- Email: [email protected]
SEA SHORES AND TIDAL FLATS
] 72
G61tenboth, Timotius, Milan and Margraf
SEA SHORES AND TIDAL FLATS
03 ~ O 9
E
03 03 9 03 ~J 9 o C~ o
o
N
9
~
~
m
"
L,)
,x::
Ecology of Insular SE Asia 9 The Indonesian Archipelago
~
e'~
~.~~~ 9' ~
0
~
~~~~~
]~
Ecotones and Special Ecosystems
] ~7~
G61tenboth, Timotius, Milan and Margraf
SEA SHORES AND TIDAL FLATS
=
,_,= O ,_Q
E
9
o o
9
0
e,l
cO
c~
"~
~J c~
o c~
o
v
~ ~.~
N o
x:
_~
] ~
Ecosystems
~
~
and Special
.~
c~
:~o 9
~
~
o
~'~ ~ ~ ~~'~ ~
c~
E c o l o g y of Insular SE Asia ~ The Indonesian A r c h i p e l a g o
c~
r~
~176
~
9
Ecotones
176
G61tenboth, Timotius, Milan and Margraf
species only occupying narrow areas (Tsuchy and Lirwitayapasit, 1986). In most of the Indonesian islands, distinct zones within the eulittoral and infralittoral could be identified which show a typical distribution oftidal organisms. It is possible and important to distinguish the three major types of shorelines: 9 Rocky shorelines. 9 Sandy shorelines ofvolcanic or other terrestrial origin. 9 Sandy shorelines exclusively built from the biogenetic materials of reefs, the so-called coral cays. The vast majority of the 17,508 islands of the Indonesian archipelago are most likely coral cays or low islands (Tomascik et al., 1997). Among the best known examples ofthis kind of islands with splendid white shores are the about 120 islands of Kepulauan Seribu Thousand Islands off the Jakarta Bay in the Java Sea. Coral cay sediments are biogenic, originating mainly from the skeletal material of numerous calcifying plants and animals which built and live in the reef. During the early stages of development, most sand cays are rather unstable systems. Once sand cays reach a certain mass, the movements become much less pronounced. Stabilization is greatly enhanced by the colonization of plants whose seeds arrrive by atmospheric transport, water currents or on floatsam, or carried bybirds. In addition to the reef sediments, some cays located close to the mainland of large islands (e.g. Java) or active volcanoes (e.g. Banda Neira in the Moluccas) may have non-carbonate sediments incorporated into the cays (Yomascik et al., 1997). The sandy shorelines of the majority of the larger islands are mostly of pure terrestrial origin with often volcanic or jurassic sediments and sands or a mixture of carbonic and non-carbonic sediments.
SEA SHORES AND TIDAL FLATS
Rocky shores occur along the coastlines of many Indonesian islands: 9 The west coast of Sumatra is lined by rocky beaches. 9 Most of the Lesser Sunda Islands and the Moluccas consist of volcanic or limestone shorelines with little or no beach formation.
ECOSYSTEM FUNCTIONS Shorelines absorb the energy of wave action and hence, prevent erosion in coastal areas. Depending on tidal regime, tidal zones can be sinks or sources of nutrients. Like mangroves, they can hold among the most productive soils in the tropics. Shorelines are habitats o fa specialized community of organisms, some ofthem being useful to man like different species of algae, sea cucumbers, and molluscs. Thousands of shorebirds coming from temperate and arctic zones of both hemispheres greatly depend on different types of shorelines as feeding grounds. Species of endangered sea turtles use sandy beaches as nesting ground. Sand bars or rocky shores are breeding grounds for an array of shorebirds. Abiosis The composition of communities of organisms along shorelines is strongly dependent on the type of substrate. Shoreline habitats can range from steep cliffs, over rocky, boulder and gravel shores, sandy beaches, to silty and muddy flats. The different sizes of particles do not only influence the water content of the substrate and the interstitial space between the particles, but also the stability of the surface as a whole. Mud bottoms consist of very fine particles. Space between particles is small so that these substrates usually contain only small amount of water. Silty and clayey bottoms are more stable than most other substrates. Hence, it is possible for the infauna or such animals of the sediment beaches which rarely emerge onto the surface, in contrast to
Ecology of Insular SE Asia 9 The Indonesian Archipelago
the epifauna or animals that spend at least some time on the surface, to build permanent burrows. Sandy substrates consist of mineral and calcareous components of larger size. The latter usually are of organic origin. Sand surfaces are highly influenced by water movement and sometimes form characteristic ripple patterns. Interstitial spaces are inhabited by a unique microfauna. Like in the clayey or silty substrates, the infauna is dominant. However, permanent burrrows are rare, because of the instability of the substrate. The type of substrate strongly depends on the current regime of the respective area. Hard-bottom substrates occur in areas with strong erosive forces, soft-bottoms only can form in situations where deposition is stronger than erosion. Low waves with a long wavelength are carrying material to the shore, whereas high waves with short wavelength have erosive effects (Morton and Morton, 1983). Also wind can have a major impact in the forming of seashore landscapes. Sand dunes are a restricted feature in Indonesia. They only occur near Pangumbahan along the south coast of West Java and the north coast of Irian Jaya. The dunes form parallel to the coastline with the younger more mobile ones closer to the beach. As initial stage of the forming of a dune, drifting sand is caught on the leeward of small obstacles, like vegetation or debris. Sand is blown over the ridge off this small obstacle. Subsequently, the sand bakes together and vegetation establishes. Consequently, the older dunes in the hinterlands are less, but still are growing (Siringan and Pataray, 1996). Shorelines are strongly influenced FIGURE by the tides. The latter are caused by the rising and falling of the world's ocean's
]77
water body due to the gravitational influence of the moon and the sun upon the earth (Fig. 9.3). Two tidal protrusions of water permanently exist, one facing the moon and one on the opposite site of the globe. These bulges result in two high tides with two low tides in-between during one lunar day lasting 24 hours and 50 minutes. Together with the rotation time of the earth, a cycle of 24 hours and 21 minutes duration results. The impact ofthe sun on tidal forces is lower than that of the moon. However, it has a modifying influence. Spring tides occur, with considerable differences in water level during low
9.3.
Forming of hypothetical tidal bulges under the influence of the moon and the sun during: 1 New moon, 2 Last quarter, 3 Full moon, 4 First quarter.
