Algal morphology, flow, and spatially variable recruitment of surf grass Phyllospadix torreyi

Causes of spatial variation in recruitment of the surfgrass Phyllospadix to rreyi were explored. Recruitment of seedlings in this dominant, late succes sional seagrass is dependent on the presence of host plants (mainly branche d algae), which serve as substrates for seed attachment and anchor the seed ling during its early growth stages. Field observations indicated that atta chment of seeds and seedlings to host plants was not always proportional to abundance of host plants. To explore mechanisms causing this pattern we me asured seed attachment to 7 algal species and surfgrass in the laboratory u nder 3 water flow regimes. Laboratory experiments were conducted using a no w 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 effic iency of a host varied with flow velocity. The flow tank experiments reveal ed significant effects of both host species and flow regime, and no signifi cant interaction between host species and now. Seed attachment to all speci es was greatest at a moderate flow (max. velocity 85 cm s(-1)) and was redu ced 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 intertid al red alga Chondracanthus canaliculatus (Harv.) Guiry and the subtidal cor alline alga Lithothrix aspergillum Gray. Differences in seed attachment amo ng the remaining 6 species were generally small; on average these plants co llected about half as many seeds as C. canaliculatus and L, aspergillum. Da ta collected from field surveys were generally consistent with results obta ined in the laboratory. The field abundance of host species having high see d-attachment potential in the laboratory explained a significant amount of spatial variation in seedling density at 3 of 4 intertidal sites (numbers o f seedlings at 2 subtidal sites were very low and precluded similar analyse s). Nonetheless, a substantial amount of variability in seedling recruitmen t remained unexplained by the abundance and species composition of host pla nts. Data obtained from standardized seed collectors placed in the field we re consistent with this observation as the numbers of seeds attached were h ighly variable both within and among sites. This suggested that factors tha t influence seed production, dispersal and seedling survivorship also contr ibute to spatial variability in the density of seedlings in nature.