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Seagrass Ecosystems МОРСЬКИЙ ЕКОЛОГІЧНИЙ ЖУРНАЛ УДК 574.5:582.271/.275 SEAGRASS ECOSYSTEMS © R. C. Phillips, N. A. Milchakova A. O. Kovalevsky Institute of Biology of the southern Seas, National Academy of Sciences of Ukraine, Sevastopol, Ukraine Поступила 3 февраля 2003 г. The seagrass ecosystem is defined as a unit of biological organization comprised of interacting biotic and abiotic components. The structural components are shelter and food and feeding pathways and biodiversity. Functional components include the rate of nutrient cycling, the rate of energy flow, and biological regulation. Healthy intact seagrass ecosystems provide services since they relate to the health, stability and well-being of the environment in which they live, but also to that of human populations. Key words: seagrass, ecosystem, production, nutrient cycling, energy flow, biological regulation, ecosystem services Дано определение экосистемы морских трав как единицы биологической организации, состоящей из взаимо- действующих биотических и абиотических компонентов. К структурным компонентам относятся местооби- тания, пищевые цепи и биоразнообразие. Функциональные компоненты включают в себя интенсивность кру- говорота питательных веществ, скорость энергетического потока и биологическую регуляцию. Целостные экосистемы морских трав обеспечивают стабильность и благополучие не только своей собственной среды обитания, но и той, в которой обитает человечество. Ключевые слова: морские травы, экосистема, поток энергии, биологическая регуляция, экосистем- ные услуги Seagrasses are monocotyledonous vascu- semble fields of terrestrial grasses. There are ap- lar flowering plants that live in coastal and estua- proximately 50 species of seagrasses in 12 gen- rine areas of the world. They are unique in that era. Seven genera are considered tropical, while they are: 1) usually totally submerged in the wa- the remaining five are more or less confined to ter, 2) they possess a root system with stems bur- temperate waters [35]. In almost all cases, sea- ied within a soft substrate, 3) they have vegetative grass communities are multispecific in the trop- and sexual reproduction, and, 4) have flowers ics, but tend to be unispecific in many parts of the fertilized by water-borne pollen. temperate zones. Seagrasses are not truly grasses. They be- Seagrasses are found along most coast- long to three families of monocotyledonous lines of all continents. They appear to be ex- plants, but are called seagrasses since they usually cluded generally north of the Arctic Circle and form extensive underwater meadows, which re- south of the Antarctic Circle [48]. Zostera marina Морський екологічний журнал, № 2, Т. II. 2003 29 R. C. Phillips, N. A. Milchakova L. (eelgrass) extends north of the Arctic Circle Wood et al. [61] summarized the func- only in northern Russia, presumably due to the tions of the seagrass ecosystem: 1) relatively high warming influence of the Gulf Stream [47, 48]. organic production. For some species, this pro- Seagrass beds have long been recognized duction rivals or exceeds that of subsidized crops; as critical coastal nursery habitat for estuarine 2) high standing crops which produce large fisheries and wildlife [2, 3, 19 – 21, 23, 26 – 31, amounts of dissolved and particulate detritus 39, 40, 44, 46, 50, 56 – 60, 63, 65]. They function which form the basis of important food chains, as direct food sources for fish, waterfowl, du- both within the seagrass ecosystem and shore- gongs and manatees, and sea turtles. They are ward and offshore as the materials are washed also participants in nutrient cycling processes, away from the seagrasses; 3) leaves and erect and are stabilizing agents in coastal sedimentation shoots present surfaces for epibiotic organisms to and erosion processes. In recent years, they have attach to. This increases both primary and secon- received attention as biological indicators of es- dary productivity, as well as providing a large tuarine water quality and ecosystem health as a amount of food sources for fish and invertebrates; result of their sensitivity to nutrient enrichment 4) because the seagrasses are rooted in their sub- and eutrophication [14]. strate and produce shoots with leaf bundles, they Description of seagrass ecosystem. An stabilize their habitat. The leaves form a baffle, ecosystem has been defined as a unit of biological which slows and retards current and wave activ- organization comprised of interacting biotic and ity, which promotes sedimentation of particles as abiotic components [47] (Fig. 1). well as inhibiting resuspension of organic and Up to 1973, most seagrass research was inorganic materials. The roots and rhizomes also either autecological or ecophysiological in ap- form a complex, interlocking matrix