Seagrass Ecosystems in the Western Indian Ocean

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Seagrass Ecosystems in the Western Indian OceanAuthor(s): Martin Gullström, Maricela de la Torre Castro, Salomão O. Bandeira, Mats Björk, MattisDahlberg, Nils Kautsky, Patrik Rönnbäck, and Marcus C. ÖhmanSource: AMBIO: A Journal of the Human Environment, 31(7):588-596. 2002.Published By: Royal Swedish Academy of SciencesDOI: http://dx.doi.org/10.1579/0044-7447-31.7.588URL: http://www.bioone.org/doi/full/10.1579/0044-7447-31.7.588

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588 © Royal Swedish Academy of Sciences 2002 Ambio Vol. 31 No. 7-8, Dec. 2002http://www.ambio.kva.se

INTRODUCTIONSeagrass ecosystems constitute an essential part of marine habi-tats in continental shelf waters throughout the world. The dis-tribution of seagrasses ranges from high intertidal to shallowsubtidal soft bottoms, i.e. sandy bays, mud flats, lagoons and es-tuaries, where they often form extensive mono- and multispecificmeadows. In the tropics seagrass beds are commonly found ad-jacent to coral reefs and mangroves. Seagrass beds are amongthe most productive aquatic ecosystems in the biosphere (1) andmay increase biodiversity of associated organisms (e.g. 2–4).They are important as nursery grounds, foraging areas and pre-dation refuges for numerous fish and invertebrate populations

(5–7) and provide great benefits for commercial, subsistence andrecreational fisheries (8, 9). Due to the complex architecture ofthe leaf canopy in combination with the dense network of rootsand rhizomes, seagrass beds stabilize bottom sediments (10) andserve as effective hydrodynamic barriers reducing wave energyand current velocity (11), thereby reducing turbidity (12) andcoastal erosion (13). Further, seagrass beds trap large amountsof nutrients and organic matter in the bottom sediment (14, 15).Through microbial decomposition, seagrass biomass may enterthe marine food web as detritus and thus support productivitythrough recycling of nutrients and carbon (16, 17).

Seagrass ecosystems in the Western Indian Ocean (WIO) re-gion have received limited scientific attention compared to man-groves and coral reefs. The major part of seagrass research hasbeen conducted in Kenya, Tanzania, Mozambique, and easternSouth Africa, and deals mainly with seagrass diversity and ecol-ogy (e.g. 18–22). Few reports deal with how seagrass beds areused as natural resources (e.g. 23–26) or how they are affectedby human disturbance (e.g. 27, 28). Still they do play an impor-tant role for the benefits of the local communities in the region,especially in terms of fisheries (29).

This paper discusses and illustrates the ecological significanceof seagrass ecosystems in the WIO region, mainly from the per-spective of fish and fisheries. Further, we present a case studyfrom Inhaca Island, Mozambique, that is one of the first quanti-tative surveys of fish communities in seagrass beds in the WIOregion.

GEOGRAPHYThe WIO region, which has been characterized as a biogeo-graphic subregion (30, 31), is a province of the Indian Oceanencompassing the African east coast from Somalia to South Af-rica (32) (Fig. 1). There are 10 states situated in the area of whichSomalia, Kenya, Tanzania, Mozambique, and South Africa be-long to the mainland countries and the Seychelles, Comoros,Reunion (belonging to France), Mauritius and Madagascar arethe Island States. The extension of the WIO coastline includingthe Island States is about 12 000 km. A mosaic of different habi-tats and substrates runs along the coastline, e.g. estuaries, coastallagoons, mangrove forests, coral reefs, seagrass beds, mud flats,algal beds, barrier islands, and sandy and rocky beaches (33).The average tidal range across the region varies from 2–4 m andis semidiurnal (32, 34).

The climate and pattern of currents in the WIO are complexand strongly influenced by the monsoonal circulation. Two dif-ferent monsoon periods affect the region (34, 35). The South-east monsoon (Apr–Oct) is distinguished by lower air tempera-tures, strong winds and cool water with low productivity. TheNortheast monsoon (Nov–Mar) presents higher air temperaturesand weak winds (35).

