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Article Excerpt
ABSTRACT: Growth of newly settled winter flounder, Pseudopleuronectes americanus, and abundance of potential prey were measured during May and June of 1998 to evaluate flounder recruitment habitat across an eelgrass (Zostera marina) vegetated--unvegetated edge. Flounder growth in standard length (mm) and weight (g) was assessed through ten--day caging experiments in 1) unvegetated habitat, and from 2) edge and 3) interior portions of Z. marina. Winter flounder had greater mean growth in length ([G.sub.length]) in unvegetated habitat (0.016 [day.sup.-1]) compared to Z. marina edge (0.014 [day.sup.-1]) and interior (0.011 [day.sup.-1] and greater growth in weight ([G.sub.weight]) in unvegetated habitat (0.057 [day.sup.-1]), compared to Z. marina edge (0.05 [day.sup.-1]) and interior (0.047 [day.sup.-1]) habitats. Pseudopleuronectes americanus also showed decreasing growth rates during successive caging trials, with growth rate significantly reduced by the end of June. Benthic core samples showed that densities of large potential prey (0.85-4 mm) were significantly more abundant at Z. marina edge, reduced in the interior, and were lowest in samples from unvegetated sand (P < 0.001). Densities of small potential prey (0.25-0.85 mm) showed no consistent pattern of abundance among habitats. Plant biomass and shoot density were greatest within interior Z. marina. Increases in habitat structural complexity among the habitats tested may have affected P. americanus growth by decreasing foraging success, and thus explains the differences in relative growth between unvegetated and vegetated habitats. The relatively high prey abundance and reduced plant structural complexity at eelgrass edges may account for the increased flounder growth rates observed at the edge compared to the interior of Z. marina.
KEY WORDS: Winter Flounder, Eelgrass, Zostera, Growth, Edge Effect
INTRODUCTION
Seagrasses are common in coastal tropical and temperate regions as well as in portions of the sub--Arctic. Many commercially important fish and invertebrate species use seagrass during early development, consequently, these vegetated habitats are vital nursery grounds, and provide both food and refuge for many associated species (Heck and Thoman, 1984; Heck et al., 1995; Heck et al., 1997). Winter flounder (Pseudopleuronectes americanus) is an important member of the northeastern United States coastal Atlantic fish community and is important in recreational and commercial fisheries. Previous work in Little Egg Harbor, New Jersey, has shown that winter flounder settle in and use several estuarine habitats, including eelgrass (Sogard and Able, 1991). It has been shown that the density of benthic invertebrates is significantly greater in seagrass compared to adjacent unvegetated habitats (Lewis and Stoner, 1983; Virnstein et al., 1983; Sogard and Able, 1991). These invertebrates are important prey for juvenile w inter flounder (Sogard, 1992; Vivian et al., 2000).
Seagrass structure is important in coastal regions because it dampens wave energy and reduces water velocity (Fonseca et al., 1982). These changes in the physical regime create a gradient of sediment size from the edge to the interior of a bed (Orth, 1977). Therefore, due to the differences in physical regime and sediment composition, two habitats may exist within eelgrass beds: 'edge' and 'interior'. Edge habitats have been studied extensively in terrestrial systems (see Forman, 1995, and references within), but few such investigations have been made in seagrass communities (but see Bologna and Heck in press). The distribution of fauna among seagrass species and across geographical regions has been studied, but relationships between within--habitat variability in plant structural components (e.g., plant biomass) and potential impacts on the fauna are poorly known (but see Bologna and Heck, 1999). Recent research by Bologna and Heck (in press) has indicated dramatic differences in plant structural complexity , faunal density and secondary production associated with edges of seagrass beds. They showed significantly greater densities of amphipods, decapods, and polychaetes at turtle grass edge, leading to elevated secondary production. However, plant biomass was less at the bed edge. This has important implications for trophic transfer to higher--level consumers, as elevated secondary production at seagrass bed edges may be a conduit of trophic resources between herbivores and secondary consumers in these communities. Increased structural complexity of seagrasses may serve as refuge for fishes. Fishes are known to select habitats which do not maximize growth, but afford greater survival from predators (Sogard, 1994). Consequently, winter flounder, which forage at eelgrass edges, may encounter elevated prey densities and relatively reduced plant complexity compared to the...
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