Interannual and small-scale spatial variability in sexual reproduction of the seagrasses Posidonia coriacea and Heterozostera tasmanica, southwestern Australia

Sexual reproduction in Posidonia coriacea Kuo and Cambridge and Heterozostera tasmanica (Aschers.) Dandy on Success Bank, Western Australia, was assessed over a 3-year period (1996-1998). Interannual and spatial variation in inflorescence, flower and seed density and the probability of flowers setting seed were examined for both species of seagrass. Flower and seed densities of R coriacea showed that sexual reproduction could contribute to the maintenance of this population on Success Bank. Substantial interannual variation in flowering intensity indicates that the relative contribution of sexual reproduction to meadow maintenance varies among years. Flower densities of H. tasmanica in 1997 pointed to potential sexual reproduction as a contribution to the meadow, but the absence of flowers in the 2 other years of the study, and the absence of a seed bank within the sediment, suggested that sexual reproduction did not contribute to the maintenance of this population on Success Bank. Seed production determines the upper bounds of the potential for a species to recruit. We have quantified these upper bounds for P. coriacea and H. tasmanica on Success Bank, southwestern Australia at 15 +/- 3 and 60 +/- 11 m(-2) per year (mean +/- S.E, n = 110), respectively. These upper bounds are comparatively low relative to vegetative shoot recruitment at 240 +/- 84 and 360 +/- 86 m(-2) per year (mean +/- S.E., n = 6) for R coriacea and H. tasmanica, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.