Annual patterns of nutrients and chlorophyll in a subtropical coastal lagoon under the upwelling influence (SW of Baja-California Peninsula)

The coastal lagoon of Bahía-Magdalena, located on the west coast of the Peninsula of Baja-California, is a subtropical ecosystem with an arid climate and very little freshwater input. During the 2005-2011 period the thermohaline properties varied between cold and warm half-yearly periods. They were influenced by the Transitional Water mass transported by the South California Current from February to July and by the Subtropical Surface Water from August to January. The nutrient concentrations increased (viz up to 16 μM of nitrate) from March to June, when the upwelling index was the highest. Similarly, the inter-annual variation of chlorophyll-a showed a six-monthly pattern with the highest average monthly concentrations being found in June (5 mg m^-3 in situ or 8 mg m^-3 based on satellite information) and the lowest in December-January. A spatial zoning was also observed in the lagoon with a shallow inner zone that is warmer and richer in chlorophyll-a than the deeper closed mouth area. In the Bahía-Magdalena lagoon a spatial-temporal division into two zones and two seasons was repeated year after year with only minor differences. During the first semester in the outer zone, years 2006 and 2007 were colder and nutrient rich while 2010 was warmer, according to the upwelling conditions in the Southern California Region. Hence, among the coastal lagoons that present a prevailing marine influence, the coastal system of Bahía-Magdalena corresponds to an unusual type of subtropical coastal lagoon where the nutrient input is mainly due to upwelling phenomena.