Hydrographic surveys were conducted during May and November 2013 and January and August 2014 in Puyuhuapi Fjord and Jacaf Channel (Fig. 2, Table 1). These profiles were obtained with a SeaBird 25 CTDO, sampling at 8 Hz with a descent rate of ∼1 m s-1. The data collected, whose nominal vertical resolution was ∼12 cm, were averaged into 1 m bins, following SeaBird recommendations. The conservative temperature (∘C) and absolute salinity (g kg-1) were calculated according to the Thermodynamic Equation of Seawater 2010 (IOC et al., 2010). Additionally, nitrate samples were taken using a Niskin bottle at various depths and analyzed spectrophotometrically following the methods of Strickland and Parsons (1968). To validate CTDO measurements, in situ oxygen samples were analyzed using the Winkler method (Strickland and Parsons, 1968), carried out using a Metrohm burette (Dosimat plus 865) and an automatic visual end-point detection (AULOX Measurement System).

Microstructure measurements were collected using a vertical microstructure profiler (VMP-250, Rockland Scientific, Inc.). The VMP-250 is equipped with two airfoil shear probes and two fast-response FP07 thermistors, which allowed for data recording at 512 Hz with a descending free fall speed of ∼0.7 m s-1. The micro-shear measurements permitted a direct measurement of the dissipation rate of turbulent kinetic energy (ε) for isotropic turbulence, according to Lueck et al. (2002), Eq. (1), ε=7.5ν∂u′∂z2‾, where ν is the kinematic viscosity, u is the horizontal velocity, z is the vertical coordinate axis and therefore ∂u′∂z2‾ is the shear variance.

Using the values of ε, the diapycnal eddy diffusivity (Kρ) was calculated. The most used formulation was proposed by Osborn (1980), Kρ=ΓεN2, where Γ is the mixing efficiency, generally set to 0.2 (Thorpe, 2005), and N is the buoyancy frequency. Shih et al. (2005) noted that when the ratio ε/νN2 is greater than 100, Eq. (2) results in an overestimation. Therefore, they proposed a new parameterization for this case given by

Kρ=2ν(ενN2)1/2.

More recently, Cuypers et al. (2012) used Eq. (2) when ε/νN2>100, Eq. (2) when 7<ε/νN2<100, and considered null eddy diffusivity when ε/νN2<7. This approach was followed in this study. The correlation between the dissipation rate of turbulent kinetic energy and the abundance of major zooplankton groups throughout the water column was accomplished by using a quadratic polynomial curve fit between these data sets (explained in detail in Sect. 4.6). These analyses were only applied to measurements collected at the fixed station in Puyuhuapi Fjord, because the VMP-250 was not available during the measurement campaign in Jacaf Channel.

Three types of acoustic data were collected: ADCP, single-frequency echo-sounder and dual-frequency echo-sounder data. ADCP measurements were obtained with two 307.7 kHz Teledyne RDI Workhorse ADCPs, moored upwards at depths of ∼50 m (ADCP-1) and ∼100 m (ADCP-2), both moored at the same location in north-central Puyuhuapi Fjord but during different time periods (Table 1, Fig. 1). Data were collected hourly with a vertical bin size of 1 m, over periods of austral autumn (ADCP-1: May 2013) and spring–summer (ADCP-2: January 2014). During the final ADCP-2 mooring deployment, single-frequency data were also collected along the Puyuhuapi Fjord using a SIMRAD EK60 scientific echo sounder, running a 38 kHz transducer (ES38B), during daytime and nighttime hours, from 22 to 25 January 2014 (black line in Fig. 1). These ADCP and single-frequency echo-sounder measurements were complemented by in situ zooplankton sampling (see Sect. 3.3 for details) carried out on 23–24 January 2014, at a fixed station close to the ADCP mooring location, over a period of 36 h (Fig. 1).

