Marine habitats and biodiversity of Singapore’s coastal waters: A review

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Abstract

Rapid and sustained coastal development, land reclamation, and intense use of surrounding waters by shipping, have changed the original land- and seascape of Singapore in the last two centuries. Reclaimed land now account for more than 30% of the current land area, with a concomitant loss of original intertidal and subtidal habitats across most of the southern shoreline of Singapore mainland as well as other offshore islands. The extent of coral reefs, mangroves, mudflats, seagrass beds, estuarine reefs, sandy and rocky shores has diminished considerably, so much so that man-made habitats such as seawalls, tidal canals, swimming lagoons and other artificial structures now form significant marine habitats in their own right. These remarkable changes in the marine environment have affected marine organisms to a greater or lesser extent, based on the very limited information available on marine habitats in Singapore prior to large-scale reclamation. However, the present extent and diversity of marine life that can be observed in Singapore today is still impressive. Much remains to be discovered and deciphered in terms of their biology and ecology. New records and species new to science continue to be described, even as new coastlines are built, and organisms continually adapt to a changing environment characterized by chronic disturbance. This review serves as a snapshot of the current state of knowledge of marine habitats and biodiversity in Singapore based on existing literature. Key threats and knowledge gaps are also highlighted.

Introduction

Whether considered a small island lying immediately south of peninsular Malaysia, or as the northern limit of the Riau archipelago, Singapore is part of a gateway between the Malacca Strait and the South China Sea (Bird et al., 2006). In biogeographical terms, Singapore is juxtapositioned between two of the largest marine eco-regions in the world, i.e., the Western Indo-Pacific and Central Indo-Pacific regions (Spalding et al., 2007). However, Singapore is also squarely on the shallow Sunda Shelf that is contiguous with the Gulf of Thailand, the Malacca Strait, and the Malayan archipelago. Its maritime position, considered together with its proximity to the two large landmasses comprising peninsular Malaysia to the north and Sumatra to the west, is probably responsible for the unique marine ecosystems that can be seen even today around Singapore. At the same time, the waters surrounding the city-state of Singapore are some of the most intensively utilized in the world. To sustain a population exceeding 5 million with a high standard of living, the Singapore economy relies on being a very busy container and bunkering port, as well as having significant petrochemical, ship-repair and offshore rig construction industries. These coastal and offshore facilities increasingly use seawater for cooling. However, with a limited land area to harvest rainwater, industrial and domestic demand for freshwater is also being augmented by reverse osmosis of seawater and wastewater. Offshore islands and coastlines provide public recreational amenities, exclusive residential and military training areas, as well as marine nature reserves. The large number of vessels present requires suitable anchorages and room to manoeuver without interfering with floating aquaculture farms. All of these activities take place in designated areas that are less than 50 km from each other.

The coastal environment in Singapore (see reviews by Chuang, 1961; Hill, 1973; Tham, 1973a; Chia et al., 1988; Chia and Chou, 1991; Chou and Chia, 1991; Chia, 1992; Gupta and Pitts, 1992; Hilton and Manning, 1995; Turner and Yong, 1999; Lu et al., 2005; Ng et al., 2007; Hutomo et al., 2010; Tan et al., 2010; Davison et al., 2012; Sin et al., 2016) is a fairly benign one. Air temperatures throughout the day and year range between 24 to 35 °C, with humidity levels nearly always well above 70%. Wave action, compared to other seashores in the vicinity, is minimal (Chew, 1974). Waves buffeting the coastlines are more often caused by wakes generated by heavy boat and ferry traffic than by local squalls (also known as barat), which are often short-lived (typically lasting not more than 2 h) but can generate winds up in excess of 100 km per hour (Choi, 2000). However, coastal habitats are subjected to the harsh heating and drying effects of the tropical sun, particularly those above the mid-tide level of the shore. On a typical sunny day, rock surfaces directly exposed to the sun can reach temperatures exceeding 45 °C (Nguyen et al., 2011), but cool down rapidly after sunset to reach air temperatures of between 24 and 28 °C at night. Seawater temperatures typically range between 27 ° and 31 °C depending on depth and time of year. Having an annual tidal range of about 3.3 m, the lower of the two low tides during spring semi-diurnal tides in Singapore (Wyrtki, 1961) is generally confined either to the early morning (May through August) or late afternoon (October through March). Hence the lower littoral zone is generally spared the more stressful conditions caused by exposure to the sun. There is also no distinct wet or dry season that is often typical of regions elsewhere in Southeast Asia, since Singapore is affected both by the Southwest Monsoon (typically May through September) and Northeast Monsoon (typically November through March), although mean monthly temperatures may be slightly lower during November and December each year (Nieuwolt, 1973).

