Coral recruits on concrete hemispherical artificial reef (HSAR) with different composition in Pasir Putih, Situbondo, Indonesia

Artificial reefs (AR) are artificial habitats laid in seabed by mimicking some of the characteristics of natural reefs, hence can be used as natural substrates for recruitment process for coral larvae. One of well-developed AR is concrete hemispherical artificial reef (HSAR) which serve as a solid platform for settlement and recruitment processes of coral larvae. In this study, we constructed HSAR made of concrete with crushed shell waste (CSW) and fly ash-bottom ash (FABA) as substitutes for coarse and/or fine aggregates. The purpose of the research is to compare the diversity, density and level of coral recruitment on HSAR with different composition; namely C1), conventional concrete composed by a mixture of Portland cement, sand and gravel (1:3:2); C2), concrete with CSW as substitutes for gravel or coarse aggregates (1:3:2); and C3) concrete composed by Portland cement, FABA, sand and CSW (1:2:2:1). Observation of coral recruits conducted in-situ in Pasir Putih beach, Situbondo, East Java, Indonesia. All observed ambient environmental parameters (temperature, salinity, lucidity, level of dissolved oxygen and sedimentation level) are in range for tolerance level of coral growth and development as well as to support larval settlement and recruitment. Results of one-way Anova and LSD test (p = 0.05) showed that there was no difference in term of density of recruits in C1 (9.5±2.07 colonies/m2) and C2 (8.7±2.5 colonies/m2), yet significantly differed with C3 (6.5±1.43 colonies/m2). We identified at least 12 genera of coral recruits grown on the surface of HSAR, dominated by Acropora (60.08%), Porites (26.98%) and Seriatopora (17.06%), respectively; with relatively similar genera composition on each AR. These results suggest that addition of CSW to concrete AR may not have significant positive effect on the coral larval settlement. Furthermore, concrete AR contain FABA was presumably less suitable for coral recruitment process.


Introduction
In 2015, the target of Indonesia shellfish production was 233,700 tons but achieved only at 39,124.06 tons [1].However, it is estimated that shellfish production will continue to increase in line with population growth and consumer demand.As the results, increased shellfish production will have impact on increasing of the shell waste which reaches 90% from the total wet weight of the shellfish [2] and therefore will require serious handling.Generally, shell wastes will be thrown away or maybe 1250 (2023) 012013 IOP Publishing doi:10.1088/1755-1315/1250/1/012013 2 used as land filling material [2], or made into handicrafts, abrasive components in toothpaste, animal feed mixtures, absorbents for water management and other minor utilization [3].In addition, shell waste also contains calcium carbonate, make it potentially utilized as a secondary base material for applications in a circular economy perspective, for example as a substitute material in cement mixtures [2,4].This concept has also been developed and applied in the manufacture of green artificial reefs (GARs) [3,4] where shellfish waste is used as a substitute of coarse aggregate for concrete artificial reefs (AR) [3].Calcium content in concrete AR will leached when submerged in seawater [5] and is estimated to be a source of calcium for coral biomineralization processes.AR is man-made structure deliberately introduced or submerged on the seafloor for the purposes to provide suitable habitat for marine organism and enhance fisheries as well as for shoreline protection [6,7].In addition to shell waste, fly ash-bottom ash (FABA) resulted from coal combustion can also be used as an additional or substitute material in the cement mixture for AR; either as substitutes for aggregates or the cement itself.In Indonesia, FABA is no longer regarded as hazardous and toxic waste [8] even though contains various heavy metals such as As, Cd, Pb, Zn, Cu, Hg, etc [9].Many of those trace elements are known to inhibit growth and development or even cause the death to the corals [10,11,12].Research on the use of shell waste as a substitute of coarse aggregate, as well as FABA as a substitute for fine aggregate in concrete AR has been carried out by [3].In the study, it was found that Acropora muricata coral fragments transplanted on AR containing 25% of FABA had a lower growth rate than coral fragments in conventional concrete or concrete with shell waste.Study by [3] focused only on the growth rate of transplanted coral fragments, whereas study on how the effect of addition of those substitutes into concrete AR on coral recruitment has not been conducted.

Artificial reef design
The hemispherical artificial reefs (HSAR, Figure 2) [13,14] were 50 cm in diameter and height, used as platform for coral settlement and recruitment process [15].Three types or mixtures were selected: C1), conventional concrete which composed by a mixture of Portland cement, sand, and gravel (composition was 1:3:2); C2), a mixture of Portland cement, sand, and crushed bivalve's shells or CSW (composition was 1:3:2); and C3), a mixture of Portland cement, FABA, sand, and CSW (composition was 1:3:3:2), respectively.The HSAR were deployed in 2017 at the depth of 4-5 meter and placed in empty spaces among life coral colonies.

Observation of coral recruits and data analysis
Coral recruits or juvenile coral colonies (less than 5 cm in width) [16] were observed in-situ on the surface of each type of HSAR.Each coral recuits was counted to determinate the density (per square meter) and identified to the level of genus [17].The density of coral recruits then categorized based on the coral recruitment rate according to [18].A one-way analysis of variance (Anova) followed by Tukey's HSD test (at α = 0.05) were performed to compare the density on each of HSAR.

Environmental parameters
The success of corals recruitment process and early growth are determined by the availability of coral larvae, availability of substrate for larval attachment and suitable environmental conditions.In this study, abiotic environmental factors measured during field observation were temperature, salinity, current velocity, alkalinity (pH), level of dissolved oxygen (DO), visibility (lucidity) and sedimentation rate with the results shown in Table 1.All the parametes are considered suitable for the growth of coral and to support larval settlement, based on Indonesian quality standard parameter for coral reef and other previous research.

