Variability of hydrographic factors, biomass and structure of the phytoplankton community at the entrance to the Gulf of California (spring 2013)

Abstract

This study reports observations of the horizontal and vertical variation of chlorophyll a (Chla) and the taxonomic composition of phytoplankton at the entrance to the Gulf of California (GC) and its relationship with physicochemical processes. This region is part of the northern boundary of one of the largest shallow Oxygen Minimum Zones in the world, characterized by marked physicochemical gradients. Therefore, understanding the prevailing seasonal patterns and their relationship with primary producers is considered particularly relevant. The study period, April 2013, was characterized by mesoscale eddies and fronts associated with several water masses (California Current Water - CCW, Gulf of California Water - GCW, and Subtropical Subsurface Water - StSsW), which influenced the horizontal and vertical distribution of Chla between 0 and 150 m. Superficial southward flow of CCW was observed near the west coast of Baja California, turning toward the GC flowing between 30 and 60 m depth. At the entrance to the GC, the currents circulated anticyclonically, flowing into the GC near Baja California, while GCW flowed out of the GC near the mainland coast. The vertical distribution of Chla featured a subsurface maximum layer (SCML) associated with the 25 kg m-3 isopycnal, highlighting the importance of stratification processes as drivers of the vertical distribution of phytoplankton biomass. In turn, this favored different associations of phytoplankton groups. In surface waters, the phytoplankton community was dominated by picocyanobacteria (Synechococcus and Prochlorococcus) and Prymnesiophytes, while at the SCML, Prymnesiophytes coexist mainly with Prasinophytes, Chrysophytes, and Cryptophytes. The prevalence of these flagellates (and the absence of autotrophic dinoflagellates) suggests the importance of a multivorous food web in this region, typical of environments where carbon recycling predominates in the pelagic zone. Diatoms made no significant contribution to Chla, although their abundance increased in the SCML associated with the influence of the CCW. The low N:P (8.3) and Si:N (0.86) ratios indicate that nitrates and silica are potentially limiting nutrients for the development of this group.