ekosistem padang lamun
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1 EKOSISTEM PADANG LAMUN Daerah perairan pantai adalah wilayah perairan yang berada antara ujung paparan benua dengan kedalaman laut sekitar 200 m sampai pantai yang didalamnya terdapat ekosistem mangrove, terumbu karang, estuari, padang lamun, sumber hayati dan nonhayati, serta fasilitas-fasilitas seperti pelabuhan dan pemukiman dan panorama pesisir. Seagrass Ekosistem yang terabaikan Sering dapat dilihat hamparan hijau pada dasar laut di pinggir pantai yang menyerupai padang rumput hijau, yang tidak lain adalah padang-lamun atau yang populer dikenal dengan seagrass. Seagrass adalah tempat hidup bagi banyak organisme, seperti ikan, kepiting, udang, lobster, seaurchin (bulu babi), dan lainnya. Sebagian besar organisma pantai (ikan, udang, kepiting dll) mempunyai hubungan ekologis dengan habitat lamun. Sebagai habitat yang ditumbuhi berbagai spesies lamun, padang lamun memberikan tempat yang sangat strategis bagi perlindungan ikan-ikan kecil dari "pengejaran" beberapa predator, juga tempat hidup dan mencari makan bagi beberapa jenis udang dan kepiting. ian.umces.edu/discforum/index.php?topic=108.0 Seagrass bukan "rumput laut" Habitat Lamun atau yang lebih di kenal dengan kata seagrass merupakan habitat pantai yang sangat unik. dengan di tumbuhi oleh lamun 2 (golongan macrophyte) yang dapat beradaptasi dengan kondisi pantai yang labil, tumbuhan lamun memebrikan amat sanagt banyak fungsi ekologis bagi organisma yang berasosiasi dengannya. Banyak organisme yang secara ecologis dan biologis sangat tergantung pada keberadaan lamun. Banyak orang awam mengenal kata seagrass sebagai "rumput laut" yang konotasinya ke arah seaweed. Namun jika dirunut lebih jauh tentang kedua tumbuhan ini akan sangat jauh perbedaanya. Sebagai tumbuhan sejati, seagrass merupakan tumbuhan yang mempunyai akar (Ryzome dan serabut akar), batang, daun, bunga dan beberapa spesies berbuah. Berbeda dengan seaweed yang merupakan alga besar (Macroalga) yang tidak mempunyai akar, batang dan daun sejati. Sebagai tumbuhan tingkat tinggi, seagrass mempunyai sistem reproduksi dan pertumbuhan yang khas. research.myfwc.com/.../view_article.asp?id=20720 Seagrasses are submerged flowering plants found in shallow marine waters, such as bays and lagoons and along the continental shelf in the Gulf of Mexico. A vital part of the marine ecosystem due to their productivity level, seagrasses provide food, habitat, and nursery areas for numerous vertebrate and invertebrate species. The vast biodiversity and sensitivity to changes in water quality inherent in seagrass communities makes seagrasses an important species to help determine the overall health of coastal ecosystems. Seagrasses perform numerous functions: Stabilizing the sea bottom Providing food and habitat for other marine organisms Maintaining water quality Supporting local economies Seperti layaknya padang rumput, seagrass dapat menyebar dengan perpanjangan ryzome (batang akar). Penyebaran seagrass terlihat sedikit unik dengan pola penyebaran yang sangat tergantung pada topografi dasar pantai, kandungan nutrient dasar perairan (substrate) dan beberapa faktor 3 fisik dan kimia lainnya. Kadang terlihat pola penyebaran yang tidak merata dengan kepadatan yang relative rendah dan bahkan terdapat semacang ruang-ruang kosong di tengah padang lamun yang tidak tertumbuhi oleh lamun. Kadang-kadang terlihat pola penyebaran yang berkelompokkelompok, namun ada juga pola penyebaran yang merata tumbuh hampir pada seluruh garis pantai landai dengan kepadatan yang sedang dan bahkan tinggi. Berbeda dengan seaweed, yang umunnya sangat memerlukan benda keras di dasar perairan sebagai susbtrat untuk melekat. namun memang ada juga banyak yang tumbuh dan menyebar secara alami dengan substrat dasar yang lunak. www.seagrasswatch.org/seagrass.html A number of environmental parameters are critical to whether seagrass will grow and persist. These include physical parameters that regulate the physiological activity of seagrasses (temperature, salinity, waves, currents, depth, substrate and day length), natural phenomena that limit the photosynthetic activity of the plants (light, nutrients, epiphytes and diseases), and anthropogenic inputs that inhibit access to available light for growth (nutrient and sediment loading). Various combinations of these parameters will permit, encourage or eliminate seagrass from a specific location. Seagrasses occupy a variety of coastal habitats. Seagrass meadows typically occur in most shallow, sheltered soft-bottomed marine coastlines and estuaries. These meadows may be monospecific or may consist of multispecies communities, sometimes with up to 12 species present within one location. 4 The depth range of seagrass is usually controlled at its deepest edge by the availability of light for photosynthesis. Exposure at low tide, wave action and associated turbidity and low salinity from fresh water inflow determine seagrass species survival at the shallow edge. Seagrasses survive in the intertidal zone especially in sites sheltered from wave action or where there is entrapment of water at low tide, (e.g., reef platforms and tide pools), protecting the seagrasses from exposure (to heat, drying) at low tide. The habitat complexity within seagrass meadows enhances the diversity and abundance of animals. Seagrasses on reef flats and near estuaries are also nutrient sinks, buffering or filtering nutrient and chemical inputs to the marine environment. They also stabilise coastal sediments. They also provide food and shelter for many organisms, and are a nursery ground for