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The Trophic Index (TRIX) was computed monthly, from January 2015 to December 2018, for ten sitesin Bahia Magdalena, Baja California Sur. Hydrobiological variables showed interannual and seasonalvariations in the shallow and deep zones of the lagoon. The chlorophyll-a (Chla) was the variable withfewest significant correlations (P < 0.05) during the study period. A significant decrease in TRIX wasobserved in 2015, compared to all other years, likely related to the effects of warming events such as‘‘El Niño’’ and ‘‘The Blob’’ (defined as a large mass of relatively warm water in the Pacific Ocean offthe coast of North America) in the region. TRIX values in the winter–spring (January to June) weregenerally higher than those in summer-autumn (July–December) due to the prevailing NW winds thatcause coastal upwelling and shallow water mixing, making nutrients available in the lagoon. There wasa marked stratification of the water column in the second half of the year that made the surface layernutrient-poor. We found a high and significant correlation (r = 0.66, P < 0.05) between TRIX valuesand water relative density in the deep zone of the lagoon, denoting the effect of subsurface oceanicwater on nutrient concentrations. At the local scale, TRIX values in the shallow zone were generallylower than in the deep zone, but these differences reached statistical significance only in 2018. Themonthly average TRIX (5.9 ± 0.2) denotes a high trophic level and mediocre water quality. AlthoughTRIX values might have some shortcomings, these results can be used as an early warning for therelevant stakeholders and authorities

Palabras clave: Bahia Magdalena coastal lagoon; Ramsar site; eutrophication; nutrients; Chlorophyll a

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