Ecotones and Special Ecosystems
] 78
G61tenboth, Timotius, Milan and Margraf
and high tide, when the moon, the earth and the sun are situated in line (during full or new moon). The sun's gravity pull diminishes that of the moon, when the position of the sun is on a right angle to that of the moon. Subsequently, neap tides, with only minor changes of water levels between the tides become prevalent. Each constellation occurs twice during a lunar month. In reality, tidal regimes follow a much more complicated pattern, since the world oceans are disrupted by land masses which influence the tidal bulges considerably. This change between marine and terrestrial conditions is the single most important factor that limits the diversity of the community of organisms in this ecosystem. Tidal cycles do not only influence the diurnal activity patterns of virtually all organisms of the tidal zones, but also sometimes function as triggers for their longer term life cycles, for example through determining time of mating or migrating. Like most ecotones, shorelines are subjected to extremely variable abiotic factors. Especially the water bodies in tidal pools can vary considerably regarding temperature and oxygen content. When exposed to the sun, evaporation increases the salinity, while when exposed to rain, it decreases. In the zone where the surf breaks, organisms are exposed to strong physical forces. Action ofwaves is influenced by the speed of the wind and the distance over which the wind operates (Morton and Morton, 1983). Algae occurring in tidal zones have developed thick cuticulas to avoid desiccation and structures to stand the physical forces ofwave action like strong rhizoids and flexible thalloids. Animals adapt to these factors by evolving hard shells like molluscs. Others hide in crevices or the substrate where they can endure when conditions get unfavorable like some echinoderms or annelids. These organisms are usually referred to as 'cryptofauna' (Evans, 1949). Other mobile organisms search actively for their fitting surrounding like crustaceans or some blenniid or gobiid fish, which can move over dry land for some distance.
SEA SHORES AND TIDAL FLATS
Biodiversity Littorial communities can be divided into two categories: 9 Free living species including fish, molluscs, crabs, sea urchins, sea stars, and polychaetes. 9 Sessil species including algae, barnacles, sea anemones, and corals.
Producers Main producers in tidal zones are different forms of algae. On soft bottoms diatoms (Fam. Chrysophyceae) form a yellow to green cover during low tide which partly is removed during high tide. Those microalgae are those producers contributing most to the food chain. Some varieties ofseagrasses are also able to dwell on tidal zones. On hard bottoms, the pinkish to red calcareous algae Corallina sp. form encrusting covers. Euryoecious macro algae like Padina sp. are capable of growing on hard bottoms in tidal pools. Higher plants are able to establish on beaches.Pioneering species have different tolerance to saltwater exposure following germination, which results in a loose, but discernable zonation ( Tomascik et a1.,1997). On coral cay and other limestone beaches, the most tolerant species to salt water exposure is Sporobolus fertilis . Just above the highest inundation line, the creeping Vigna marina and the fast growing Spinifex litoreus can be found. Parts of the shoreline only affected by sea spray are frequently dominated by the grasses Thaurea involuta and Lepturus repens. A typical pioneering species is the Goat's foot vine Ipomoea pes-caprae that can form dense vegetation mats on sandy substrates. It does not only prevent erosion but also acts as trap for nutrients with its root system. The bright pink flowers are very conspicuous and the common name is derived from the characteristically shape ofthe leaves which resembles the hoofofa goat (Fig. 9.4). It is often pollinated by Xylocopa sp.( Hymenoptera).They produce , like many others of the beach plants, floating seeds. The
Ecology of Insular SE Asia
9 The Indonesian Archipelago
179
portulacustrum (Fam. Aizoaceae), Euphorbia chaissonis (Fam. Euphorbiaceae), Vigna marina (F am. F ab ace ae), Triumfetta repens (Fam.Tiliaceae), and Vitex trifolia (Faro. Verbenaceae). Different species of grasses and sedges can also settle on barren sand. Common are the sedge Cyperus pedunculatum (Faro. Cyperaceae) and the grass Spinifex littoreus var. longifolius (Fam. Poaceae) which further enhance the stability of the substrate. Eventually this vegetation can be replaced by woody species which may develop into beach forest. On very sandy stretches, pure stands of Casuarina equisetifolia can develop followed by a full Barringtonia community.
FIGURE 9.4.
A) Pes-caprae formation on a sandy beach. B) Goat's foot vine Ipomoeapes-caprae(Fam. Convolvulaceae).
whole plant community with this habit is named after this species as Pes-caprae-formation.This formation is an open community of low, sand-binding herbs, grasses and sedges behind the drift line. Endemics are absent and many species have a pantropical distribution. Often associated with the former species is Canavalia maritima (Faro. Fabaceae). Superficially the vegetative parts resemble that of the goat's foot vine with fleshy shoots that form a tangle of vegetation. Other associated plants are Wedelia biflora (Fam. Compositae), Sessuvium
Consumers The community of animals inhabiting tidal zones and beaches is dominated by the following guilds: 9 Sessile or hemisessile planctivores. 9 Grazing herbivores which feed on the algal turf growing on the rocks and the layer of diatoms on the soft bottoms. 9 'Grazing' carnivores which feed on benthic invertebrates. 9 Detritivores which feed on the dead organic matter brought by high tide. 9 Carnivores and scavengers are particularly abundant in supralittoral beaches. The majority of animals belong to three groups concerning their feeding habit: 9 They filter plankton from the seawater and are therefore filter feeders. 9 They suck organic deposits and microorganisms off the sediment surface. 9 They sort edible particles of sediments and are therefore deposit feeder. In addition, beach organisms are frequently divided into three sediment-size related groups (Carter, 1988): 9 Attached microfauna 9 Interstitial meiofauna
Ecotones and Special Ecosystems
180
G61tenboth,Timotius, Milan and Margraf
9 Borrowing or loosly attached macrofauna. The nearshore waters support usually rich populations ofphytoplankton and zooplankton, which derive great benefit from turbulent waters. Diatoms are the most abundant phytoplankton group. The diatom Chaetoceros sp. thrives particularly in the surf zone of high energy beaches, like in Java and Bali, where nutrients are available from land runoff and strong coastal upwelling. They have the ability to attach themselves to bubbles which allow them to remain at the surface in maximum light (Carter, 1988). A study on a tidal reef flat yielded a very high abundance ofmeiofauna and small macrofauna, the nematodes, harpactoids and polychaetes being the three most important groups. Planctivores like some cnidarians, polychaetes and bryozoans are most abundant in tidal pools that never or rarely become dry. The primitive Chiton Acanthopleura sp. is commonly found attached to rocks splashed with sea water. Oysters (Fam.Ostreidae) have solid shells and are permanently attached to hard substrates and therefore can withstand strong waves, as well as being exposed to the sun during low tide. Barnacles also have developed shellqike structures which can be closed to avoid dessication. Despite superficial resemblence with molluscs, they are highly specialised crustaceans. Their legs have been modified to cirri that function as filtering organs. Common in intertidal zones are Pollicipes mitella, Chthalamalus sp., Tetrar sp., Tetrar sp. (Rosell, 1986). Other species live attached to animals, especially crustaceans, but also on drifting woods and other hard substrates. Higher crustaceans are relatively diverse along shorelines. The most conspicuous crustaceans on mud flats are the Fiddler crabs Uca spp. The males have one enlarged claw with which they signal other males their position and that the respective territory is occupied. Before their burrows get flooded, they cut out round pieces of mud from the surface, retreat
