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South African Journal of Botany 2002, 68: 191–198 Copyright © NISC Pty Ltd Printed in South — All rights reserved SOUTH AFRICAN JOURNAL OF BOTANY ISSN 0254–6299

Diversity and distribution of around Island, southern

SO Bandeira

Department of Marine Botany, Göteborg University, PO Box 461, SE 405 30 Göteborg, Sweden Present address: Department of Biological Sciences, Universidade Eduardo Mondlane, PO Box 257, , Mozambique e-mail: [email protected]

Received 21 September 2000, accepted in revised form 15 August 2001

Nine species were identified around Inhaca as tion key is presented. Seagrasses covered half of the well as a more narrow form of cilia- whole intertidal area around Inhaca Island and diversity tum (Forsk.) den Hartog occurring in rocky pools at the was also high at community level. Transects at macro north east coast. Seagrasses were mapped and and micro scales showed zonation of species which grouped in seven distinct community types (in order of may be due to tidal gradients or topographic variation. areas covered): Thalassia hemprichii/Halodule wrightii, Cluster analysis indicated ecological dissimilarities capensis, /Cymodocea between co-dominant species. A decline of Zostera serrulata, Thalassodendron ciliatum/seaweeds, capensis has recently been seen outside the local vil- Cymodocea rotundata/Halodule wrightii, Cymodocea lage. Baseline studies like this are important for coastal serrulata and Halophila ovalis /Halodule wrightii; with zone management in developing countries, where large the ninth species occurring in future changes in seagrass cover can be expected. three of these communities. A dichotomous identifica-