The water column thus experiences little temperature variation either diurnally or seasonally. Similarly, salinity is generally constant in the Singapore Strait, ranging between 27 and 32 psu, essentially due to constant mixing created by the regular east–west tidal current flow experienced in that area. In contrast, the salinity in the Johor Straits ranges between 21 and 31 psu with lower salinities predominating on either side of the Causeway with relatively poor exchange (Lim, 1983, Lim, 1984a, Lim, 1984b), often resulting in stratification. Such estuarine conditions are primarily caused by the combination of riverine input and tidal influence into the two narrow cul-de-sacs, formed by the causeway (see Fig. 1) linking Johor Bahru and Singapore island. Dissolved oxygen ranges between 5 and 8 mg L1, but supersaturation can occur during plankton blooms in the day in the Johor Straits. Mean values of total suspended solids range between 15 mg L1 (Gin et al., 2000) to 23 mg L1 (Tkalich and Sundarambal, 2003) and may exceed 100 mg L1 during a thunderstorm (resulting from terrestrial runoff; Chatterjea, 1998; Van Maren et al., 2014) or in the vicinity of reclamation activities.

Section snippets

Marine ecosystems and associated biodiversity

Key marine habitats in Singapore were recently reviewed by Chou (2006) and Tan et al. (2010). Broadly speaking it is possible to recognize nine major natural marine ecosystems in Singapore, and a further three artificial ecosystems (see Fig. 1 for location names). Coral reefs are located exclusively in the Singapore Straits, either as fringing or patch reefs in the vicinity of the Southern Islands. In contrast, mangroves (or mangal, which includes the fauna) are most extensive on the shores

Coral reefs

Despite being one of the busiest container ports in the world with significant oil refining and bunkering industries within the same confines, coral reefs are not uncommon on the shores of many islands in the Singapore Strait. Coral reefs in Singapore can generally be divided into two types: fringing and patch reefs (Chou, 1988a, Wong and Sin, 2013). The fringing reefs present in the offshore islands south of Singapore island are generally narrow (Chuang, 1977), but can be divided spatially

Mangroves

Mangroves comprised about 13% (about 75km2) of the total land area of Singapore Island when it was first established as a British trading post in the early 19th century (Corlett, 1991). By 1978 this was reduced to some 24km2 (Khoo, 1980) and today, less than 1.5km2 of the original mangrove remains (Turner and Yong, 1999, Liow, 2000, Yang et al., 2013), although a more recent study has estimated the total mangrove area is now about 6km2 (Lai et al., 2015). Demand for firewood, shrimp and fish

Mudflats

Intertidal mudflats in Singapore are generally located adjacent and beyond the seaward edges of mangrove forests. These are best developed in the sheltered waters of the Johor Straits, although significant expanses of mud are also found on a number of larger islands in the Singapore Strait (e.g., Pulau Semakau, P. Pawai). Floral and faunal studies of mudflats have been fairly limited in scope due to their inaccessibility. However, the community of organisms is expected to bear similarities to

Seagrass

The term “seagrass” generally refers to flowering plants growing in shallow marine environments (Kuo and den Hartog, 2001) and excludes marine algae, mangrove trees and shrubs. Of a total of twelve genera and 60 species of recognized seagrass species in four monocotyledonous families worldwide, up to 12 seagrass species in two families Cymodoceae and Hydrocharitaceae are recorded from Singapore (Loo et al., 1995, Keng et al., 1998, Yaakub et al., 2013, Yaakub et al., 2014). The two families

Subtidal benthos

The soft bottom subtidal benthos comprises the largest contiguous marine ecosystem in Singapore. The seafloor off the Singapore coastline is generally flat or gently sloping, with depths ranging between 5 and 50 m, although in the Singapore Strait towards the southern port limits there are several areas where depths exceed 100 m. The deepest area of about 200 m (The Singapore ‘Deeps’; Hill, 1968 and Bird et al., 2006) is a small basin measuring 2 nautical miles in length and 1 nautical mile in