SEAGRASS DISTRIBUTION AND ECOLOGYIN THE WIO

Distribution

Extensive seagrass beds are found in all countries of the WIOregion. About 50 seagrass species have been described in theworld (36, 37), and the coastal zones of the WIO encompass 13

Seagrass Ecosystems in the Western IndianOcean

Martin Gullström, Maricela de la Torre Castro, Salomão O. Bandeira, Mats Björk,Mattis Dahlberg, Nils Kautsky, Patrik Rönnbäck and Marcus C. Öhman

Seagrasses are marine angiosperms widely distributed inboth tropical and temperate coastal waters creating oneof the most productive aquatic ecosystems on earth. In theWestern Indian Ocean (WIO) region, with its 13 reportedseagrass species, these ecosystems cover wide areas ofnear-shore soft bottoms through the 12 000 km coastline.Seagrass beds are found intertidally as well as subtidally,sometimes down to about 40 m, and do often occur inclose connection to coral reefs and mangroves. Due to thehigh primary production and a complex habitat structure,seagrass beds support a variety of benthic, demersal andpelagic organisms. Many fish and shellfish species, includ-ing those of commercial interest, are attracted to seagrasshabitats for foraging and shelter, especially during theirjuvenile life stages. Examples of abundant and widespreadfish species associated to seagrass beds in the WIObelong to the families Apogonidae, Blenniidae, Centris-cidae, Gerreidae, Gobiidae, Labridae, Lethrinidae Lutja-nidae, Monacanthidae, Scaridae, Scorpaenidae, Sigani-dae, Syngnathidae and Teraponidae. Consequently, sea-grass ecosystems in the WIO are valuable resources forfisheries at both local and regional scales. Still, seagrassresearch in the WIO is scarce compared to other regionsand it is mainly focusing on botanic diversity and ecology.This article reviews the research status of seagrass bedsin the WIO with particular emphasis on fish and fisheries.Most research on this topic has been conducted along theEast African coast, i.e. in Kenya, Tanzania, Mozambiqueand eastern South Africa, while less research was carriedout in Somalia and the Island States of the WIO (Sey-chelles, Comoros, Reunion (France), Mauritius and Mada-gascar). Published papers on seagrass fish ecology in theregion are few and mainly descriptive. Hence, there is aneed of more scientific knowledge in the form of describingpatterns and processes through both field and experi-mental work. Quantitative seagrass fish community studiesin the WIO such as the case study presented in this paperare negligible, but necessitated for the perspective offisheries management. It is also highlighted that the pres-sure on seagrass beds in the region is increasing due togrowing coastal populations and human disturbance frome.g. pollution, eutrophication, sedimentation, fishing activi-ties and collection of invertebrates, and its effect are littleunderstood. Thus, there is a demand for more researchthat will generate information useful for sustainablemanagement of seagrass ecosystems in the WIO.

589Ambio Vol. 31 No. 7-8, Dec. 2002 © Royal Swedish Academy of Sciences 2002http://www.ambio.kva.se

A seagrasscommunityat Zanzibar,Tanzania.Photo:K. Österlund Björk.

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tats. Very common in the region are also Halophila ovalis (R.Br.) Hook. f., Cymodocea rotundata Ehrenb. et Hempr. exAschers., Cymodocea serrulata (R. Br.) Aschers. et Magnus,Syringodium isoetifolium (Ascherson) Dandy and Haloduleuninervis (Forsk.) Aschers. in Bossier, whereas Halodulewrightii Ascherson is mainly reported from Kenya and Tanza-nia (39, Bandeira, Björk and S. Beer, unpubl. data). Occurrenceof Enhalus acoroides (L.f.) Royle, Halophila stipulacea (Forsk.)Aschers. and Halophila minor (Zoll.) den Hartog is mainly re-ported from northern Mozambique to Tanzania and in some lo-cations in Kenya (40). Zostera capensis Setchell is only com-mon in southern Mozambique and South Africa where largemonospecific stands may occur (38, 41), but the species has beenfound also in Kenya (40). Ruppia maritima L. (which most au-thors consider a seagrass) is found from eastern South Africanorthwards in the WIO region. In South Africa, R. maritima isquite common in estuaries (42). In Mozambique this species oc-curs mainly in brackish waters of the coastal lakes in the south-ern part of the country and in Madagascar the species was de-scribed as a member of the Madagascar floristic diversity byHumbert and Jumelle (43).

Habitat

Many tropical seagrasses inhabit intertidal regions. As desicca-tion resistance is limited in seagrass they must rely on ways ofavoiding desiccation rather than enduring it (37, 44, 45). Thedepth limits of seagrasses are set by the light penetration.Thalassodendron ciliatum has been reported to grow down to40 m in clearer waters in the WIO (46). The seagrasses in theWIO all need soft substrates like sand or mud except T. ciliatumthat has been reported to grow on bare rock in some exposedlocalities in southern Mozambique (47). The seagrass habitatsin Kenya, Tanzania and northern Mozambique generally con-sist of sediments from coral limestone, while the coastline insouthern Mozambique is principally made of sand (48, 49). Theorganic loading of the sediment might be a critical factor forseagrasses, because it affects the oxygen content. To battle an-oxia, seagrasses have evolved ways of leading oxygen from theshoot to the roots via the lacunae. This allows seagrasses to pro-liferate in anoxic sediments but only up to a certain level. Thislevel has in Southeast Asian seagrass beds been given as 6% or-

ganic matter (DW) in the sediment (37).

