Hydrobiologia 208: 55-62, 1990. P. Kilham and K. M. Mavuti (eds), Comparative Ecology of Freshwater and Coastal Marine Ecosystems. 0 1990 Kluwer Academic Publishers. Printed in Belgium.

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Coastal marine ecosystems of Mauritius

I. Fagoonee School of Agriculture, University of Mauritius, Reduit, A4.

Key words: lagoons, coral reefs, mangroves, biota, sediment processes

Abstract

The coastal ecosystems of Mauritius island (1850 km2, 20” S and 57” E, Indian Ocean) consist of lagoons, reefs, estuaries, mangroves, salt water wetlands, and sheltered bays. Fringing reefs almost completely surround the volcanic island, enclosing a series of lagoons of variable depths (l-4 m). Tides are not important but wave heights can reach 3 m. The zonation of lagoons in a transect from the beach towards the open sea is given, together with the zonal distribution of the flora and fauna, the types of corals and sediments, the detrital sediment composition, the different hydrodynamic processes acting on the benthic sediments, as well as the geochemical (redox) processes. Commercial exploitation of the lagoonal fisheries and other anthropogenic factors have contributed to reef stress and degradation. A rich algal biomass is present which shows seasonal variation. Two species of mangroves, Rhizophora mucronata and Bruguiera gymnorrhiza exist. Fish and shellfish culture is practised in enclosed ponds (barachois).

Introduction

Mauritius is a tiny island with a surface area of 1850 km2, situated at 20” S and 57” E, some 1000 km east of Madagascar. It is of volcanic origin, being formed by a series of collapsed calderas filled with doleritic basalts (lava) during three lava flows (Baxter, 1976): older, inter- mediate, and younger series which have been K-Ar isotopically dated at 7.8 to 5.5, 3.5 to 1.9 and 0.70 to 0.17 million years ago, respectively by McDougall & Chamalaun (1969). The coastline was therefore built by the seaward flow of the lava which, over time, has been undergoing constant erosion by hydrodynamic forces and biological and chemical processes. The island forms part of the Mascarene Archipelago with La Reunion, Rodrigues, St. Brandon and a few other smaller

islands. There is, however, no continental shelf proper, the water reaching depths of 3,000 m within 20 km from the coastline towards the east, west and south of the island, with a shallower prolongation towards the north.

There is quite a diversity of coastal habitats from east to west and from north to south. For instance, on parts of the south coast directly sub- jected to wave action, the coastline is generally smooth and the substrata are exposed as basaltic cliffs, revealing basalt boulders.

Otherwise, the coastline provides numerous sandy beaches and some large bays; other inden- tations are at river mouths.

Studies on the coastal ecosystems proper are virtually nonexistant, except for rocky-shore flora and fauna (Hodgkin, 1959 and Hodgkin & Michel, 1963). A bibliography on marine biology

56

in general has been prepared by Michel (1974). There is now strong interest in coastal ecosystem studies, in particular lagoons, reefs, estuaries and mangroves. A survey was recently carried out on the possibilities offered by the coastal marine resources of the country (Fagoonee, 1984).

Types of coastal habitats

The characteristics of the island’s coastal habitats are mainly determined by its volcanic origin, the existence of coral reefs and the mouths of rivers. These habitats comprise beaches, lagoons, coral reefs, estuaries, saltwater wetlands, mangroves, and sheltered bays. Coral reefs of the fringing type almost completely surround the island (Fig. 1) but are discontinuous, giving rise to a series of lagoons of varying widths (0 to 8 km) which are wider towards the east.

General physical characteristics of the lagoons

The lagoon waters are shallow, 1 to 4 m deep. Some of the reefs are very exposed at low tide (for example, at Flit-en-Flat on the west coast and at Riambel towards the south). Deep channels exist where reefs are absent. These channels are asso- ciated with some of the rivers or underground springs crossing the lagoons.

Tides are not important, with an amplitude of 0.5 m at neap tides and 0.6 m at spring tides. In fact, the seawater level depends more on meteorological conditions, such as winds and barometric pressures. Waves are broken by the reefs. The waves are stronger towards the wind- ward (eastern) side and the south due to strong easterlies (south east trade winds) than they are on the western and northern sides of the island. Wave heights can exceed 3 m and the zero crossing periods (i.e. the mean period of the waves) can reach 10 s (Saha & Jugessur, 1983). The mean value of wave heights range from 1.67 m in summer months to 2.86 m in winter months.

