How do seagrasses grow and spread?

Seagrass colonisation and meadow maintenance proceeds by patch stablishement, from seed germination and plant fragments, and clonal growth. Knowledge on seagrass growth rates and success of reproductive effort is crucial to manage seagrass ecosystems, particularly to derive expectations on the recolonisation times required to recover seagrass meadows.

By Núria Marbà (IMEDEA), Carlos M. Duarte (IMEDEA), Ana Alexandra (CCMAR) and Susana Cabaço (CCMAR)

Seagrasses are clonal plants sharing a similar essential to predict the time scales of seagrass architecture and presenting a highly organised colonisation and, thus, recovery. growth. Seagrass growth relies on the reiteration In this chapter we aim to provide an of ramets, which are composed of modules (i.e. understanding of seagrass growth processes and leaves, piece of rhizome, roots, flowers or their rates, as well as mechanisms of seagrasses inflorescences). The understanding of the design to spread. We do so by describing seagrass of seagrasses provides insight on their growth architectural features and the wide repertoire of patterns. Despite the similar architecture of module addition and growth rates, and discussing seagrasses, plant size and growth rate vary some the mechanisms and rates of seagrass orders of magnitude across species. To a large colonisation. Four sections of this chapter are extent, variability in rates between seagrass dedicated to the growth pattern and spreading species reflects differences in plant size, with mechanisms of those seagrass species present smaller species growing faster than larger ones. along European coasts. At the end we discuss the In addition, seagrass growth is able to adapt to implications of European seagrass growth and environmental change, and it exhibits substantial spread for management. plasticity. Knowledge on seagrass growth rates allows assessment of meadow productivity and seagrass health, as well as forecasting their Seagrass achitecture capacity to survive disturbances. In addition, seagrass rhizome growth responses to Seagrasses share a common architecture, all disturbances remain imprinted on the plant species being clonal, rhizomatous plants. allowing reconstruction of past disturbance Rhizomes are stems extending horizontally below dynamics. the sediment surface or vertically, raising the Vegetative proliferation is the main mechanism of leaves towards, or above, the sediment surface. seagrasses to occupy habitat space, and thus it is Seagrasses are modular plants composed of units a critical process for seagrass meadows to spread repeated during clonal growth. Each unit is and persist. Most ramets in seagrass populations composed of a set of modules: a piece of are produced as rhizomes elongate. Rhizome rhizome, which can be either horizontal or vertical; growth is the process that regulates the rate of a bundle of leaves attached to the rhizome; and a formation and the spatial distribution of ramets root system (see chapter 1). In addition, the units (and, thus, modules) within seagrass meadows, may hold flowers or fruits, depending on the and, thus, it constrains the development of their timing of observation. The morphology of populations. The spread, and maintenance, of seagrasses does not present any peculiar seagrass meadows also depend on sexual deviations relative to those of other terrestrial reproduction since it is the main mechanism monocotyledons. regulating patch formation. Hence, information The rhizome is responsible for the extension of about the effort and success of seagrass the clone in space, as well as for connecting reproduction and rhizome growth patterns are neighbouring ramets, thereby maintaining