SEA SHORES AND TIDAL FLATS
back in their burrows and seal the hollow tight with the piece of mud used as a cover. On sandy beaches, land hermit crabs Coenobita sp., and sand crabs Ocypode sp. are particularly common. They are predominantely noctumal to avoid predation in a habitat that does not provide much shelter. The soldier crabs Mictyris sp. are closely related to the latter. Like their relatives they live in sand holes. In contrast to the larger sand crabs, they often forage in large aggreggations, possibly as a means to avoid predation. The crabs of the genus Dotilla leave behind very conspicuous signs of feeding. Their holes are surrounded by concentric walls of sand 'pebbles'. These are formed when the crab is screening the sand for food around its hole. The walls of sand pebbles surrounding the central hole are not completely closed to allow the crab a fast retreat in case of danger.The environmental effects of these burrows lay in the increase ofthe penetration depth of oxygen and bringing lower sediments to the surface. The deposits of the suspension feeders in form of fecal pellets are food for the deposit feeders, while the deposit feeders resuspend their deposits in the sea water. Members of the genus Grapsus are typical for rocky shores. With their flattened carapaces and their strong legs, they can withstand the waves without being washed away. Insects are becoming increasingly more abundant going from the seaward to the landward side of shorelines. Tidal zones are the only ecosystems strongly influenced by the sea where some insects are able to successfully compete with crustaceans. Water skippers (Fam. Gerridae) dwell on the surfaces of tidal pools and some species of aphids, beetles and ants live in soft bottom tidal flats. To avoid flooding oftheir burrows, the openings are not made wider than 3mm. This is enough to hold back the air due to adhesive forces, even during high tide (Maitland and Maitland, 1994). Sandy beaches are often inhabited by tiger beetles (Fam. Cicindelidae). Agile predators with
Ecology of Insular SE Asia 9 The Indonesian Archipelago
strong mandibles prey on small invertebrates on the barren ground. Large wasps of the families Pompilidae and Sphecidaehunt spiders and caterpillars, respecively, by immobilizing them with their poisonous sting. The still living prey is buried alive in sand holes and serves as food for the wasp larvae. Molluscs are the most common herbivore grazers on tidal flats. Patellid snails even can survive on rocks which are exposed to very strong wave action. They have cone-shaped shells and attach with their sucker-like foot to the rocky surface always returning to the same spot after grazing. Subsequently, a cavity forms which follows the outlines of the shell. Hence, the snail is able to seal of its body from the unfortunate microclimate during low tide. Prosobranch snails like Littoraria sp. can close their shell tight with a calcareous structure at its foot called operculum. The zonation of organisms on rocky shores usually follows a typical pattern with three major zones: Lower intertidal; middle intertidal; and upper intertidal. Each zone is characterized by a key group oforgnisms. Space is a key limiting factor and inter- and intraspecies competition can be intense. Brittle stars (Ophiuroidea) are the most common echinoderms to be encountered on tidal flats. They can be easily spotted bythe vivid movements oftheir arms to filter out detritus from the water surface. However, the whole animal is difficult to see, since it usually hides in a crevice. Most sea urchins that most commonly graze on algae growing on hard bottoms are not adapted to be exposed to the air and have to retreat to rock pools or water-filled crevices during low tide. Predators among the molluscs are some cowries (Fam. Cypraeidae) and cone snails (Faro. Conidae). The latter are killing their prey, with toxins that in some species can even be fatal to humans. Nekton like schools of goatfishes (Fam. MuUidae) or gray mullets (Fam. Mugilidae) enter tidal zones only during high tide to utilize the
] 81
productive feeding grounds. In tidal pools, only few fishes permanently use the open water body like some damselfishes, when still being immature. However, the fish community in the intertidal is dominated by benthic families with the Gobiidae and Blenniidae being the most diverse. Some species are even able to leave the water and live on land for a limited time. The mudskippers Periophtalmus sp. are well known amphibious fishes which commonly live in mangroves. The leaping blennyAlticus saliens is typical for more open tidal zones. Marine turtles are only temporary visitors of sandy beaches where they lay their eggs. Mainly four species are recorded: the largest and rarest is the Leatherback Dermochelys coriacea. It can reach a length of 3 m and weights up to 750 kg. In contrast to its smaller relatives, its upper shell consists of bony plates that are embedded in the skin with seven conspicuous ridges. Its main diet consists of jellyfish and it has been reported that more and more animals die because they swallow discarded plastic bags which they mistake for their prey. The other species all have bony shells that are completely covered with horny scales. With some exercise, the different species can be identified by the distinct scale patterns. The Green turtle Chelonia mydas is mainly vegetarian and is found in seagrass beds and stands of algae in shallow water. The Loggerhead Lepidochelys olivacea feeds on fish and pelagic invertebrates, whereas the smallest species, the Hawksbill Eretmochelys imbricata feeds on benthic invertebrates. Alcala (1980) has conducted a threeyear study on the ecology of this species. It was found to be most common in shallow coral reefs, reef drop offs and channels between islands. The preferred nesting beaches have generally fine gray sand. Laying of eggs presumably occurs year-round. However, the single females do not lay eggs every year, but seem to follow three-year nesting cycles. Clutch sizes are reported to range between 112 to 130 eggs. Each female produces two or even more
Ecotones and Special Ecosystems
182
G61tenboth,Timotius, Milan and Margraf