Reproduction

Seagrasses in the WIO generally seem to have a vegetativepropagation since most species only rarely have been seen flow-ering. Out of the 13 species occurring in the region, floweringhas been sporadically observed in Cymodocea serrulata, Enhalusacoroides, Halodule uninervis, Halophila minor, and H.stipulacea (41, 50). Frequent flowering occurs in a few speciessuch as Halophila ovalis, Syringodium isoetifolium, Thalassiahemprichii and Thalassodendron ciliatum (Bandeira and Björk,pers. comm.).

Structure and Function

Structural measurements on seagrasses of the mainland part ofthe WIO region have mainly been performed in Thalassiahemprichii, Thalassodendron ciliatum and Zostera capensis spe-cies (e.g. 38, 51–54). These measurements included principally

Figure 1. Map showing the Western Indian Ocean region and thelocation of Inhaca Island, Mozambique (the case study area).

known species (21) (Fig. 2). Mixedseagrass beds with a high diversityare common, up to 8 or 10 speciesat the same locality have been re-ported for Mozambique (38) andTanzania (Bandeira, Björk and S.Beer, unpubl. data).

Two of the most common speciesare Thalassia hemprichii (Ehren-berg) Asherson and Thalassoden-dron ciliatum (formerly Cymodoceaciliata) (Forsk.) den Hartog, bothforming extensive beds in mostparts of the region. T. hemprichiimay usually be found in more pro-tected habitats or on intertidal flats,whereas T. ciliatum normally inhab-its exposed or semi-exposed habi-


biomass, leaf area, size measurements as well as C, N and P con-tents. To date, the highest seagrass biomass in the WIO regionhas been recorded for T. ciliatum at Inhaca Island with the totalbiomass of 1070 g DW m–2 (47). Functional aspects where meas-ured in Kenya and Mozambique (e.g. 19, 47, 55) and includedmainly growth dynamics and nutrient resorption efficiency. T.ciliatum leaf growth rate was estimated at 25 g DW m–2 day–1

and N and P resorption efficiency up to 32.4% and 34.3%, re-spectively (38, 47). T. hemprichii resorption efficiency was upto 3.0% for N and 0.3% for P (56). Ingram and Dawson (28),

species of crustaceans (harpacticoid copepods, amphipods, andostracods), bivalves, polychaetes, nematods, cumaceans, holo-thuroids and phoronoids (62).

Seagrasses attract an assortment of organisms to proliferateattached to the stem and leaf canopy. Tropical seagrass plantsare often inhabited by colonies of sessile fauna like bryozoansand hydroids (60), in association with epiphytic algae and de-tritus (63, 64). The fouling community also includes a likenumber of motile meiofaunal organisms such as amphipods,harpacticoid copepods, ostracods, nematodes, turbellarians,

Figure 2. Seagrass speciesof the Western IndianOcean region. Illustrationsare modified fromRichmond (34).

one of the few works dealingwith aspects of structural eco-logical measurements in theSeychelles, studied leaf areaand shoot density in Cymo-docea serrulata, Syringodiumisoetifolium and T. hemprichii.

ANIMAL COMMUNITIES

General Description

Seagrass beds in tropical re-gions support a large varietyof associated faunal organismsof different taxa with severalecological characteristics (37).Generally they contain greaterbiodiversity and density ofanimals than adjacent unvege-tated habitats (e.g. 2). Sea-grass habitats provide food,shelter and nurseries for sev-eral animals, including manycommercially important fishand shellfish species (8), andcreate remarkably high ratesof secondary productivity (e.g.57–59). The 3-dimensionalstructure of seagrass beds con-tains a broad spectrum ofmicrohabitats and niches mak-ing them convenient as per-manent and transient resi-dences for various benthic,demersal and pelagic organ-isms (60, 61). Animals livingwithin the bottom sedimentare dominated by invertebrate

Hydrocharytaceae Zosteraceae Cymodoceaceae Ruppiaceae

Cymodocea rotundataRuppia maritima

Enhalus acoroides

Halophila minor

Halophila ovalis

Halophila stipulacea

Thalassia hemprichii

Zostera capensis

Cymodocea serrulata

Halodule sp.

Syringodium isotifolium

Thalssodendron ciliatum

H. uninervis H. wrightii


polychaetes, foraminiferans, and gastropods (60, 63, 65). Occa-sionally, substantial quantities of suspension feeding ascidiansare attached to seagrass leaves (66). The major taxa living onthe sediment surface in seagrass beds are echinoderms (starfish,sea urchins, brittle stars and sea cucumbers), crustaceans (crabs)and mollusks (bivalves and snails) (67, 68). The epibenthic faunaalso consist of mobile animals inhabiting the water over and un-der the seagrass leaf canopy, and includes fish, decapod crusta-ceans (prawns and shrimps) and cephalopods, as well as smallcrustaceans like mysids, copepods (cyclopoids and calanoids),amphipods and isopods (60, 69, 70). Many species spend theirpostlarvae and juvenile stages in the seagrass beds before theymigrate into other habitats. The presence of juveniles of com-mercially important penaeid prawns, with peak abundances dur-ing relatively short periods of the year, have been reported in anumber of studies (e.g. 71–75).

