Ecology of seaweed beds on two types of artificial reef

Chang Geun Choi1, Yashuo Takeuchi2, Toshinobu Terawaki3, Yukihiko Serisawa4, MasaoOhno5 and Chul Hyun Sohn1,*1Department of Aquaculture, Pukyong National University, Pusan, 608-737, Korea; 2Shikoku ResearchInstitute Inc., Yashima-nishimachi Takamatsu, Kagawa, 761-0192, Japan; 3National Research Institute ofFisheries and Environment of Inland Sea, Fisheries Agency, 2-17-5 Maruishi, Ohno, Saeki, Hiroshima, 739,Japan; 4Marine Biosystems Research Center, Chiba University, Amatu-kominato, Awa-gun, Chiba, 299-5502,Japan; 5Usa Marine Biological Institute, Kochi University, Inoshiri, Usa, Tosa, Kochi, 781-1164, Japan;*Author for correspondence (e-mail: chsohn@nuri.net; phone +82-51-620-6133; fax +82-51-628-7430)

Received 1 February 2002; accepted in revised form 13 May 2002

Key words: Artificial reef, Community, Ecklonia karume, Enteromorpha intestinalis, Roughness, Sargassum,Settlement, Succession

Abstract

Artificial reefs for the development of valuable fishery resources were placed on sandy substrates at 8, 10 and 13m depths in Muronohana, Ikata, Japan, and observed monthly or bimonthly from February 1999 to June 2001.Enteromorpha intestinalis and Colpomenia sinuosa were the primary dominants in spring. The number of sea-weed species at all the sites gradually increased in winter. Each reef reached a climax stage of Sargassum spp.,Ecklonia kurome and Padina arborescens within 18 months. Overall, 38 seaweed species were found during thestudy. The settlement of kelp, such as E. kurome, was promoted by reduced sand cover as a result of turbulence.Large-scale surface roughness could be important in maintaining the communities after initial establishment.

Introduction

Seaweeds provide a habitat and serve as spawning-beds, breeding and feeding grounds for coastal fishand shellfish, as well as other important fisheries re-sources (Terawaki et al. 2001), which are commer-cially important in themselves (Watanuki and Yama-moto 1990). Seaweed beds of rocky bottoms are animportant source of drifting plants that provide foodand shelter for coastal marine life, including econom-ically important fish (Largo and Ohno 1993). In orderto achieve sustainable coastal development, seaweedforest formation has been emphasized as one methodto conserve a healthy coastal environment (Choi2001).

An “M type” artificial reef has been constructedfor the purpose of increasing abalone numbers. The“RF type” of artificial reef has become a popular tech-nique for the enhancement of spiny lobster popula-tions. These reefs are thought to improve fisheries by

concentrating fish and also increasing the natural pro-duction of biological resources.

Many ecological studies have been undertaken re-garding the re-establishment of kelp forests, with spe-cial attention to the use of artificial reefs to recoverlost seaweed beds (Choi et al. 2000a,b). Seaweed bedcreation has become an important aspect of conserva-tion of the marine ecosystem and of coastal zonemanagement (Terawaki et al. 2001). The aims of thepresent study were to describe successional and sea-sonal trends of seaweed components on concrete reefsplaced at different depths at a site in southern Japan.

Materials and methods

The study was conducted on artificial reefs placed atdepths of 8, 10 and 13 m in Muronahana, Ikata,Shikoku islands, Japan on 16 February 1999. Tworeefs of different sizes were placed at depths of 8 (siteA), 10 (site B) and 13 m (site C) (Figure 1). The “M

343Journal of Applied Phycology 14: 343–349, 2002.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

type” artificial reef placed at sites A and B had a vol-ume of 3.47 m3 (2.5 × 1.5 × 1.25 m). The “RF type”reef at sites A and C was 4.8 m3 (2 × 2 × 1.2 m) (Fig-ure 1I, 1II). A large number of perennial seaweeds,such as Sargassum horneri, S. macrocarpum and Eck-lonia kurome grow on natural substrata around thestudy site.

The succession and growth of various marine al-gae were observed by scuba diving monthly or bi-monthly over a 15-month period from March 1999 toJune 2001. Photographs were taken by digital videocamera, video camera and 35 mm camera, includingones of seaweeds on adjacent natural substrata.

Water temperature, salinity and transparency (Sec-chi disk method) were measured at the study site. Sa-

linity was measured with a digital salinometer (Model3-G, Tsurumi Seiki, Yokohama).