commercially important prawn and fish species. The high primary production rates of seagrasses are closely linked to the high production rates of associated fisheries. These plants support numerous herbivore- and detritivore-based food chains, and are considered very productive pastures of the sea. The associated economic values of seagrass meadows are very large, although not always easy to quantify. Seagrass/algae beds are rated the 3rd most valuable ecosystem globally (on a per hectare basis), only preceded by estuaries and wetlands. The average global value of seagrasses for their nutrient cycling services and the raw product they provide has been estimated at 1994US$ 19,004 ha-1 yr-1 (Costanza et al. 1997). This value would be significantly greater if the habitat/refugia and food production services of seagrasses were included. www.seagrasswatch.org/seagrass.html 5 Identifikasi Seagrass Seagrasses are aquatic flowering plants that form meadows in nearshore brackish or marine waters in temperate and tropical regions. Australia has the highest diversity of seagrasses in the world , comprising more than half of the world's species, and all but one genus. At the broadest level, seagrasses are differentiated into temperate and tropical species. Seagrass species can also differ in terms of the breadth of their distributional ranges (broad vs restricted), their reproductive strategies (e.g. rapid seeding, seed banks and vegetative reproduction), the degree of their persistence (ephemeral vs persistent), physiology (e.g. growth dynamics, nutrient cycling and response to disturbance) and in their ecological interactions (e.g. influence of grazing, leaf canopy structure, detritus production and epiphyte production). Assemblages of seagrass species give rise to a series of dynamic and temporally and spatially variable seagrass meadows. Changes in the species composition of seagrass meadows may indicate slow but important changes in the environment, and are a suggested indicator for State of the Environment . www.ozcoasts.org.au/.../seagrass_species.jsp Coastal seagrass habitats support high levels of primary productivity and are the most biodiverse of all the seagrass habitats. The primary controls on the species composition of these habitats are tidal range (intertidal or subtidal), physical disturbance caused by storms and cyclone related swell and waves, sediment movement, and the extent of grazing by macroherbivores (e.g. turtles and dugongs). The seagrass communities of reefs support a high level of biodiversity and are highly productive. The species composition of these communities reflect low nutrient availability, 6 unstable sediment and fluctuating water temperature and salinity. Deep water seagrasses (15 - 58m) often form monospecific meadows. The primary control on deep water seagrass meadows are low light levels (and changes in spectral composition) caused by the refraction and absorption of light in the water column and pulsed turbidity events. Syringodium isoetifolium Habitat Tumbuhan laut sejati, tumbuh pada rataan terumbu bersubtsrat pasir, tumbuh sampai kedalaman 3 m, pada zona yang tidak terlalu lama terekspos udara pada saat surut maksimal atau pada teluk yang bersubstrat pasir terrigenous, tidak pernah menyusun padang lamun monospesifik namun tumbuh bersama-sama dengan jenis lamun yang lain. Karakteristik Morfologi Daun silindris dengan panjang mencapai 25 cm dan lebar 2 mm. Lokasi Tumbuh Pantai Bama Taman Nasional Baluran Situbondo Jawa Timur Indonesia 7 Wide leaved species found in small numbers in the bay. A tropical species found no further south. Grows in small areas of high productivity with a number of other species creating a food source for dugongs. 8 Posidonia oceanica Posidonia form complete underwater ecosystems providing a crucial sanctuary and feeding ground for sea turtles. They enable a unique habitat to take form on the seafloor providing an array of nutrients and covering a total surface area of roughly 20.000 square nautical miles. Posidonia oceanica is a slow growing seagrass found at depths of 5 to 35 meters along the Mediterranean coastline. It plays an important role in oxygenating and clarifying coastal waters, provides a habitat for a rich diversity of plants and animals, acts as a safe breeding-area for many species, and protects beaches from erosion. These meadows also act as a "carbon sink" absorbing carbon dioxide from the atmosphere. Due to the tough lignin that covers its cells, it is only grazed by animals that have special microorganisms in their intestinal tract to help them digest it. One such animal is the endangered green sea turtle, Chelonia mydas. Posidonia thrive at depths of 3-5 metres all the way to 30-40 metres in pristine conditions. They can be found on rocky as well as soft substrate, in temperatures between 15 and 20 degrees Celsius, and at stable salinity levels. Posidonia meadows are succeptible to even the slightest human activity. Posidonia reproduce by vegetative reproduction (the breaking off of fragments and replanting itself) , or through the dispersal of seeds. The fruit of the plant is extremely light. This enables the fruit of the plant to detatch itself and freely float to the surface where it will be taken away by waves through the current. Eventually, the fruit will dissintegrate, allowing the heavy seeds to sink to the seafloor and find a new home. 9 Halophila spinulosa Below depths of about 8 metres forms very dense stands of up to 800 shoots per square metre. Fern like ; Leaves arranged in opposite pairs ; Erect shoot up to 15cm long ; Found at subtidal depths (>10m) . 