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integration within the clone (see below). The Formation of leaves, rhizomes and rhizomes of most seagrass species are flexible, roots: clonal growth whereas those of Posidonia oceanica are hihgly lignified and explain the persistence of dead The rate of formation of seagrass leaves, tissues in the sediments, which extends for rhizomes and roots, and, therefore, the spread of millenia in Posidonia oceanica. Seagrass the clone, depends on the activity of meristems, rhizomes are composed of internodes, the which are the areas where active cell division, and rhizome fragments in between two nodes, which therefore, growth takes place. The division of the are the insertion points of leaves. The distinct meristems is a rather continuous process, lines identifying the nodes after leaf abscission responsible for the maintenance and expansion of are also referred to as “leaf scars”. Rhizome seagrass clones. Small seagrass species, such as internodes range widely in size among seagrass Zostera noltii, produce new leaves much faster species (Table 3.1). (13.71 days) than species with large leaves, such Cymodocea nodosa and Posidonia oceanica have as Posidonia oceanica (50.68 days). Roots are both horizontal and vertical rhizomes, whereas typically formed in the internodes of rhizomes, Zostera species have only horizontal rhizomes. both horizontal and vertical. The death of the Horizontal rhizomes can revert into vertical meristems results in the discontinuity of the rhizomes, which leads to the cessation of their production of new modules (leaves, internodes, horizontal growth. In turn, vertical rhizomes can etc.). The production of new rhizome material, branch to produce horizontal rhizomes when the which leads to the development of new shoots apical meristem of the original horizontal rhizome and roots, as well as new branches, is the basis of dies, thereby resuming the capacity for horizontal the growth of seagrass clones. Clonal growth is, growth. Zostera species bear a leaf on each therefore, a fundamental component of the horizontal rhizome node, and Posidonia oceanica production and space occupation of seagrasses, and Cymodocea nodosa bear leaves on the particularly during the colonization of new habitats nodes of both horizontal and vertical rhizomes. or their recovery from disturbance. European seagrasses have long and relatively Meristematic death is also associated to sexual narrow strap-like leaves, ranging in size, from the reproduction in Zostera spp, which have terminal small leaf areas of Zostera noltii to the large leaf inflorescences. Meristematic death is followed by areas of Posidonia oceanica (Table 3.1). The the loss of functionality of the modules, which may leaves are often present in bundles on shoots, be subsequently shed, thereby avoiding with up to 8 leaves per shoot in Posidonia respiratory losses by non-functional organs. The oceanica and 2 to 5 leaves in the other species. life span of seagrass shoots, leaves and roots, Seagrass roots provide the necessary anchoring which reflects the life span of their associated and nutrient acquisition, and vary greatly in size meristems, differs greatly among species, and is across the European species (Table 3.1). scaled to their size, with small species having Seagrass flowers are often inconspicuous and short leaf life spans and larger species having very simple, for they do not rely on animals for longer leaf life spans. Leaf life span ranges from a pollination. Seagrass flowers, seeds and fruits few days in Zostera noltii to almost a year in range greatly in size from the minute flowers of Posidonia oceanica (Table 3.1). Available Zostera noltii, which contain multiple ovaries and information suggests that roots are longer-lived seeds to the large infloresences of Posidonia, than leaves, remaining attached to the plants which yield large fruits known as sea olives. longer than leaves do. The life span of the shoots Cymodocea nodosa has separate male and of European seagrasses ranges from weeks female clones, unlike the other European (Zostera noltii) to decades for Posidonia oceanica seagrasses, which are hermafrodites. (Table 3.1). The life span of the meristems of Cymodocea nodosa shoots produce two seeds, horizontal rhizomes is presently unknown, except attached to the base of the shoot, Posidonia for species, such as Zostera spp, for which sexual oceanica produces half a dozen seeds per shoot, reproduction is a terminal event. and flowering shoots of Zostera noltii and Z. marina produce hundreds of seeds. Seagrass growth rate

Leaf growth rates New leaves are produced centrally in the leaf bundles by the meristems. Once the leaf has

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Table 3.1 Average architectural features and growth rates of European seagrass species. nd: no data. Range of values are indicated within brackets when available. Data from Duarte 1991, Marbà et al 1996, Duarte et al 1998, Marbà and Duarte 1998, and Hemminga et al 1999. Cymodocea Posidonia Zostera Zostera nodosa oceanica marina noltii Leaf surface (cm2) 9 82.8 34.6 1.15 Shoot mass (mgDW) 82.8 731 272.5 6.5 Fruit size (mm3) 48 523.6 18 2.8 Horizontal internodal length (mm) 25 (6-53) 3 (1-4) 11 (9-12) 12 (3-20) Vertical internodal length (mm) 1.4 (0.1-2.5) 1 (0.4-2) nd nd Root length (cm) 21.3 43.1 nd 3.2 Shoot elongation rate (cm shoot -1 d-1) 1.3 0.8 3.2 0.7 Horizontal rhizome elongation rate -1 -1 40 (7-204) 2 (1-6) 26 (22-31) 68 (10-127) (cm apex yr )

