Demography and biomass of the seagrass Zostera marina in a Mexican coastallagoon

From January 1987 to February 1988 the annual biomass cycle and demography of the seagrass Zostera marina were assessed in San Quintin Bay, a shallow coastal lagoon on the Pacific coast of Baja California, Mexico. Shoot densi ty and aboveground biomass were sampled monthly along two intertidal transe cts parallel to the shore. Belowground biomass was sampled every 2 mo. Shoo t density differed between transects, ranging from 929 +/- 71 (SE) in July to 279 +/- 80 shoots m(-2) in December, at the deeper transect (I). At the shallow transect (II) there was not a significant difference through time, and a mean of 737 shoots m(-2) was calculated. Lateral shoots were present year round and represented between 1% and 30% of total density at transect I and between 3% and 25% at transect II. Reproductive shoots were present f rom March to September at both transects, with a density range of 77 +/- 28 shoots m(-2) (March) to 9 +/- 3 shoots m(-2) (September), and represented 5% of total shoot density. Neither aboveground biomass nor LAI (Leaf Area I ndex) differed between transects, with values ranging between 77 +/- 14.5 g dry wt m(-2) (October 1987) and 13 +/- 2.4 g dry wt m(-2) (February 1988) for aboveground biomass, and between 0.6 +/- 0.2 m(2) leaves m(-2) substrat e January) and 2.7 +/- 0.3 m(2) leaves m(-2) substrate (September) for LAI. Neither root biomass nor rhizome biomass differed between transects, or as a function of time; the mean value for roots was 17 g dry wt m(-2) and for rhizomes 29 g dry wt m(-2). Belowground biomass represented 54% of total b iomass. We found a significant correlation between aboveground biomass and LAI (r = 0.949 for transect I, and 0.926 for transect II) as web as between total biomass (aboveground and belowground) and LAI (r = 0.814), which all ows us to consider using LAI as a predictor of these variables. Biomass cha nges were related to changes in shoot weight (r = 0.676 at transect I; 0.58 2 at transect II), more than to changes in shoot number. Water temperature was found to be the driver of biomass changes in the aboveground compartmen t.