clutches that are laid in three-week intervals in one nesting season. After laying eggs, the turtles are known to return to the beaches where they have been born. However, it is still not fully understood what features make a suitable nesting beach. Depending on the temperature, propagation time is around 60 days. Only very few of the hatchlings survive the first years. Already the eggs are heavily predated by monitor lizards, pigs, dogs, rats and man. Additional predators of the young when they try to reach the sea are crabs and larger seabirds. In the sea, sharks and other predatory fishes and even large jellyfishes are known to decimate the numbers of the young turtles. However, the main threat to turtles is the capture of adult animals by man. Especially in demand are the Hawksbill and the Green turtles, because of their meat, the bones and the shells which are used for handicraft. Now on the brink ofextintion, all marine turtles are protected by law in Indonesia and conservation projects are launched also to secure the nesting beaches. One of the most important reserves for marine turtles in the country is Pulau Sangalaki in East Kalimantan where 30-45 turtles dig their nests every night of the year (Tomascik et al., 1997). The main nesting sites for the Green turtle in South East Asia are the islands of Penyu, Lucipara and Am (Fig.9.5). The skink Emoia atrocostata (Fam. Scincidae) is a relative frequent visitor of the Pes-caprae formation. Resident sea birds like boobies (Fam. Sulidae), frigate birds (Fam. Fregatidae), tobicbirds (Fam. Phaetonidae), and noddies (Farn. Laridae) and migrant shore birds relay heavily on shorelines as feeding grounds. Most of them are winter visitors coming from Siberia orNorth East Asia. The different species of waders (Charadriiformes) have adapted to different feeding niches to avoid competition. The Sanderling Crocethia alba forages near the water's edge of sandy beaches on small invertebrates washed ashore by the surf. The Mongolian sandpiper Charadrius mongolus feeds on open sand or mud
SEA SHORES AND TIDAL FLATS
flats, whereas the Ruddy turnstone Arenaria interpres is more often encountered on shores with gravels or crushed corals where it preys on animals by turning around the stones under which they hide. The Common sandpiper Actitis hypoleucos is the wader most likely to be encountered in Indonesia feeding on soft bottom, hard bottom and boulder shores alike. It has been recorded that the biomass density of prey for these tactile feeders varies widely between 1-37 g.m -2around Java. Due to their long beaks the Asian dowitcher Limnodromus semipalmatus and especially the Whimbrel Numeniusphaeopus are able to utilize food sources that are hidden so deep in the substrate that most other waders cannot reach them. Several species of herons and egrets (Fam. Ardeidae) can be regularly observed in tidal zones, but only the Reef egret Egretta sacra seems to be restricted to it (Fig. 9.6). The only mammal that usually can be found in tidal zones is the Small-clawed otter Aonyx cinerea which is widespread in Southeast Asia. Decomposers Since particulate organic matter (POM) is brought to the system twice a day, the guild of decomposers is rich in species. Especially the echinoderms like brittle stars and sea cucumbers are abundant, as are different species of crustaceans and annelid worms. As much as 27 species of holothurians have been recorded from intertidal areas (Tan Tiu, 1981). Since POM is a resource with spotty distribution, most detritivores are mobile or hemisessile. One exception is the spaghetti wormLoimia medusa that has tentacles which can cover up to lm 2 around its burrow in search for food (Storch and Rosito, 1981 and Fig. 9.1).
NUTRIENT AND ENERGY FLOW Since solar radiation is high and nutrients are usually abundant, shorelines where sedimentation takes place are productive ecosystems. Every high tide brings
Ecology of Insular SE Asia 9 The Indonesian Archipelago
F I G U R E 9.5.
] 83
The most conspicous species of marine turtles recorded from Indonesian waters. 1 Leatherback (Dermochelys coriacea) 2 Green turtle (Chelonia mydas) 3 Loggerhead (Caretta carctta)
4 Hawksbill (Eretmochelys imbricata) 5 Olive or Pacific ridley (Lepidochelys olivacea)
Ecotones and Special Ecosystems
184
G61tenboth,Timotius, Milan and Margraf
new nutrients to the system. Losses of nutrients are caused by currents in the first place. Pelagic fishes or terrestrial animals which are exploiting the resources of tidal flats during high or low tide, respectively, are also devoiding the system ofnutrients (Fig. 9.7). Most of the nutrients originate from the neritic zone. Primary production is virtually non-existent. For rocky tidal zones with strong waves eroding the stone, the only source of nutrients is the sea water during high tide and possibly terrestrial run-off. During low tide, these nutrients are usually not disposed in form of POM like in the tidal flats, but washed away by the surf.
FIGURE 9.6.
Different species of shore birds feeding on tidal flats. 1 2 3 4 5
Mongolian plover (Charadrius mongolus) Sanderling (Crocethia alba) Common sandpiper (Actitis hypoleucos) Ruddy turnstone (Arenaria interpres) Asian dowitcher (Limnodromus
semipalmatus) 6 Whimbrel (Numenius phaeopus) 7 Reef egret (Egretta sacra)
SEA SHORES AND TIDAL FLATS
HUMAN INFLUENCE AND ECOLOGICAL STATUS Most probably, extensive gleaning represents the most severe human impact on shorelines. However, restriction of access is difficult, since most of the people involved can not afford expensive fishing equipment and therefore, have no other access to marine sources of protein. On small islands, predominately women are involved in gleaning activities. Most important collected organisms are molluscs (41 species), followed by echinoderms (7 species), algae (2-3 species), fish (22 species) and one species each of swimming crab and one sea anemone (Schoppe et al., 1997, Schoppe, 2000). Gathering of sand, gravel or rocks from tidal flats is commonplace. It is not known how these activities affect the biocoenosis of this ecosystem. In tidal flats with soft bottoms, pollutants can easily enrich with the effect ofthe whole community of organisms being destroyed through poisoning. Even when some organisms can withstand these hazards, they will possibly accumulate the pollutant and probably poison organisms feeding on them. Even man can be affected by this, when collecting intertidal organisms in contaminated areas like the Bay of Jakarta where extremely high concentrations of heavy metals can be found in sediments.
Ecology of Insular SE Asia 9 The Indonesian Archipelago
FIGURE 9.7.
18~
Simplified model of the nutrient and energy flows on a sandy beach. Most of the nutrients originate from the neritic zone (after Ott, 1988). AM ARM BPP CA DOM
anerobic meiofauna with microbial symbionts anerobic meiofauna benthic pelagic predators chemoautotroph microorganisms dissolved organic matter
Shorelines are sometimes affected by construction of harbors, moles or fishponds.
IMPLICATION FOR EIA STUDIES Commercial exploitation of organisms inhabiting tidal zones should be based on data of population dynamics of the respective species to ensure sustainable harvest.
M microorganisms POM particulate organic matter RPD redox potential discontinuity
Contents of toxicants in soft bottom sediments, especially of heavy metals and organo halogenides, should be monitored carefully in the vicinity of larger human settlements, near industrial facilities, or in areas with intensive agriculture. Construction in tidal flats should not cut off the area from the tides, nor change the tidal regimes considerably. The gathering of sand or gravel in
Ecotones and Special Ecosystems
186
G61tenboth,Timotius, Milan and Margraf
beach areas should be monitored carefully since it can lead to serious erosion. SUMMARY
Shorelines form a marine-terrestrial ecotone. Parameters of some abiotic factors are subjected to extreme variations. Only a limited set of species can survive in this environment by adapting to these fluctuations or by avoiding them through mobility. To avoid competition, the single species have adapted to separate niches which can be quite narrow in space and time. Succession is fast and easy to follow, especially the sessile organisms are easily manipulated, making tidal zones a convenient object for ecological studies (Morton, 1990). Tidal zones are heavily exploited by man for their organisms since they are easily accessible and not many pieces of equipment are required for gleaning.
