Annual life cycles of two Zostera marina L. populations in the Gulf of california: Contrasts in seasonality and reproductive effort

Populations of the annual form of Zostera marina at Punta Chueca (PC) and Punta Viboras (PV), in the Midriff Islands region of the Gulf of California, were followed monthly from November 1996 to November 1997 to confirm their annual habit. PC is a shallow bed located at -2 m with respect to mean low low water (MLLW), and PV a deep bed located at -5 m MLLW. Bottom water temperature and irradiance were also measured monthly. Shoot density, shoot length above and belowground biomass, percentage of reproductive shoots, abundance and size of reproductive structures, and number of seeds per spathe were also measured. At PC shoots were present from November to July, while at PV this was from December to April. Significant differences between sites (p < 0.05) existed for shoot density, size and belowground biomass, with corresponding seasonal means of: I 622 ± 425 shoots m^-2 (PC), and 476 ± 174 (PV); 14 ± 2.9 cm (PC), and 54 ± 15 (PV); 51 ± 17 g DW m^-2 (PC), and 22 ± 6.4 (PV). Aboveground biomass did not differ between stations (p > 0.05), presenting seasonal means of 222 ± 85 g DW m^-2 (PC), and 194 ± 100 (PV). Maximum biomass was significantly correlated with shoot density (r² = 0.820 at PC, and 0.812 at PV). The reproductive phase was found to start in February at the deep station (PV) and one month later at the shallow site (PC). Reproductive structures were larger and more abundant at PV (p < 0.05), with the exception of number of spathes per rhipidia. Average seed abundance on eelgrass shoots was 100 376 ± 18765 and 41998 ± 7218 seeds m^-2 for PC and PV, respectively. Bottom irradiance and hydrodynamism seem to regulate the shorter annual life cycle at the deep site.