10 Halophila ovalis Narrow leaved species. Can grow on the intertidal flats in sparse mixed stands with Halodule uninervis and Halophila minor. These stands are the preferred grazing area for dugongs. Grows rapidly. Oval shaped leaves in pairs 8 or more cross veins No hairs on leaf surface Preferred dugong food Common early colonising species Found from intertidal to subtidal depths 11 Halophila minor Narrow leaved species. Mixed stands with H. ovalis and Halodule uninervis are preferred dugong food source. Less than 8 pairs of cross veins Small oval leaves occurring in pairs Wedge-shaped leaf sheath Found on shallow/intertidal sand flats 12 13 Halophila decipiens Has small translucent oval leaves. Found in low densities in Shark Bay. Small oval leaf blade 1-2.5cm long 6-8 cross veins Leaf hairs on both sides Leaves usually longer than wider Found at subtidal depths (>10m). 14 Halodule uninervis Apart from the 2 species above it is the only other species in the bay to form monospecific stands eg at mouth of Wooramel delta, an important summer feeding ground for dugongs. Rhizome of this species is rich in starches. Can be found at high salinities of 62 parts per thousand but in low numbers. Grows rapidly. 15 Halodule pinifolia Short leaved = (mean length range 49.70-57.43 mm, length range 40-80mm) . Long leaved = (mean length range 83.91-102.52 mm, length range 60-166 mm). Environment/Habitat : Inhabits the areas from the lower intertidal to the upper subtidal zone with sandy and muddy bottoms in sheltered bays and coral reefs. It has also been found in mangrove swamps. Commonly found in soft mud together with Halophila ovalis and on compact mud together with Cymodocea rotundata (den Hartog 1970). 16 Halophila tricostata (HT) Erect shoots 8-18cm long Leaves with 3 veins 2-3 leaves at each node Leaves “whorl” around stem Found at subtidal depths (>10m) Endemic to Queensland, Australia 17 Cymodocea serrulata Linear strap-like leaves, 5-9mm wide Serrated leaf tip Leaf sheath is broadly triangular with a narrow base Leaf scars do not form a continuous ring around the stem Found on shallow subtidal reef flats and sand banks 18 Cymodocea rotundata Flat, strap-like leaves 2-4mm wide Rounded, smooth leaf tip Smooth rhizome Scars from well developed leaf sheaths form a continuous ring around the stem Found on shallow reef flats . 19 Thalassia hemprichii (TH) Short black bars of tannin cells in leaf blade Thick rhizome with scars between shoots Hooked/curved shaped leaves Leaves 10-40cm long Common on shallow reef flats. Thalassodendron ciliatum (TC) Cluster of ribbon-like curved leaves at the end of an erect stem Round, serrated leaf tip Tough, woody rhizomes with scars from successive shoots Very coiled, branched roots Typically found in rocky areas with strong reef crests 20 Thalassia testudinum 21 Enhalus acoroides Very long ribbon-like leaves 30-150 cm long Leaves with inrolled leaf margins Thick rhizome with long black bristles and cord-like roots Found on shallow/intertidal sand/mud banks (often adjacent to mangrove forests) 22 Zostera capricorni (ZC) Long strap-shaped leaves 5 longitudinal veins Cross veins which form a mesh across leaf blade Rounded leaf tip Leaf grows straight from rhizome ie. no stem Found on shallow and intertidal mud/sand flats 23 Habitat Lamun atau yang lebih di kenal dengan kata seagrass merupakan habitat pantai yang sangat unik, dengan di tumbuhi oleh lamun (golongan macrophyte) yang dapat beradaptasi dengan kondisi pantai yang labil, tumbuhan lamun memberikan amat sangat banyak fungsi ekologis bagi organisma yang berasosiasi dengannya. Seperti layaknya padang rumput, seagrass dapat menyebar dengan perpanjangan ryzome (batang akar). Penyebaran seagrass terlihat sedikit unik dengan pola penyebaran yang sangat tergantung pada topografi dasar pantai, kandungan nutrient dasar perairan (substrate) dan beberapa faktor fisik dan kimia lainnya. Kadang terlihat pola penyebaran yang tidak merata dengan kepadatan yang relative rendah dan bahkan terdapat semacam ruang-ruang kosong di tengah padang lamun yang tidak tertumbuhi oleh lamun, kadang terlihat pola penyebaran yang berkelompokkelompok. Namun ada juga pola penyebaran yang merata tumbuh hampir pada seluruh garis pantai landai dengan kepadatan yang sedang dan bahkan tinggi. Berbeda dengan seaweed, yang umunnya sangat memerlukan benda keras di dasar perairan sebagai susbtrat untuk melekat. namun memang ada juga banyak yang tumbuh dan menyebar secara alami dengan substrat dasar yang lunak. Di pesisir pantai Indonesia ada tiga tipe ekosistem yang penting, yakni terumbu karang, mangrove, dan padang lamun. Di antara ketiganya, padang lamun paling sedikit dikenal. Kurangnya perhatian kepada padang lamun, antara lain, disebabkan padang lamun sering disalahpahami sebagai lingkungan yang tak ada gunanya, tak memberikan manfaat bagi kehidupan manusia. Di kalangan akademisi pun masalah padang lamun baru mulai banyak dibicarakan setelah tahun 2000. Lamun Lamun (seagrass) adalah tumbuhan berbunga yang telah menyesuaikan diri hidup terbenam di dalam laut dangkal. Lamun berbeda 24 dengan rumput laut (seaweed) yang dikenal juga sebagai makroalga. Lamun berbunga (jantan dan betina) dan berbuah di dalam air. Produksi serbuk sari dan penyerbukan sampai pembuahan semuanya terjadi dalam medium air laut. Lamun mempunyai akar dan rimpang (rhizome) yang mencengkeram dasar laut sehingga dapat membantu pertahanan pantai dari gerusan ombak dan gelombang. Dari sekitar 60 jenis lamun yang dikenal di dunia, Indonesia mempunyai sekitar 13 jenis. kurrawa.gbrmpa.gov.au/.../1seagrasses.html Suatu hamparan laut dangkal yang