..~'..~'~i~iiiiiiiili!iliiiiii~,,~, ....~'..~.!~!i!i~!~!!~!ii!'!ii,
SEA S H O R E S A N D T I D A L FLATS
G61tenboth, Timotius, Milan and Margraf
SEA SHORES AND TIDAL FLATS
03 ~ O 9
E
03 03 9 03 ~J 9 o C~ o
o
N
9
~
~
m
"
L,)
,x::
Ecology of Insular SE Asia 9 The Indonesian Archipelago
~
e'~
~.~~~ 9' ~
0
~
~~~~~
]~
Ecotones and Special Ecosystems
] ~7~
G61tenboth, Timotius, Milan and Margraf
SEA SHORES AND TIDAL FLATS
=
,_,= O ,_Q
E
9
o o
9
0
e,l
cO
c~
"~
~J c~
o c~
o
v
~ ~.~
N o
x:
_~
] ~
Ecosystems
~
~
and Special
.~
c~
:~o 9
~
~
o
~'~ ~ ~ ~~'~ ~
c~
E c o l o g y of Insular SE Asia ~ The Indonesian A r c h i p e l a g o
c~
r~
~176
~
9
Ecotones
176
G61tenboth, Timotius, Milan and Margraf
species only occupying narrow areas (Tsuchy and Lirwitayapasit, 1986). In most of the Indonesian islands, distinct zones within the eulittoral and infralittoral could be identified which show a typical distribution oftidal organisms. It is possible and important to distinguish the three major types of shorelines: 9 Rocky shorelines. 9 Sandy shorelines ofvolcanic or other terrestrial origin. 9 Sandy shorelines exclusively built from the biogenetic materials of reefs, the so-called coral cays. The vast majority of the 17,508 islands of the Indonesian archipelago are most likely coral cays or low islands (Tomascik et al., 1997). Among the best known examples ofthis kind of islands with splendid white shores are the about 120 islands of Kepulauan Seribu Thousand Islands off the Jakarta Bay in the Java Sea. Coral cay sediments are biogenic, originating mainly from the skeletal material of numerous calcifying plants and animals which built and live in the reef. During the early stages of development, most sand cays are rather unstable systems. Once sand cays reach a certain mass, the movements become much less pronounced. Stabilization is greatly enhanced by the colonization of plants whose seeds arrrive by atmospheric transport, water currents or on floatsam, or carried bybirds. In addition to the reef sediments, some cays located close to the mainland of large islands (e.g. Java) or active volcanoes (e.g. Banda Neira in the Moluccas) may have non-carbonate sediments incorporated into the cays (Yomascik et al., 1997). The sandy shorelines of the majority of the larger islands are mostly of pure terrestrial origin with often volcanic or jurassic sediments and sands or a mixture of carbonic and non-carbonic sediments.
SEA SHORES AND TIDAL FLATS
Rocky shores occur along the coastlines of many Indonesian islands: 9 The west coast of Sumatra is lined by rocky beaches. 9 Most of the Lesser Sunda Islands and the Moluccas consist of volcanic or limestone shorelines with little or no beach formation.
ECOSYSTEM FUNCTIONS Shorelines absorb the energy of wave action and hence, prevent erosion in coastal areas. Depending on tidal regime, tidal zones can be sinks or sources of nutrients. Like mangroves, they can hold among the most productive soils in the tropics. Shorelines are habitats o fa specialized community of organisms, some ofthem being useful to man like different species of algae, sea cucumbers, and molluscs. Thousands of shorebirds coming from temperate and arctic zones of both hemispheres greatly depend on different types of shorelines as feeding grounds. Species of endangered sea turtles use sandy beaches as nesting ground. Sand bars or rocky shores are breeding grounds for an array of shorebirds. Abiosis The composition of communities of organisms along shorelines is strongly dependent on the type of substrate. Shoreline habitats can range from steep cliffs, over rocky, boulder and gravel shores, sandy beaches, to silty and muddy flats. The different sizes of particles do not only influence the water content of the substrate and the interstitial space between the particles, but also the stability of the surface as a whole. Mud bottoms consist of very fine particles. Space between particles is small so that these substrates usually contain only small amount of water. Silty and clayey bottoms are more stable than most other substrates. Hence, it is possible for the infauna or such animals of the sediment beaches which rarely emerge onto the surface, in contrast to
Ecology of Insular SE Asia 9 The Indonesian Archipelago
the epifauna or animals that spend at least some time on the surface, to build permanent burrows. Sandy substrates consist of mineral and calcareous components of larger size. The latter usually are of organic origin. Sand surfaces are highly influenced by water movement and sometimes form characteristic ripple patterns. Interstitial spaces are inhabited by a unique microfauna. Like in the clayey or silty substrates, the infauna is dominant. However, permanent burrrows are rare, because of the instability of the substrate. The type of substrate strongly depends on the current regime of the respective area. Hard-bottom substrates occur in areas with strong erosive forces, soft-bottoms only can form in situations where deposition is stronger than erosion. Low waves with a long wavelength are carrying material to the shore, whereas high waves with short wavelength have erosive effects (Morton and Morton, 1983). Also wind can have a major impact in the forming of seashore landscapes. Sand dunes are a restricted feature in Indonesia. They only occur near Pangumbahan along the south coast of West Java and the north coast of Irian Jaya. The dunes form parallel to the coastline with the younger more mobile ones closer to the beach. As initial stage of the forming of a dune, drifting sand is caught on the leeward of small obstacles, like vegetation or debris. Sand is blown over the ridge off this small obstacle. Subsequently, the sand bakes together and vegetation establishes. Consequently, the older dunes in the hinterlands are less, but still are growing (Siringan and Pataray, 1996). Shorelines are strongly influenced FIGURE by the tides. The latter are caused by the rising and falling of the world's ocean's
]77
water body due to the gravitational influence of the moon and the sun upon the earth (Fig. 9.3). Two tidal protrusions of water permanently exist, one facing the moon and one on the opposite site of the globe. These bulges result in two high tides with two low tides in-between during one lunar day lasting 24 hours and 50 minutes. Together with the rotation time of the earth, a cycle of 24 hours and 21 minutes duration results. The impact ofthe sun on tidal forces is lower than that of the moon. However, it has a modifying influence. Spring tides occur, with considerable differences in water level during low
9.3.
Forming of hypothetical tidal bulges under the influence of the moon and the sun during: 1 New moon, 2 Last quarter, 3 Full moon, 4 First quarter.
