BATHYMETRIC DISTRIBUTION, BIOMASS AND GROWTH DYNAMICS OF INTERTIDAL PHYLLOSPADIX SCOULERI AND PHYLLOSPADIX TORREYI IN BAJA-CALIFORNIA (MEXICO)

The bathymetric distribution, biomass, growth dynamics and production of surf grass species in Baja California (NW Mexico) were examined. Th e maximum cover of Phyllospadix scouleri (16 +/- 3.6%) was found betwe en 40 and 50 cm below MLWL (mean low water level), whereas P. torreyi showed continuous cover (100%) at the lower intertidal (80 to 90 cm be low MLWL). Both species showed similar seasonal patterns, with the low est rate of production of new leaves and biomass observed in April, wh en the daytime exposure to air of the intertidal area studied was long est. The production of surf grass was very high, exceeding 8000 g DW m (-2) yr(-1) and the rhizome elongation rate ranged from 0.24 to 0.58 m m d(-1): resulting in an annual rhizome growth rate of about 8.6 cm yr (-1). The growth and biomass of both species tended to decline with in creasing exposure to air during day hours, but this decline was strong est for P. torreyi. The leaves of P. torreyi desiccated faster than th ose of P. scouleri when exposed to air, and the net photosynthesis rat e of P. torreyi was more affected than that of P. scouleri after being exposed to air for similar lengths of time. The shoot size of P. torr eyi was more affected than that of P. scouleri under the same regime o f air exposure. These results suggest that P. torreyi is more sensitiv e to desiccation than P. scouleri, and point to bath species as the mo st productive seagrass communities yet studied.