A second scientific campaign was conducted on 17 and 19 August 2014, which included a dual-frequency echo-sounder survey and a third ADCP mooring (ADCP-3) located in Jacaf Channel. This time, the echo-sounder survey coverage was extended to eastern Jacaf Channel (Fig. 1, red line) and a second 120 kHz transducer (ES120-7C) was added to the 38 kHz transducer used in the first survey. Several day/night transects were completed across Puyuhuapi Fjord and Jacaf Channel, with special attention paid to Jacaf sill (only the most representative echograms area shown in Figs. 5, 7 and 8). To determine the statistical relationship (R2) between acoustic data from the 38 kHz echo sounder with hydrographic properties of the fjords (temperature, salinity and dissolved oxygen), a quadratic polynomial curve was also applied between these data sets. During this survey, two RDI Workhorse ADCP with 614.4 kHz frequency (referenced hereafter as ADCP-3) and was moored at ∼30 m depth in the vicinity of the Jacaf sill. The near-surface placement of ADCP-3 allowed for near-surface currents to be adequately quantified.

Vessel speed during all echo-sounder surveys was maintained between 8 and 10 knots. Echo sounders were operated using a variable ping rate of 0.3–2.0 pings per second, a pulse duration of 1.024 ms and output powers of 2 and 0.5 kW for the 38 and 120 kHz frequencies, respectively. Calibration was made using copper spheres and standard procedures (Foote et al., 1987).

In situ mesozooplankton samples were collected with a WP2 net (60 cm diameter mouth opening, 300 µm mesh, flowmeter mounted in the net frame) towed vertically from 50 m to the surface in May 2013, and with a Tucker Trawl (1 m2 mouth opening, 300 µm mesh with flowmeter) used to obtain stratified oblique tows in January and August 2014 (Table 1). All samples were preserved in a 5 % formaldehyde solution. Zooplankton abundances were standardized to individuals per m3 of filtered seawater. WP2 vertical tows consisted of five depth intervals from surface to 50 m, every 10 m (0–10, 10–20, 20–30, 30–40, 40–50 m).

Stratified Tucker tows considered four depth strata: 0–10, 10–20, 20–50 and 50–100 m in the Puyuhuapi Fjord. In Jacaf Channel, the stratified sampling included five depth strata: 0–10, 10–20, 20–50, 50–100 and 100–150 m. The hauling speed for both nets was between 2–3 knots. Sampling occurred during a 36 h period every 3 h from 22 to 24 January 2014 (Puyuhuapi Fjord) and every 5–6 h from 18 to 19 August 2016 (Jacaf Channel) (Fig. 1, red dots). At all sites and dates, zooplankton species were identified, sorted into functional groups, measured (length) and classified into size classes using a 5 mm length threshold. To determine the correlation (R2) between the Sv records from the 38 kHz transducer and the major macrozooplankton groups (Siphonophores, Chaetognaths and Euphausiids), a quadratic polynomial curve was also applied between these data sets (further details in Sect. 4.3).

The tidal constituents were computed using HOBO U20 water level loggers and the pressure sensor from ADCP-3 (Tables 1–2, Fig. 1). A tidal harmonic analysis was applied to the sea level time series according to Pawlowicz et al. (2002), which considers the algorithms of Godin (1972) and Foreman (1977, 1978). We classified tides by the dominant period of the observed tide based on the form factor (F), defined by the ratio between the sum of the amplitudes of the two main diurnal constituents (principal lunar declinational, O1 and luni-solar declinational, K1) and the sum of the amplitudes of the two main semidiurnal constituents (principal lunar, M2, and principal solar, S2), F=(O1+K1)/(M2+S2) (Bearman, 1989; where F<0.25 semidiurnal, 0.25<F<1.5 mixed semidiurnal and F>3.0 diurnal).

