Sexual reproduction and seed dispersal pattern of annual and perennial Zostera marina in a heterogeneous habitat

The sexual reproduction of annual and perennial Zostera marina was investigated in Moon Lake, Shandong, China. Based on the disturbance and stress regimes, the Z. marina beds were classified into five types: intertidal annual (IA) and perennial (IP) eelgrass patches, subtidal patch area (PA), meadow margin (MM) and meadow center (MC). Seed dispersal was investigated using artificial seagrass units in the five areas and another two areas [adjacent bare area and Zostera japonica meadow (Zj)]. Total and flowering shoot density and aboveground biomass of flowering shoots per unit area were higher in PA and MM, and lower in IA and IP, whereas the total biomass per unit area in MC showed the highest value. Reproductive effort (RE) in IA showed negative response to intertidal stress, while in perennial IP, PA and MM it showed significantly positive response to anthropogenic or natural disturbances. The density-based RE in perennial IP, PA and MM was 1.1-, 5.1- and 5.1-fold higher than that in MC, while in annual IA it was 0.46-fold lower. Additionally, the biomass-based RE in IP, PA and MM was 1.8-, 3.5- and 3.8-fold higher than that in MC, while the RE in IA was 0.84-fold lower. The estimated seed production per unit area was much greater in PA (60,793 +/- A 9,843 seeds m(-2)) and MM (43,414 +/- A 8,718 seeds m(-2)) than in IA (416 +/- A 83 seeds m(-2)), IP (3,820 +/- A 1,470 seeds m(-2)) and MC (9,779 +/- A 631 seeds m(-2)), while the seed density ranged from 24 +/- A 6 to 584 +/- A 56 seeds m(-2). Results suggested that in response to disturbances and stress, Z. marina in subtidal areas increased their RE and seed production and thus seeds were available to be dispersed into areas where seed production was limited.