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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.
Highlights
The Ballenas Channel (BC), located in the Midriff Region of the Gulf of California (GC), is considered a unique oceanographic province because of its distinct dynamics and physicochemical characteristics that are different from the rest of the gulf (Lavín et al 1997, Lavín and Marinone 2003)
We present evidence that the temporal variability of nutrient concentrations in the surface layer of BC, on an annual scale, is the result of seasonal variations in water column mixing, variations in the vertical flow of Subtropical Subsurface Water, and the biological activity that occurs in this layer
The 35.0 isohaline indicated that Gulf of California Water (GCW: S ≥ 35 and T > 12 oC) occupied the surface layer and that Subtropical Subsurface Water (SSW: 34.5 < S < 35.0 and 9.0 oC ≤ T < 18.0 oC) lay below this layer and filled the deep basin
Summary
The Ballenas Channel (BC), located in the Midriff Region of the Gulf of California (GC), is considered a unique oceanographic province because of its distinct dynamics and physicochemical characteristics that are different from the rest of the gulf (Lavín et al 1997, Lavín and Marinone 2003). On the other hand, Marinone (2008) used a three-dimensional numerical model to simulate deep circulation around Ángel de la Guarda Island, including BC, and reproduced the deep water flows that enter and converge in BC and diverge at the surface, numerically corroborating the conceptual model proposed by López et al (2006). Both approaches suggest that vertical input of subsurface water may be responsible for the low sea surface temperature in BC throughout the year
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