Temperature profiles collected in Puyuhuapi Fjord and Jacaf Channel showed a similar structure during the winter and summer campaigns (Fig. 2a, b). The largest temperature gradients were found between the surface and ∼70 m depth, ranging from 8.5 to 17 ∘C. A thin, fresh layer (salinity values varied from 11 to 29 g kg-1) was found in the first ∼10 m of the water column below which salinity varied little (29 to ∼34.2 g kg-1), as a result of the presence of Modified Sub-Antarctic Water (MSAAW, salinity between 31 and 33 g kg-1), the Sub-Antarctic Water (SAAW, salinity between 33 and 33.8 g kg-1) and the Equatorial Subsurface Water (ESSW, salinity >33.8 g kg-1) (Fig. 2c, d). Hypoxic conditions (dissolved oxygen below 2 mL L-1 and ∼30 % saturation) were detected in Puyuhuapi Fjord below 100 m depth, with oxygen concentrations between 1 and 2 mL L-1 (Fig. 2e). Deep water in Jacaf Channel was more ventilated, with dissolved oxygen values above hypoxic conditions throughout the water column (Fig. 2f). The hypoxic layer was located over the depth range of the ESSW and oxygen-rich water (3–6 mL L-1) was observed at depths occupied by MSAAW and SAAW. Below 10 m depth, high nitrate concentrations were measured in Puyuhuapi Fjord, but concentrations in the winter (August, 2014) were higher than in fall (May, 2013) and summer (January, 2014) (Fig. 2g). Along with the in situ hydrographic sampling, in situ zooplankton samples were collected and will now be discussed.

Volume backscatter (Sv) from ADCP-1 (50 m depth, May 2013) showed large variability, ranging from high (-90 to -75 dB re 1 m-1) to low (-115 to -100 dB re 1 m-1) (Fig. 3a). The highest Sv values (>-90 dB re 1 m-1) were recorded during the night hours (∼ 18:00 to ∼ 07:00 LT, local time; with all remaining times for in situ sampling expressed in local time), while minimum Sv values were observed in the daytime (∼ 07:00 to ∼ 18:00) suggesting that vertically migrating organisms from deeper waters (below ADCP-1 mooring depth of 50 m) migrate upwards during nighttime hours. From the in situ measurements of macrozooplankton collected at various depth strata in May 2013, the most abundant groups were siphonophores, chaetognaths and medusae (Fig. 3c–f). A marked change in vertical distribution and in total abundance of the macrozooplankton groups in the water column was observed from the first sampling hour (Fig. 3c) to the night sampling time (∼18:00 h), revealing the start of the nocturnal migration to the surface (Fig. 3d) coincident with a DVM pattern as seen in the ADCP-1 backscatter data (Fig. 3a, b).

Data from the ADCP-2 mooring (positioned deeper but at the same location as ADCP-1) from 22 to 24 January 2014 also showed a strong macrozooplankton DVM pattern, which extended down to ∼100 m depth (Fig. 4a). During daylight hours (8:00–18:00), dense aggregations were observed between 80 and 100 m depth, which started to ascend from 18:00 to 21:00, concentrated close to the surface at night, and began to descend at ∼06:00. In situ stratified sampling showed the most abundant macrozooplankton groups were euphausiids, siphonophores, chaetognaths, decapods and medusae (Fig. 4b–f). Euphausiids and siphonophores showed higher abundance close to a surface layer (10–20 m) during night hours (Fig. 4c, f) and at deeper layers during the daytime (Fig. 4d, e). However, euphausiids showed the clearest diel vertical migration with maximum abundance between 10 and 20 m layer during night hours, and at ∼100 m depth during the daytime (Fig. 4c–f). The in situ zooplankton samples were complemented by echo-sounder measurements collected along the fjord systems during the summertime and the wintertime. These measurements will now be discussed.

To examine relationships between the distribution of biological scattering and water column properties, Sv values quantified from the 38 kHz acoustic profiler were matched to the consecutive time at which CTD and DO data were captured. This was done in Puyuhuapi Channel and Jacaf Channel during the summer and winter seasons, respectively. The relationship between water temperature and Sv was weak during summer (R2=0.30) and winter (R2=0.41), with maximum Sv values occurring between 8 and 10 ∘C. A weak relationship was found between Sv and salinity in Puyuhuapi Fjord (R2=0.29) and Jacaf Channel (R2=0.35), with higher Sv values found in the MSAAW and SAAW water masses (salinity >31 g kg-1). Both in Puyuhuapi Fjord and Jacaf Channel, Sv with DO and oxygen saturation showed the highest R2 values (R2∼0.6). Hence, only 20.4 % of total Sv>-110 dB re 1 m-1 were in the hypoxic layer of Puyuhuapi Fjord, while just 1.2 % were in the hypoxic layer in Jacaf Channel. Now the turbulent kinetic energy dissipation will be discussed to relate macrozooplankton assemblages to vertical mixing in the water column.