didominasi oleh tumbuhan lamun dikenal sebagai padang lamun. Padang lamun dapat terdiri dari vegetasi lamun jenis tunggal ataupun jenis campuran. Padang lamun merupakan tempat berbagai jenis ikan berlindung, mencari makan, bertelur, dan membesarkan anaknya. Ikan baronang, misalnya, adalah salah satu jenis ikan yang hidup di padang lamun. Amat banyak jenis biota laut lainnya hidup berasosiasi dengan lamun, seperti teripang, bintang laut, bulu babi, kerang, udang, dan kepiting. Duyung (Dugong dugon) adalah mamalia laut yang hidupnya amat bergantung pada makanannya berupa lamun. Penyu hijau (Chelonia mydas) juga dikenal sebagai pemakan lamun yang penting. Karena itu, rusak atau hilangnya habitat padang lamun akan menimbulkan dampak lingkungan yang luas. Padang lamun sering dijumpai berdampingan atau tumpang tindih dengan ekosistem mangrove dan terumbu karang. Bahkan, terdapat interkoneksi antarketiganya. Karena fungsi lamun tak banyak dipahami, banyak padang lamun yang rusak oleh berbagai aktivitas manusia. Luas total padang lamun di Indonesia semula diperkirakan 30.000 kilometer persegi, tetapi diperkirakan kini telah menyusut 30-40 persen. Kerusakan ekosistem lamun, antara lain, karena reklamasi dan pembangunan fisik di garis pantai, pencemaran, penangkapan ikan dengan cara destruktif (bom, sianida, pukat dasar), dan tangkap lebih (over-fishing). Pembangunan pelabuhan dan industri di Teluk Banten, misalnya, telah melenyapkan ratusan hektar padang lamun. Tutupan lamun di Pulau Pari 25 (DKI Jakarta) telah berkurang sekitar 25 persen dari tahun 1999 hingga 2004. Mengingat ancaman terhadap padang lamun semakin meningkat, akhir-akhir ini mulailah timbul perhatian untuk menyelamatkan padang lamun. Undang-Undang Nomor 27 Tahun 2007 tentang Pengelolaan Pesisir dan Pulau-pulau Kecil juga telah mengamanatkan perlunya penyelamatan dan pengelolaan padang lamun sebagai bagian dari pengelolaan terpadu ekosistem pesisir dan pulau-pulau kecil. Program pengelolaan padang lamun berbasis masyarakat yang pertama di Indonesia adalah Program Trismades (Trikora Seagrass Management Demonstration Site) di pantai timur Pulau Bintan, Kepulauan Riau, yang mendapat dukungan pendanaan dari Program Lingkungan Perserikatan Bangsa-Bangsa (UNEP) dan baru dimulai tahun 2008. ”Blue Carbon” Awal Oktober 2009, tiga badan PBB, yakni UNEP, FAO, dan UNESCO, berkolaborasi meluncurkan laporan yang dikenal sebagai Blue Carbon Report. Laporan ini menggarisbawahi peranan laut sebagai pengikat karbon (blue carbon), sebagai tandingan terhadap peranan hutan daratan (green carbon) yang selama ini sangat mendominasi wacana dalam masalah pengikatan karbon dari atmosfer. Di seluruh laut terdapat tumbuhan yang dapat menyerap karbon dari atmosfer lewat fotosintesis, baik berupa plankton yang mikroskopis maupun yang berupa tumbuhan yang hanya hidup di pantai seperti di hutan mangrove, padang lamun, ataupun rawa payau (salt marsh). Meskipun tumbuhan pantai (mangrove, padang lamun, dan rawa payau) luas totalnya kurang dari setengah persen dari luas seluruh laut, ketiganya dapat mengunci lebih dari separuh karbon laut ke sedimen dasar laut. Keseluruhan tumbuhan mangrove, lamun, dan rawa payau dapat mengikat 235-450 juta ton karbon per tahun, setara hampir setengah dari emisi karbon lewat transportasi di seluruh dunia. Dengan demikian, penyelamatan ekosistem padang lamun sangat penting, dan tidak kalah strategis, dibandingkan dengan pengelolaan ekosistem terumbu karang yang sudah mulai mendunia dengan Coral Triangle Initiative atau ekosistem mangrove dengan Mangrove for the Future. Ekosistem Lamun Lamun (seagrass) merupakan satu-satunya tumbuhan berbunga (Angiospermae) yang memiliki dan memiliki rhizoma, daun, dan akar sejati yang hidup terendam di dalam laut beradaptasi secara penuh di perairan yang salinitasnya cukup tinggi atau hidup terbenam di dalam air, beberapa ahli juga mendefinisikan lamun (Seagrass) sebagai tumbuhan air berbunga, hidup di dalam air laut, berpembuluh, berdaun, berimpang, berakar, serta berbiak dengan biji dan tunas. Karena pola hidup lamun sering berupa hamparan maka dikenal juga istilah padang lamun (Seagrass bed) yaitu hamparan vegetasi lamun yang menutup suatu area pesisir/laut dangkal, terbentuk dari satu jenis atau lebih dengan kerapatan padat atau jarang. Lamun umumnya membentuk padang lamun yang luas di dasar laut yang 26 masih dapat dijangkau oleh cahaya matahari yang memadai bagi pertumbuhannya. Lamun hidup di perairan yang dangkal dan jernih, dengan sirkulasi air yang baik. Air yang bersirkulasi diperlukan untuk menghantarkan zat-zat hara dan oksigen, serta mengangkut hasil metabolisme lamun ke luar daerah padang lamun. Hampir semua tipe substrat dapat ditumbuhi lamun, mulai dari substrat berlumpur sampai berbatu. Namun padang lamun yang luas lebih sering ditemukan di substrat lumpur-berpasir yang tebal antara hutan rawa mangrove dan terumbu karang. Sedangkan sistem (organisasi) ekologi padang lamun yang terdiri dari komponen biotik dan abiotik disebut Ekosistem Lamun (Seagrass ecosystem).Habitat tempat hidup lamun adalah perairan dangkal agak berpasir dan sering juga dijumpai di terumbu karang. Di seluruh dunia diperkirakan terdapat sebanyak 52 jenis lamun, di mana di Indonesia ditemukan sekitar 15 jenis yang termasuk ke dalam 2 famili: (1) Hydrocharitaceae, dan (2) Potamogetonaceae. Jenis yang membentuk komunitas padang lamun tunggal, antara lain: Thalassia hemprichii, Enhalus acoroides, Halophila ovalis, Cymodocea serrulata, dan Thallassodendron ciliatum. Padang lamun merupakan ekosistem yang tinggi produktivitas organiknya, dengan keanekaragaman biota yang juga cukup tinggi. Pada