Land–ocean gradient in haline stratification and its effects on plankton dynamics and trophic carbon fluxes in Chilean Patagonian fjords (47–50°S)

Patagonian fjord systems, and in particular the fjords and channels associated with the Baker/Pascua Rivers, are currently under conspicuous natural and anthropogenic perturbations. These systems display very high variability, where limnetic and oceanic features overlap generating strong vertical and horizontal physicochemical gradients. The CIMAR 14-Fiordos cruise was conducted in the Chilean fjords located between 47° and 50°S during the spring (October–November) of 2008. The main objectives were to study vertical and horizontal gradients in physical, chemical and biological characteristics of the water column, and to assess plankton dynamics and trophic carbon fluxes in the fjords and channels of central-south Patagonia.The water column was strongly stratified, with a pycnocline at ca. 20 m depth separating a surface layer of silicic acid-rich freshwater discharged by rivers, from the underlying nitrate- and orthophosphate-rich Subantarctic waters. The outflows from the Baker and Pascua Rivers, which range annually between 500 and 1500 m3 s−1, generate the strong land–ocean gradient in salinity (1–32 psu) and inorganic nutrient concentrations (2–8 and 2–24 μM in nitrate and silicic-acid, respectively) we observed along the Baker Fjord. The POC:chl-a ratio fluctuated from 1087 near the fjord’s head to 175 at its oceanic end in the Penas Gulf. This change was mainly due to an increase in diatom dominance and a concurrent decrease in allochthonous POC towards the ocean.Depth-integrated net primary production (NPP) and bacterial secondary production (BSP) fluctuated between 49 and 1215 and 36 and 150 mg C m−2 d−1, respectively, with higher rates in oceanic waters. At a time series station located close to the Baker River mouth, the average NPP was lower (average 360 mg C m−2 d−1) than at more oceanic stations (average 1063 mg C m−2 d−1), and numerically dominated (45%) by the picoplankton (<2 μm) and nanoplankton (2–20 μm) size fractions. The high average vertical carbon flux (234 mg m−2 d−1) and high export production (65% of the NPP) support the idea that Patagonian fjords may behave as a net sink for CO2 during the productive (spring) season. Trophic fluxes near the head of the fjords, with oligotrophic low-salinity waters, were dominated by heterotrophic nanoflagellates (HNF) and small copepods (52 mg C m−2 d−1, each), suggesting that the microbial food web is the main trophic pathway in these environments.