the test blocks retrieved in September 1994, nine months after artificial reef installation. A total of 387 recruits were recorded. Fig. 4 shows the population dynamics of the recruits from nine to 24 months post-deployment of test blocks. Results of a one-way general factorial ANOVA (Table 2) showed that the numbers of O. crispata recruits were significantly different between the period of block immersion and settlement orientation (p-values < 0:05) but not between block type (p ¼ 0:45). The SNK test grouped the subsets in terms of mean number of coral recruits according to the period in water in ascending order as (1) 3, 6, and 9 months and (2) 24, 12, 14, 18, and 21 months. The subsets for surface orientation were (1) sheltered and top; (2) exposed and (3) bottom.
Fig. 3. Density of coral recruit (recruit m2 ) on the test blocks retrieved from the experimental artificial reef, at Hoi Ha Wan. Density was calculated as pooled number of recruit over total studied surface, i.e. (15 15 64) cm2 .
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two variables for coral recruits on the bottom (df ¼ 282, r ¼ 0:0153, p > 0:05), exposed (df ¼ 77, r ¼ 0:0737, p > 0:05) and top surfaces (df ¼ 21, r ¼ 0:161, p > 0:05). Fig. 5 showed the data for the distances of the coral recruits from the block edge on the top, exposed and bottom surfaces. A Pearson correlation coefficient showed that the frequency of coral recruits and their distance from the block edge was not significantly correlated with the exposed (df ¼ 77, r ¼ 0:081, p > 0:05) and sheltered surfaces (df ¼ 21, r ¼ 0:15, p > 0:05). Distance from the block edge was, however, highly negatively correlated with both the frequency of the coral recruits on the bottom surface (df ¼ 284, r ¼ 0:65, p < 0:01) and the log transformed frequency (df ¼ 74, r ¼ 0:8, p < 0:01). Coral recruitment onto the sheltered surfaces was excluded from the analysis of edge effect because only one was recorded, at a distance of 1 mm. Analysis of the linear-regression residuals showed that the relationship between the two variables, log transformed frequency of coral recruits and distance from the bottom surface of the block edge, was nonlinear (Zar, 1984). Results of a curve estimation by SPSS (version 7.0) software indicated that the relationship was best described by an exponential decay curve model, as follows: X
Y ¼ a ec : In this model, the parameter a describes an hypothetical maximum recruitment frequency, and c describes the rate at which distance from the block edge affects the frequency of coral recruits, i.e., the edge effect. Non-linear regression analysis using this model explained the pattern of coral distribution on the bottom surface as a function of edge effect, i.e., Y ¼ 1:317 e0:0217X ðr2 ¼ 0:71; p < 0:01Þ (Fig. 6). 3.3. Culicia japonica recruits
Fig. 4. Size frequency distribution of O. crispata recruits on 1.44 m2 of test block surface for different periods of block immersion.
When the size of the coral recruits and the distance of the recruit from the edge of the block were tested using the Pearson correlation coefficient (2-tailed, at p ¼ 0:05 significance), no correlation was obtained between these
Only 30 C. japonica recruits were recorded throughout the two-year study period. The results of the ANOVA showed that the number of C. japonica recruits was significantly different between periods of immersion of the test blocks (p < 0:05), block type (p < 0:05) and their surface orientation (p < 0:05). The periods were grouped by the SNK test into two subsets with respect to a similar mean number of C. japonica recruits, in an ascending order as follows: (1) 3, 6, 9, 21, 14, and 18 months and (2) 21, 14, 18, 12, and 24 months. In general, more C. japonica recruits were recorded on test blocks with an increased period of immersion. The SNK test grouped the block type factor into two subsets, i.e.: (1) block types A, B and C and (2) block types C and D. The subsets for surface orientation were (1) sheltered, top and bottom and (2) exposed.
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Table 2 One-way ANOVA analysis of number of coral recruits between period of immersion, block type and surface orientations, on test blocks retrieved from the experimental artificial reef at Hoi Ha Wan Source of variation O. crispata Period Between groups Within groups Total SNK test groupings in Block Between groups Within groups Total
df
Mean square
F
Significance
Remarks
7 25.06 7.89 0 * 504 3.18 511 an ascending order of means: 3-month; 6-month; 9-month < 24-month; 12-month; 14-month; 18-month; 21-month 3 508 511
3.11 3.48
0.89
0.45
Ns
0
*
Orientation Between groups Within groups Total SNK test groupings in
3 130.17 47.71 508 2.73 511 an ascending order of means: sheltered; top < exposed < bottom
C. japonica Period Between groups Within groups Total SNK test groupings in
7 0.2 3.16 0.003 * 504 0.06 511 an ascending order of means: 12-month; 14-month; 18-month; 6-month; 24-month; 3-month; 21-month; 9-month
Block Between groups Within groups Total SNK test groupings in
3 0.18 2.78 508 0.07 511 an ascending order of means: Type A; Type B; Type C; Type D
0.04
*
Orientation Between groups Within groups Total SNK test groupings in
3 0.55 8.8 508 0.06 511 an ascending order of means: top; bottom < exposed; sheltered
0
*
Computed using a ¼ 0:05. Period ¼ time since deployment, block ¼ test block type, orientation ¼ block surface orientation, ¼ significant at p < 0:05, Ns ¼ not significant.