which binds proach. In 1973, the U.S. National Science Foun- the sediment and retards erosion; 5) the dense leaf dation sponsored an International Seagrass Work- baffle forms shelter for an extremely diverse shop in Leiden, The Netherlands, attended by 38 fauna for all trophic levels; and, 6) since the leaf scientists from 11 countries. This group assem- baffle generates and entrains autochthonous, as bled all known information on seagrasses, dis- well as allochthonous organic material, an active covered what were the major gaps in our knowl- environment is created for decomposition and edge, and formulated recommendations as to fu- nutrient cycling. ture research. Among these recommendations was In this paper, we describe the seagrass one to view the seagrass community as an ecosys- systems in terms of their structural and functional tem and that research on seagrasses be conducted components. The study of structural or functional from an ecosystem point of view. components of a system alone is too static, since Over a period of years, it was found, that communities undergo change. The complex rela- even though water temperatures and adjacent tions between the various components of an eco- communities were different in the tropics and system have developed over long time periods temperate zones, e.g., mangroves, marshes, and and are the result of evolution, selection, and coral reefs in the tropics, and marshes in the tem- adaptational processes within each component perate zones, all seagrass communities displayed [13]. similar structures and functions. 30 Морський екологічний журнал, № 2, Т. II. 2003 Seagrass ecosystems Figure. Conceptual model of a seagrass ecosystem Рис. Концептуальная модель экосистемы морских водорослей Морський екологічний журнал, № 2, Т. II. 2003 31 R. C. Phillips, N. A. Milchakova Structural Components. Structure con- arrangement of the organisms in space and time; sists of three major subcomponents that are inter- and, 3) interrelationships within the community related: 1) floristic and faunistic composition; 2) and with the abiotic environment (Table 1) [47]. Table 1. Representative Seagrass Productivities Таблица 1. Продуктивность некоторых видов морских трав Species Location Productivity (gC/m2/day) Source Cymodocea nodosa Mediterranean 5.5 – 18.5 [17] Halodule wrightii North Carolina 0.5 – 2.0 [15] Syringodium filiforme Florida 0.8 [unpublished, Zieman] Thalassia testudinum Texas 0.6 – 9.0 [34, 38] Florida east coast 0.9 – 16.0 [25, 62, 64] Cuba 0.6 – 7.2 [7, 8] Puerto Rico 2.4 – 4.5 [38] Jamaica 1.9 – 3.0 [18] Barbados 0.5 – 3.0 [41, 42] Bermuda 5.6 - 7.2 (leaves - total [42] plant) Zostera marina Denmark 1.72 - 0.45 (leaves- rhi- [51] zomes) Rhode Island 0.4 – 2.9 [9] North Carolina 0.9 – 1.23 (leaves) [57] 0.9 – 1.04 (leaves) [unpublished, Kenworthy] 0.15 – 0.28 (rhizomes) Washington 0.7 – 4.0 [45] Alaska 3.3 – 8.0 [32 – 33] Japan 0.3 – 1.8 (leaves) [1, 37] France 1.06 - 0.5 (leaves- [24] rhizomes) The spatial arrangement of seagrasses structured than are the climax communities. So displays a number of characteristic patterns that far, no elaborate studies are available in which the can be divided into vertical, horizontal, and three- framework consists of more than one species. It is dimensional. Vertical patterns are characterized not known whether the various coexisting sea- by zonation and stratification. Horizontal patterns grass species in a mixed community are suffi- may be observed over geographic distances or ciently different in their anatomical and phyto- may be due to differences in bottom configuration chemical properties to cause further differentia- or prevailing hydrodynamic conditions. The tion in the form of development of species- three-dimensional pattern is the way in which the specific epiphytic and epizoic associations [13]. species fill up the available space. This is proba- А. Shelter. bly the structural characteristic most decisive for The structural function of seagrasses is a the function of the community [13]. result of the morphology of the seagrass plants. Not all seagrass communities contain all The seagrass plants possess an extensive under- these structural elements. The pioneering com- ground rhizome/root system with erect shoots munities, e.g., Halodule wrightii, are more simply with bundles of leaves, which extend into the 32 Морський екологічний журнал, № 2, Т. II. 2003 Seagrass ecosystems water column. As a result, they create a highly resulting currents and waves; 3) mineral and or- structured ecosystem from a relatively unstruc- ganic particles sink to the bottom in the less tur- tured one. Stauffer [55] classified the diversity of bulent water, creating relatively clear water; and habitats within the seagrass bed as: 1) on the 4) the leaf mass
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