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. Theaggregation of plankton species along fjords can be linked to physicalproperties and processes such as stratification, turbulence and oxygenconcentration. The goal of this study is to determine how water columnproperties and turbulent mixing affect the horizontal and verticaldistributions of macrozooplankton along the only northern Patagonian fjordknown to date, where hypoxic conditions occur in the water column. AcousticDoppler current profiler moorings, scientific echo-sounder transects andin situ plankton abundance measurements were used to study macrozooplanktonassemblages and migration patterns along Puyuhuapi Fjord and Jacaf Channel inChilean Patagonia. The dissipation of turbulent kinetic energy was quantifiedthrough vertical microstructure profiles collected throughout time in areaswith high macrozooplankton concentrations. The acoustic records and in situmacrozooplankton data revealed diel vertical migrations (DVM) ofsiphonophores, chaetognaths and euphausiids. In particular, a densebiological backscattering layer was observed along Puyuhuapi Fjord betweenthe surface and the top of the hypoxic boundary layer ( ∼ 100 m), whichlimited the vertical distribution of most macrozooplankton and their DVM,generating a significant reduction of habitat. Aggregations ofmacrozooplankton and fishes were most abundant around a submarine sill inJacaf Channel. In this location macrozooplankton were distributed throughoutthe water column (0 to ∼200  m), with no evidence of a hypoxic boundarydue to the intense mixing near the sill. In particular, turbulencemeasurements taken near the sill indicated high dissipation rates ofturbulent kinetic energy ( e ∼ 10 - 5  W kg −1 ) andvertical diapycnal eddy diffusivity ( K ρ ∼ 10 - 3  m 2  s −1 ). The elevated vertical mixing ensures that thewater column is well oxygenated (3–6 mL L −1 , 60 %–80 %saturation), creating a suitable environment for macrozooplankton and fishaggregations. Turbulence induced by tidal flow over the sill apparentlyenhances the interchange of nutrients and oxygen concentrations with thesurface layer, creating a productive environment for many marine species,where the prey–predator relationship might be favored.

Turbulence and hypoxia contribute to dense biological scattering layers in a Patagonian fjord system