The level of seagrass herbivory is thought to be very low ow-ing to poor nutritional values, high C/N ratios and high cellu-lose contents in the leaves of seagrasses (e.g. 76). Valentine andHeck (77) suggested herbivory of seagrasses being underesti-mated and reported that there is a need for more research in thisfield. Cebrián and Duarte (78) showed that the herbivory pres-sure could vary considerably among seagrass species. However,a substantial fraction of tropical seagrass production may be di-rectly consumed by herbivorous fish species and sea urchins (79,80) as well as providing the main food of larger animals suchas dugongs and green turtles (81, 82).

Fish

Seagrass beds provide habitats for a variety of fish species (7,69). It has been widely regarded that they support a higher di-versity and abundance of associated fish than adjacentunvegetated habitats (e.g. 83–86), although there are some con-tradictions (e.g. 6, 87). Seagrass beds play an important role asnursery areas for fish with a number of species that directly de-pend on the seagrass habitat for their survival (e.g. 69; 88–90),while other species have more general preferences (e.g. 91, 92).According to Hemminga and Duarte (37), fish species livingwithin seagrass beds can be distinguished by their residence sta-tus: i) permanent residents are species that spend their entire lifein seagrass beds; ii) temporary residents are species present sea-sonally or during parts of their life in these habitats; iii) regularvisitors are species that frequently visit seagrass beds, e.g.through diurnally migrations from an adjacent coral reef; iv) oc-casional visitors are species that migrate to the beds sporadi-cally.

In the WIO, few studies in fish ecology deal with fish bio-diversity associated to seagrass beds. In reports from Kenya (23),Tanzania (93), Mozambique (26, 94, this study) and Madagas-car (95–97) typical seagrass associated fish communities havebeen characterized. The most common species found belong tothe families Apogonidae, Blenniidae, Centriscidae, Gerreidae,Gobiidae, Labridae, Lethrinidae Lutjanidae, Monacanthidae,Scaridae, Scorpaenidae, Siganidae, Syngnathidae, and Terapo-nidae. Some taxa were more restricted in their distribution, in-cluding species belonging to Plotosidae in Kenya, Atherinidaeand Portunidae in Tanzania, and Pomacentridae and Tetra-odontidae in Mozambique. Pollard (69) showed in a review onthe ecology of seagrass fish communities that the WIO regionwas similar to other areas in terms of fish family composition.In particular Blenniidae, Gerreidae, Gobiidae, Labridae, Mona-canthidae, Sciaenidae, Scorpaenidae, Sparidae, Syngnathidae,and Tetraodontidae were dominant throughout most seagrasshabitats and geographical areas.

A few studies in the WIO deal with feeding preferences andtrophic characters of fish in seagrass beds (e. g 25, 80, 98, 99).Often the most commonly represented trophic category inseagrass habitats is carnivorous fish (e.g. 98), even though vari-

ous important species prefer herbivorous diet (80, 99). The feed-ing preferences may also differ between locations and seasons(98).

FISHERIES ASSOCIATED WITH SEAGRASSES INTHE WIO REGIONAs a consequence of the high primary productivity and habitatcomplexity, the secondary productivity in seagrass beds is sig-nificant. Many of the species using seagrasses during their lifestages have a high commercial value (e.g. 100). The local popu-lation of the coastal zone in the WIO region exploits seagrassbeds in two principal ways: by collection or fishing (e.g. 25, 29,101–103).

Collectors that are active during low tides target invertebratessuch as cockles, cowries, octopus and other mollusks. Crabs andlobsters are highly demanded as well as sea urchins and sea stars.The most extensive commercial activity is the collection of seacucumbers. The fishery over seagrass beds in the WIO is per-formed by artisanal fishermen with beach seine nets and trawl-ing being the most common techniques. In addition, traps andgillnets might be used.

Unfortunately, there is little documentation available that per-mits an evaluation of the size and importance of those fisheriesin ecological, social and economic terms. Information on theseagrass fisheries from the WIO is either scarce or difficult toaccess as it may be in report form at local institutions or authori-ties. However, Gell (29) and Gell and Whittington (26) havedocumented the seagrass fishery and the diversity of fishes inseagrass beds of Montepuez Bay in the north of Mozambique.The results showed that the seagrass fishery was very importantat local levels. Seagrass fishery sustains over 400 fishermen inthe bay. The total fish catch from an area of 35 km2 covered byseagrass was estimated at about 500 t yr–1 (or 14.3 t km–2 yr–1),with a market value of approximately USD 120 000. Part of thecatch went to direct consumption and part was traded. A posi-tive correlation was found using catch per unit effort and totalseagrass cover as variables. According to the authors, this re-sult indicates that seagrass coverage may influence fish biomassand fishery productivity.

