Specific shoot mortality rates range greatly both across seagrass species (Hemminga and Duarte,
2000) and across meadows for any one species (Marba et al., 1996b; Peterson and Fourqurean,
2001), from lowest values of 0.06 year-1(i.e. 6% of shoots dying in a year) for a stand of the long-lived Mediterranean seagrass Posidonia oceanica to a maximum estimated mortality rate of 4.47 year-1 for Cymodocea nodosa (Fig. 4). These shoot mortality rates incorporate two additive components, a baseline mortality corresponding to an internally-controlled mortality rate necessary to maintain shoot turnover, and a component derived from stresses and disturbances to the meadows.
Shoot mortality is not only a prominent component of the dynamics of seagrass meadows, but is indeed a necessary one. In an established, steady meadow, the continuous recruitment of seagrass shoots resulting from branching processes cannot be sustained without a parallel mortality of shoots, as crowding would otherwise impare recruitment. Shoot mortality is, however, insufficiently understood, and the causes of shoot mortality have not been elucidated as yet. Shoot mortality is a necessary component of the maintenance of stable seagrass meadows, so that the presence of a stress factor need not be invoked to account for shoot mortality. These thoughts suggest that, to some extent, shoot mortality should be considered a component of clonal integration, such that a clone may selectively 'decide' to cease the activity of a particular leaf-producing meristem, thereby causing shoot death. Whereas the activation of seagrass meristems in response to disturbance, such as increased branching rates (i.e. shoot production) in response to clipping of apical rhizome meristems (Terrados et al., 1997), have been examined, the internal controls on loss of meristematic activity have not been addressed, as yet. More importantly, there is a need to examine what factors may cause the death of apical meristems, which would reduce shoot recruitment. The understanding and capacity to predict meristematic activity may provide the capacity to detect stress and forecast mortality before this is reflected in shoot density changes.
Hence, most knowledge on the controls on shoot mortality derives from examination of stress and disturbance factors. Reduced water and sediment quality leads to shoot mortality, often resulting in catastrophic seagrass loss through multiple factors. Deterioration of water quality leads to seagrass mortality through light limitation and unbalanced plant carbon budgets (e.g. Gordon et al., 1994; Ruiz and Romero, 2001). Shoot mortality as a consequence of reduced light penetration has been reported at the depth limit of seagrass meadows (Krause-Jensen et al., 2000), and confirmed by shading experiments (Gordon et al., 1994; Ruiz and Romero, 2001). Increased nutrient inputs have also been shown to be associated to high mortality rates (Perez et al., 1994). Deterioration of sediment conditions, such as increased sediment anoxia and sulfide production has been shown to lead to seagrass mortality, although the responses vary greatly across species (Terrados et al., 1999). Water column hypoxia, also derived from excessive organic inputs, has also been identified as a factor affecting the health of leaf-bearing meristems, eventually causing shoot death (Greve et al., 2003). Sediment disturbance, such as excessive burial and sediment erosion, also causes shoot death by killing meristems, altering clonal integration, and, when extreme, creating topographical barriers (Marba and Duarte, 1994, 1995; Duarte et al.,
1997a). Physical disturbance is also an important source of shoot mortality, through uprooting of the plants during storms or due to human activities such as anchoring, dredging, anchor damage, and trawling (Duarte, 2002). Biological disturbance may also generate substantial seagrass mortality (e.g. Orth, 1975).
