MEPS 184:119-128 (1999)  -  doi:10.3354/meps184119

ABSTRACT: Causes of spatial variation in recruitment of the surfgrass Phyllospadix torreyi were explored. Recruitment of seedlings in this dominant, late successional seagrass is dependent on the presence of host plants (mainly branched algae), which serve as substrates for seed attachment and anchor the seedling during its early growth stages. Field observations indicated that attachment of seeds and seedlings to host plants was not always proportional to abundance of host plants. To explore mechanisms causing this pattern we measured seed attachment to 7 algal species and surfgrass in the laboratory under 3 water flow regimes. Laboratory experiments were conducted using a flow tank designed to produce oscillatory flow. The primary objectives of our laboratory experiments were to determine the relative seed catching ability of different host species, and the extent to which the seed catching efficiency of a host varied with flow velocity. The flow tank experiments revealed significant effects of both host species and flow regime, and no significant interaction between host species and flow. Seed attachment to all species was greatest at a moderate flow (max. velocity 85 cm s^-1) and was reduced significantly at both lower (max. velocity 45 cm s^-1) and higher (max. velocity 180 cm s^-1) flows. Seed attachment was highest on the intertidal red alga Chondracanthus canaliculatus (Harv.) Guiry and the subtidal coralline alga Lithothrix aspergillum Gray. Differences in seed attachment among the remaining 6 species were generally small; on average these plants collected about half as many seeds as C. canaliculatus and L. aspergillum. Data collected from field surveys were generally consistent with results obtained in the laboratory. The field abundance of host species having high seed-attachment potential in the laboratory explained a significant amount of spatial variation in seedling density at 3 of 4 intertidal sites (numbers of seedlings at 2 subtidal sites were very low and precluded similar analyses). Nonetheless, a substantial amount of variability in seedling recruitment remained unexplained by the abundance and species composition of host plants. Data obtained from standardized seed collectors placed in the field were consistent with this observation as the numbers of seeds attached were highly variable both within and among sites. This suggested that factors that influence seed production, dispersal and seedling survivorship also contribute to spatial variability in the density of seedlings in nature.

KEY WORDS: Seagrass · Seedling · Algal host · Recruitment · Phyllospadix torreyi · Marine ecology · Oscillatory water flow