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

Rafael Cervantes-Duarte,Ricardo Prego,Silverio López-López,Fernando Aguirre-Bahena,Natalia Ospina-Alvarez

doi:10.1016/j.ecss.2013.01.020

Rafael Cervantes-Duarte, Ricardo Prego + Show 3 more

Open Access

https://doi.org/10.1016/j.ecss.2013.01.020

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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 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−3in 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.

Highlights

Continental shelf waters represent 8% of the Earth’s oceanic surface but it is where nearly25% 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
Thermohaline and biogeochemical variables were able to identify an inner zone that is associated with depths lower than 20 m in the subtropical lagoon of Bahía-Magdalena
According to the explanation given by Zaitsev et al (2010) it has to do with the main physical processes affecting the thermohaline structure, such as tidal transport and surface heat in Bahia-Magdalena

Summary

- INTRODUCTION

Continental shelf waters represent 8% of the Earth’s oceanic surface but it is where nearly. In relation to nutrient input and phytoplankton activity (from chlorophyll levels), in the coastal lagoon of Bahía-Magdalena it is hypothesized that, as result of a prevailing marine influence, the fertilization process inside this lagoon is mainly due to coastal upwelling In this way, the objectives are as follows: (i) to identify and characterize the main intra-annual periods highlighting the influence of the water masses that yearly are present in Bahia-Magdalena, in particular, Sub-Arctic Water, Equatorial Subsurface Water and the mixing of both of them, the Transitional Water; and (ii) to describe the inter-annual changes in the lagoon and to explore their relationship with the California Current System. Data were used to analyze the seasonal and interannual variations for a period of seven years: from 2005 to 2011

- MATERIAL AND METHODS

– RESULTS

Findings

- DISCUSSION