Introduction

Although diversity and occurrence of seagrasses are gener- both distribution and other ecological performances. ally known on a global scale (Den Hartog 1970, Phillips and Recently, De Boer (2000) emphasised the importance of Menez 1988, Kuo and McComb 1989), very few studies seagrasses as a nutrient source in the southern bay at have been published from the east African region and Inhaca. Distribution and factors influencing their zonation Mozambique. In coastal areas of developing countries, one patterns in other areas of the western were can expect large future changes as tourism, modern fishing mostly carried out in and (Isaac 1968, Uku techniques, aquaculture, shipping and industrialisation et al. 1996, Björk et al. 1997, 1999). Despite all this, and the increase. Thus, baseline studies such as the present one importance those communities have as nursery and foraging are needed to document the original pristine and natural dis- areas for the fauna, including many commercial fishes tribution of dominant species and communities before (Larkum et al. 1989), there is still little research focussed on anthropogenic impact and changes in water quality have the distribution of seagrass communities and their zonation grown too large. Results of such studies should then be in the region. This paper updates and revises earlier taken into consideration for coastal zone management and research, emphasising the distribution and zonation of the the protection of biodiversity. seagrass communities around Inhaca Island. In Mozambique, the first publications of seagrasses were by Moss (1937) and Cohen (1939); they were followed later Methods by a number of studies performed by scientists based in (Kalk 1959, Macnae and Kalk 1962, Macnae Inhaca island is situated north east of between 1969, Munday and Forbes 1979) dealing mostly with sea- latitudes 25°58’S and 26°05’S and longitudes 32°55’ and grasses of Inhaca Island but including also other areas of 33°00’E. The intertidal zones around this island have two the country. Later, marine botanical work covering southern contrasting features. The east side is exposed to the open Mozambique and a taxonomical compilation on seagrasses sea, to the dominant south east wind (Kalk 1995), and to an in the western Indian Ocean was published (Bandeira 1995, abrupt depth increase at the beach line. The west side, fac- 1997). Martins (1997) produced a first account on Zostera ing Maputo bay, is relatively protected having a gentle topo- capensis of Inhaca Island and the Maputo area covering graphic slope reaching in some areas a distance of 1km 192 Bandeira between the coast and the low spring tide level. The tidal Results amplitude vary between 0.1m and 3.9m. Inshore water tem- perature around Inhaca island varies within the extremes of Species diversity and taxonomical key 20–39°C, and the salinity within 30–39ppt (mean 35ppt) while being constant at 35ppt at the northern end of Inhaca Nine seagrass species distributed in three families were Island (Bandeira unpublished data). identified for Inhaca: Cymodocea rotundata Ehrenb. et Specimens of seagrasses were collected and identified Hempr. ex Aschers., C. serrulata (R. Br.) Aschers. et using literature (Macnae and Kalk 1969, Den Hartog 1970, Magnus, Halodule uninervis (Forsk.) Aschers. in Bossier, H. Larkum et al. 1989, Simpson 1989). Identification was wrightii Aschers., Syringodium isoetifolium (Aschers.) checked with herbarium material at Eduardo Mondlane Dandy, Thalassodendron ciliatum (Forsk.) den Hartog University Herbarium (LMU). A dichotomous identification (), Halophila ovalis (R.Br.) Hook. f., key was made based on characters from both fresh and Thalassia hemprichii (Ehrenb.) Aschers. in Petermann dried material as well as from literature (Isaac 1968, Macnae (Hydrocharitaceae) and Zostera capensis Setchell and Kalk 1969, Den Hartog 1970, Phillips and Menez 1988, (). An identification key for these species is Larkum et al. 1989). given (Appendix 1). T. ciliatum occurs in two forms: in sandy To map the seagrass communities around Inhaca two substrate with broad/wide leaves and in rock pools with nar- main steps were followed. Firstly, the species composition row, shorter leaves. Different size-forms of leaf width were was determined semi-quantitatively by applying a nominal observed in Halophila ovalis varying from 1.5–14mm in scale of frequency of species occurrence: highly frequent, width. Size forms and leaf tip variants were also observed in frequent and present. The communities were then charac- Halodule wrightii. Such morphological variations also occur terised by the number of highly frequent species. Secondly, in Brazilian H. wrightii. (Creed 1997). ground truthing (walking and snorkelling during spring low tide) all over the island with a map 1:10 000 was made in Mapping order to delineate the contours of the seagrass communi- ties; a reference point on the island, for precision of map- Seven seagrass community types were identified around ping, was used (Kirkman 1996). Black and white aerial Inhaca (listed in order of areas covers): Thalassia photographs were also used to visualise inner and outer hemprichii/Halodule wrightii, Zostera capensis, contours of the seagrass beds. The community types were Thalassodendron ciliatum/Cymodocea serrulata, named after the most dominant species. For this, visual esti- Thalassodendron ciliatum/seaweeds, Cymodocea rotunda- mates of percentage cover based on density classes ta/Halodule wrightii, Cymodocea serrulata and Halophila (Tomasko et al. 1993) were also used. The area of each ovalis /Halodule wrightii (Table 1). The boundaries of each community type was calculated by weighing paper with the community type were drawn and the seagrass map was traced area. then established (Figure 1). Some of the seagrass communities show macro-scale Overall, seagrasses covered around 50% of the entire and micro-scale variation in species composition, which may intertidal area around Inhaca. The three largest community be derived from the tidal gradient and small scale topo- types: T. hemprichii/H. wrightii, Z. capensis and T. cilia- graphic variation. Therefore, macrotransects and microtran- tum/C. serrulata covers 88% of the seagrass beds around sects were performed to study the zonation patterns. Six the island (Table 2). T. hemprichii/H. wrightii is the most macrotransects, 1m wide, running from the coastline to the species-rich community type, where all nine seagrass low water spring tide level were marked and a sample col- lected in a 1m2 quadrat at each 10m of the transect. These macrotransects were outlined in three community types Table 1: Species composition in each seagrass community type (with higher species diversity): Thalassia hemprichii/Halodule wrightii, Thalassodendron ciliatum/ Community type Species Cymodocea serrulata and Zostera capensis. Two microtran- Th Tc Cs Cr Hw Hu Si Zc Ho sects showing species diversity on a small-scale depth gra- Th/Hw *** * * ** *** ** * ** ** dient were analysed during low tide in the T. hemprichii/H. Tc/Cs * *** *** * ** ** ** ** wrightii community type located at the south bay of the Cs *** * * * island. At 1m long microtransects, presence/absence of Cr/Hw *** *** * species were recorded at each cm. The reason for choosing Ho/Hw *** * *** only the T. hemprichii/H. wrightii community for closer analy- Zc * * *** sis was that this community shows a high species diversity Tc/seaweeds *** at a small scale. Also, most of its topography has small Cr–Cymodocea rotundata ***–highly frequent depressions and elevations intermingled with smooth sur- Cs–Cymodocea serrulata **–frequent faces. Data on macrotransects were subjected to cluster Hu–Halodule uninervis *–present analyses using the computer programme STATISTICA 5.5, Hw–Halodule wrightii to ascertain possible ecological affinities between seagrass Si–Syringodium isoetifolium species (Shepherd and Womersley 1981). Tc–Thalassodendron ciliatum Th–Thalassia hemprichii Zc–Zostera capensis Ho–Halophila ovalis South African Journal of Botany 2002, 68: 191–198 193