The harmonic analysis carried out with the sea level time series obtained in Puyuhuapi Fjord and Jacaf Channel, denoted the dominance (in terms of amplitude) of the semidiurnal constituents (M2 and S2; Table 2). Diurnal constituents (O1 and K1) were also important, specifically at the Jacaf ADCP-3 station located close to the Jacaf sill region (Table 2 and Fig. 1). The contribution of diurnal constituents added the mixed character to the tidal regime in the study area. The spectral analysis implemented at all sea-level stations showed maximum energy in the semidiurnal band (Table 2), with the highest spectral energy (57.29 m2 cph-1, where cph is cycles per hour) at Jacaf sill (Jacaf ADCP-3 station), which could be due to the extreme convergence of the channel at this location accelerating the tidal flows.

Turbulence measurements collected with the VMP-250 microstructure profiler showed high dissipation rates of turbulent kinetic energy (ε) in the upper 20 m of the water column in Puyuhuapi Fjord and Jacaf Channel (Fig. 10). In this layer, ε ranged from 10-7 to 10-5 W kg-1. However, below this surface layer (<20 m depth) the highest values were obtained around Jacaf sill (ε=1.2×10-7 W kg-1), as shown on 21 November 2013 at 140 m depth (Fig. 10a). In Puyuhuapi Fjord turbulent kinetic energy dissipation between 20 and 180 m was weak (10-10 to 10-7 W kg-1) (Fig. 10c, e). The dissipation rates of turbulent kinetic energy are obtained by integrating the velocity shear spectrum at each respective depth bin up to the noise limit. The noise limit is determined by comparing the measured spectra to the theoretical Naysmyth spectra and determining where the measurements begin to deviate from theory. To display how the estimates of ε were obtained at the Jacaf sill depth, the shear spectra are shown for VMP profiles collected at the Jacaf sill region (21 November 2013 at 140 m depth; Fig. 10b), and in Puyuhuapi Fjord on 22 November 2013 (at 140 m depth; Fig. 10d) and on 23 January 2014 (at 140 m depth; Fig. 10f).

In Puyuhuapi Fjord, the correlation between ε and zooplankton Sv data (38 kHz, fixed station, January 2014) was high (R2=0.65, Fig. 11a). In the same campaign, the in situ macrozooplankton density (>5 mm) was also highly correlated with ε values (R2=0.79 for ε vs. siphonophores, R2=0.66 for ε vs. chaetognaths, and R2=0.77 for ε vs. euphausiids; Fig 11b–d). Unfortunately, VMP data were not collected in Jacaf Channel in wintertime. In order to confirm the relationship between ε and various zooplankton species, additional turbulence measurements were collected in November 2013 along Jacaf sill (Fig. 12a). Results showed strong velocity shear in the horizontal velocities (Fig. 12b) accompanied by high ε values (10-7 to 10-5 W kg-1). Maximum ε was measured at the Jacaf–Puyuhuapi confluence (10 km along transect) at ∼63 m depth where ε=1.9×10-5 W kg-1 (Fig. 12b; St. 164). The diapycnal eddy diffusivity (Kρ) was also high in the same area with values of 10-4 to 10-3 m2 s-1 (Fig. 12c).