Large variations in salinity are observed;

samples taken from 30 sites (Fagoonee, unpubl.) around the island gave salinity values ranging from 2 to 38x,.

Salinity is lower at river mouths (fresh water input) and highest in rocky habitats where there is less mixing and more evaporation.

Surface water temperature varies from 22 ‘C in winter to 27 ’ C in summer. The pH is in the range of 7 to 8 (more exactly between 6.95 to 8.6).

Lagoon zonation

The lagoons vary from a simple one consisting of a sandy beach and a reef zone to a complex one with or without a beach, mangroves and sandy or muddy swamps, a lagoonal channel followed by seagrass beds, coral heads and colonies, a sandy bottom and a reef zone (Fig. 2). The reef zone is subdivided into an inner slope with detrital sub- strata, a reef flat, a groove-and-spur zone and an outer slope towards the open sea. Fig. 2 also shows the distribution of the flora and fauna, the detritus, the method of sediment transport as well geochemical processes.

Sediment characteristics

Sedimentation in the lagoons has been studied by Pichon (197 1). At river mouths, terrigenous sedi- ments are brought down by the rivers into the lagoon. The mineral salts and soil humus accumu- late as erosional and run-off materials near estuaries, thus favouring growth of algae and other brackish water plants.

The most characteristic bottom deposits con- sist of: a) coarse gravel, composed of broken fragments

of branching corals, and b) more or less homogeneous fine sand, which is

especially abundant in the east coast of the lagoon (Pichon, 197 1).

The biodetritus consists of benthic organisms, whose distribution has been studied by

570 30’E 57O

20” 30’S ---

57

‘O’E

Fig. 1. Island topography of Mauritius showing distribution of main coastal ecosystems: reefs, lagoons, mangroves and estuaries.

58

&nation: Beach or

.sedimmt Biota

Forminifera Corals - SerpulidS

wel&esioideae

Traction Suspension

Processes !i-------Ckidation Redxtion 6

Fig. 2. Typical transect of a reef and lagoon, showing their zonation and the distribution of sediment biota, the mode of sediment transport and redox processes in each of the zones.

Montaggioni & Mahe (1980). Sponges and Halimeda occur in the swampy and grassbed bottom, whereas crustaceans are found among coral patches. Molluscs extend from the beach to the inner slope, with serpulids, echinoderms and melobesioideae in sandy bottoms and the detrital zone of the inner reef. Some coral heads occur in the channel but are abundant in the reef and the Acropora and Pavona zones. However, foramini- fera and bryozoans extend from the reef flat to the outer slope, and alcyonarians remain localized in the outer slope (Fig. 2).

Sediment deposition depends upon the hydro- dynamic forces prevailing in the different zones, namely suspension in high energy environments, saltation in medium energy environments and bedload in low energy environments. Deposition also depends on geochemical processes, i.e. oxidation-reduction taking place in each zone (Montaggioni & Mahe, lot. cit.) (Fig. 2).

The reef ecosystem

The distribution of reefs around the lagoons is illustrated in Fig. 1. In some places, they meet the coastline, e.g. at Flit-en-Flat in the west. The outer slope harbours live corals, whereas rubble and some live outcrops are found in the lagoon. Three main types of fringing reefs are described by Salm (1976): peripheral fringing reefs, sheltered fringing reefs and lagoonal coral patches. The reef flat is exposed at low tide. The reefs are more developed eastward of the island, e.g. near Mahebourg (Pichon, 1967), and tend to have barrier reef characteristics (Stoddart, 1973) with a channel in their lagoon.

At shallow depths (O-20 m), the spur-and- groove zone of the outer slope is sometimes re- placed by dead coral flagstone. At deeper depths (20-50 m), one finds volcanic flagstone with a thin layer of coral and a buttress-and-valley zone

59

of volcanic origin (Faure & Montaggioni, 1976). Owing to its extreme narrowness (less than

10 m wide), the reef flat does not accumulate sedi- ment deposits. Where reefs are absent, sclerac- tinian corals have become established on very recent volcanic flows whose walls plunge verti- cally into the sea. It is not certain whether they represent the embryonic stage of a future reef. According to Pichon (1971), they are merely a veneer of carbonate-secreting organisms.