Figure 1: Map of Inhaca Island showing the seagrass communities 194 Bandeira

Table 2: Area occupied per each community type ly subtidal, and could not be mapped, thus increasing the total cover of seagrass beds around the island. At the north- ernmost point of the island (Ponta Mazónduè), a community Community Type Area(km2) Percentage of total of T. ciliatum and seaweeds has established in rocky pools and creeks. This is an area of high hydrodynamics with huge Thalassia hemprichii/Halodule wrightii 20.10 43.72 Zostera capensis 10.63 23.12 surf waves and strong currents. Here, T. ciliatum and sea- Thalassodendron ciliatum/ weeds are strongly attached to the substrate, and grow Cymodocea serrulata 9.54 20.75 directly on rocks. The most common seaweed species Thalassodendron ciliatum/seaweeds 3.49 7.60 observed here were: Anadyomene wrightii Harvey ex J.E. Cymbocea rotundata/Halodule wrightii 1.83 3.98 Gray, Colpomenia sinuosa (Martens ex Roth) Derbès et Cymodocea serrulata 0.27 0.59 Solier, Chamaedoris delphinii (Hariot) Feldmann et Halophila ovalis/Halodule wrightii 0.11 0.24 Boergesen, Dictyota spp. Digenea simplex (Wulf.) C. Ag., Gracilaria spp., Halimeda cuneata Hering in Krauss, Hormophysa cuneiformis (J.F. Gmelin) P.C. Silva, Hypnea species listed for Inhaca are represented, closely followed rosea Papenfuss, Neomeris vanbosseae M. Howe, by the T. ciliatum/C. serrulata community with eight species Pseudocodium de-vriesii Weber van Bosse, Padina boryana (Table 1). Z. capensis (the second largest community type, Thivy, Sargassum spp., Valoniopsis pachynema (G. located at the south bay and at Portinho) dominates in areas Martens) Boergesen and Valonia macrophysa Kützing. An with high content of organic matter and, apparently, with high extensive list of seaweeds occurring at Inhaca island mostly quantity of ooze and mud compared with the other commu- at Ponta Mazónduè is provided by Critchley et al. (1997). nity types (cf. Martins 1997). This community is mostly locat- ed in an area surrounded by mangroves both at Inhaca Zonation Island and the mainland peninsula in the south. Recently, a decline of Z. capensis was observed at the Harbour area The species Thalassia hemprichii (Figure 2a) and (Portinho, Figure 1, personal observation). This decline Thalassodendron ciliatum (Figure 3a) tends to occupy deep- might be due to increasing density of boats, shallow water er areas far from the coastline whereas, Halodule wrightii fishing, and in addition, increasing trampling of the area at (Figure 2a) and Cymodocea serrulata (Figure 3a) tends to low tide. The T. ciliatum/C. serrulata community occurs in an occupy shallow areas closer to the coastline. T. ciliatum, in extensive area with gentle topographic slope. The real area the subtidal fringe, occurs in homogenic stands except in cover of this community type is higher than what was calcu- some areas where it is also accompanied by a band of lated since a rather extensive area of T. ciliatum is constant- Syringodium isoetifolium parallel to the coastline. The rela-