Ecotones and Special Ecosystems
] 78
G61tenboth, Timotius, Milan and Margraf
and high tide, when the moon, the earth and the sun are situated in line (during full or new moon). The sun's gravity pull diminishes that of the moon, when the position of the sun is on a right angle to that of the moon. Subsequently, neap tides, with only minor changes of water levels between the tides become prevalent. Each constellation occurs twice during a lunar month. In reality, tidal regimes follow a much more complicated pattern, since the world oceans are disrupted by land masses which influence the tidal bulges considerably. This change between marine and terrestrial conditions is the single most important factor that limits the diversity of the community of organisms in this ecosystem. Tidal cycles do not only influence the diurnal activity patterns of virtually all organisms of the tidal zones, but also sometimes function as triggers for their longer term life cycles, for example through determining time of mating or migrating. Like most ecotones, shorelines are subjected to extremely variable abiotic factors. Especially the water bodies in tidal pools can vary considerably regarding temperature and oxygen content. When exposed to the sun, evaporation increases the salinity, while when exposed to rain, it decreases. In the zone where the surf breaks, organisms are exposed to strong physical forces. Action ofwaves is influenced by the speed of the wind and the distance over which the wind operates (Morton and Morton, 1983). Algae occurring in tidal zones have developed thick cuticulas to avoid desiccation and structures to stand the physical forces ofwave action like strong rhizoids and flexible thalloids. Animals adapt to these factors by evolving hard shells like molluscs. Others hide in crevices or the substrate where they can endure when conditions get unfavorable like some echinoderms or annelids. These organisms are usually referred to as 'cryptofauna' (Evans, 1949). Other mobile organisms search actively for their fitting surrounding like crustaceans or some blenniid or gobiid fish, which can move over dry land for some distance.
SEA SHORES AND TIDAL FLATS
Biodiversity Littorial communities can be divided into two categories: 9 Free living species including fish, molluscs, crabs, sea urchins, sea stars, and polychaetes. 9 Sessil species including algae, barnacles, sea anemones, and corals.
Producers Main producers in tidal zones are different forms of algae. On soft bottoms diatoms (Fam. Chrysophyceae) form a yellow to green cover during low tide which partly is removed during high tide. Those microalgae are those producers contributing most to the food chain. Some varieties ofseagrasses are also able to dwell on tidal zones. On hard bottoms, the pinkish to red calcareous algae Corallina sp. form encrusting covers. Euryoecious macro algae like Padina sp. are capable of growing on hard bottoms in tidal pools. Higher plants are able to establish on beaches.Pioneering species have different tolerance to saltwater exposure following germination, which results in a loose, but discernable zonation ( Tomascik et a1.,1997). On coral cay and other limestone beaches, the most tolerant species to salt water exposure is Sporobolus fertilis . Just above the highest inundation line, the creeping Vigna marina and the fast growing Spinifex litoreus can be found. Parts of the shoreline only affected by sea spray are frequently dominated by the grasses Thaurea involuta and Lepturus repens. A typical pioneering species is the Goat's foot vine Ipomoea pes-caprae that can form dense vegetation mats on sandy substrates. It does not only prevent erosion but also acts as trap for nutrients with its root system. The bright pink flowers are very conspicuous and the common name is derived from the characteristically shape ofthe leaves which resembles the hoofofa goat (Fig. 9.4). It is often pollinated by Xylocopa sp.( Hymenoptera).They produce , like many others of the beach plants, floating seeds. The
Ecology of Insular SE Asia
9 The Indonesian Archipelago
179
portulacustrum (Fam. Aizoaceae), Euphorbia chaissonis (Fam. Euphorbiaceae), Vigna marina (F am. F ab ace ae), Triumfetta repens (Fam.Tiliaceae), and Vitex trifolia (Faro. Verbenaceae). Different species of grasses and sedges can also settle on barren sand. Common are the sedge Cyperus pedunculatum (Faro. Cyperaceae) and the grass Spinifex littoreus var. longifolius (Fam. Poaceae) which further enhance the stability of the substrate. Eventually this vegetation can be replaced by woody species which may develop into beach forest. On very sandy stretches, pure stands of Casuarina equisetifolia can develop followed by a full Barringtonia community.
FIGURE 9.4.
A) Pes-caprae formation on a sandy beach. B) Goat's foot vine Ipomoeapes-caprae(Fam. Convolvulaceae).
whole plant community with this habit is named after this species as Pes-caprae-formation.This formation is an open community of low, sand-binding herbs, grasses and sedges behind the drift line. Endemics are absent and many species have a pantropical distribution. Often associated with the former species is Canavalia maritima (Faro. Fabaceae). Superficially the vegetative parts resemble that of the goat's foot vine with fleshy shoots that form a tangle of vegetation. Other associated plants are Wedelia biflora (Fam. Compositae), Sessuvium
Consumers The community of animals inhabiting tidal zones and beaches is dominated by the following guilds: 9 Sessile or hemisessile planctivores. 9 Grazing herbivores which feed on the algal turf growing on the rocks and the layer of diatoms on the soft bottoms. 9 'Grazing' carnivores which feed on benthic invertebrates. 9 Detritivores which feed on the dead organic matter brought by high tide. 9 Carnivores and scavengers are particularly abundant in supralittoral beaches. The majority of animals belong to three groups concerning their feeding habit: 9 They filter plankton from the seawater and are therefore filter feeders. 9 They suck organic deposits and microorganisms off the sediment surface. 9 They sort edible particles of sediments and are therefore deposit feeder. In addition, beach organisms are frequently divided into three sediment-size related groups (Carter, 1988): 9 Attached microfauna 9 Interstitial meiofauna
Ecotones and Special Ecosystems
180
G61tenboth,Timotius, Milan and Margraf
9 Borrowing or loosly attached macrofauna. The nearshore waters support usually rich populations ofphytoplankton and zooplankton, which derive great benefit from turbulent waters. Diatoms are the most abundant phytoplankton group. The diatom Chaetoceros sp. thrives particularly in the surf zone of high energy beaches, like in Java and Bali, where nutrients are available from land runoff and strong coastal upwelling. They have the ability to attach themselves to bubbles which allow them to remain at the surface in maximum light (Carter, 1988). A study on a tidal reef flat yielded a very high abundance ofmeiofauna and small macrofauna, the nematodes, harpactoids and polychaetes being the three most important groups. Planctivores like some cnidarians, polychaetes and bryozoans are most abundant in tidal pools that never or rarely become dry. The primitive Chiton Acanthopleura sp. is commonly found attached to rocks splashed with sea water. Oysters (Fam.Ostreidae) have solid shells and are permanently attached to hard substrates and therefore can withstand strong waves, as well as being exposed to the sun during low tide. Barnacles also have developed shellqike structures which can be closed to avoid dessication. Despite superficial resemblence with molluscs, they are highly specialised crustaceans. Their legs have been modified to cirri that function as filtering organs. Common in intertidal zones are Pollicipes mitella, Chthalamalus sp., Tetrar sp., Tetrar sp. (Rosell, 1986). Other species live attached to animals, especially crustaceans, but also on drifting woods and other hard substrates. Higher crustaceans are relatively diverse along shorelines. The most conspicuous crustaceans on mud flats are the Fiddler crabs Uca spp. The males have one enlarged claw with which they signal other males their position and that the respective territory is occupied. Before their burrows get flooded, they cut out round pieces of mud from the surface, retreat