Reef biota

The flora and fauna of the reefs have been reviewed by Rosen (197 1) & Stoddart (1973). The dead coral flagstones have a rich biota consisting of algae, scleractinian corals, Porifera, hydroids, and gorgonians. In the deeper zone, Antipatharia and hydrocorals are found, whereas the upper zone abounds in Millepora platyphylla (a non- scleractinian coral). The reef edge is also rich in scleractinian corals, alcyonarians (Lobophytum), and zooantharians (Polythoae), as well as melo- besioidean algae (Porolithon) in places where the sea is rough. The groove-and-spur zone (Fig. 2) has many encrusting corals: Pocillopora, Millepora platyphylla, Montipora, Hydnophora, Oxypora, Echinopora, Leptoporia, Favia, Favites, Alveopora, Porites, as well as soft corals, namely Saruephyton and Lobophytum. The boulder zone has three dominant species (see Fig. 2) viz. Turbinaria ornata, Sargassum, sp. and Echinometra mathaei, whereas the Acroporu zone has four dominant species : Acropora pharonis, Galaxea fascicularis, Montipora foliosa and Fungia.

The scleractinian coral communities are a horizontal extension of the Puvona communities and of the branching Acroporu communities. These formations are not constructed by corals. The fauna of the scleractinian coral community has been catalogued by both Pichon (197 1) & Rosen (1971). There are about 600 fish species, 244 crab species, 190 echinoderm species, 170 pelecypod species, and some 1150 species of gas- tropods. Harrnelin - Vivien (1976) has estab- lished a list of the coral reef fish species. Most of

these belong to the Indo-West Pacific fauna, ex- tending from the east coast of Africa to the Pacific islands, e.g. Hawaii. In fact, many crab, bivalve and sea urchin species are common to both Mauritius and Hawaii.

Coral reef degradation

The reefs have been subject to degradation owing to: a) illegal use of explosives for fishing in the past; b) coral collections for sale to tourists; c) massive predation by the giant thorny starfish

Acanthaster planci; d) pollution, and e) careless anchoring of boats by fishermen, re-

sulting in coral heads destruction.

There have been serious outbreaks of coral predation by the crown of thorns starfish, A. planci (Fagoonee & West, 1983; Fagoonee, et al., 1985) over the last 15 years or so. The problem posed by this voracious coral eater had assumed alarming proportions in Mauritian waters in the early eighties.

Surveys carried out in 1971 and 1980 indicated a 13.5fold increase of the starfish, from 30 to 416 starfish per 10000 m2. The main predator of A. planciis the gastropod mollusc Charonia tritonis whose population has seriously declined, owing to extensive collections of its shell (the T&on’s Trumpet), and possibly owing also to pollution (e.g. by pesticides).

Lagoon flora

The lagoon is rich in algae, though grass beds do occur to a lesser extent. Hodgkin & Michel(l963) have studied the zonation of some rocky shores in terms of the flora and fauna. This was comple- mented by a recent taxonomic survey of the lagoon algal resources (Mshigeni, 1985). The rela- tive abundance of algal species at 12 selected sites is now known; the southwest, south and north- east regions have the highest species diversity.

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Limited biomass estimates yielded a biomass of up to 3 83 g dry weight m - ’ of Gracilaria crassa at one site, Pointe aux Roches. Algae which occur in abundance and which could be exploited on an industrial scale for various phycocolloids and other products include species of Rhodophytes, i.e. Eucheuma, Enteromorpha, as well as some species of Cyanophytes (Fagoonee et al., 1985). The biomass of these algae is higher during summer months than during winter months.

Protective management of coral reefs is therefore essential.

Pollution of the lagoon ecosystems

Lagoon fishes

The main fishes caught by some 2500 fishermen who fish daily in the lagoon include groupers, rabbitfish, siganids, emperors (Lethrinids), Na- sidae, goat and parrot fishes and mullets. Octopus, squids and coastal mud crabs are also caught. The mean fish catch over the last 15 years has been around 1882 metric tonnes (Fagoonee et al., 1985).

Industrial development and agricultural activities are accompanied by effluent discharges and soil runoff, inorganic pollution by heavy metals, elIluents from chemical and other industries, sewage, thermal pollution, etc. This is further ag- gravated by careless dumping of garbage at sea. From time to time, oil spills from tankers in the open sea reach the lagoons and beaches. Fringing reefs are particularly vulnerable to pollutants and sediments washed from the land. Pollution should be viewed with concern. Public awareness is being aroused to safeguard the coastal environment.