(a)

Th Tc Cs Cr Hw Zc Ho

0 100 200 300 400 500 600 700 800 900 1 000 DISTANCE (m) (b)

Th Tc Ho Cs Cr Zc Hw

Figure 2: (a) Macrotransect of the Thalassia hemprichii/Halodule wrightii community type from northern bay (0m corresponds to the level of low water spring tide); (b) cluster analysis of the same transect. For species symbols see Table 1 South African Journal of Botany 2002, 68: 191–198 195

(a)

Th

Tc

Cs

Hw

Hu

Si

Zc

0 100 200 300 400 500

DISTANCE (m)

(b)

Th

Si

Hw

Hu

Zc

Cs

Tc

Figure 3: Macrotransect of the Thalassodendron ciliatum/Cymodocea serrulata and Zostera capensis community types at Portinho area (0m corresponds to the level of low water spring tide); (b) cluster analysis of the same transect. For species symbols see Table 1 tionship between T. hemprichii and H. wrightii in the T. observed in northern Mozambique and Tanzania (Bandeira hemprichii/H. wrightii community type is repeated at small and António 1996 and personal observation). scale variation in the microtransect where most T. hemprichii The taxonomic key presented is based on distinctive and occups depression areas and most H. wrightii was found in practical features to differentiate the species. The key can elevated areas (Figure 4) all observed at spring low tide, be used for either fresh or dried specimens. Halophila ovalis when this was only covered by residual showed large leaf variability, with broad, narrow and inter- water. This microtransect also showed that Halodule unin- mediate leaf forms; the shape of the leaf blade being obo- ervis has a tendency to occur in submerged areas, while vate-ovate, oblong-elliptic up to linear. Very narrow forms Cymodocea rotundata occurs in both immersed and were described as H. ovalis (R. Br.) Hook. f. ssp. linearis emersed areas (Figure 4). Those two species have broader (Den Hartog) Den Hartog (Den Hartog 1970, Kuo and leaves, especially if compared to H. wrightii and Zostera McComb 1989). Plasticity of H. ovalis has also been capensis. Cluster analysis revealed a high dissimilarity index observed elsewhere (McMillan 1983) and some variability between the co-dominant species in each community type, seems related to different salinities (Benjamin et al. 1999). as the two co-dominants seem to explore different zonation The seagrass community types observed in this study are patterns (Figures 2b, 3b). similar to those observed along other parts of the Mozambican coast such as the north beach of Maputo city, Discussion where Zostera capensis is the dominant species and coex- ists with Halodule wrightii (Martins 1997); at Bazaruto Island Nine seagrass species occur around Inhaca, making up (21°34’S, 36°00’E) where Thalassodendron ciliatum was 75% of the total number of seagrass species occurring in observed as dominant to the west coast; at Inhassoro Mozambique and 16% of the 58 world seagrass species (21°32’S, 35°12’E) where Thalassia hemprichii, coodomi- (Den Hartog 1970, Kuo and McComb 1989). This diversity is nant with H. wrightii and T. ciliatum, occurs in subtidal areas high for such a small area as Inhaca Island. Furthermore, all (personal observation). At Mecúfi (northern Mozambique nine species could be found within only a small area of (13°17’S, 40°34’E), a coral limestone area), the seagrasses Thalassia hemprichii/Halodule wrightii community type. One often occur together with seaweeds (Bandeira and António of the highest species diversity observed in the world was 1996). Coppejans et al. (1992) reported the existence of reported by Walker et al. (1988) for Shark Bay, Western beds dominated by T. hemprichii, T. ciliatum and Enhalus , with 12 species in an area 84 times bigger than acoroides L. (Royle) in Kenya. Hemminga et al. (1995) that of Inhaca island. Twelve seagrass species occur along found that T. ciliatum was the dominant species in subtidal the whole eastern African coast and 26 along western areas in Kenya. Stands of E. acoroides were observed by Australian waters (Isaac 1968, Kuo and McComb 1989, the author at Quissanga Bay and Quirimbas Archipelago Bandeira 1997). Extensive areas with seagrasses were also (northern Mozambique, around 12°22’S, 40°35’E); Z. capen- 196 Bandeira