Only the exposed surfaces were tested for an edge effect as C. japonica settled on this side only. The distance of C. japonica recruits from the block edge ranged from 3 to 73 mm, with a mean of 38:97 19:86 mm (Fig. 7). A Pearson correlation coefficient between frequency of C. japonica recruits and distance from the block edge was )0.052 (df ¼ 28; p > 0:01), indicating that the two were not correlated.
4. Discussion The absence of coral recruits on the test blocks collected and replaced trimonthly for two years, and the appearance on the test blocks deployed nine months after reef immersion, have shown that ÔconditioningÕ is required for the substratum surface prior to coral larval settlement and subsequent metamorphosis. Colonization by bacteria, diatoms and algae are necessary for the development of settlement cues (Morse et al., 1996).
This is consistent with past studies which have shown that the conditioning period for coral recruitment varies from six months to more than one year (Birkeland et al., 1982; Wallace, 1985; Benayahu and Loya, 1987; Harriott and Fisk, 1987; Bailey-Brock, 1989; Edwards and Clark, 1992; Wittenberg and Hunte, 1992). Also, sexual reproduction of O. crispata in Hong Kong occurs between July and September (Lam, 2000a). The combination of conditioning and availability of larvae, which is directly associated with the timing of spawning, explain why corals were recruited in this study only onto test blocks which had been immersed for a period exceeding nine months. The number of recruits on the 12 to 24th-month samples were the same, implied that recruitment onto the PFA–concrete blocks had reached an equilibrium after the 12th month of immersion, i.e., the rate of settlement more or less equalled the rate of recruit mortality. A decrease in settlement and increase in mortality with time may be attributed to diminished surface
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Fig. 5. The frequency of O. crispata recruits as a function of distance from the edges of the A, top, B, exposed and C, bottom surfaces of the test blocks on the experimental artificial reef at Hoi Ha Wan.
through occupation by other benthic macroinvertebrates and grazing activity (Lam, in press a). The recruitment density of O. crispata on the experimental artificial reef is due to both sexual and asexual reproduction. O. crispata spawns between July and September (Nakano and Yamazato, 1992; Lam, 2000a). It also releases asexual planulae in Hong Kong during the resting phase of its annual gametogenic cycle as well as in Japan (Nakano and Yamazato, 1992). This can partly account for the first appearance of recruits in September 1994. There was, however, no distinct increase in accumulated recruitment from June to
September 1995. Such asexual planulae were most likely released by the O. crispata individuals that had settled on the artificial reef in the previous year, i.e., 1994. Only one coral recruit was recorded from the sheltered surfaces. Coral recruitment onto this surface was much lower as compared with the exposed and bottom ones that had totals of 79 and 284 spat, respectively. Contrasting results of coral recruitment onto cryptic habitats created by artificial substrata have been described previously. Harriott and Fisk (1987, 1988) showed that Great Barrier Reef coral planulae prefer a
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Fig. 6. Regression analysis using an exponential decay model to explain the pattern of coral distribution as a function of distance from the edge of the test blocks on the experimental artificial reef at Hoi Ha Wan.