Estuaries

The lagoon fishery is already being exploited to the maximum and may be overexploited, as shown by a decreasing trend in the annual catch over the years (Table 1): from 2200 tonnes in 1970 to 1375 tonnes in 1984.

This trend can be explained by: a) the capture of undersized fish using ‘sardine’

nets;

Of the 51 rivers and streams entering the sea, not all have developed estuaries at their entrance into the lagoon. The more developed estuaries corre- spond to the larger and longer rivers (Fig. l), namely the Grand River South East, the Grand River North West, not far from Port Louis Harbour, the Poste de Flacq and Trou d’Eau Deuce rivers forming the Flacq estuarine com- plex, and ten others.

b) slow recruitment, and; Most of these estuaries offer the potential for c) lower than expected production, indicating de- aquaculture. For example, the Black River estuary

gradation of the reef and lagoonal ecosystems. is quite rich in various types of small fish such as

Table 1. Fish catch in the lagoon and adjacent waters since 1970. Fagoonee et al. (1985).

Catch: (metric tonnes) 1) Commercial and

subsistence fishing

2) Sport & amateur fishing (estimates)

Year

1970

2200

250

1975 1980 1984

2100 1235 1375

600 700 700

Mean & SD. (1970-84)

1882 f 489

547 + 212

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tilapia and flatfishes. Other estuarine species also abound. A variety of brackish water plants can be found, including mangroves. No studies have been carried out so far on the local estuaries.

Saltwater wetlands

Saltwater wetlands occur at various places around the coast, especially in bays and estuaries. These sites harbour crabs (Scylla serrata) which are the object of extensive (and fruitful) nocturnal digging activities (for example, at Case Noyale and La Gaulette in Black River on the southwest coast, and near Mahebourg and Pointe d’Esny on the southeast coast).

One major marshland at Fort Williams near Port Louis harbour has been completely exca- vated, filled with rocks and paved to build modern transhipment and automated sugar loading (VRAC) terminals. These and other ports facilities have contributed significantly towards making Port Louis Harbour one of the most modem harbours in this part of the Indian Ocean. Mer Rouge, also near Port Louis, is currently undergoing similar reclamation to cater to increased cargo handling and industrial develop- ment.

In general, the other marshlands have mostly gone untouched and their distribution, ecology and biota have not yet been studied.

Mangroves

Not much attention has yet been given to man- groves and the mangrove ecosystem. Sauer (1962) identified two mangrove species (Rhizo- phoraceae), namely Bruguiera gymnorrhiza (L.) Lam. and Rhizophora mucronata Lam. ; with the latter being the dominant species. These man- groves grow up to 1.5-3 m tall along quiet shores that are sandy, rocky or muddy.

They occur mostly in the eastern lagoons of the island (Fig. 1) extending from Blue Bay and Pointe d’Esny northwards towards Mahebourg, Petit Be1 Air, Riviere des Creoles, Petit Sable,

Grand Sable, Trou d’Eau Deuce, Ile aux Cerfs and Pointe Lascar. The total area covered by mangroves, the mangrove biomass, rate of man- grove growth and the associated floral and fauna1 communities need to be investigated. In some places, mangroves have been cut for fuelwood, and it would be worthwhile to monitor their regeneration and regrowth.

Barachois fish ponds)

These are areas of lagoons enclosed by stone walls with a view to mariculture. There are 31 barachois around the island, particularly on the east coast, with a total area of about 300 ha. The barachois vary from 0.6 to 51 ha in size. There is continuous exchange of estuarine, brackish and lagoon waters across the barachois walls. At present, fintish and, to a lesser extent, shellfish such as crabs and oysters, are raised on a small scale. Fingerlings are caught by seine, and young crabs are collected by dip nets and introduced in the barachois.

Seed oysters are collected from mangrove roots. However, one of the factors limiting pro- ductivity is the number of predators invading the barachois: barracudas (Sphyraena barracuda), Caranx spp., eels (Conger spp., Gymnothorax spp., Thyrsoidea spp.), lizard fish (Synodus varie- gatus), Tylosurus crocodilus and Ablennes hians.

Conclusion

From the foregoing, it is clear that there is a paucity of qualitative and quantitative data on the marine coastal ecosystems of the island. Unless these data become available, it is not possible to formulate a comprehensive strategy for develop- ment of the coastal marine resources and optimize utilization of the estuaries, bays, mangroves and reefs. Coral reefs play an important role in coastline protection and, together with man- groves, constitute the most productive ecosys- tems of the tropical lagoons.

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