It is possible to estimate the age of individual shoots of most seagrass species because there is a relatively constant rate of production of new leaves on a shoot, called the plastochron interval. Each leaf leaves a distinctive scar on the short shoot at the node, so it is possible to count the number of leaves produced over the lifespan of an excavated shoot and multiply this number of leaves by the plastochron interval to estimate the age of the shoot (Patriquin, 1973; Duarte et al., 1994). Once recruited into the population, shoots of different species have different average lifespans. Shoots of the small, fast-spreading species, like Halophila spp., have an average lifespan of only a month or so, and a maximum age of a few months (Table 1). In contrast, the shoots of the larger, slower-spreading species like Posidonia spp. and Thalassia spp. have average life expectancies of a few years, with some shoots surviving for decades. A genetically individual plant may be much older than individual short shoots, since most sea-grasses exhibit monopodial or sympodial growth. As a rhizome grows through the soil and produces new shoots, each successive shoot is necessarily younger than the previous shoots. Older shoots may eventually senesce, but their progeny shoots may continue to thrive and extend away from the point where a seedling originally produced the genetically individual plant. Theoretically, genetic individuals could be as old as the origin of the species, even though individual shoots can only survive a few decades at most.
Seagrasses, as angiosperms, are all capable of sexual reproduction through flowering and seed production (although sexual structures have not been observed for all species, e.g. Jewett-Smith et al., 1997). As long as seeds result from the fertilization of an ovule by pollen from another genetically distinct individual, the plant originating from that seed is genetically distinct from others in the population. Once a seedling becomes established in a seagrass meadow, it begins to grow up by the production of photosynthetic leaves, but also out by the production of new plant modules consisting of a length of rhizome, associated roots, and a shoot. The branching pattern created by the production of new modules varies from many-branched plants that expand almost equally in two dimensions (e.g. Posidonia oceanica) to plants that extend almost exclusively linearly through space (e.g. Thalassia testudinum). Eventually, through the action of either senescence of modules or disturbance, these individuals can become physically separated so that what was once one plant can become many isolated plants—but all of these plants are genetically identical—i.e. they are parts of the same genetic individual (i.e. genet).
So, when studying the dynamics of seagrass populations, it is important to keep in mind that what appears above the sediments as a shoot is likely connected to many more shoots underground. And, merely because two shoots do not share a common connection somewhere under the sediments is no indication that these shoots are genetically different. In fact, there is molecular evidence for genetically identical shoots of T. testudinum separated by over 3 km in an otherwise genetically diverse, continuous seagrass bed (Davis et al., 1999). A more thorough discussion on this topic is provided in Waycott et al. (Chapter 2).
New genets can enter a population not just through successful seedlings, but also as adult plant fragments that may drift into a population from some distant source (Setchell, 1929). Seagrasses can float and survive for extended periods out of the sediment; apparently viable modules of the tropical seagrass Tha-lassia testudinum can occasionally be found on the temperate beaches of the North Carolina in the US (JWF, pers. observ), over 1000 km from the nearest known T. testudinum populations. Seagrass shoots can survive for months in the water column, but the ability of detached shoots to survive when transplanted decreases with time in the water column, limiting the potential of drifting adult plants to establish new seagrass beds (Ewanchuk and Williams, 1996). Floating seagrass shoots not only have some potential to become reestablished and expand via asexual reproduction, but they can also carry viable seeds (Harwell and Orth, 2002; Orth et al., Chapter 5) and epiphytes (Worcester, 1994) to distant locations. The role of vegetative fragments as vectors for colonization has likely been underestimated in seagrass ecology, as these are rare events, that chal lenge direct observation, although direct evidence of widespread establishment by fragments has been recently reported (Campbell, 2003).
Although there are mechanisms to provide genetically unique recruits to seagrass populations, the importance of these mechanisms in producing new shoots in seagrass beds is considered low compared to the asexual ramification of plant modules by clones already extant in populations (Tomlin-son, 1974). For most species, observations of successful seedling recruitment are rare (Orth et al., Chapter 5). However, the study of sexual recruitment in established populations is complicated by the difficulty in distinguishing whether shoots are derived from a single seed or from fragmentation of a larger clone (cf. Waycott et al., Chapter 2). Moreover, it is possible that the perception that successful seedling recruitment is a rare event may be dependent on insufficient observational effort, as this process may occur over significant spatial and temporal scales that challenge conventional sampling strategies.
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