cryptic, low-light, microhabitat for settlement while Collinson (1997) believed the reverse is true locally. Suboptimal light regimes attributed to increased sedimentation would make gap habitats unsuitable for coral recruitment (Maida et al., 1994), as demonstrated in both studies at Hoi Ha Wan (Collinson, 1997; Lam, 1999). Edge effect shaped the recruitment pattern in the zooxanthellate O. crispata but not in the azoxanthellate C. japonica. O. crispata recruitment onto the bottom surface, as described by the exponential decay curve, is similar to a previous finding on the Great Barrier Reef
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(Maida et al., 1994) in which a sigmoid curve model described the recruitment of zooxanthellate, hermatypic, corals as a function of distance from the plate edge. The edge effect of zooxanthellate coral recruitment onto the lower surfaces was further shown to be significantly related to light intensity (Maida et al., 1994). O. crispata seems to have a stronger tolerance to low light intensity than other species as it can locally be found on shaded boulders and in areas with high turbidity where no other corals are present. Its zooxanthellate polyps usually expand fully, even during the daytime, in contrast to other hermatypic corals (Kawaguti and Sakumoto, 1952; personal observations). This behaviour probably enables it to increase its photosynthetic surface and thus inhabit areas with low light intensity. Green fluorescent pigment is present in the epidermis of this species and is thought to be essential for converting short-wavelength light into a suitable light for zooxanthellae photosynthesis (Kawaguti, 1973). The results suggested that the larvae are, however, still selective in choosing a more favourable light regime for settlement. O. crispata recruitment onto PFA–concrete blocks (types B, C and D) was similar to that on concrete blocks (type A). C. japonica preferred to settle on types D and C blocks, i.e., blocks with a higher PFA content. These results indicate that the surface texture and chemistry of the PFA–concrete blocks were suitable for coral larval settlement. Macroinvertebrate colonization on these block types were also the same (Lam, 2000b). Some studies have shown a higher settlement, in terms of both species richness and percentage cover, on ash blocks in comparison with concrete ones because the former had a more friable surface (Relini et al., 1995). The friable PFA–concrete surfaces are, probably, preferred by the larvae of some corals such as C. japonica. Chou and Lim (1986) have also shown that concrete pillars, in comparison with natural substrata on an
Fig. 7. The frequency of C. japonica recruits as a function of distance from the edges of the exposed surfaces of the test blocks on the experimental artificial reef at Hoi Ha Wan.
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adjacent reef slope, were preferred for coral settlement. Greater numbers of coral colonies and size and depth distribution ranges were also recorded on such concrete structures. While coral recruitment has been recorded on artificial reefs constructed from oil ash blocks (Savercool, 1988) and concrete modules (Raymond, 1975; Bailey-Brock, 1989; Clark and Edwards, 1994), there is little or no documented information on coral recruitment onto PFA blocks. This is due to the fact that most coal ash reefs have been deployed in either temperate waters or estuaries (Lam, 1998), where corals are absent naturally. There have been limited studies concerned with coral settlement and recruitment in Hong Kong. Only Platygyra sinensis has been recorded from the pilings of the pier at Ping Chau, Mirs Bay, after an unknown period of submergence (Huang et al., 1992). A recent study by Collinson (1997) showed that four coral species, i.e., O. crispata, C. japonica, Oulangia stokesiana and Porites lobata were recruited onto PVC plates after two years at Hoi Ha Wan. Corals growing within five years on dollos of the High Island Reservoir Dam, which were deployed between February and August 1974, included Montipora informis, Cyphastrea microphthalma, Acropora pruinosa, M. venosa, Cyphastrea serailia, P. lobata and trace additional species (Scott, 1984). Although a smaller number of species (O. crispata and C. japonica) were identified in this study, it is still the first successful attempt, locally, to identify in detail coral recruitment onto a bare artificial substratum. The above studies demonstrate that recruitment only occurs at sites where natural coral communities were present and is limited by a lack of viable larvae (Collinson, 1997), in contrast to the situation on other coral reefs, where recruitment is restricted by suitable substrata rather than larval supply (Fisk and Harriott, 1990). In comparison to other studies in different locations, coral recruitment on artificial substrata in Hong Kong is generally low, in terms of both the number of species and recruit density (Alcala et al., 1982; Birkeland et al., 1982; Wallace and Bull, 1982; Fisk and Harriott, 1990; Latypov, 1991; Harriott, 1992; Maida et al., 1994). The experiments undertaken in this study show that PFA–concrete blocks, like concrete, is a substratum potentially suitable for coral recruitment. The low species composition of scleractinian recruits onto the artificial reef, i.e., O. crispata and C. japonica only, may be due to either the unavailability of coral planula-larvae or environmental conditions at the site at the time of artificial reef deployment. For the same bay, Collinson (1997) showed that only four species of scleractinian corals recruited (all in low densities) onto his settlement plates over a period of approximately 24 months. In conclusion, therefore, coral recruitment onto artificial substrata in Hoi Ha Wan was low, despite the natural diversity of 26 coral species present (Thompson and