A number of authors suggest seagrass fisheries in the WIOregion to be substantial. McClanahan and Young (104), Ngoileand Lindén (33) and Hinrichsen (105) have mentioned fisheryactivities in the whole region, while other authors discuss theimportance of seagrass fisheries in specific countries, e.g. inKenya (106), Tanzania (102, 107) and Mozambique (27, 38).Concrete examples of seagrass fisheries described in the litera-ture are found in Chwaka Bay, Zanzibar (108); Kigomani, Zan-zibar (109); Matemwe and Mkokotoni, Zanzibar (110); InhacaBay (103); Mafia Island and Jibondo (102); the coasts of Tan-zania, Zanzibar and Mafia Island (111). However, only few au-thors, e.g. de Boer and Longamane (101), Gell (24) and de Boeret al. (25), have made comprehensive studies on seagrass fish-ery in the WIO. Numerous studies done in Australia, Europe,and North America have shown the importance of seagrasses forfisheries production (e.g. 75, 90, 112, 113).

ECONOMIC SIGNIFICANCE OF SEAGRASSESSeagrasses are important economic assets in the WIO on bothregional and local scales. Some species of fish and shrimps areexport products that bring foreign income fundamental for theeconomic development of the region. Seagrasses are valuable atlocal levels since they contribute to the provision of protein andcash income to the different human populations. Moreover,seagrasses provide a range of goods and services apart from fish-eries production. Some of the most important ecological serv-ices identified for seagrass habitats are primary and secondary


production, enhancement of biodiversity and erosion control (e.g.37, 114, 115). Despite these facts, few efforts on economic valu-ation of seagrasses have been done not only in the WIO, but alsoin general.

Costanza et al. (116) reviewed the global economical valueof 17 ecosystems services for 16 biomes and calculated thatseagrass/algal beds are estimated to generate gross financial ben-efits amounting 19 000 USD ha–1 yr–1. This is the third highesttotal value of the biomes involved (close to the valuesof estuaries and floodplains) and about 10 times the estimatedvalue of tropical rain forests. Watson et al. (117) estimated thenursery function of the seagrasses for prawn fisheries in CairnsHarbor, Australia. The area studied was dominated by a mix ofthe seagrass species Zostera capricorni and Halodule pinifolia.The approximate area was 876 ha and the value calculated wasbetween 365 and 1324 USD ha–1 yr–1.

Once seagrasses have been damaged, restoration costs can bevery high (118). Restoration of seagrasses has to be planned inappropriate places and a large set of different conditions has tobe fulfilled to achieve success (e.g. 119, 120). Further, the hy-drodynamics of particular areas are significant as to seagrass lossand recovery (e.g. 121). There is little information about the suc-cess of restoration in terms of recovering the whole complexityof the original system (115).

The economic importance of seagrasses is obvious becausethey are essential ecosystems for fisheries production. Fish prod-ucts are important economic assets not only in local but also ininternational markets. Since the fisheries fraction of the totalvalue of seagrass beds is high, the benefit of beds including allecological functions can be substantial.

THREATSDuring the last decades seagrass degradation have received in-creased attention worldwide (122). Widespread losses of seagrasshabitats are reported from many coastal areas including NorthAmerica (123), Australia (124), Europe (125, 126) and Africa(Gullström, unpubl. data). Seagrass demise might be induced bynatural events such as storms (127) or diseases (128). Seagrassloss, however, mainly occurs due to human impacts and the mostgeneral explanation for reduction of seagrass is excessive nutri-ent enrichment, i.e. eutrophication, of coastal waters (e.g. 129–131). Effluent disposal (132) and changes in land-use patterns(133) are other important anthropogenic disturbances thatthreaten seagrass populations. As mentioned, human pressure isimportant and one of the driving forces shaping the coast of WIOis the rapid demographic growth. High nativity rates and migra-tion from inlands to the coastal zone have contributed to an in-creased coastal population which are now supporting about 35million people or about one third of the total population of theEast African region (134).

Environmental impact studies of the human activities onseagrasses are scarce in the WIO. Factors like deforestation, col-lection of invertebrates, destructive fishing practices, sedimentalteration, waste disposal (domestic and industrial), constructionwork activities, changing water regimes through damming ordeviation of rivers and estuaries, unsuitable farming methods andwastes from oil tankers have, however, been mentioned as ac-tivities that threaten seagrasses (27, 135).

Municipal waste problems may vary between countries in theWIO. One important issue is the low capacity and outdated tech-nology of most sewage systems in the region. The sewage sys-tem of Dar es Salaam is planned to be replaced and modernizedduring the coming 8 years. The systems in Mozambique are verydeficient in both rural and urban areas, while Somalia and Mada-gascar have no systems at all (105). In the Seychelles, Ingramand Dawson (28) investigated the effect of untreated river ef-fluents. Their results indicated that sedimentation, salinity and

water quality deriving from discharges were the most importantfactors affecting seagrass growth.

