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Bahia Magdalena (BM), known for its high primary productivity, is one of the largest tidal lagoons on theMexican Pacific coast of the Baja California Peninsula. BM is located in an area of active coastal upwellingand significant tides with a maximum range of about 2.4 m. Dissolved inorganic nutrients upwelled fromthe depths are transported by tidal water exchange into the lagoon, contributing to its fertilisation. Toestimate the magnitude and mechanisms driving the tidal exchange of water and nutrients, field observationsof the nutrient content were made in the inlet area and on the adjacent shelf during March2003, December 2004 and June, August and November 2005. In March 2003, the research vessel El Pumacarried out a complete hydrological study of the area using Seabird-19 CTD profiler. At the same time, acurrent meter with a tide gauge was installed in the BM inlet, and multiple measurements of currentswere made on a section across the inlet with a SonTek hull-mounted Acoustic Doppler Current Profiler(ADCP). Field studies were complemented by numerical experiments with the hydrodynamic modelECOM 3D. Analysis of the currents in the inlet area, deriving from both the ADCP data and the numericalsimulation, indicates that the water volume transported during a semidiurnal tidal cycle through theinlet varied from 0.3 km³ for neap tide to 0.82 km³ for spring tide. Net nitrate mass intakes to the lagoonderiving from currents in the mouth can be estimated as 7.0 x
10³ kg for neap tides and 20.0 x
10³ kg forspring tides, and the maximum phosphate contribution was estimated at 2.5
10³ kg and 8.5
x 10³ kg,respectively. Taking into account that fluvial contribution in the lagoon is practically absent, unexpandedmangroves are distributed mainly at its northern part, and organic sediment decomposition is potentiallyevaluated as low, we can thus conclude that the coupled effect of upwelling and tidal currents play animportant role in fertilising the BM. Naturally, this mechanism works only in periods of upwelling activity,namely, from March to June, and to a lesser extent from September to October.

Palabras clave: Upwelling; Tidal currents; nutrients; Tidal lagoon; Bahía Magdalena

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