Cope, 1982; Cope and Morton, 1988; Collinson, 1997). Both findings represent a picture of early succession by a coral community, with O. crispata and C. japonica being the pioneer species. Juveniles of three other faviids, Favites abdita, Goniastrea aspera and Favia speciosa, with maximum diameter of 5 cm, were observed on the reef modules during a site revisit in 1999. These were estimated to have recruited after this study. With increasing substratum immersion time, a greater diversity of coral species can be expected. There is, however, no previous study on estimating generation time of a new coral community onto an artificial substratum. The time required for the artificial reef at Hoi Ha Wan to recreate an original coral community comparable to that at Coral Beach in Hoi Ha Wan (Cope and Morton, 1988; Collinson, 1997) will, as a result, not to be accomplished within a decade. It has been shown that artificial substrata, e.g., plastic, appear to mainly attract coral recruits of genera usually regarded as opportunists, such as Pocillopora, Seriatopora and Acropora, while a far wider range of genera is recruited onto natural substrata (Wallace and Bull, 1982). If the species composition of coral recruits on natural substrata and PFA–concrete could be further compared, the feasibility of such PFA–concrete artificial reef installations in rehabilitating coral reefs would be better determined. Certain planulators, such as Stylophora pistillata (Loya, 1976), Pocillopora damicornis (Harrigan, 1972) and O. crispata (this study) are classified as pioneer species, that is, they are the first to colonize an open area. Coral colonization probably also follows a successional pattern, in which different species appear in the community as it ages. The herein identified coral recruitment at Hoi Ha Wan, which obtained a single pioneer species, has probably only identified an initial stage, i.e., within the first two years, of a longer term pattern of coral community development. This could be confirmed, if coral recruitment status on the artificial reef were to be studied for a longer time. The choice of any substrata for use in artificial reef construction must satisfy four criteria: durability, safety, functionality and economy (Grove et al., 1991). For durability, the material used can be tested for its structural integrity pre-deployment. The Japanese standard for reef material durability is a minimum 30-year life span without deterioration (Grove et al., 1991). The last criterion can be predicted easily by calculations of costeffectiveness. The second criterion involves studies on leaching of potentially harmful materials, such as trace metals, from the substratum. Preliminary results from studies in the USA (Hockley and Van der Sloot, 1991) and the UK (Collins et al., 1990, 1994) have indicated that trace metal leaching from PFA–cement blocks is insignificant, even after long periods of immersion in seawater because of the formation of a surface salt barrier. The amount and rate of trace metal released
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from blocks used in the present study are not expected to influence adversely the existing flora and fauna of Hoi Ha Wan (Leung et al., 1997). The leaching of metals, i.e., chromium, copper, nickel and zinc, from the Hoi Ha Wan artificial reef structural blocks has been demonstrated to be insignificant (Leung et al., 1997). Any toxicity to the attached biota can be evaluated by studying the accumulation of leachates in it. The material used should also have satisfactory organism-aggregating abilities. This can be tested by comparing the population dynamics and behaviour of individual species between the artificial and natural substrata. The present study shows that PFA-containing concrete (25%, 50% and 75% of PFA by volume), does not differ from true concrete in terms of species diversity, recruitment and succession, coral recruitment, growth of coral transplants and O. crispata growth studies (Lam, 2000b,c,d, in press b, this study). Of the 28 recorded attaching species, only Acrochaetium robustum, Watersipora cucullata, Serpula vermicularis, Spirobranchus tricornis, Pomatoleios kraussii, Corophium crassicorne and Chirona tenuis, showed variations in recruitment between block types. Other studies comparing epifaunal recruitment onto substrata with different PFA contents have also shown that certain species associate better with coal ash concrete blocks than concrete ones, and vice versa (Hatcher, 1993; Sampaolo and Relini, 1994; Relini et al., 1995). In past studies, the higher settlement of biota (in terms of the number of species and individuals) on ash as compared with concrete blocks was suggested to be because of the more friable surface of the former (Relini et al., 1995). The chemical properties of the block surfaces may, therefore, be suitable for algal and invertebrate settlement. Coral recruit densities on such PFA materials, i.e., 48.6 recruit m2 , is fourfold that on PVC plates, i.e., 11.9 recruit m2 (Collinson, 1997). Concrete may be a better substratum for coral settlement than a natural reef slope (Chou and Lim, 1986). These suggest that PFA–concrete is an appropriate material for artificial reef construction, especially when such structures aim at restoring the coral community through natural recruitment. The feasibility of using PFA (up to 75% of PFA by volume) in artificial reef construction for this purpose is, thus, confirmed by this study. This experimental artificial reef deployment study evaluated the use of a concrete artificial reef in habitat restoration of similar marine habitats lacking a hard substratum, especially those previously destroyed by either terrestrial runoff or dredging. Past researches have produced supporting findings for the use of power station wastes in building artificial reefs (Pickering, 1996). The two Hong Kong coal-fired power plants produce approximately one million tonnes of coal waste each year. Land fills and its use in some civil engineering projects are the only available disposal options at pre-
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sent. Such cement-stabilised coal ash could, however, be also used to construct artificial reefs and provide an alternative channel for disposal.
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