The southern coast of Chile, also known as the Chilean Patagonia (41 to 55° S), constitutes one of the most extensive fjord and channel systems in the world (Figure 1 ). This system has a rugged bathymetry, dissected coastline, and strong but highly variable water column stratification. Marked salinity gradients exhibit seasonal and latitudinal patterns determined by heavy riverine inflow from ice melting in late spring (November–December) and persistent rainfall with an average of 2700 mm y−1 and up to 5000 mm in exceptional years [72 (link),73 (link)]. Summer heating breaks the winter–spring thermal inversion and generates stronger thermohaline gradients [59 (link)].
Circulation in the Patagonian fjords is of two-layered estuarine-type, with a variable (5–10 m) estuarine surface water (EW) and a more uniform saltier lower layer, the Subantarctic Water (SAAW, S > 33) reaching 150 m depth [74 ]. Mixing of the two layers at the interface generates Modified Subantarctic Water (MSAAW, S = 31–33) [59 (link)]. Depending on freshwater inputs, different water masses can be identified within the estuarine surface water: Estuarine Fresh Water (EFW, S = 11–21), Estuarine Saline Water (ESW, S = 21–31). When salinity is less than 11, the water is classified as Fresh Water (FW) [52 (link)]. The Pitipalena (PIT) (~ 43°S), and Puyuhuapi (PUY) (~ 44° S) fjords, in the Aysén region, form part of this great fjord system. PUY (100 km) is much longer than PIT (22 km), the latter being more semi-enclosed [59 (link),60 (link),75 (link)]. Unlike PIT fjord, PUY has two connections with oceanic waters, one through the Moraleda Channel in the mouth and another through the Jacaf Channel close to the head [59 (link)]. In the two fjords, the main freshwater inputs come from riverine inflows and rainfall. The main river flowing into PUY (Cisnes, average river discharge 218 m3s−1) has its mouth located by the middle reaches of this fjord [75 (link),76 (link)]. In contrast, in PIT, the Palena river, with a four-fold average river discharge (800 m3s−1), is located at the mouth [60 (link)]. These characteristics affect hydrodynamic conditions, including stratification and water residence time, which is maximal in PUY (~250 days) compared to PIT fjord (~200 days) [77 ,78 ], directly promoting phytoplankton retention and HAB development.
Circulation in the Patagonian fjords is of two-layered estuarine-type, with a variable (5–10 m) estuarine surface water (EW) and a more uniform saltier lower layer, the Subantarctic Water (SAAW, S > 33) reaching 150 m depth [74 ]. Mixing of the two layers at the interface generates Modified Subantarctic Water (MSAAW, S = 31–33) [59 (link)]. Depending on freshwater inputs, different water masses can be identified within the estuarine surface water: Estuarine Fresh Water (EFW, S = 11–21), Estuarine Saline Water (ESW, S = 21–31). When salinity is less than 11, the water is classified as Fresh Water (FW) [52 (link)]. The Pitipalena (PIT) (~ 43°S), and Puyuhuapi (PUY) (~ 44° S) fjords, in the Aysén region, form part of this great fjord system. PUY (100 km) is much longer than PIT (22 km), the latter being more semi-enclosed [59 (link),60 (link),75 (link)]. Unlike PIT fjord, PUY has two connections with oceanic waters, one through the Moraleda Channel in the mouth and another through the Jacaf Channel close to the head [59 (link)]. In the two fjords, the main freshwater inputs come from riverine inflows and rainfall. The main river flowing into PUY (Cisnes, average river discharge 218 m3s−1) has its mouth located by the middle reaches of this fjord [75 (link),76 (link)]. In contrast, in PIT, the Palena river, with a four-fold average river discharge (800 m3s−1), is located at the mouth [60 (link)]. These characteristics affect hydrodynamic conditions, including stratification and water residence time, which is maximal in PUY (~250 days) compared to PIT fjord (~200 days) [77 ,78 ], directly promoting phytoplankton retention and HAB development.
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