A freshwater lid on a saltwater world
The Aysén Fjord is a classic two-layered estuary, a permanent feature of Patagonian fjord systems. Massive volumes of freshwater, rich in glacial sediment, pour into the fjord from rivers like the Aysén and Baker. The Baker River alone has a mean annual discharge of 1,133 cubic meters per second. This cold, fresh water is less dense than the salty Subantarctic Water from the Pacific Ocean, so it spreads out to form a distinct surface layer.
This buoyant brackish layer can be 5 to 20 meters deep. The boundary between the fresh top and salty bottom is a sharp density gradient called a pycnocline, which is a physical barrier. It dramatically slows the mixing of the two water masses, creating long residence times for the deep water, sometimes exceeding 500 days. This stratification is so pronounced that salinity can jump from near zero at the surface to over 31 practical salinity units (psu) in the deep water. While the surface layer is rich in silicates from glacial melt, the deep oceanic water contains high concentrations of nitrates and phosphates. The freshwater lid effectively traps these important nutrients in the dark, deeper parts of the fjord, isolating them from the sunlit surface where phytoplankton would normally thrive.
An ecosystem out of balance
The natural stratification of the Aysén Fjord creates a unique but delicate ecosystem. The low-salinity surface layer and the nutrient-rich deep layer support different biological communities. This balance is profoundly disrupted by intensive salmon aquaculture. Chile is the world's second-largest producer of farmed salmon, and much of this activity is concentrated in the Aysén Region.
The thousands of tons of uneaten feed and fish waste sink below the net pens, adding large amounts of organic matter and nutrients—primarily nitrogen and phosphorus—into the deep, slow-moving water layer. The decomposition of this waste by bacteria consumes large quantities of dissolved oxygen, leading to hypoxic (low-oxygen) and sometimes anoxic (no-oxygen) conditions on the seafloor. These "dead zones" can cause a significant loss of benthic biodiversity under the farms.
This nutrient pollution also fuels harmful algal blooms (HABs). In the austral summer of 2016, a massive bloom of the microalga Pseudochattonella verruculosa devastated the region's aquaculture. The event, linked to unusually warm and stratified water conditions, caused the death of an estimated 27 million salmon and trout, resulting in economic losses of around $800 million. The bloom suffocated fish by damaging their gills and potentially depleting oxygen.