SEA SHORES AND TIDAL FLATS
back in their burrows and seal the hollow tight with the piece of mud used as a cover. On sandy beaches, land hermit crabs Coenobita sp., and sand crabs Ocypode sp. are particularly common. They are predominantely noctumal to avoid predation in a habitat that does not provide much shelter. The soldier crabs Mictyris sp. are closely related to the latter. Like their relatives they live in sand holes. In contrast to the larger sand crabs, they often forage in large aggreggations, possibly as a means to avoid predation. The crabs of the genus Dotilla leave behind very conspicuous signs of feeding. Their holes are surrounded by concentric walls of sand 'pebbles'. These are formed when the crab is screening the sand for food around its hole. The walls of sand pebbles surrounding the central hole are not completely closed to allow the crab a fast retreat in case of danger.The environmental effects of these burrows lay in the increase ofthe penetration depth of oxygen and bringing lower sediments to the surface. The deposits of the suspension feeders in form of fecal pellets are food for the deposit feeders, while the deposit feeders resuspend their deposits in the sea water. Members of the genus Grapsus are typical for rocky shores. With their flattened carapaces and their strong legs, they can withstand the waves without being washed away. Insects are becoming increasingly more abundant going from the seaward to the landward side of shorelines. Tidal zones are the only ecosystems strongly influenced by the sea where some insects are able to successfully compete with crustaceans. Water skippers (Fam. Gerridae) dwell on the surfaces of tidal pools and some species of aphids, beetles and ants live in soft bottom tidal flats. To avoid flooding oftheir burrows, the openings are not made wider than 3mm. This is enough to hold back the air due to adhesive forces, even during high tide (Maitland and Maitland, 1994). Sandy beaches are often inhabited by tiger beetles (Fam. Cicindelidae). Agile predators with
Ecology of Insular SE Asia 9 The Indonesian Archipelago
strong mandibles prey on small invertebrates on the barren ground. Large wasps of the families Pompilidae and Sphecidaehunt spiders and caterpillars, respecively, by immobilizing them with their poisonous sting. The still living prey is buried alive in sand holes and serves as food for the wasp larvae. Molluscs are the most common herbivore grazers on tidal flats. Patellid snails even can survive on rocks which are exposed to very strong wave action. They have cone-shaped shells and attach with their sucker-like foot to the rocky surface always returning to the same spot after grazing. Subsequently, a cavity forms which follows the outlines of the shell. Hence, the snail is able to seal of its body from the unfortunate microclimate during low tide. Prosobranch snails like Littoraria sp. can close their shell tight with a calcareous structure at its foot called operculum. The zonation of organisms on rocky shores usually follows a typical pattern with three major zones: Lower intertidal; middle intertidal; and upper intertidal. Each zone is characterized by a key group oforgnisms. Space is a key limiting factor and inter- and intraspecies competition can be intense. Brittle stars (Ophiuroidea) are the most common echinoderms to be encountered on tidal flats. They can be easily spotted bythe vivid movements oftheir arms to filter out detritus from the water surface. However, the whole animal is difficult to see, since it usually hides in a crevice. Most sea urchins that most commonly graze on algae growing on hard bottoms are not adapted to be exposed to the air and have to retreat to rock pools or water-filled crevices during low tide. Predators among the molluscs are some cowries (Fam. Cypraeidae) and cone snails (Faro. Conidae). The latter are killing their prey, with toxins that in some species can even be fatal to humans. Nekton like schools of goatfishes (Fam. MuUidae) or gray mullets (Fam. Mugilidae) enter tidal zones only during high tide to utilize the
] 81
productive feeding grounds. In tidal pools, only few fishes permanently use the open water body like some damselfishes, when still being immature. However, the fish community in the intertidal is dominated by benthic families with the Gobiidae and Blenniidae being the most diverse. Some species are even able to leave the water and live on land for a limited time. The mudskippers Periophtalmus sp. are well known amphibious fishes which commonly live in mangroves. The leaping blennyAlticus saliens is typical for more open tidal zones. Marine turtles are only temporary visitors of sandy beaches where they lay their eggs. Mainly four species are recorded: the largest and rarest is the Leatherback Dermochelys coriacea. It can reach a length of 3 m and weights up to 750 kg. In contrast to its smaller relatives, its upper shell consists of bony plates that are embedded in the skin with seven conspicuous ridges. Its main diet consists of jellyfish and it has been reported that more and more animals die because they swallow discarded plastic bags which they mistake for their prey. The other species all have bony shells that are completely covered with horny scales. With some exercise, the different species can be identified by the distinct scale patterns. The Green turtle Chelonia mydas is mainly vegetarian and is found in seagrass beds and stands of algae in shallow water. The Loggerhead Lepidochelys olivacea feeds on fish and pelagic invertebrates, whereas the smallest species, the Hawksbill Eretmochelys imbricata feeds on benthic invertebrates. Alcala (1980) has conducted a threeyear study on the ecology of this species. It was found to be most common in shallow coral reefs, reef drop offs and channels between islands. The preferred nesting beaches have generally fine gray sand. Laying of eggs presumably occurs year-round. However, the single females do not lay eggs every year, but seem to follow three-year nesting cycles. Clutch sizes are reported to range between 112 to 130 eggs. Each female produces two or even more
Ecotones and Special Ecosystems
182
G61tenboth,Timotius, Milan and Margraf