The industrialization of the WIO region is still low, but grow-ing. Agricultural activities, sugar mills and factories producinga variety of products such as textiles, soaps and plastics havebeen reported as sources of pollutants into coastal waters (104,105, 136). The effects of industrial wastes on seagrass beds havebeen little explored. However, studies have demonstrated theability of seagrasses to bioaccumulate heavy metals (122). Theeffects of organic loads from fish farms on the seagrassPosidonia oceanica have been investigated (137, 138). A reduc-tion in water transparency and an increase in dissolved nutrientsand organic content in sediments reduced the shoot size, leafgrowth rates and leaves per shoot of seagrasses. Mechanicaldamage through clearing of seagrasses to make open space fortourism and aquaculture as well as coastal development in theform of housing and harbor structures might also be a threat.Certain species are vulnerable to physical damage and have prob-lems in recovering over a reasonable time span. However, it isunclear to what extent the collection of invertebrates by localsmay damage seagrasses. Boese (139) investigated the effects ofrecreational clam harvesting on Zostera marina in Yaquina Bay,Newport, USA. The study showed that benthic infaunal com-munities were not affected while seagrass biomass was reduced.The author suggested that the results should be taken with cau-tion since no long-term effects were considered.

One of the most negative impacts on seagrasses is thedestabilization of sediments due to the impossibility for the ma-jority of seagrasses to root in high dynamic and mobile sedimentenvironments (e.g. 37), so if adjacent ecosystems such as man-groves and coral reefs are disturbed the sedimentation and en-ergy patterns might change and seagrasses will be affected (e.g.100). Seagrass loss is generally not a gradual slow rate process;rather it seems to be a rapid self-accelerating chain effect proc-ess (140). Losses of mangroves and coral reef areas could alsohave a negative impact on adjacent seagrass beds (141, 142).Other causes of water clarity reduction are sediment loading inthe coasts through land-use changes, mangrove cutting anddredging activities causing higher sediment transport. Some con-struction-related activities like tourism, housing, commerce andmining might contribute to sediment loading as well.

MANAGEMENT ISSUESIn the WIO region seagrass beds have been little considered inmanagement plans. Indirectly, they have been taken into accountwhen protected areas or conservation projects are implemented,but no direct attention has been given to seagrass ecosystems.

The efforts to adopt Integrated Coastal Zone Management(ICZM) in the WIO started in the 1980s and in 1993 the ArushaResolution on Integrated Coastal Zone Management was signedin response to the UN Rio Conference in 1992 (143). Regionalnongovernmental organizations have taken initiatives to promoteICZM and the Western Indian Ocean Marine Scientist Associa-tion (WIOMSA) is an example of an organization, where scien-tists participate with research for development and management.

Programs based on community development and participationhad been the most successful activities in the region (144). Withthis background, the future efforts trying to manage and protectthe extensive seagrass beds in the WIO have to be designed.Seagrasses play an important role in the livelihoods of coastalcommunities, even though the relationship between seagrassesand welfare might not be directly apparent to managers and de-cision makers. Fishermen take large catches from seagrass beds,collectors of invertebrates depend on the seagrass habitat to findtheir products, and in time of crisis probably the local knowl-edge of eating the seeds of seagrasses is still present. To high-light these links to the local population is an important action


to manage seagrass beds. A combined strategy of local partici-pation and education, while speeding up the acquisition ofknowledge and baseline data of seagrasses in the region, seemsto be appropriated at present. The socioeconomic importance andthe design of proper institutions (formal and informal) for ef-fective management of seagrasses also need to be highlighted.Finally, seagrasses must be seen as part of the seascape con-tinuum, where a number of ecosystems interact with each otherand respond to human activities in the coastal zone.

A CASE STUDY AT INHACA ISLAND, MOZAMBIQUEThe dynamics of fish communities in seagrass beds have beenstudied in many tropical coastal waters (e.g. 69, 145–147). Inthe Western Indian Ocean (WIO) region, however, studies ofseagrass fish communities are few and deal mainly with speciescomposition and relative abundance (e.g. 23, 26, 93–97). Thestudy presented here examines density, biomass and spatial dis-tribution of fish assemblages in different seagrass habitats andis one of the first investigations that reveal quantitative fish datafrom seagrass beds in the WIO. A more detailed description onspecies level will be given by Gullström et al. (unpubl.).

