Hydrobiology of a salt pan from the Peninsula of Baja California, Mexico

The study site, a salt pan in the northwest of Baja California, Mexico, has a surface area of 33.2 ha and water capacity of 216,072 m3. It is intermittently isolated from the adjacent ocean by a sand bar, so that flooding via the coastal plain is a low frequency event associated with extreme storm conditions. Temporal changes in water volume were represented by a particular expression of the mass conservation equation having as variables the volumetric capacity of the system, water inputs by pluvial precipitation, runoff and infiltration, and water loss by evaporation; the numerical model explained over 80 per cent of observed fluctuations. The hydrobiological study covered a three-year period and two hydrological cycles; the first one was of mixed marine and pluvial origin, and the second, continental. Throughout the first cycle, phosphate and nitrate concentrations were up to one order of magnitude higher as were total particulate protein, chlorophyll a and phytoplankton abundance. In general, nannoplankton abundance was up to three orders of magnitude greater than the microphlanktonic fraction. In the former, 29 different cell forms were observed; two halophytic bacteria constituted 60–80 per cent of that fraction. In the microplankton, there were 27 different forms; most were benthic diatoms. The zooplanktonic assemblages of the first cycle had marine and continental elements; at 141 per mille,Artemia sp. first appeared; at 240 per mille, it was the only faunistic element in the system. During the second cycle,Artemia was always present, co-existing with other continental species up to a salinity of 99 per mille. The morphology and morphometry ofArtemia were similar to those ofArtemia franciscana.