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 surfgrass species in Baja California (NW Mexico) were examined. The maximum cover of Phyllospadix scouleri (16 ± 3.6%) was found between 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 below MLWL). Both species showed similar seasonal patterns, with the lowest rate of production of new leaves and biomass observed in April, when the daytime exposure to air of the intertidal area studied was longest. The production of surfgrass was very high, exceeding 8000 g DW m^-2 yr^-1, and the rhizome elongation rate ranged from 0.24 to 0.58 mm 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 increasing exposure to air during day hours, but this decline was strongest for P. torreyi. The leaves of P. torreyi desiccated faster than those of P. scouleri when exposed to air, and the net photosynthesis rate 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. torreyi was more affected than that of P. scouleri under the same regime of air exposure. These results suggest that P. torreyi is more sensitive to desiccation than P. scouleri, and point to both species as the most productive seagrass communities yet studied.