Meadow fragmentation and reproductive output of the SE Asian seagrass Enhalus acoroides

Abstract

Flower and fruit production of the abundant, tall, long-lived, dioecious, surface-pollinating seagrass species Enhalus acoroides (L.) Royle were estimated at seven sites in the reef flats off Bolinao (NW Luzon, The Philippines) featuring different fragmentation of the seagrass meadows. Fragmentation of the seagrass meadow was quantified as cover of E. acoroides and all seagrass species present in 20×20 m plots. E. acoroides and overall seagrass cover were correlated positively. The proportion of female flowers of E. acoroides that developed a fruit increased sharply as overall seagrass cover was around 50%. Apparent sex ratio bore no relationship with overall seagrass cover. This threshold-type of relationship suggests that fragmentation of seagrass meadows can have a major effect on the reproductive output of this species. A possible mechanism underlying these results would be a non-linear increase of the efficiency of trapping the surface-dispersed pollen with increasing seagrass canopy density. This provides the first evidence based on real data that fragmentation can affect the population dynamics of seagrass species.

Introduction

Seagrasses are rooted angiosperms inhabiting unconsolidated sediments of shallow coastal waters where physical disturbance by waves and currents is a natural phenomenon (Marbà and Duarte, 1995, Vermaat et al., 1997). Such disturbance has been related to the spatial structure and fragmentation of seagrass meadows (Patriquin, 1975, Fonseca and Kenworthy, 1987, Fonseca and Bell, 1998). Human activities in coastal waters have also resulted in increased fragmentation of seagrass meadows (Walker et al., 1989, Pasqualini et al., 1999, Bell et al., 2002). Large-scale declines of seagrass meadows (Short and Wyllie-Echeverria, 1996) call for in-depth understanding of the ecological processes involved in seagrass recovery. Although some seagrasses can show a high recolonisation capacity through vegetative growth (Kendrick et al., 1999, Hemminga and Duarte, 2000), seeds are considered crucial for the recovery and recolonisation of declining or disappeared seagrass beds (Orth et al., 1994, Orth et al., 2000).

Several environmental factors may indirectly affect the reproductive success of seagrasses through their effect on plant growth, for example light and nutrient availability (Agawin et al., 1996, Rollon, 1998). Pre-and post-dispersal seed predation can reduce the reproductive success of seagrasses (Fishman and Orth, 1996, Nakaoka, 2002, Orth et al., 2002, Balestri and Cinelli, 2003). Seagrass canopies modify hydrodynamics (Fonseca, 1996, Koch and Gust, 1999, Verduin and Backhaus, 2000) with consequences for habitat conditions and processes (Ward et al., 1984, Koch, 1999a, Gacia and Duarte, 2001, Gacia et al., 2002) which can affect plant growth (Koch, 1994, Koch, 1999b) and possibly sexual reproduction output. Additionally, pollen dispersal, pollination efficiency and fruit production might be directly influenced by the effect of seagrass canopies on water motion (cf. Lovett Doust and Laporte, 1991, Verduin et al., 1996). Hence, increased fragmentation of seagrass meadows may affect their reproductive output and, consequently, their capacity for long-term maintenance.

The large, long-lived, dioecious seagrass Enhalus acoroides (L.) Royle is abundant in SE Asian seagrass meadows (Johnstone, 1979, Nienhuis et al., 1989, Chansang and Poovachiranon, 1994, Terrados et al., 1998). Contrary to most seagrass species, E. acoroides invests substantial resources in reproduction (Duarte et al., 1997: 20% of annual aboveground productivity), and pollination occurs at the water surface (Troll, 1931, Den Hartog, 1970, Pettitt, 1984, Cox, 1988). The length of the female peduncle limits successful pollination to low tide depths shallower than ca. 0.5 m (Den Hartog, 1970). Vegetatively, the species occurs substantially deeper (Rollon, 1998). Rhizome branching frequency and longevity of shoots and rhizomes give rise to a patchy distribution of clonal fragments (Marbà and Duarte, 1998), with the gaps filled by other seagrasses, macroalgae or bare sediment. The natural patchiness of E. acoroides meadows and its abundant production of fruits (Duarte et al., 1997) make this species an appropriate model for our purpose: to elucidate if reproductive output of E. acoroides was affected by fragmentation of the seagrass meadows.