The role of the low-salinity, high-nitrate layer as a novel lateral mechanism for nutrient supply into the San Jorge Gulf (Patagonian Shelf) during mid-summer

Abstract

The San Jorge Gulf (Patagonian Shelf) is productive in the surface layer mainly in three regions during summer associated with small-scale coastal fronts controlled by interaction with islands to the NE, with coastal upwelling to the SW due to the wind, and with the dynamics of the Southern Tidal Front at the gulf mouth. In early February 2014, a time series of the water column characteristics were performed with a Conductivity-Temperature-Depth (CTD)/rosette package and a vessel hull-mounted acoustic Doppler current profiler (ADCP)over two complete semidiurnal cycles in a quasi-fixed location of the San Jorge Gulf mouth in order to evaluate interaction mechanisms between the surface mixed layer and interior layers that condition the distribution of properties. The pycnocline thickness defined by 24.90 kg m−3 < 𝜎0 < 25.50 kg m−3 is driven by the semidiurnal tide, with a mean modulation of 32.7 m amplitude during a 2.75 m sea-level oscillation (R2 = 0.75). Patchy cores of minimum salinity (𝑆𝑃 ∼ 33.10) with high nitrate concentrations (9.9 mmol m−3, on average) were found at the pycnocline centre (38.0 m ± 4.5 m) during the flood as pulses of sub-surface water betweenconsecutive high tides. Also, a high correlation (R2 = 0.88) was found between the thickness of the low-salinity layer (𝑆𝑃 < 33.20) and the pycnocline thickness. A clear mode-1 internal tide of semidiurnal frequency was identified during the time series with some non-linear features in the first tidal cycle. As a consequence, a difference in the reversal of the internal tide was seen during both tidal cycles. The water column decreases its stratification when the low-salinity water enters the gulf due to a widening of the pycnocline thickness. At the same time, moderately high nitrate concentrations are observed in the surface mixed layer, suggesting a novel lateral mechanism to maintain the surface chlorophyll layer observed during mid-summer.