clutches that are laid in three-week intervals in one nesting season. After laying eggs, the turtles are known to return to the beaches where they have been born. However, it is still not fully understood what features make a suitable nesting beach. Depending on the temperature, propagation time is around 60 days. Only very few of the hatchlings survive the first years. Already the eggs are heavily predated by monitor lizards, pigs, dogs, rats and man. Additional predators of the young when they try to reach the sea are crabs and larger seabirds. In the sea, sharks and other predatory fishes and even large jellyfishes are known to decimate the numbers of the young turtles. However, the main threat to turtles is the capture of adult animals by man. Especially in demand are the Hawksbill and the Green turtles, because of their meat, the bones and the shells which are used for handicraft. Now on the brink ofextintion, all marine turtles are protected by law in Indonesia and conservation projects are launched also to secure the nesting beaches. One of the most important reserves for marine turtles in the country is Pulau Sangalaki in East Kalimantan where 30-45 turtles dig their nests every night of the year (Tomascik et al., 1997). The main nesting sites for the Green turtle in South East Asia are the islands of Penyu, Lucipara and Am (Fig.9.5). The skink Emoia atrocostata (Fam. Scincidae) is a relative frequent visitor of the Pes-caprae formation. Resident sea birds like boobies (Fam. Sulidae), frigate birds (Fam. Fregatidae), tobicbirds (Fam. Phaetonidae), and noddies (Farn. Laridae) and migrant shore birds relay heavily on shorelines as feeding grounds. Most of them are winter visitors coming from Siberia orNorth East Asia. The different species of waders (Charadriiformes) have adapted to different feeding niches to avoid competition. The Sanderling Crocethia alba forages near the water's edge of sandy beaches on small invertebrates washed ashore by the surf. The Mongolian sandpiper Charadrius mongolus feeds on open sand or mud
SEA SHORES AND TIDAL FLATS
flats, whereas the Ruddy turnstone Arenaria interpres is more often encountered on shores with gravels or crushed corals where it preys on animals by turning around the stones under which they hide. The Common sandpiper Actitis hypoleucos is the wader most likely to be encountered in Indonesia feeding on soft bottom, hard bottom and boulder shores alike. It has been recorded that the biomass density of prey for these tactile feeders varies widely between 1-37 g.m -2around Java. Due to their long beaks the Asian dowitcher Limnodromus semipalmatus and especially the Whimbrel Numeniusphaeopus are able to utilize food sources that are hidden so deep in the substrate that most other waders cannot reach them. Several species of herons and egrets (Fam. Ardeidae) can be regularly observed in tidal zones, but only the Reef egret Egretta sacra seems to be restricted to it (Fig. 9.6). The only mammal that usually can be found in tidal zones is the Small-clawed otter Aonyx cinerea which is widespread in Southeast Asia. Decomposers Since particulate organic matter (POM) is brought to the system twice a day, the guild of decomposers is rich in species. Especially the echinoderms like brittle stars and sea cucumbers are abundant, as are different species of crustaceans and annelid worms. As much as 27 species of holothurians have been recorded from intertidal areas (Tan Tiu, 1981). Since POM is a resource with spotty distribution, most detritivores are mobile or hemisessile. One exception is the spaghetti wormLoimia medusa that has tentacles which can cover up to lm 2 around its burrow in search for food (Storch and Rosito, 1981 and Fig. 9.1).
NUTRIENT AND ENERGY FLOW Since solar radiation is high and nutrients are usually abundant, shorelines where sedimentation takes place are productive ecosystems. Every high tide brings
Ecology of Insular SE Asia 9 The Indonesian Archipelago
F I G U R E 9.5.
] 83
The most conspicous species of marine turtles recorded from Indonesian waters. 1 Leatherback (Dermochelys coriacea) 2 Green turtle (Chelonia mydas) 3 Loggerhead (Caretta carctta)
4 Hawksbill (Eretmochelys imbricata) 5 Olive or Pacific ridley (Lepidochelys olivacea)
Ecotones and Special Ecosystems
184
G61tenboth,Timotius, Milan and Margraf
new nutrients to the system. Losses of nutrients are caused by currents in the first place. Pelagic fishes or terrestrial animals which are exploiting the resources of tidal flats during high or low tide, respectively, are also devoiding the system ofnutrients (Fig. 9.7). Most of the nutrients originate from the neritic zone. Primary production is virtually non-existent. For rocky tidal zones with strong waves eroding the stone, the only source of nutrients is the sea water during high tide and possibly terrestrial run-off. During low tide, these nutrients are usually not disposed in form of POM like in the tidal flats, but washed away by the surf.
FIGURE 9.6.
Different species of shore birds feeding on tidal flats. 1 2 3 4 5
Mongolian plover (Charadrius mongolus) Sanderling (Crocethia alba) Common sandpiper (Actitis hypoleucos) Ruddy turnstone (Arenaria interpres) Asian dowitcher (Limnodromus
semipalmatus) 6 Whimbrel (Numenius phaeopus) 7 Reef egret (Egretta sacra)
SEA SHORES AND TIDAL FLATS
HUMAN INFLUENCE AND ECOLOGICAL STATUS Most probably, extensive gleaning represents the most severe human impact on shorelines. However, restriction of access is difficult, since most of the people involved can not afford expensive fishing equipment and therefore, have no other access to marine sources of protein. On small islands, predominately women are involved in gleaning activities. Most important collected organisms are molluscs (41 species), followed by echinoderms (7 species), algae (2-3 species), fish (22 species) and one species each of swimming crab and one sea anemone (Schoppe et al., 1997, Schoppe, 2000). Gathering of sand, gravel or rocks from tidal flats is commonplace. It is not known how these activities affect the biocoenosis of this ecosystem. In tidal flats with soft bottoms, pollutants can easily enrich with the effect ofthe whole community of organisms being destroyed through poisoning. Even when some organisms can withstand these hazards, they will possibly accumulate the pollutant and probably poison organisms feeding on them. Even man can be affected by this, when collecting intertidal organisms in contaminated areas like the Bay of Jakarta where extremely high concentrations of heavy metals can be found in sediments.
Ecology of Insular SE Asia 9 The Indonesian Archipelago
FIGURE 9.7.
18~
Simplified model of the nutrient and energy flows on a sandy beach. Most of the nutrients originate from the neritic zone (after Ott, 1988). AM ARM BPP CA DOM
anerobic meiofauna with microbial symbionts anerobic meiofauna benthic pelagic predators chemoautotroph microorganisms dissolved organic matter
Shorelines are sometimes affected by construction of harbors, moles or fishponds.
IMPLICATION FOR EIA STUDIES Commercial exploitation of organisms inhabiting tidal zones should be based on data of population dynamics of the respective species to ensure sustainable harvest.
M microorganisms POM particulate organic matter RPD redox potential discontinuity
Contents of toxicants in soft bottom sediments, especially of heavy metals and organo halogenides, should be monitored carefully in the vicinity of larger human settlements, near industrial facilities, or in areas with intensive agriculture. Construction in tidal flats should not cut off the area from the tides, nor change the tidal regimes considerably. The gathering of sand or gravel in
Ecotones and Special Ecosystems
186
G61tenboth,Timotius, Milan and Margraf
beach areas should be monitored carefully since it can lead to serious erosion. SUMMARY
Shorelines form a marine-terrestrial ecotone. Parameters of some abiotic factors are subjected to extreme variations. Only a limited set of species can survive in this environment by adapting to these fluctuations or by avoiding them through mobility. To avoid competition, the single species have adapted to separate niches which can be quite narrow in space and time. Succession is fast and easy to follow, especially the sessile organisms are easily manipulated, making tidal zones a convenient object for ecological studies (Morton, 1990). Tidal zones are heavily exploited by man for their organisms since they are easily accessible and not many pieces of equipment are required for gleaning.
..~'..~'~i~iiiiiiiili!iliiiiii~,,~, ....~'..~.!~!i!i~!~!!~!ii!'!ii,
SEA S H O R E S A N D T I D A L FLATS