Interannual variability in the trophic status and water quality of Bahía Magdalena, Mexico, during the 2015–2018 period: TRIX

The Trophic Index (TRIX) was computed monthly, from January 2015 to December 2018, for ten sites in Bahia Magdalena, Baja California Sur. Hydrobiological variables showed interannual and seasonal variations in the shallow and deep zones of the lagoon. The chlorophyll-a (Chla) was the variable with fewest significant correlations (P < 0.05) during the study period. A significant decrease in TRIX was observed 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 off the coast of North America) in the region. TRIX values in the winter–spring (January to June) were generally higher than those in summer-autumn (July–December) due to the prevailing NW winds that cause coastal upwelling and shallow water mixing, making nutrients available in the lagoon. There was a marked stratification of the water column in the second half of the year that made the surface layer nutrient-poor. We found a high and significant correlation (r=0.66, P < 0.05) between TRIX values and water relative density in the deep zone of the lagoon, denoting the effect of subsurface oceanic water on nutrient concentrations. At the local scale, TRIX values in the shallow zone were generally lower than in the deep zone, but these differences reached statistical significance only in 2018. The monthly average TRIX (5.9 ± 0.2) denotes a high trophic level and mediocre water quality. Although TRIX values might have some shortcomings, these results can be used as an early warning for the relevant stakeholders and authorities.