Density of coral recruits
The application of HSAR in the study is to provide a sturdy and suitable platform for larval settlement [3].The addition of CSW in C2 and C3 intended as a substitute for coarse aggregate (gravel) [3,4]; even though this will decrease the compresive and split tensile strengh of the concrete [7,22] and increase the porosity of the concrete [7] thus increasing contact between the concrete surface and the seawater.Shell waste also contains calcium carbonate and is estimated to be a source of calcium for coral biomineralization or calcification processes.Density of coral recruits in each type of AR (C1, C2 and C3) are shown in Table 2.It is initially expected that, compared to the C1 model, the C2 and C3 AR models will be more attractive to coral larvae to settle and grow on the surface of the concrete; since both AR contain more calcium carbonate and have more porous structure; also with higher concentration of silica in C3.
However, results of one-way Anova followed by LSD test (both at p = 0.05) showed that there was no difference in term of density of recruits in C1 (9.5±2.07 colonies/m 2 ) and C2 (8.7±2.5 colonies/m 2 ).The average density in C2 seem to be lower, yet the density of coral recruit ranges from 7 to 14 colonies on both AR.Based on this result, the addition of CSW may not have significant positive effect on the coral larval settlement.A relatively similar finding was shown by [7], in which porous structure may be beneficial to biological attachment only in short term and the reverse result occurred in the long term.
In case of the C3, the density of coral recruit (6.5±1.43 colonies/m 2 ) are significantly lower compared to C1 and C2.Addition of FABA containing heavy metals into the concrete mixture may causing decrease of coral recruits in C3.In low concentration, certain heavy metals in FABA such as Cu and Pb have sublethal effect on the larvae and adult corals.Unfortunately, the data of concentration of those heavy metals in the concrete are not available in this study.Different coral species may show different responses when exposed to Cu.In a study by [23], settlement of Acropora tenuis larvae reached 43.5% in seawater with 20μg/L of Cu and decreased to 13.1% when concentration of Cu is 50 μg/L.It is also stated that larval settlement and metamophosis stages are more sensitive to Cu compared to the response of mortality.Sublethal effect of Cu and Pb on coral larvae was also investigated by [10], showing significant effect of both elements on motility and larval survival.Another experiment by [24] showed that Pocillopora damicornis larvae exhibit significant decrease of settlement in 1000μg/L of nickel.Under ambient temperature, adults and larvae of P. damicornis appear to tolerate exposure to unusually high levels of Cu and Pb; but warmer condition will reduce the tolerance to Cu toxicity [25].Similar to [24], coral P. damicornis are more sensitive to copper compared to Pb [25].Thus, it can be considered that Cu is relatively more toxic compared to other metals in marine environment.

Species composition
During the research we identified 12 genera of corals that settled and growth on the surface of concrete HSAR as shown in Table 2 and Fig. 3.In term of number of genera, there is no difference between different type of HSAR as indicated by result of one-way Anova (not shown).However, concrete with CSW (C2 and C3) have slighty higher diversity with 8 and 7 genera, compared to C1 (without CSW) with 6 genera.In general, composition of coral genera on each type of concrete are similar, mostly dominated by Acropora (F.Acroporidae) with relative density of 38.95-40.68%,followed by Porites (F.Poritidae, 18.39-27.53%)and Seriatopora (F.Pocilloporidae, 13.68-21.84%),respectively.
C1 concrete HSAR C2 concrete HSAR C3 concrete HSAR The ultimate factor affecting the presence and composition of coral recruits is the occurrence on adult coral colonies that produced the larvae [26].The reef flat surrounding study site is dominated by submassive and branching Acropora [3], as well as massive or submassive colonies of Porites, Favia, Favites, Goniastrea, Montastrea and other genera from the families of Merulinidae.Acroporid is the most diverse and abundant coral in shallow and clear water of Indonesia or West Indo-Pacific [17].Poritid are well known as corals with high ability to withstand in fluctuated environmental condition and tolerant to various substrates such as rubble, dead corals and sand; therefore, usually found in high abundance.Seriatopora and other Pocilloporid are able to spawn throughout the year; they are also pioneer corals to colonize new substrate.Several genera of coral recruits in this study were only found in certain types of concrete.Galaxea, Goniastrea and Montastrea are only found in C2; Heliofungia is only found in C3 while Merulina is only found in C1. Adult colonies of those genera also occur around the study site but are less common than other members of Acroporidae, Pocilloporidae and Poritidae; so it is possible that the recruitment of these genera is lower and only occurs in certain types of AR.

Conclusion
After comparing the density of coral recuits on each type of concrete HSAR with different composition, we conclude that addition of the addition of CSW may not have significant positive effect on the coral larval settlement yet concrete with CSW have higher number of coral genera.Adversely, addition of FABA on the concrete was presumably have negative impact on the larval settlement due to presence of heavy metals (mainly Cu or copper) in the FABA; however, this assumption need to be further investigated.

Acknowledgement
We

Figure 2 .
Figure 2. Design view of concrete hemispherical artificial reef (HSAR) used as artificial substrate for coral's larval settlement in the study

Figure 3 .
Figure 3. Composition and relative density of coral genera found on each type of HSAR

Figure 4 .
Figure 4. Photographs of several genera of coral recuits in the study

Table 2 .
Density of coral recruits (mean ± standard deviation) on each type of HSAR (density value followed by same superscript symbol show no significant difference based on results of one-way Anova and Tukey's HSD test at α = 0.05) thank Ministry of Education, Culture, Research, and Technology of Republic of Indonesia for the research grant schemed Penelitian Disertasi Doktor (Doctoral Thesis Research) 2022; also to Directorate of Research and Community Services (DRPM) of Institut Teknologi Sepuluh Nopember and local community (Slolop Marine Conservation) in Pasir Putih, Situbondo for invaluable support during the research.