quently only in Thalassodendron ciliatum (but not in the rock pools), Thalassia hemprichii and Halophila ovalis. At Mecúfi, frequent flowering of Syringodium isoetifolium has also been observed. In Kenya, apart from the above species, flowering was seen once or a few times, or only flower fragments were collected, in Cymodocea serrulata, Enhalus acoroides, Halodule uninervis, Halophila minor and Halophila stipu- lacea (Isaac 1968, McMillan 1980). C. serrulata, H. stipu- lacea, S. isoetifolium and Zostera capensis from Kenya did, however, flower under experimental environmental manipu- lations in (USA) in controlled conditions of tempera- ture, salinity, day/night period, nutrients with synthetic sea- Figure 4: Changes in species composition with depth along a one metre microtransect of the Thalassia hemprichii/Halodule wrightii water (McMillan 1980). community type The technique for mapping seagrasses could be improved for more accuracy and possibly for better comparison with other studies carried out elsewhere. Remote sensing tech- niques such as photo-interpretation using digitised colour sis which forms rather pure stands in the Maputo Bay area aerial photographs or videos could be used (Long et al. and at Inhaca, was only observed in a mangrove area of 1994, Chauvaud et al. 1998, Ramage and Schiel 1999). Mecúfi (Bandeira and António 1996). In the Indo-Pacific Landsat images could also be used (Kirkman 1996, Ward et region the seagrasses T. hemprichii, T. ciliatum and E. al. 1997). The information collected can be loaded in a acoroides dominate in many areas (Brouns 1985a, b, Geographical Information System (GIS) (Kirkman 1996). Brouns and Heijs 1986, Miller and Sluka 1999). Cluster Such data gathered in GIS, including maps, could be useful analysis tended to reflect ecological affinities (Shepherd and for the study of losses and increase of seagrass beds (Lee Womersley 1981, Bandeira and António 1996). Different Long et al. 1996, Garrabou 1998). A combination of different zonation patterns between T. hemprichii and H. wrightii and methods, involving both simple and high-tech methods, between T. ciliatum and C. serrulata have also been combining e.g. digitised aerial photographs, GIS and observed elsewhere (Coppejans et al. 1992). ground-truthing, would be ideal to get a better picture of sea- Thalassodendron ciliatum in rocky pools, at Ponta grass mapping, diversity, zonation, as well as for under- Mazónduè, have adapted to strong waves and water-swept standing possible dynamics in distribution (Kirkman habitat similar to other rocky areas in southern Mozambique 1990, 1996, Chauvaud et al. 1998). and eastern South African coast (Barnabas 1982, 1991, The ecological demands of the seagrass species making Bandeira 1995). A closer inspection of this community up the different communities at Inhaca could not be showed that T. ciliatum accumulates shell debris and some- analysed. Tidal gradient associated with species tolerance times large grained sand amongst roots and rhizomes. This to desiccation might be among the main factors inducing may help anchorage of the . It also seems that in rainy zonation. Björk et al. (1999) did not find any apparent rela- seasons (October–March), when winds are stronger, some tion between zonation and desiccation in tropical seagrass- rocky pools accumulate some sand. T. ciliatum in rocky habi- es from Zanzibar, as the shallow intertidal species were, in tat seems to have adapted similarly to Phyllospadix spp., a general, more sensitive to desiccation than the deeper occurring only on rocks on water-swept Pacific species. The species growing highest in the intertidal zone, shores (Den Hartog 1970, Turner 1985, Cooper and McRoy e.g. Halophila ovalis and Halodule wrightii, did not seem to 1988, Ramírez-García et al. 1998). desiccate much in situ during low tide because their leaves North of Inhaca island, east of the small Portuguese island lie flat on the moist sand (Björk et al. 1999). Björk et al. and between these two islands at the silted channel, new (1997) showed that intertidal species growing mostly at shal- sand is often brought in (Hatton and Couto 1992) and these low depths, e.g. H. wrightii, had a higher affinity for the form – new areas are colonised by Cymodocea serrulata which of carbon (HCO3 ) commonly available in desiccating envi- acts as a pioneer species. C. serrulata is reported to be ronments contrarily to species occurring more in subtidal highly tolerant to siltation (Bach et al. 1998). Halophila ovalis regions/waters, e.g. T. ciliatum, which showed low affinity to ssp. linearis and Halodule wrightii have also been found as this inorganic carbon species. At saturating irradiances the pioneers at the bare areas close to the coastline, at the west seagrass Cymodocea serrulata, which grew in the intertidal coast of Inhaca. These findings agree with those reported by areas of Inhaca island, showed almost twice as high net Coppejans et al. (1992) who stated that H. ovalis and H. photosynthetic rate as that of T. ciliatum (Johnson et al. wrightii are pioneer species in Gazi Bay, Kenya. These 1993). Nutrient competition between species as well as species have been observed as pioneers also elsewhere growth pattern may also affect the species composition and (Gallegos et al. 1994, Bach et al. 1998). Recently, in 2000, it successions in seagrass beds as was shown to occur was observed that part of the sand bank, in the northern bay between H. wrightii and Thalassia testudinum in , of the island, never became submerged giving rise to USA (Fourqurean et al. 1992). Sangala Island some 1 500m in length and 40m in width (Figure 2, personal observation). Acknowledgements — I thank Prof Inger Wallentinus (Department Flowering of seagrasses at Inhaca has been seen fre- of Marine Botany, Göteborg University, Sweden) and Dr John Hatton (Department of Biological Sciences, Universidade Eduardo South African Journal of Botany 2002, 68: 191–198 197

