Silicic acid enrichment of subantarctic surface water from continental inputs along the Patagonian archipelago interior sea (41–56°S)

We estimated Si∗, the surplus or deficit of orthosilicic acid (DSi) relative to nitrate available for diatom growth, in the Chilean Patagonian Archipelago Interior Sea (PAIS). Si∗ and salinity were negatively correlated in the PAIS because of the mixing of high nitrate, low DSi subantarctic surface water and high DSi, low nitrate continental freshwater runoff. Both the slope and the intercept of this relationship decreased from northern to southern Patagonia, which was likely a consequence of reduced DSi inputs from several overlapping hydrological, biological and geological drivers along this gradient. In general, lower freshwater DSi concentrations were expected below 46°S, and a lower total DSi load was expected from reduced runoff below 51°S. The north–south decreasing DSi concentration trend may be linked to dilutions from a higher proportion of runoff in latitudes with higher precipitation rates (45–53°S), the transition to more resistant granitic rocks and glacial melt-water from the Northern and Southern Patagonia Ice Fields (46–51°S) and a reduced density of volcanoes active during the Holocene (48–56°S). The intensification of a southward DSi deficit may be a forcing factor involved in the reported southward reductions in plankton biomass and a more frequent occurrence of non-diatom blooms in southern PAIS.