Results

Water temperature, salinity and transparency are pre-sented in Figure 2. Water temperature ranged between10.0 °C to 28.3 °C and salinity between 32.96 and35.49‰ during the study period. Transparency rangedfrom 6 to 8 m at site A, to 10 m at site B and to 12 mat site C.

Table 1 shows the monthly abundance of algae onthe artificial reefs over the study period. A total of 38species of seaweed (6 Chlorophyta, 18 Phaeophyta,

Figure 1. Schematic representation and arrangements of “M type” (I) and “RF type” reefs (II) placed at the different sites. III, IV: Growth ofseaweeds on artificial reefs.

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14 Rhodophyta) were identified during the study pe-riod. Diatom colonization on all reefs achieved 100%cover within one month of placement. Within threemonths after placement, Enteromorpha intestinalisand Colpomenia sinuosa appeared in abundance onthe reefs, although they did not grow on the rocks inthe study area. Thalli of E. intestinalis and C. sinuosadecayed over the summer, at which time coralline redalgal species started to grow on the reefs. From Au-gust to October 1999, coralline algae covered almost100% of the reefs at the study site. Diatoms and cor-alline algae had greatly increased their colonizationof the reefs from October to December 1999.

Brown algae such as Sargassum spp. and Eckloniakurome covered over 20% of the reef during the win-ter season. The number of species tended to increaseon the reefs during the winter and spring seasons(February to June 2000). In February 2000, C. sinu-osa re-appeared and covered 20–60% of the artificialreefs until June 2000. The dominant and subdominantspecies were E. kurome, Padina arborescens, Sargas-

sum spp. and Jania adhaerens from June 2000 to June2001.

Table 2 shows the cover of Sargassum spp. andEcklonia kurome attached to the tops and sides of Mtype and RF type reefs during June to August 2001.In June, Sargassum spp. covered approximately 50–60% of the top surfaces. In June and August, the Sar-gassum community was most abundant on the topsurface of M type and RF type reefs than the sides.Ecklonia kurome covered approximately 50% of theside part of the reefs in June and August. However,coverage of the top surface did not exceed 23%within the same reefs. The cover of E. kurome tendedto be the same on the sides during June to August.

Within 5 months of reef placement, green algaewere more abundant at the shallow site (A) than atdeeper sites (B and C; both the M and RF type reefs).Brown algae at the shallower site (A) appeared ear-lier and were more abundant than at the deeper sites.In contrast, the cover ratio of red algae did not in-

Figure 2. Seasonal changes in environmental factors at the study sites.

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crease as fast and to such an extent at site A whencompared to sites B and C.

Algal biomass on the reefs averaged 1698g wet wt m−2 at site A and averaged 620.6g wet wt m−2 and 583.1 g wet wt m−2 at sites B andC, respectively in June, (Figure 3). The biomass atsite A was mostly due to Sargassum spp. and Col-

pomenia sinuosa. The Sargassum community bio-mass was 1214 g wet wt m−2 at site A, 203.7g wet wt m−2 at site B, and 74.1 g wet wt m−2 at siteC in June 2000. Sargassum spp. biomass decreasedremarkably in July and August, i.e. 439.1g wet wt m−2 to 226.9 g wet wt m−2 at site A. In con-trast Padina arborescens gradually increased on the

Table 1. Monthly abundance of algae on the artificial reefs at the study site.

Species 1999 2000 2001

M M J J A O N D F M A J J A O D M A J

Diatom ‰ ‰ ‰ ‰ ‰

Enteromorpha interstinalis ‰ ‰ ‰ � � �

Enteromorpha sp. �

Ulva pertusa � �

Cladophora sp. �

Codium fragile � � � �

C. tenue �

Ectocarpus sp. © © �

Colpomenia bullosa �

C. sinuosa ‰ © ‰ © ‰ © � ©

Hydroclathrus clathratus © � ©

Cutleria multifida � �

Ecklonia kurome � � © © © © © © © ©

Dictyota dichotoma �

D. linearis � ©

D. patens � � � �

Pachydictyon coriaceum � � © �

Padina arborescens � � � � © © © © © � � ©

Spatoglossum pacificum �

Sargassum hemiphyllum � © © � � � ©

S. horneri � � � � © � �

S. macrocarpum � � © © � � � © © © © © © � ©

S. patens � � � � �

S. piluliferum � © � � © � ©

Sargassum sp. � �

Scinaia japonica �

S. latifrons � �

Asparagopsis taxiformis � �

Gelidium sp. � �

Amphiroa dilatata � � ©

A. zonata �

Jania adhaerens � © ‰ ‰ ‰ ‰

Jania sp. ©

Prionitis sp. �

Hypnea japonica �

Gigartina sp. �

Lomentaria catenata �

Ceramium kondoi �

Coralline algae © ‰ ‰ ‰ ‰ ‰ ‰

‰: Dominant (more than 40% cover), ©: Subdominant (10–40% cover), �: Rare (less than 10% cover)

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artificial reefs from 103.5 to 206.6 g wet wt m−2. Thebiomass of seaweed was found to decay and decreasesignificantly on the artificial reefs at all study areasduring the summer season (July to August 2000).