ekosistem ini hidup beraneka ragam biota laut (Gambar 17), seperti ikan, krustasea, moluska (Pinna sp., Lambis sp., Strombus sp.), Ekinodermata (Holothuria sp., Synapta sp., Diadema sp., Archaster sp., Linckia sp.), dan cacing Polikaeta. ian.umces.edu/.../displayimage-topn-0-790.html 27 Lamun atau secara internasional dikenal sebagai seagrass merupakan tumbuhan tingkat tinggi dan berbunga (Angiospermae) yang sudah sepenuhnya menyesuaikan diri hidup terbenam di dalam laut. Keberadaan bunga dan buah ini adalah faktor utama yang membedakan lamun dengan jenis tumbuhan lainnya yang hidup terbenam dalam laut lainnya, seperti rumput laut (seaweed). Hamparan lamun sebagai ekosistem utama pada suatu kawasan pesisir disebut sebagai padang lamun (seagrass bed). Pada ekosistem padang lamun berasosiasi berbagai jenis biota laut yang bernilai penting dengan tingkat keragaman yang sangat tinggi. Padang lamun yang merupakan salah satu ekosistem di wilayah pesisir memiliki keanekaragaman-hayati yang kaya dan merupakan penyumbang nutrisi yang sangat potensial bagi perairan disekitarnya mengingat produktivitasnya yang tinggi. Perannya sebagai pelindung pantai, daerah asuhan bagi ikan, teripang, kuda laut dan udang, stabilisator dan penangkap sedimen sangat penting bagi ekosistem lainnya seperti ekosistem terumbu karang dan mangrove. Daun lamun yang lepas akan mengendap di perairan sekitarnya dan dihanyutkan ke ekosistem atau perairan lainnya. Daun lamun yang mengendap akan didekomposisi oleh bakteri dan biota bentik pemakan serasah. feww.wordpress.com/2009/06/ 28 Padang lamun memiliki fungsi ekologis dan nilai ekonomis yang sangat penting bagi manusia. Menurut Nybakken (1988), fungsi ekologis padang lamun adalah: (1) sumber utama produktivitas primer, (2) sumber makanan bagi organisme dalam bentuk detritus, (3) penstabil dasar perairan dengan sistem perakarannya yang dapat menangkap sediment (trapping sediment), (4) tempat berlindung bagi biota laut, (5) tempat perkembangbiakan (spawning ground), pengasuhan (nursery ground), serta sumber makanan (feeding ground) bagi biota-biota perairan laut, (6) pelindung pantai dengan cara meredam arus, (7) penghasil oksigen dan mereduksi CO2 di dasar perairan. Nilai ekonomi dan ekologi padang lamun (manfaat ekonomi total), terkait dengan biota yang hidupnya tergantung dengan ekosistem padang lamun sebesar U$ 412.325 per ha per tahun atau 11,3 milyar rupiah per hektar per tahun (Fortes, 1990). Terdapat hingga 360 spesies ikan (seperti ikan baronang), 117 jenis makro-alga, 24 jenis moluska, 70 jenis krustasea, dan 45 jenis ekinodermata (seperti teripang) yang hidupnya didukung oleh ekosistem padang lamun di Indonesia. Disamping itu, padang lamun telah dimanfaatkan secara langsung oleh manusia untuk memenuhi kebutuhan hidupnya, seperti untuk makanan, pupuk, obat-obatan. www.aims.gov.au/.../apnet-seagrasses01.html They are called 'seagrass' because most have ribbon-like, grassy leaves, but none is a true grass. There are many different kinds of seagrasses and some do not look like grass at all. For example, they may have oval leaves. Seagrasses have roots, stems and leaves. They also form tiny flowers, fruits and seeds. Most seagrasses reproduce by pollination - the pollen is transported to other plants by water. The roots and horizontal stems (rhizomes), often buried in sand or mud, anchor the grasses and absorb nutrients. Leaves, usually green, are produced on vertical branches and also absorb nutrients. The stems and leaves of seagrasses contain veins and air channels so they can carry fluid and absorb gases. Seagrasses rely on light to convert carbon dioxide into oxygen (photosynthesis). The oxygen is then available for use by other living organisms. 29 SEAGRASSES AND NUTRIENTS: Seagrasses: Why are they important? Seagrass beds are of considerable ecological importance in coastal and marine ecosystems where they play a significant role in the processes and resources of nearshore coastal ecosystems. They are among the most productive and dynamic elements of an aquatic ecosystem. The growth and survival of seagrass communities is of major importance to coastal waters as seagrasses are: a. primary producers that contribute large quantities of fixed carbon (the basis of all food chains) to coastal ecosystems; b. important in stabilising bottom sediments because they slow water movement which promotes sedimentation of particulate matter; c. part of the nutrient cycle in the aquatic system; d. important in supplying shelter and refuge for adult and juvenile animals and contributing large amounts of substrate for encrusting animals and plants; and e. essential food for dugongs (Dugong dugon) and green turtles (Chelonia mydas). Where do they grow in the Great Barrier Reef? Seagrasses grow in shallow-water ecosystems, notably the inshore lagoon of the Great Barrier Reef. The Great Barrier Reef lagoon is a largely sheltered area and offers special protection to seagrass beds within the reef itself and on the lee or landward side of reefs or embayments. Surveys conducted of seagrasses between Cape York and Hervey Bay show that they are most often found in areas that receive shelter from the prevailing winds, such as in bays, behind northerly facing peninsulas, behind islands, reefs and shoals, and on some reef platforms and fringing reefs. The regional contribution of these seagrass beds to primary production and as habitat for marine fauna is likely to be extremely important. The large majority of seagrasses found in the Great Barrier Reef Region grow in the inshore lagoon in waters no deeper than 10 metres and no greater than 10 kilometres