Vertical rhizome elongation rate -1 -1 1.4 (0.1-16) 1 (0.1-4) nd nd (cm apex yr ) Leaf life span (days) 79 (50-155) 295 (256-345) 88 (33-164) 86 (46-125) Shoot life span (days) 876 4373 554.8 nd been produced, it elongates from its basal part, where the leaf meristem is located, until it attains Environmental and internal controls on growth the length characteristic of the species and rates enforced by the habitat conditions. Seagrass Seagrasss growth rates, to large extend, are shoots produce new leaves while the standing species-specific and scaled to plant size. The ones are still growing. When summed over the negative relationship between seagrass growth leaves present on any given shoot, the total daily and size derives from the increasing construction leaf elongation rate per shoot tends to be on the costs of seagrass modules the larger they are. In order of one or a few centimetres addition, seagrasses modulate their growth in response to environmental (e.g. climate, nutrients, Horizontal and vertical growth rates sediment quality) and population (shoot density) conditions. Leaf and rhizome growth of European The growth rates of seagrass rhizomes vary seagrasses, except horizontal rhizome growth of greatly, from a few centimetres per year in P. oceanica, exhibits wide seasonal fluctuations in Posidonia oceanica to more than 2 m yr-1 in response to changes in temperature and/or Cymodocea nodosa (Table 3.1). The vertical irradiance. Growth of European seagrasses is rhizomes of P. oceanica and C. nodosa also maximal during summer and minimal during extend, but at slow rates of a few centimetres per winter, when in most species (C. nodosa, Z. year. Seagrass horizontal rhizomes branch, marina, Z. noltii) it almost ceases. Seagrass accelerating the occupation of space. Horizontal response to seasonality remains imprinted on the branching is more profuse in small species (e.g. length of rhizome internodes, allowing Zostera noltii may branch at every node), than in retrospective quantification of rhizome growth large seagrass species (e.g. P. oceanica over the time scale equal to rhizome longevity (i.e. rhizomes produce, on average, a branch every 30 from few months in Z. noltii to decades in P. years). In addition, the horizontal branching angle oceanica). Seagrass growth requires light varies across species, small seagrasses (e.g. conditions being at least 11 % of surface Zostera noltii) branching with angles close to 90º irradiance. During periods of fast seagrass and large ones (e.g. P. oceanica) branching with growth, ambient nutrient availability may constrain angles about 40 º. Small seagrass species seagrass growth. Seagrasses with leaf nitrogen elongate and branch their rhizomes at much faster and phosphorous concentrations below 1.8 % rates than large ones. Hence, small seagrass plant dry weight and 0.2 % plant dry weight, spread in two dimensions at a greater rate than respectively, are susceptible to encounter growth large seagrass species do, so that small seagrass limitations, and they often respond to nutrient species play a pioneer role and are better able to sediment additions by increasing leaf growth (e.g. recover from disturbance. C. nodosa and P. oceanica). Seagrass

