Abstract. 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 halfyearly 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 DecemberJanuary. 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.Keywords: inter-annual, intra-annual, biogeochemistry, California Current, chocked lagoon, Bahía-Magdalena, Pacific coast, Mexico. -INTRODUCTIONContinental shelf waters represent 8% of the Earth's oceanic surface but it is where nearly 25% of the primary sea production occurs (Walsh, 1989) and where a variety of types of coastal systems, which determine the exchange between land and sea, are found. In these systems, coastal lagoons, making up 13% of the coastal areas worldwide, are predominant (Kjerve 1994). Coastal lagoons are inland water bodies, usually oriented parallel to the coast, separated from the sea by a barrier and connected to the ocean by one or more restricted inlets (Kjerfve and Magill 1989). In the subtropical climate region bordering, as delimited by Perillo et al. (1999), the Tropics of Cancer and Capricorn (23.5° latitude) and the temperate region (40° latitude), coastal lagoons are subject to a wide climatic diversity ranging from monsoons to desert-like conditions. In all of these climate types the availability of nutrients restricts biological richness (Salomons et al., 2005). Despite this fact, subtropical coastal lagoons have not been studied extensively from a biogeochemical viewpoint, rather it is the ecological aspects that have undergone a more indepth examination (Kjerfve, 1994).The wide variety of subtropical coastal lagoons have been classified according to the type of entrance channel into choked, restricted or leaky lago...

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Relevancia de fuentes nitrogenadas nuevas y regeneradas en la columna de agua en Bahía Magdalena (SO Península de Baja California), MéxicoAbstract.-Six bimonthly surveys during 2007 were carried out in Magdalena Bay (Mexico) during spring tide conditions. Spatiotemporal variations of temperature, salinity, nitrite, nitrate, ammonium, phosphate and chlorophyll-a were analyzed in the Bay. During spring-early summer (April-June) seawaters were colder and richer in new nutrients and chlorophyll-a than autumn and winter (February and October-December). The coastal upwelling during spring-early summer contributes with new nutrients to the Bay, whereas bottom-recycled nutrients should be the main input during the autumn-winter period. Thus, nutrients availability, dissolved inorganic nitrogen and phosphate, promote a seasonal pattern of phytoplankton biomass in Magdalena Bay.

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