Sandy shores

In Singapore, many of the natural sandy shores that were present along the southeastern coast from Tanjung Rhu to Changi, and also along the southwestern coast from Teluk Ayer to Pasir Panjang, have been reclaimed extensively. A small stretch of beach is preserved in the Labrador Nature Reserve (Lim et al., 1994, Todd and Chou, 2005) and other short lengths of sandy areas remain along the estuarine northern shores of Singapore (e.g., Seletar, Kranji) as well as on various offshore islands

Rocky shores

Only a small proportion of original rocky shores are left in Singapore, and most have largely been either reclaimed or replaced by seawalls and breakwaters. The only natural rocky shore left on Singapore island is at Labrador Beach (Lim et al., 1994, Todd and Chou, 2005) but short stretches remain on St John’s Island, Pulau Tekukor, Pulau Semakau, and Pulau Pawai, which mostly comprise sedimentary rock, in contrast to those of Pulau Ubin, where the intertidal rocks are granitic in composition (

Estuarine reefs

Estuarine reefs at Beting Bronok and Tanjung Chek Jawa at the mouth of Sungei Johor in the East Johor Strait are characterized by a high diversity of echinoderms, including starfish (Protoreaster), sea urchins (Salmacis) and sea cucumbers (see e.g.,  Tan and Yeo, 2003). Extensive sea-grass beds and sponge gardens also occur. While the number of species of scleractinian corals is lower than on coral reefs of the southern islands, there is a fair diversity of octocorals including soft corals, sea

Water column

The seas surrounding Singapore that is within its port limits occupy an area of about 600km2 that is mostly shallow (<50 m) with waters that are generally turbid and eutrophic. Typical values for total suspended solids (TSS): 100 mg L1 (e.g., high values can exceed 1000 mg L1); turbidity (formadazin turbidity unit, FTU): typically below 20 FTU, but can exceed 150 FTU; dissolved oxygen (DO): typically between 2 and 8 mg L1 although algal blooms can cause DO to reach in excess of 18 ppt, as

Seawalls

Seawalls are a dominant feature of the much-reclaimed Singapore coastline (Lai et al., 2015), and have a surprisingly diverse flora and fauna associated with this artificial habitat, with some 30 autotrophic and 66 invertebrate taxa observed in a study carried out on two islets in the Singapore Strait (Lee et al., 2009, Lee and Sin, 2009). Despite their apparent physical uniformity, biological composition appears to depend on the location as well as the slope of the seawall. Seawalls in

Tidal canals

While much of the original landscape of Singapore have given way to buildings, bridges, roads and flyovers, many of the original waterways still remain, although in nearly all cases, they have been canalized and rerouted in one way or another. New canals are also apparent on reclaimed land. Because these monsoon (or storm) drains are often connected the sea, they are influenced by the tides. The concrete walls and floor of these canals support a surprisingly diverse estuarine community that can

Artificial lagoons

Numerous artificial swimming lagoons for recreation were constructed in the early and late 1970s on many of the islands south of Singapore island. These included Sentosa (formerly Pulau Blakang Mati), Kusu, St John’s, Sisters’ Islands, Pulau Hantu, Sultan Shoal, Pulau Sudong, and Pulau Satumu, all of which now have shallow sandy lagoons that are often intertidal (i.e., the lagoon floor is exposed during spring low tides) with a variety of marine plants and animals.

Some of the more conspicuous

Key threats and knowledge gaps

The continued sustenance of biodiversity and natural heritage in the face of urbanization is probably one of the key challenges facing Singapore society today (Sin et al., 2016). Both terrestrial and aquatic habitats are equally threatened, but the marine realm is particularly vulnerable, as much lies under water and its components are not directly visible. Coastal transformation and reclamation have directly erased many natural habitats, which have now been substituted by concrete, granite

Summary

There are probably very few places in the tropics where high marine biodiversity interacts so closely with coastal development. As a city-state on an island with a very dense population and high standard of living, the marine environment has undergone an ecological transformation not unlike that seen on land (Corlett, 1992). Increasing urbanization has obliterated many natural habitats during the nation’s development but those remaining have received increasing support from the public for their

Acknowledgements

This review would not have been possible without the inputs of mentors, colleagues and students at the Tropical Marine Science Institute, Lee Kong Chian Natural History Museum, and Department of Biological Sciences, National University of Singapore over the last 30 years. Their contributions form the essence of what we now know and understand about our marine diversity in Singapore. TKS is also grateful to the various government agencies in the Ministries of National Development and Transport

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