from the coast (Lee Long et al. 1993; Larkum et al. 1989). Large areas of deepwater seagrass (in waters of between 10 and 30 metres depth) have also recently been found in the Great Barrier Reef. Seagrasses in close proximity to land are more likely to be affected by material flowing from land and vulnerable to changes in coastal processes. Recent studies of the factors contributing to seagrass decline have shown that increased anthropogenic inputs to the coastal zone are often linked to seagrass loss. The species of seagrass found between Cape York and Hervey Bay are common throughout northern Australia, including the Gulf of Carpentaria and Torres Strait. Fourteen species have been recorded from the seagrass habitats of north-eastern Australia (Larkum et al. 1989). Tropical Australia supports well-developed seagrass communities and a large proportion of all known seagrass species (> 22%). Tropical Australia 30 has a greater diversity of seagrass species than elsewhere in the tropical Indo-West Pacific. Seagrasses are an ecological group, not a taxonomic group, of angiosperms (flowering plants), i.e. various seagrass families do not necessary have to be closely related. www.seagrasswatch.org/id_seagrass.html Key environmental factors The distribution and growth of seagrasses is regulated by a variety of water quality factors such as temperature, salinity, nutrient availability, substratum characteristics, turbidity and submarine irradiance. For example, it is well known from overseas and temperate Australian studies that the availability of nutrient resources affects the growth, distribution, morphology and seasonal cycling of seagrass communities. In addition, seagrasses depend on an adequate degree of water clarity to sustain productivity in their submerged environment. Increased turbidity and sedimentation reduce water clarity, which can affect the health and productivity of seagrass communities. Although natural events have been responsible for both large-scale and local losses of seagrass habitat, recent evidence suggests that human population expansion is now the most serious cause of seagrass habitat loss. Increasing anthropogenic inputs to the coastal oceans are primarily responsible for the enhanced nutrient input from the land and the worldwide decline in sea grasses. Human activities that most affect seagrasses are those that alter water quality or clarity. These activities can include nutrient and sediment loading from agricultural run-off and sewage disposal, 31 dredging and filling, urban stormwater, upland development, and certain fishing practices. How do increased nutrients affect seagrass survival? Nutrient loading is the primary factor responsible for both reduction of water quality and stimulation of algal growth in coastal marine waters (Short et al. 1996; Short and Wyllie-Echeverria 1996). Several studies (Neverauskas 1987; Johansson and Lewis 1992; Phillips and Menez 1988; Short et al. 1996) have related the decline of seagrass distribution to the degree of nutrient loading within various catchments. Causes of seagrass degradation include various forms of nutrient loading, including sewage enrichment, enrichment of groundwater supplies and run-off from agricultural lands. Loss of seagrasses in Cockburn Sound in Western Australia is strongly correlated with the increase of discharge rich in plant nutrients over a period of increasing industrial development. Once impacted, seagrass colonisation and regrowth can be very slow, or nonexistent, because of possible ongoing impacts and poor dispersal capabilities of most seagrass species (Preen et al. 1995; Dennison and Kirkman 1996). Loss of seagrasses can bring about a change in the marine food chain with an accompanied shift in main primary producers from benthic to planktonic and a reduction in leaf detritus production. Continued seagrass loss can result in an ecosystem shift to a lagoonal system dominated by high turbidity and algal growth or bare sandy/silty substrate which may remain after the decline of the seagrass beds. This change results in a considerable loss of diversity. Seagrasses respond to changes at both a global and local scale but, for the scope of this paper, only local or regional changes in the environmental nutrient regime will be considered. At a regional scale, increases in nutrient loading associated with eutrophication and changes in light quality can adversely affect seagrass beds, resulting in either their reduction or disappearance. Effects on seagrasses can be evident in four different stages, these being structural impacts, diseases, reduced photosynthesis (directly linked with reduced light) and ecosystem shifts. Structural impacts Under conditions of high nutrient loading, seagrasses take up additional nutrients from the water. This can cause stress in the plant as there is little intercellular tissue space available for nitrate accumulation. As a consequence, high quantities of nitrate will be converted into ammonia, either immediately, or following vascular storage (Brown 1993). Ammonia production, in turn, requires the plant to divert substantial carbon resources for immediate conversion into amino acids. After an extended period of elevated nutrient uptake, the plant, even with abundant carbon available, will not have the capacity to fix enough carbon to meet its total carbon demand. Lack of carbon in the cellular tissue ultimately severely affects the structural integrity of the seagrass, and results in the death of the plant. Diseases Physiological stresses imposed by nutrient supply imbalances may also affect weakened plants by enhancing