Abstract. The
aggregation of plankton species along fjords can be linked to physical
properties and processes such as stratification, turbulence and oxygen
concentration. The goal of this study is to determine how water column
properties and turbulent mixing affect the horizontal and vertical
distributions of macrozooplankton along the only northern Patagonian fjord
known to date, where hypoxic conditions occur in the water column. Acoustic
Doppler current profiler moorings, scientific echo-sounder transects and
in situ plankton abundance measurements were used to study macrozooplankton
assemblages and migration patterns along Puyuhuapi Fjord and Jacaf Channel in
Chilean Patagonia. The dissipation of turbulent kinetic energy was quantified
through vertical microstructure profiles collected throughout time in areas
with high macrozooplankton concentrations. The acoustic records and in situ
macrozooplankton data revealed diel vertical migrations (DVM) of
siphonophores, chaetognaths and euphausiids. In particular, a dense
biological backscattering layer was observed along Puyuhuapi Fjord between
the surface and the top of the hypoxic boundary layer (∼100 m), which
limited the vertical distribution of most macrozooplankton and their DVM,
generating a significant reduction of habitat. Aggregations of
macrozooplankton and fishes were most abundant around a submarine sill in
Jacaf Channel. In this location macrozooplankton were distributed throughout
the water column (0 to ∼200 m), with no evidence of a hypoxic boundary
due to the intense mixing near the sill. In particular, turbulence
measurements taken near the sill indicated high dissipation rates of
turbulent kinetic energy (ε∼10-5 W kg−1) and
vertical diapycnal eddy diffusivity (Kρ∼10-3 m2 s−1). The elevated vertical mixing ensures that the
water column is well oxygenated (3–6 mL L−1, 60 %–80 %
saturation), creating a suitable environment for macrozooplankton and fish
aggregations. Turbulence induced by tidal flow over the sill apparently
enhances the interchange of nutrients and oxygen concentrations with the
surface 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
