Wintertime enrichment of inorganic nutrients in the Ballenas Channel, Gulf of California

Abstract

Three oceanographic cruises were carried out in the region of the Ballenas Channel (BC) during winter 2002, spring 2003, and summer 2004, in order to study the impact of vertical mixing of the water column (ф) on the surface concentration of dissolved inorganic nutrients (NO₃, PO₄, and Si(OH)₄). Mean nutrient concentrations integrated over the upper 100 m of BC showed a clear seasonal variation, with higher values in winter (18.2 ± 1.1 ± µM NO₃, 2.36 ± 0.08 µM PO₄, and 35.4 ± 3.2 µM Si(OH)₄), associated with a well-mixed water column (ф = 0.7–16 J m^–3), and low values in summer (7.1 ± 1.0 µM NO₃, 1.22 ± 0.06 µM PO₄, and 16.0 ± 2.1 µM Si(OH)₄), related to vertically stratified water column conditions (ф = 150–223 J m^–3). In contrast, below 200 m the seasonal variability of the three nutrient concentrations was reduced. The hydrographic analysis indicated that the wintertime surface nutrient enrichment was due to an intensification of the water column mixing, which vertically transported Subtropical Subsurface Water (SSW, cold and nutrient-enriched) to the surface of BC. In contrast, nutrient impoverishment in summertime was due to biological consumption combined with the stratification of the water column, which reduced the vertical flow of SSW and, consequently, the supply of nutrients to the surface of this system. The N:P and Si:N ratios and an excess of PO₄ > 0.65 µM in surface waters indicated that NO₃ was the potential limiting nutrient for primary production in this oceanographic province of the Gulf of California.