susceptibility to opportunistic 32 pathogens (i.e. wasting disease). This may be due to a decrease in the production of anti-microbial compounds under conditions of enriched nitrate. Reduced photosynthesis A reduction in light reaching the seagrass can be brought about by increased turbidity arising from living or non-living particulates in the water, or increased shading by the deposition of silt on photosynthetic tissue (Larkum et al. 1989). Elevated algal growth on the leaf surfaces or stems, resulting from the uptake of additional nutrients by the epiphytic algae, can also limit the amount of light reaching the underlying seagrasses. A reduction in light reaching the seagrass chloroplast precludes effective seagrass photosynthesis. Loss of structural integrity and increased incidence of disease may be exacerbated by reduced photosynthesis. Many documented cases of seagrass loss have followed eutrophication of coastal embayments where enhanced nutrients have resulted in a reduction in light penetration of the water column, or a reduction in light reaching seagrass levels due to its interception by epiphytic algae. In enhanced nutrient regimes of coastal areas, there is a strong potential for interactions between water-column nitrate and suspended sediment loading (or other sources of light reduction, such as macroalgal overgrowth). Ecosystem shifts Nutrient enrichment can enhance the growth of macroscopic and microscopic algae on seagrass leaf surfaces. Nutrients are required for seagrass growth but the concentrations in tissues are lower than in macroalgae. Due to differences in the carbon:nitrogen:phosphorus ratio, macroalgae can dominate seagrasses under conditions of marked eutrophication, both as epiphytes and as free-floating species which may originate as attached epiphytes. Increased epiphytic growth results in shading of seagrass leaves by up to 65%, which reduces the photosynthetic rate and leaf densities. Nutrient concentrations and seagrasses in Great Barrier Reef waters Eutrophication effects on seagrass beds are most severe in sheltered habitats with reduced tidal flushing, where nutrient loadings are both concentrated and frequent, and where temperatures fluctuate more widely than in areas with greater water exchange. Shallow seagrass beds found in the inshore Great Barrier Reef lagoon are exposed to potentially higher nutrient inputs, infrequent flushing and temperature variability, making them vulnerable to any changes in the nutrient and light regime. In protected waters similar to those facing northward along the Queensland coast, epiphytes and macroalgae respond so quickly to watercolumn enrichment that they can seasonally outgrow grazing pressure, leading to severe light reduction and decline of the underlying seagrass. Recent studies have shown that under conditions simulating poorly flushed coastal habitats, even low levels of nitrate enrichment can promote the decline of seagrasses. Growth and survival of seagrass species 33 significantly decreased at all enrichment levels, with the most rapid decline occurring at the highest nitrate loading. Plant death was preceded by loss of structural integrity in above-ground tissues. Laboratory studies have found that the seagrass species Zostera marina declined under low to moderate (3.5-7.0 mM) water-column nitrate enrichment (Short et al. 1995; Burkholder et al. 1992). Long-term nitrate additions cause severe seagrass decline, likely to be enhanced by increasing temperatures and light reduction. Enriched levels of ammonia (1.85-5.41 mM) and phosphate (0.22-0.50 mM) lead to a reduction in shoot density and biomass of the seagrass population (Short et al. 1995). Conversely, laboratory studies on Great Barrier Reef algae have demonstrated increased algal growth associated with nutrient enrichment (Schaeffelke and Klumpp 1997). Growth of epiphytic algae is also likely to be promoted by excess water column nutrients. Small increases in water column nutrient concentration can also result in increased growth of seagrasses. This has occurred around Green Island following prolonged discharge of untreated sewage. On nearshore reefs, the water column nutrients are highly variable, ranging from non-detectable to levels indicative of a eutrophication state (Schaeffelke and Klumpp 1997; Bell 1992). Approximate ranges for (nonflood) inshore water quality concentrations have been measured between non-detectable and 2 mM for dissolved inorganic nitrogen (predominantly ammonia) and non-detectable and 0.2 mM for phosphate (Furnas et al. 1995; Furnas and Brodie 1997; Devlin et al. 1997; Schaeffelke and Klumpp 1997). Nutrients and suspended particulate concentrations associated with cyclones and floods are the highest that most Great Barrier Reef communities are likely to be exposed to. Inshore seagrass communities are episodically subjected to high dissolved nutrient and suspended loads more typical of a eutrophic system. Water samples taken in flood plumes have consistently recorded elevated ammonia (0.6-4.2 mM), nitrate-nitrite (0.2414.36 mM) and phosphate (0.13-1.98 mM) (Steven et al. 1997). In large flood events, nutrient levels have remained high in the inshore lagoon for a number of days to weeks. 