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rhizospheres tend to be denser, with more integrated over time scales ranging from, at least, branched root networks when they grow in more less than one month in small species (Z. noltii) nutrient-poorsurroundings. On the contrary, and several years in large ones (P. oceanica). excess of ambient ammonia has been The greater capacity of large seagrasses to store demonstrated to be detrimental for seagrass and mobilize resources in their clones than small (Zostera marina) survival. Deterioration of ones allows them to uncouple their growth more sediment quality, reflected by strongly reducing from ambient resource heterogeneity, and, sediment conditions and high concentrations of thereby, buffer their growth responses. toxic compounds (e.g. sulphide), has been shown to suppress seagrass growth. Sediment dynamics also alters seagrass growth. Sand erosion and Seagrass spreading deposition on seagrass beds decrease shoot survival. However, seagrass species with vertical rhizomes are able to cope with sand deposition Vegetative vs. sexual spreading because the growth of vertical rhizomes, and The reproductive biology of seagrass species has leaves, of surviving shoots is enhanced with interested naturalists for about two centuries. moderate sand burial, increasing at proportional Flowering of seagrasses is often controlled by rates as the height of sand accreted. Similarly, temperature and often occurs simultaneously they reduce vertical rhizome and leaf growth to across large spatial scales. European seagrass minimum rates when sand is eroded. The species flower in late spring , , and some of them response of seagrasses to sand burial is triggered (Zostera spp) throughout the summer as well, by darkness around vertical rhizome meristems. when irradiance improves and water temperature The seagrass response to sediment deposition increases, except for the Mediterranean species remains imprinted on the vertical rhizomes as long Posidonia oceanica, which flowers in the fall internodes, allowing retrospective identification of (October). Flowering is a rare event for most burial/erosional events. Seagrass clonal growth is seagrass species, where typically < 10 % of the also dependent on the density of neighbouring shoots flower each year. Yet, flowering is profuse shoots. Horizontal rhizome elongation and in annual Z. marina populations developing at the branching rate of at least C. nodosa and Z. marina intertidal zone. The reproductive effort of decrease with increasing shoot density around the seagrasses can be highly variable between years growing rhizome apex. Because rhizome growth and among populations, and episodic mass regulates shoot proliferation in seagrass beds, flowering can occur in connection to climatic regulation of clonal growth rate in response to extremes, such as the massive flowering of neighbouring shoot density is an important Posidonia oceanica in connection to extreme mechanism to avoid shoot density-mortality under summer temperatures in 2003. Disturbances, crowding conditions, and, thus, intra-specific such as burial derived from the migration of sand competition. waves may also enhance seagrass flowering. The response of seagrass growth to Because of the low probability of flowering, sexual environmental and population changes is species- reproduction is a negligible component of the specific as a consequence of differences in carbon allocation of seagrasses, involving < 10 % sensitivity to environmental forcing and capacity to of the annual production for most species. All uncouple plant growth from ambient conditions European seagrass species have hydrophilous among species. For instance, seasonality in C. pollination, in which pollen grains are released in the water column to fertilise the female flower. nodosa growth is highly dependent on -2 temperature, whereas that in Z. marina growth is Seed production can reach thousands of seed m for Zostera species, whereas it is in the order of, mainly forced by light conditions. The high -2 sensitivity of C. nodosa to temperature conditions at most, tens of seeds m for Cymodocea nodosa has been attributed to the tropical origin of this and Posidonia oceanica. A significant percent of seagrass genus. Seagrasses store resources the seeds seagrass produce are lost before being (carbon and nutrients) in their belowground released due to predation by invertebrates and organs that they use during periods when ambient fish. resource availability does not suffice to fulfil Seagrasses can disperse through sexual resource demand. Resource storage capacity propagules as well as through detached or drifting largely depends on plant size and plant longevity, rhizome fragments. Dispersal by fragments was both features being strongly species dependent. considered to be rare, but new evidence suggests In addition, the architecture of seagrasses allows that the importance of this mechanism may have the ramets of a clone to remain physiologically been underestimated. Dispersal can also occur