At the early stage (March to July 1999), E. intes-tinalis and C. sinuosa covered 30–100% of the avail-able surface. In June and July, the cover of E. intes-tinalis at sites A and B was nearly 100% (Table 3).The thalli of E. intestinalis disappeared in July at siteC, whereas at sites A and B it only disappeared inSeptember and August, respectively.

Coralline algae dominated from August to Decem-ber 1999. Sargassum spp. appeared on the reefs at siteA after 6 months, it only appeared after 16 months atsites B and C. In June 2000, P. arborescens, E.kurome and J. adhaerens gradually covered approxi-mately 4–60% of the reefs. At site C, the cover of J.adhaerens tended to be comparatively greater thansites A and B (about 36–100% on the reefs from Juneto December 2000).

Discussion

The succession on the artificial reefs described hereshows a similar pattern to that for other artificial reefs(Serisawa and Ohno 1995a,b; Ohno 1993). In the ini-tial stage, diatoms colonized all reefs at 100% cover.Commercially valuable shellfish such as turban shelland abalone appeared on the artificial reefs after onemonth of establishment and grazed on the diatom andseaweed communities. Yamada et al. (1992) reportedcolonization of small annuals and crustaceous algaeon artificial concrete reefs. In this study, E. intestina-lis and C. sinuosa were found to be the primary colo-nists. Early colonization by these species might bedue to the annual release cycle of their reproductivecells.

Several species of the Melobesioideae dominatedin summer and autumn when water temperatures werehigher. During autumn and winter, when water tem-peratures declined, the cover of brown algae wascomparatively higher. The course of succession wasclearly affected by season and temperature.

Seaweeds present on the artificial reefs in wintermatured in May or June (Ohno et al. 1990). The totalnumber of species found on all the reefs during theentire study period was 38, gradually increasing fromFebruary 2000, with the highest number in winter.Serisawa et al. (1998) reported that within a year fol-lowing placement, Sargassum spp. was dominant onall plates. In our studies Sargassum spp., E. kuromeand P. arborescens dominated in late successional(climax) stages of placement.

The stability of the substrate is very important formaintaining seaweed beds (Watanuki and Yamamoto1990) and in this study all reefs resisted waves andcurrents. Grazers, such as sea urchins, did not colo-nize the sites during the study.

Foster (1975) reported that Macrocystis pyriferacolonization was greatest near the upper horizontal

Table 2. Cover of Sargassum spp. and Ecklonia kurome attached to surface and side parts of M type and RFtype reefs at the study site during June to August 2001.

(Unit: %)

June August

Sargassum spp. Ecklonia kurome Sargassum spp. Ecklonia kurome

M RF M RF M RF M RF

Surface 60 50 20 13 30 23 23 15

Side 40 20 50 44 5 13 50 50

Figure 3. Mean biomass of seaweed attached to the different typesof artificial reef at the study sites during July to August 2000.Other: Other algae, C. sin: Colpomenia sinuosa, E. kur: Eckloniakurome, P. arb: Padina arborescens, S. spp.: Sargassum spp.

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edges of blocks and concrete prisms. He mentionedthat this ‘edge effect’ may result from a combinationof increased spore and larval settlement as well asfrom enhanced growth of plants associated with tur-bulent eddies formed around such obstructions. Forsimilar reasons, E. kurome colonization might also befacilitated. We observed little sand cover on the edgesand sides of the reefs, as a result of turbulence.

The above results suggest that if settlement of kelpspecies such as E. kurome, on artificial reefs is de-sired, then large-scale roughness and/or kelp knobs

(Terawaki et al. 2001) could help. Finally, depth iscritical for achieving maximum algal biomass and inMuronohana, artificial reefs should be established atdepths of less than 10 m.

Acknowledgements

The authors thank Dr Alan T. Critchley, Departmentof Botany, University of the Witwatersrand, Johan-

Table 3. Dominant algal species attached to the artificial reefs at sites A, B and C.

348

nesburg, South Africa, for useful suggestions in thepreparation of this manuscript.

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