34 References Abal, E.G. and Dennison W.C. 1996, Seagrass depth range and water quality in southern Moreton Bay, Queensland, Australia, Marine and Freshwater Research, 47: 763—771. Batyan, G.R. 1986, Distribution of Seagrasses in Princess Royal Harbour and Oyster Harbour on the Southern Coast of Western Australia, Technical series 1, Western Australian Department of Conservation and Environment, Perth. Bell, P.R.F. 1992, Eutrophication and coral reefs – some examples in the Great Barrier Reef lagoon, Water Research, 26: 553—568. Brodie, J. and Furnas, M. 1996, Cyclones, river flood plumes and natural water quality extremes in the central Great Barrier Reef, in Downstream Effects of Land Use, eds H.M. Hunter, A.G. Eyles and G.E. Rayment, Department of Natural Resources, Brisbane, pp. 367—374. Brown, V.M. 1993, Concepts and realities in toxicity testing for the protection of aquatic environments from wastes, Australian Biologist, 3: 133—141. Buchsbaum, R.N., Short, F.T. and Cheney, D.P. 1990, Phenolic nitrogen interactions in eelgrass (Zostera marina L.): possible implications for disease resistance, Aquatic Botany, 37: 291—297. Burkholder, J.M., Mason, K.M. and Glagow, H.B. 1992, Water-column nitrate enrichment promotes decline of eelgrass Zostera marina: evidence from seasonal mesocosm experiments, Marine Ecology Progress Series, 81: 163—178. Cambridge, M.L. and McComb, A.J. 1984, The loss of seagrasses in Cockburn Sound, Western Australia. I. The time course and magnitude of seagrass decline in relation to industrial development, Aquatic Botany, 20: 229—243. Den Hartog, C. 1996, Sudden declines of seagrass beds: wasting disease and other disasters, in Seagrass Biology: Proceedings of an International Workshop, pp. 307—314. Dennison, W.C. and Kirkman, H. 1996, Seagrass survival model, in Seagrass Biology: Proceedings of an International Workshop, pp. 341—344. Devlin, M.J., Lourey, M.J., Sweatman, H. and Ryan, D. 1997, Water quality, in Long-term Monitoring of the Great Barrier Reef, Status Report Number 2 1997, ed. H. Sweatman, Australian Institute of Marine Science, Townsville, pp. 29—61. Fonesca, M.S. and Kenworthy, J. 1987, Effects of current on photosynthesis and the distribution of seagrass, Aquatic Botany, 27: 59—78. Furnas, M., Mitchell, A. and Skuza, M. 1995, Nitrogen and Phosphorus Budgets for the Central Great Barrier Reef Shelf, Research Publication No. 36, Great Barrier Reef Marine Park Authority, Townsville. Furnas, M. and Brodie, J. 1997, Current status of nutrient levels and other water quality parameters in the Great Barrier Reef, in Downstream 35 Effects of Land Use, eds H.M. Hunter, A.G. Eyles and G.E. Rayment, Department of Natural Resources, Brisbane, pp. 9—21. Gieson, W.B.T.J. 1990, Wasting Disease and Present Eelgrass Condition, Laboratory of Aquatic Ecology, Catholic University of Nijmegen, The Netherlands. Johansson, J.O.R. and Lewis, R.R. 1992, Recent improvements of water quality and biological indicators in Hillsboro Bay, a highly impacted subdivision of Tampa Bay, Florida, USA, Science Total Environment Supplement, pp. 1199—1215. Larkum, A.W.D., McComb, A.J. and Shepard, S.A. 1989, Biology of Seagrasses: A Treatise on the Biology of Seagrasses with Special Reference to the Australian Region, Amsterdam, Elsevier. Lee Long, W.J., Mellors, J.E. and Coles, R.G. 1993, Seagrasses between Cape York and Hervey Bay, Queensland, Australia, Australian Journal of Marine and Freshwater Research, 44: 19—31. Moriarty, D.J.W. et al. 1984, Microbial biomass and productivity in seagrass beds, Geomicrobiology Journal, 4: 21—51. Neverauskas, V.P. 1987, Accumulation of periphyton biomass on artificial substrates deployed near a sewage sludge outfall in South Australia, Estuarine Coastal Shelf Science, 25: 509—517. Phillips, R.C and Menez, E.G. 1988, Seagrasses, Smithsonian Contributions to the Marine Sciences, 34: 1—104. Poiner, I.R., Conacher, C.A., Staples, D.J. and Moriarty, D.J. 1992, Moreton Bay in the balance, in Seagrasses – why are they important?, ed. O.N. Crimp, Queensland Australian Littoral Society Inc., Moorooka, pp. 41—53. Preen, A.R., Lee Long, W.J. and Coles, R.G. 1995, Flood and cyclone related loss, and partial recovery, of more than 1000 km2 of seagrass in Hervey Bay, Queensland, Australia, Aquatic Botany, 52: 3—17. Schaeffelke, B. and Klumpp, D.W. 1997, Growth of germlings of the macroalgae Sargassum baccularia (Phaeophyta) is stimulated by enhanced nutrients, in Proceedings of 8th International Coral Reef Symposium, Panama, June 24—29 1996, Volume II, eds H.A. Lessios and I.G. Macintyre, Smithsonian Tropical Research Institute, Balboa, Republic of Panama, pp. 1839—1842. Short, F.T., Burdick, D.M. and Kaldy, J.E. 1995, Mesocosm experiments quantify the effects of eutrophication on eelgrass, Zostera marina, Limnology and Oceanography, 40(4): 740—749. Short, F.T., Burdick, D.M., Granger, S. and Nixon, S.W. 1996, Long-term decline in eelgrass, Zostera marina L., linked to increased housing development, in Seagrass Biology: Proceedings of an International Workshop, pp. 291—298. Short, F.T. and Wyllie-Echeverria, S. 1996, Natural and human-induced disturbance of seagrasses, Environmental Conservation, 23: 17— 27. Steven, A.D.L., van Woesik, R. and Brodie, J. 1990, Water quality monitoring studies within the Great Barrier Reef Marine Park: case studies, in Proceedings of the 1990 Congress on Coastal and Marine Tourism, Volume 2, pp. 335—341. 36 Steven, A. et al. 1997, Spatial influence and composition of river plumes in the central Great Barrier Reef, in Downstream Effects of Land Use, eds H.M. Hunter, A.G. Eyles and G.E. Rayment, Department of Natural Resources, Brisbane, pp. 85—92. van Woesik, R., DeVantier, L.M. and Steven, A.D.L. 1990, Discharge from tourist resorts in Queensland, Australia: coral community response, in Proceedings of the 1990 Congress on Coastal and Marine Tourism, Volume 2, pp. 323—327. Walker, D.I. and McComb, A.J. 1992, Seagrass degradation in Australian coastal waters, Marine Pollution Bulletin, 25: 191—195.