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through a combination of both processes, as able to develop meadows despite the long life flowering shoots may detach and disperse, span of their modules. A simulation analysis of subsequently releasing the seeds. The mature seagrass clonal growth also showed that the seeds of Cymodocea nodosa are produced at the branching process continuously accelerates the base of the shoots, and are often positioned at, or occupation of space, such that the space just below, the sediment surface. These seeds occupied by a seagrass clone increases as the are, therefore, not likely to disperse far. Zostera third power of time for all seagrass species seeds disperse with currents, and have been simulated. Thus, branching rates and branching shown to have a relatively short dispersal range angles are even more important determinants of restricted to tens of meters. In contrast, the seeds the rate of space occupation than the linear of Posidonia oceanica remain buoyant for hours extension rate of the rhizomes, the parameter that and can be potentially dispersed across distances has received most attention to date. of tens or even hundreds of kilometres, although The growth rates of patches of European there are no direct observations to confirm seagrass species are constrained by the growth whether such potential is realised. rate of their rhizomes, with Posidonia oceanica patches showing the slowest growth (2 cm year-1) Patch formation and Cymodocea nodosa showing the fastest -1 Once in the sediments, the seeds of some growth (200 cm year ). seagrass species can remain dormant for some time before germinating, with a documented Posidonia oceanica dormancy period of about half a year for Zostera marina and 7-9 months for Cymodocea nodosa, P. oceanica is a long-living and very slow-growing thereby building a rather ephemerous seed bank. seagrass species. P. oceanica leaves may live for Seedling density is comparatively low (one or two a bit less than 1 year, vertical rhizomes for several orders of magnitude) relative to seed production, decades, and clones probably for centuries. The due to multiple aggregated losses. These losses slow horizontal rhizome elongation and branching are due to many factors, including lack of viability, rate (Table 3.1) of this species explains the physical damage, export to unsuitable areas, extremely slow spread of its clones. Simulation burial, and predation. Initiation of clonal growth models based on rhizome growth and branching (i.e. rhizome extension) will lead to the formation patters indicate that P. oceanica should spend of patches by the seedlings. Yet, most seedlings 350 yr to develop a 15 m diameter clone. P. die without ever initiating clonal growth, because oceanica vertical rhizomes elongate at rates that these require the accumulation of important may be of similar order of magnitude as horizontal amounts of resources, such as nutrients. rhizomes do, which is unusual for seagrass species with differentiated rhizomes. Because of Patch growth the relatively fast vertical growth of P. oceanica as The basic components of the clonal spreading of compared to horizontal growth, the long life span seagrasses, which leads to the formation of of the meadows and the slow decomposition of its patches or the maintenance of closed meadows, rhizomes, P. oceanica is able to develop reefs up are the rate of horizontal extension, and the to 3 m high and meadows with complex probability and angle of branching of the topography, particularly when P. oceanica rhizomes. The clonal growth of seagrasses can, colonises shallow coastal areas. P. oceanica therefore, be simulated from knowledge of this growth is particularly sensitive to deterioration of basic set of rules, an approach that has proved sediment quality and, at meadow depth limit, most useful in the examination of space water quality. occupation by clonal plants. The simulation of the P. oceanica flowers between August and space occupation by seagrass clones confirms November. The number of flowering shoots in P. the prediction that small seagrass species have a oceanica meadows is usually very low, generally less efficient, but more compact occupation of lower than 3 % per year. However, flowering space, which has been referred to as the intensity widely fluctuates between years. Massive “phalanx” strategy, whereas large, slow growing flowering events (when more than 10% shoots species have a more efficient, but looser flower) have been observed associated to occupation of the space, the “guerrilla strategy” extremely warm summers. Flowering intensity Indeed, if large species did have broader also varies with water depth, decreasing the branching angles, the time required to occupy the number of flowering shoots with increasing water space would be so long that they would not be depth, and it depends on local conditions. Many

