Zostera species are monoecious with flowers occurring clustered in spadices on branches along an extended stem floating vertically in the water column (Kuo and den Hartog, 2001; Walker et al., 2001). Male and female flowers occur within the same spadix (Fig. 4), with the anthers appearing first and pollen release occurring after germination in the spadix is complete. Pollen is released into the water column in linear strands which drifts within and between beds. Fruits and seeds develop within the spadix and are released directly from the parent plant or dispersed widely as spadices; alternatively, whole reproductive shoots drift under the influence of tidal currents and wind. Large numbers of seeds
are produced and dispersed and germination can be quite high, but only a fraction of the germinated seedlings survive to maturity.
Variations in the proportion of flowering shoots and subsequent seed production in Zostera populations have been well documented (e.g. Bulthuis, 1983; Orth and Moore, 1986; Strother and Kerr, 1990; Ramage and Schiel, 1998; Orth et al., Chapter 5), but many causal relationships between success of sexual reproduction and environmental factors are still not understood. Temperature is an important determinant of flowering in some Zostera species (De Cock, 1981), and several studies indicate that phenology of Zostera species is strongly related to latitude (Fig. 5), with the flowering sequence delayed as latitude increases (Phillips et al., 1983; Silberhorn et al., 1983; Walker et al., 2001). Temperature stress can be an important factor affecting sexual reproduction. The development of an annual phenotype of Z. marina in the Gulf of California, Mexico, has been related to high temperature (McMillan, 1983; Phillips and Backman, 1983; Meling-Lopez and Ibarra-Obando, 1999). Conversely, low temperatures and ice formation are associated with an annual form of Z. marina in Nova Scotia, Canada (Keddy and Patriquin, 1978; Robertson and Mann, 1984). Interestingly, the annual populations of Z. marina found in the Gulf of Mexico largely complete their sexual reproductive cycle before the annual high water temperatures of 30-32 ° C are reached (Meling-Lopez and Ibarra-Obando, 1999), suggesting that hot summertime conditions have resulted in the se lection of annual flowering strains of the species. Recent work investigating the reproductive potential of other intertidal and shallow subtidal species of Zostera such as Z. noltii and Z. capricorni (reported as Z. novazelandica) did not report strong relationships between potential stresses and reproductive output (Loques et al., 1988; Curiel et al., 1996; Ramage and Schiel, 1998). Ramage and Schiel (1998) observed that Z. capricorni plants growing high in the intertidal did not adopt an annual life history with a high proportion of flowering shoots. In contrast to the annual forms of Z. marina, maximum reproductive output of Z. capricorni occurred in small tide pools and low in the intertidal zone and creeks. Harrison (1993) found that intertidal populations of Z. marina growing in annually disturbed habitats in the southwestern Netherlands were annual. He suggested, however, that the annual life history may have been imposed on the population by a stressful environment, including grazing by geese and sediment disruption by winter storms that removed all remaining vegetative shoots. In contrast to the Z. marina plants, Z. noltii that co-occurred in the intertidal maintained a perennial population with no seed germination or seedling emergence detected.
Typically, light reduction reduces Zostera flowering success (Bulthuis, 1983; Dennison et al., 1987; van Lent et al., 1995), although Phillips and Backman (1983) observed 100% flowering in annual Z. marina growing across a wide depth gradient extending from the intertidal to a depth of seven meters below low water in the Sea of Cortez, Mexico. They also observed that plants growing in deep water (7 m) flowered and produced mature seeds that were released much earlier than those of plants growing in shallow water (<3 m). Plants in intertidal locations were the most delayed in their reproduction. In contrast, Orth and Moore (1983b) found little flowering in perennial Z. marina growing near its depth limits in the Chesapeake Bay.
Interactions between light availability and other factors such as nutrient availability on flowering success in Zostera have received only limited study. Short (1983) reported higher flowering rates of Z. marina growing in shallow, nutrient-poor sediments in Alaska compared to deeper, nutrient-rich sediments. van Lent et al. (1995) subsequently investigated the interaction of light and nutrients on flowering of Z. marina in The Netherlands. Here, they found that although light availability was the principal factor affecting flowering success, when sufficient light was available, sediment nutrient enrichment significantly increased flowering over un-enriched treatments.
As discussed in section II.A, genetic investigations are beginning to reveal the nature of the phylo-genetic relationships within the genus Zostera (Les et al., 2002; Kato et al., 2003), although several species in the genus are not yet fully investigated (Fig. 6). Indeed, a discussion continues about dividing the genus Zostera into two genera, Zostera and Nanozostera (Tomlinson and Posluzny, 2001; Kato et al., 2003), each with several species.
The first major geographic comparison of Z. marina populations demonstrates distinct genetic separation between clades in the eastern Atlantic, the Black Sea through Portugal, the western Atlantic, and the Pacific coast of North America (Olsen et al., 2004). These finding show an area of unexpectedly high genetic diversity for Z. marina in the North Sea-Wadden Sea-southwest Baltic Sea region. Z. marina likely "originated in the Pacific between 8 and 20" million years ago (Olsen et al., 2004); given that five Zostera species now co-occur in the northwest Pacific (Fig. 6), the genus Zostera may have originated in this region (Kato et al., 2003).
Recent advances in genetic techniques have allowed researchers to evaluate a great deal about the development, diversity, inter-connectivity and fitness of Zostera populations (Ruckelshaus, 1995, 1996, 1998; Reusch et al., 1999a,b, 2000; Reusch, 2001, 2003; Hammerli and Reusch, 2003; Olsen i— Zostera capengis i i
Zostera caiiiesceiis* Zostera asiatica*
Zostera in anna *
Fig. 6. Phylogenetic tree of the genus Zostera based on Les et al. (2002) and Kato et al. (2003). Dotted line indicates tentative species relationship; branch length is arbitrary. Family divisions within the Zosteracea are still under discussion. *Show species co-occurring in the northwest Pacific.
et al., 2004). Not only can this information be used for understanding responses of Zostera to natural and anthropogenic stresses, but possibly even more importantly, it is useful for management and restoration of diminished areas. For example, Reusch (2002) found that the area of genetic connectivity in a region of the Baltic was nearly double that of the northern Wadden Sea. Environmental stresses in the Baltic region might result in potentially broader impacts there, given the generally lower genetic diversity found in the north Baltic Sea. The relationship between population persistence in changing and stressful environments is not a simple one. Reusch and his associates (Reusch et al., 1999a) reported a single genotype clone extending over an area of approximately 160 x 40 m2, with an estimated age of more than 1000 years, suggesting a successful broad plasticity in phenotype. Given the current capacity to identify individual clones, greater understanding of their arrangement and persistence in the landscape may allow inferences about the history or disturbance regime of a site that could prove useful for management (Reusch et al., 1999b). Similarly, improved knowledge of genetic diversity and fitness as well as understanding of the importance of inbreeding and outcrossing in Zostera populations (Ruck-elshaus, 1995) can provide important information for restoration (Williams and Orth, 1998; Olsen et al., 2004).
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