Interplay between freshwater discharge and oceanic waters modulates phytoplankton size-structure in fjords and channel systems of the Chilean Patagonia

Abstract

Here, we participated in five independent research cruises that spanned almost the entire Patagonian fjords region (from 41.5 to 56.0°S) in order to determine the importance of the physical/chemical factors that influence phytoplankton size structure triggered by freshwater discharge and oceanic water intrusion. Throughout the Patagonian region phytoplankton biomass varies in association with freshwater discharge and mineral nutrient load, and to a lesser extent with surface solar radiation and photosynthesis. These correlations and the spatial domains changed depending on which size fraction was analyzed. Fresh water discharge negatively correlated with salinity and density, positively correlated with stratification and silicic acid concentration, and represents the primary influence on phytoplankton populations. More than 40% of the sites characterized as discharge-intensive locations exhibited lower total chlorophyll-a concentrations (chl-a) and phytoplankton size-structure that was dominated by small cells (<20 µm). Oceanic nutrients (nitrate and phosphate input) are the second-most important factor that control total chl-a, favoring total chlorophyll-a concentration in the southern half of the Patagonian region. Microphytoplankton contribute to more than 75% of the total community in high productivity waters with chl-a concentrations higher than 2 µg L−1 and picophytoplankton dominate when chl-a is lower than 1 µg L−1. Thus, in this extensive area, the relative success of different phytoplankton size classes may be sensitive to changes in hydrological cycles, continental runoffs, and potential anthropogenic eutrophication, modifying important ecological processes and the fate of organic matter.