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P. oceanica female flowers do not succeed to dynamics. Most of the new patches formed develop viable fruits as a consequence of fruit disappear within one year and only a few grow to abortion and, to less extent, predation. Actual effectively increase the cover of elelgrass in seed production is less than 1% of potential seed coastal zones. The time scales required for the production provided the amount of ovaries recolonisation of eelgrass meadows has been produced during flowering. Very little information estimated, provided favourable environmental is available on seedling survival and clone conditions, to be in the order of a decade. Z. initiation rate, but it should be extremely slow. marina clones, however, may persist over centuries in areas where sexual reproduction is The little investment and low success of sexual scarce (e.g. Baltic Sea). reproduction, combined with the extremely slow clonal spread of P. oceanica explains the extremely slow colonisation rate of this species. Zostera noltii Numerical models simulating the occupation of space by a P. oceanica meadow indicate that it The fast rate of rhizome elongation (68 cm year-1) would need 600 years to cover 66 % of the space and profuse great branching rate lead to a available. Similar colonisation time scales have compact space occupation. The species has a been retrospectively calculated based on patch high leaf turnover rate, i.e. as new leaves are size and patch growth rate in patchy P. oceanica formed, the older are shed in a rapid process meadows. The very long time scales for during the shoot lifetime. Besides, the leaf growth colonisation of this species indicate that recovery rate is also high, as well as the shoot production, of disturbed P. oceanica meadows, where which represent much of the production of the important plant losses have occurred, would species. As a small species, the modules of Z. involve several centuries. noltii have a short life span, with high mortality and recruitment rates, which is typical of Zostera marina colonizing seagrass species. The high rates of growth and production of Z. nolti allow this species to sustain even under considerable Zostera marina has intermediate rates of growth disturbance. and spread compared to other European seagrasses. Besides the potential for spread Beside vegetative development, these plants can derived from the horizontal growth rates of the reproduce sexually by producing flowering shoots rhizomes, Zostera marina is able to release large (Potographs 3-4) and seeds (Photograph 5). Seed numbers of seeds. At the time of reproduction, production and other events related to this eelgrass shoots produce inflorescences which can process (flowering, seed release, dispersal, and each develop large numbers of seeds. germination) are valuable to maintain genetic Reproductive shoots die off following seed set, so diversity and may be the only significant that flowering represents a terminal event for mechanism for seagrass colonization of bare eelgrass shoots. Seed production rates in Zostera sediment areas. Coupling both vegetative and marina beds reach several thousand per square reproductive patterns may therefore constitute an meter. However, they do not travel far – a few excellent survival strategy in adverse and meters at best - from the mother plant after being disturbed environments or in the establishment of released, as they are negatively buoyant and sink new areas. to the bottom. However, flowering shoots may Flowering of Z. noltii can extend from March to detach, because of disturbance, and float away, November but the flowering season may vary releasing the seeds at considerable distances from place to place, since factors such as water from the stand where they were produced, which temperature, day length, tidal amplitude and is the mechanism for long-term dispersal available fluctuating salinity regimes control the flowering to this species. In addition, swans and geese may event. The flowering shoots flourish from the ingest seeds and transport them, although this rhizome as the vegetative ones and consist of potential mechanism has not yet been several inflorescences, each containing the male investigated. A significant fraction of the seeds and female flowers. The female flowers are released are lost due to the activity of grazers, pollinated by males from different inflorescences, such as crabs, which have been shown to to avoid self-pollination. The females that were significantly reduce the seed pool produced by fertilized develop a fruit inside, which originates a eelgrass. seed. Seeds are not likely to disperse far since The germination of eelgrass seeds leads to the they are negatively buoyant. However, detached initiation of patches, which are subject to intense flowering shoots containing seeds may be