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Appendix 1: Identification key for the nine seagrass species occurring at Inhaca Island

1a. Leaves petiolate ______Halophila ovalis 1b. Leaves never petiolate, always with a leaf blade and leaf sheath ______2 2a. Leaf blade cylindric or circular in transverse section ______Syringodium isoetifolium 2b. Leaf blade flattened, strap-shaped or rectangular in transverse section, not cylindrical or circular as above ______3 3a. Leaves narrow, in general less than two mm wide; leaf apex concave, convex or obtuse______4 3b. Leaves broad, in general more than two mm (mostly more than four mm) wide; leaf apex round, often serrulate______6 4a. Leaf apex obtuse or rounded; parallel and translucent transverse veins present, parallel veins more than five; shoot and rhizome yel- lowish ______Zostera capensis 4b. Leaf apex not obtuse, always concave-bidentate or convex-tridentate; transverse veins absent, parallel veins three; shoot and rhizome not yellowish ______5 5a. Leaf blade usually less than 1 mm wide; apex concave or bidentate ______Halodule wrightii 5b. Leaf blade usually more than 1 mm wide; apex convex or tridentate ______Halodule uninervis 6a. Roots and shoots arising at each rhizome node ______7 6b. Roots and shoots usually arising at each fourth or more rhizome nodes ______8 7a. Older leaf sheaths attached on stem, leaving a shaggy mass surrounding the lower shoot; leaf pale-green, with apex entire or minute- ly serrulate; one root per each rhizome node ______Cymodocea rotundata 7b. Older leaf sheaths becoming detached leaving a naked lower stem; leaf not pale-green, apex serrulate; two roots per each rhizome node ______Cymodocea serrulata 8a. Roots and shoots usually arising in more than each fourth rhizome node; older leaf sheaths persistent, leaving a shaggy mass surround- ing the shoot; apex not serrulate; stem, rhizome and roots herbaceous ______Thalassia hemprichii 8b. Roots and shoots arising in each fourth rhizome node; older leaves becoming detached leaving a naked stem; apex strongly serrulate; stem, rhizome and roots semi-ligneous or ligneous (occurring in sandy and rocky habitats, the latter with narrow leaves, thinner stem and condensed rhizome) ______Thalassodendron ciliatum

Edited by RN Pienaar