The study was carried out at Inhaca Island situated about 35km eastward of Maputo, southern Mozambique (Lat. 25°58'–26°05'S; Long. 32°55'–33°00'E) (Fig. 1). Fish were sampled dur-ing 4 consecutive spring tide periods in October and November1999 at 4 sites in 2 seagrass communities, Thalassodendronciliatum / Cymodocea serrulata (TC) and Thalassia hemprichii/ Halodule wrightii (TH) (mapped and identified by Bandeira,2000). The sampling was conducted in daylight, 0–3 hrs beforehigh tide and at depths of 1.4–2.9 m, using a beam trawl withan opening of 1.44 x 0.43 m. The net had an unstretched meshdimension of 6 mm and a cod-end of 3 mm mesh size. In thelaboratory, all fishes were identified to the lowest taxonomiclevel possible and counted. The individuals were measured forstandard length (SL) to the nearest mm and wet weight to thenearest 0.01 g.

A total of 2102 fish individuals belonging to 56 different taxafrom 27 families were recorded during the study at Inhaca. Thefamily Siganidae (represented by only one species, i.e. Siganussutor) dominated the catch and was ranked first by overall abun-dance (23.2%) and biomass (30.7%). Labridae (21.2%),Monacanthidae (15.7%) and Teraponidae (7.9%) were also abun-dant, while high biomass was found of Teraponidae (10.3%),Labridae (9.7%), Lethrinidae (7.5%), Platycephalidae (7.0%) andScaridae (6.7%). The results showed a total fish density of 0.11± 0.02 ind. m–2 in TC and 0.02 ± 0.004 ind. m–2 in TH, and atotal fish biomass of 0.99 ± 0.21 g m–2 for TC and 0.18 ± 0.08 gm–2 for TH (Fig. 3). The differences between the two seagrasscommunities can be explained by various biotic and abiotic

mechanisms. As suggested in the literature (e.g. 91, 148, 149),the main reasons for spatial heterogeneity of fish in seagrass bedsmay be due to differences in plant morphology and structuralcomplexity, significant factors for the efficiency of shelteragainst predation and foraging success. Epiphytic algae on thestems and leaves of seagrasses might also be important for thedistribution of fish as they provide food for many marine organ-isms (150). Further, the composition of seagrass species in thetwo types of seagrass communities (38) as well as the zonationof seagrasses due to the tidal gradient around Inhaca Island mayalso influence the distribution of fish. TC occurs within or inclose connection to subtidal areas, whereas TH has its main ex-tension in the intertidal zone and, thus, longer air exposure dur-ing low tide. In addition, existing hydrodynamic conditions canalso be relevant for the fish-habitat interactions in seagrass beds.In Table 1, fish standing stock data from this study have beencompared to other studies with quantitative data in differentseagrass habitats. Both fish density and biomass seem to be quitelow, but are still within the similar range as the comparative stud-ies, where the density ranged from 0.02 to 6.08 ind. m–2 and the

Figure 3. Mean density (a) and biomass (b) ± SE of total fish catch from4 sites in 2 different seagrass community types around Inhaca Island,Mozambique. TCB = Thalassodendron ciliatum / Cymodocea serrulataat the Biological station area (n = 10); TCP = Thalassodendron ciliatum/ Cymodocea serrulata at the Porthino area (n = 3); THP = Thalassiahemprichii / Halodule wrightii at the Porthino area (n = 3); THS =Thalassia hemprichii / Halodule wrightii at the Saco da Inhaca area (n =3).

Table 1. Fish standing stock in seagrass beds.

Location Community Density Biomass Source(ind. m–2) (g m–2)

Puerto Rico Thalassia testudinum and 0.65–3.15 Martin and Cooper (152)Syringodium filiforme

Northeast Australia Seagrass areas 0.5–1.8 Blaber et al. (145)(mainly Enhalus acoroides)

Groote Eylandt, Short seagrass sites 0.57–2.21 Blaber et al. (91)northern AustraliaGroote Eylandt, Tall seagrass sites 0.16–3.84 Blaber et al. (91)northern AustraliaCairns, Australia 8 seagrass species 0.88 Coles et al. (75)

(mainly Zostera capricorni)Southern Australia Different seagrasses 3.03–6.08 1.67–2.58 Edgar et al. (112)Maine, USA Zostera marina 1.12 Mattila et al. (86)Inhaca Island, Thalassodendron ciliatum and 0.11 ± 0.02 0.99 ± 0.21 This studyMozambique Cymodocea serrulataInhaca Island, Thalassia hemprichii and 0.02 ± 0.004 0.18 ± 0.08 This studyMozambique Halodule wrightii

BIOMASS (b)

DENSITY (a)

Sampling site

Sampling site

Ind.

m–2

g m

–2

0.15

0.10

0.05

0.00

1.5

1.0

0.5

0.0

TCB TCP THP THS

TCB TCP THP THS


References and Notes1. Duarte, C.M. and Chiscano, C.L. 1999. Seagrass biomass and production: A reassess-

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Figure 4. Multidimensionalscaling (MDS) plots, basedon Bray-Curtis similaritymatrix on double squareroot transformed data, offish density (a) andbiomass (b).■ = Thalassodendronciliatum / Cymodoceaserrulata at the Biologicalstation area;● = Thalassodendronciliatum / Cymodoceaserrulata at the Porthinoarea;● = Thalassia hemprichii /Halodule wrightii at thePorthino area;▲ = Thalassia hemprichii /Halodule wrightii at theSaco da Inhaca area.