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transported by water currents over long distances. substantially the success of sexual reproduction. Observation of Z. noltii seedlings in the field is a Reproductive effort and success in C. nodosa rare event, for less than 5% of the plants originate exhibits temporal and spatial heterogeneity. from seeds. In spite of the investment in sexual Flowering intensity, for instance, has been reproduction, flowering represents less than 10% observed to increase in response to sand burial, of the shoots, which suggests that this is not the like in other seagrasses. In addition, seed main way of reproduction. production in C. nodosa should be constrained by the spatial distribution and abundance of male and female clones. The consequences of clone Cymodocea nodosa sex composition on reproductive success are evident when examining C. nodosa meadow C. nodosa growth ranks amongst the fastest ones genetic diversity. For instance, there is almost no across European seagrasses. The fast clonal genetic diversity in a C. nodosa meadow at the growth of this species allows the clones to spread Algarve (S Portugal), where no female flowers 2 across 300 m after 7 years. The life span of C. have been observed. nodosa modules and ramets is intermediate, average shoot population life-span varying The fast growth of C. nodosa clones and the between 4-22 months, and average leaf life-span relatively high patch formation rate of this species, ranging from 2 to 5 months. C. nodosa clones, when compared with the other European however, may live for at least 1 decade. C. seagrasses, indicate that C. nodosa should be nodosa growth almost exclusively occurs during able to develop a meadow within a decade, if the spring and summer. C. nodosa growth exhibits colonisation process were initiated, on bare substantial plasticity, which allows this species to sediments. The time scales for meadow recovery survive disturbances. For instance, vertical and if not all C. nodosa vegetation were lost should be horizontal rhizome growth of C. nodosa is plastic even shorter. The rapid occupation of space by C. enough for this species to colonise areas with nodosa resulting from fast clonal growth, and the intense sediment dynamics, such as bedforms relatively high patch formation rate of this species with subaqueous dunes, with an average explains the pioneering role that C. nodosa play amplitude of 20 cm (range 7-65 cm) and wave during succession process in the Mediterranean. length of 21 m (7-29 m), that migrate at average -1 velocities of 13 m yr . The close coupling Conclusion between C. nodosa vertical rhizome growth and sediment accretion has been used to quantify European seagrass flora encompasses species shallow coastal sediment dynamics impossible to with slow-growing rhizomes (Posidonia oceanica), be measured with conventional sedimentary and intermediate rhizomes expansion rates techniques. C. nodosa also exhibits substantial (Cymodocea nodosa, Zostera marina, Z. noltii), plasticity in response to ambient nutrient when compared with the range of clonal growth availability. rates displayed by seagrasses. The slow Only C. nodosa shoots older than 1 year flower, horizontal rhizome elongation and branching rates and they do so between March and June. Fruit observed in P. oceanica forecast slow (from development takes 2-3 months, although centuries to millenia) recovery time scales for this maximum density of shoots bearing fruits is species. Conversely, the rates of horizontal observed in July-August. Afterwards, fruits detach rhizome extension and branching frequency from the mother shoot and, because they have quantified for the other European seagrasses negative-buoyancy, they are rapidly buried into should allow recovery time scales of decades. In the sediment nearby the mother plant. During addition to the differences in clonal growth rules events of intense sediment dynamics (e.g. strong observed between European seagrass species, storms), however, seeds may be transported their growth is able to adapt to environmental across long distances, since there are meadows change. Seagrass plasticity, however, differs separated from the closest one by more than 300 among species, C. nodosa being amongst the km, and seeds of C. nodosa can be observed, most plastic species, and P. oceanica growth although not very often, washed on the beaches. being the least plastic. From April til June of the following year seeds Knowledge on seagrass growth rates and success germinate. C. nodosa clone formation rate has -2 -1 of reproductive effort is crucial to manage been estimated to be about 0.009 clones m yr seagrass ecosystems, particularly to derive in an area with intense sexual reproduction. expectations on the recolonisation times required However, clone mortality rate is about 50-70 % to recover seagrass meadows. Because of the during the first year of life, hence, decreasing

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low survival rate of seedlings and young patches, References they should be particularly protected. The time scale in which patches are most vulnerable Duarte CM (1991) Allometric scaling of seagrass ranges from a few months for Zostera noltii to half form and productivity. Marine Ecology a century for Posidonia oceanica. Progress Series 77: 289-300. The time required for the patches to develop Duarte CM, Merino M, Agawin NSR, Uri J, Fortes meadows ranges, across the European seagrass MD, Gallegos ME, Marbà N, Hemminga MA flora, from months to a year for Zostera noltii, to (1998) Root production and belowground less than a decade for Zostera marina and seagrass biomass. Marine Ecology Cymodocea nodosa, and several centuries for Progress Series 171: 97-108 Posidonia oceanica. The acceleration of the colonisation process along these, sometimes too Hemminga MA, Duarte CM (2000) Seagrass extended, time scales can be promoted through Ecology. Cambridge University Press the maintenance of adequate habitat conditions, Hemminga MA, Marbà N, Stapel J. (1999). Leaf including, improved light penetration and reduced nurtrient resorption, leaf life span and the organic and nutrient inputs to the waters. retention of nutrients in seagrass systems. The recovery of Zostera noltii is relatively fast, Aquatic Botany 65: 141-158Marbà N, whereas that of the other species is slow, and Duarte CM (1998) Rhizome elongation and Posidonia oceanica, in particular, does not have seagrass clonal growth. Marine Ecology the capacity to recover in opperational time Progress Series 174: 269-280 scales. Hence, the management approach to the Marbà N, Duarte CM, Cebrián J, Enríquez S, slow-recovering species should emphasize the Gallegos ME, Olesen B, Sand-Jensen K conservation and protection of the area they cover (1996) Growth and population dynamics of as to avoid losses. Posidonia oceanica in the Spanish Mediterranean coast: elucidating seagrass decline. Marine Ecology Progress Series 137: 203-213.

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