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within a seagrass bed. As the case study suggests, there is aninteraction between fish distribution and seagrass communities,likewise it might be a similar kind of interaction betweenseagrasses and other marine organisms. Eutrophication, sedimentloading, mechanical damage and effluent disposal are examplesof other human-induced threats that may have negative impactson seagrass habitats. Direct protection of seagrass ecosystemshas not been considered in the WIO, but sometimes conserva-tion occurs within systems where seagrasses are interlinked withadjacent habitats such as coral reefs and mangroves.

Besides basic research studies there is a need for studies ad-dressing ecological valuation, impact assessment, pollution in-fluences and linkages to other habitats. It is important to accel-erate the acquisition of knowledge and to try working in inter-disciplinary groups running parallel projects considering ecologi-cal and economic aspects. Thus, for a future efficient and sus-tainable coastal zone planning and management of the seagrasshabitats in the WIO, monitoring and evaluation of the ecologi-cal conditions as well as research are indispensable.

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BIOMASS(b)


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153. Acknowledgements. This study was supported by the Sida/SAREC Bilateral MarineScience Programme between Sweden and Tanzania (Sida = Swedish International De-velopment and Cooperation Agency).

Martin Gullström is a PhD student at the Department of MarineEcology, Göteborg University, Sweden. His research interestsrelate to ecology and human disturbance of seagrass ecosystemsin tropical and temperate coastal zones. His address: KristinebergMarine Research Station, SE-450 34 Fiskebäckskil, Sweden.E-mail: [email protected]

Maricela de la Torre Castro is a PhD student at the Department ofSystems Ecology, Stockholm University, Sweden. Her presentresearch focuses on seagrasses as natural resources and how thepresence of seagrasses contributes to human welfare. Heraddress: Department of Systems Ecology, Stockholm University,SE-106 91, Sweden.E-mail: [email protected]

Salomão Bandeira is a lecturer in botany at the Department ofBiological Sciences, Universidade Eduardo Mondlane,Mozambique. He obtained his PhD at the University of Göteborg,Sweden, and his main research interests cover seagrass andseaweed diversity and ecology. Salomão Bandeira is currently theHead of the Department of Biological Sciences, UniversidadeEduardo Mondlane. His address: Department of BiologicalSciences, Universidade Eduardo Mondlane, P.O. Box 257, Maputo,Mozambique.E-mail: [email protected]

Mats Björk, PhD, is associate professor in plant physiology at theBotany Department of Stockholm University. He has specialized inseaweed and seagrass physiology on relation to pollution, and isalso one of the coordinators of the bilateral Sida/SAREC Marineprogram between Sweden and Tanzania. His address: BotanyDepartment, Stockholm University, SE-106 91 Stockholm, Sweden.E-mail: [email protected]

Mattis Dahlberg, MSc in Marine biology, Göteborg University,Sweden, has been involved in seagrass research activities atInhaca Island, Mozambique. His address: Department of MarineEcology, Göteborg University, Kristineberg Marine ResearchStation, SE-450 34 Fiskebäckskil, Sweden.E-mail: [email protected]

Nils Kautsky is professor of marine ecotoxicology at Departmentof Systems Ecology, Stockholm University and Deputy Director ofThe Beijer International Institute for Ecological Economics at theSwedish Royal Academy of Sciences. He has more than 20 yearsof experience of coordinating projects on marine ecology,aquaculture and coastal area management in Africa, Asia andLatin America. His address: Department of Systems Ecology,Stockholm University, SE-106 91 Stockholm, Sweden.E-mail: [email protected]

Patrik Rönnbäck has a PhD in systems ecology from theDepartment of Systems Ecology, Stockholm University, Sweden.His research interests relate to Ecological Economics analysis offisheries, aquaculture and mangrove ecosystem. His address:Department of Systems Ecology, Stockholm University, SE-106 91Stockholm, Sweden.E-mail: [email protected]

Marcus Öhman, PhD, is a research scientist and senior lecturer atthe Department of Zoology, Stockholm University, Sweden. Hisresearch interests are in marine ecology, fisheries, disturbanceeffects on the marine biota, environmental economics and coastalzone management. He is also a coordinator and research advisorfor various Sida supported marine science projects in East Africa,South Asia and in the Caribbean. His address: Department ofZoology, Stockholm University, SE-106 91 Stockholm, Sweden.E-mail: [email protected]

Seagrass Ecosystems in the Western Indian Ocean

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