Consistent evidence from multiple echo-sounder surveys, ADCP moorings and semi-continuous in situ zooplankton measurements supported the existence of major circadian displacements of macrozooplankton during night hours between mid-depth (20–120 m) and subsurface waters in our study area. Similar DVM patterns have been found in Reloncaví Fjord (41.5∘ S), from 300 and 600 kHz ADCP data, by Valle-Levinson et al. (2014) and Días-Astudillo et al. (2017) using a 75 kHz acoustic device. Given a greater resolution, the later work was able to confirm that the DVM affected the whole water column of the fjord (∼200 m). These studies found the presence of euphausiids, decapods, mesopelagic shrimps, copepods and other groups in the Reloncaví Fjord in July and November 2006 (Valle-Levinson et al., 2014), as well as in July 2013 (Días-Astudillo et al., 2017). DVM is a common feature of many zooplankton groups, observed around the world using different ADCP and echo-sounder frequencies, e.g., at the Kattegat Channel (Buchholz et al., 1995), the northeast Atlantic (Heywood, 1996), the northwest coast of Baja California, Mexico (Robinson and Gómez-Gutiérrez, 1998), the northeastern Gulf of Mexico (Ressler, 2002), the Antarctic Peninsula (Zhou and Dorland, 2004), the Arabian Sea (Fielding et al., 2004), Funka Bay, Japan (Lee et al., 2004), south Georgia (USA), in the Atlantic sector of the Southern Ocean (Brierley et al., 2006) and Saanish Inlet, British Columbia, Canada (Sato et al., 2013). The scattering layers observed in these studies highlight the abundances of the major zooplankton species, represented by amphipods, euphausiids, siphonophores, chaetognaths, pteropods, crustaceans, small fish and gelatinous plankton. While most DVM patterns reported in these studies occurred between 0 and ∼300 m depth, the deepest DVM patters were observed in the North Atlantic Ocean, reaching depths ∼1600 m (Van Haren and Compton, 2013).

DVM patterns of zooplankton are expected to be associated with diel changes in visible light within the photic zone (from surface to ∼100 m). Thus, the zooplankton can avoid predators during daytime hours and have safe-feeding conditions at night. While only small irradiance levels, <10-7 times surface levels, can be detected beyond 600 m (Van Haren and Compton, 2013; Sato et al., 2013, 2016), zooplankton DVM can reach depths below 500 m (Van Haren and Compton, 2013). Moreover, zooplankton DVM occurs in Arctic fjords (e.g., the Kongsfjorden and Rijpfjorden fjords) even during the polar night, suggesting a high sensitivity to very low levels of solar and/or lunar light (Berge et al., 2009). Since both Puyuhuapi Fjord and Jacaf Channel are not deeper than 300 m, enough light should reach the bottom layer and stimulate zooplankton DVM across the whole water column. However, our results show that zooplankton DVM (and distribution as discussed in the next section) was limited by the hypoxic boundary layer present in the Puyuhuapi Channel (∼100 m; Fig. 5), providing indirect support to the idea that hypoxia may limit DVM in poorly ventilated Patagonian fjords and elsewhere (Ekau et al., 2010; Mass et al., 2014; Hauss et al., 2016; Seibel et al., 2016).

In Puyuhuapi Fjord, hypoxic conditions have been reported below ∼100 m depth, all year round (Schneider et al., 2014; Silva and Vargas, 2014), with sporadic deep ventilation events that increase the DO concentration from 1.4 to 2.8 mL L-1 (Pérez-Santos, 2017). These pervasive hypoxic conditions are not common in all Patagonian fjords. For instance, seasonal hydrographic data from Reloncaví Fjord showed well-ventilated conditions along the fjord, with deep, near-bottom DO values between 3 and 3.5 mL L-1 (Castillo et al., 2016).

In the current study, acoustic measurements revealed that most biological backscattering (Sv data) occurred above the hypoxic boundary layer (Fig. 5), which acted as a barrier to DVM and macrozooplankton distribution throughout the year. Similar findings were reported in Oslofjord, Norway, where hypoxic conditions dominated the water column beneath ∼60 m depth, and no fish or krill were observed below this depth (Røstad and Kaartvedt, 2013). Moreover, in the eastern South Pacific OMZ, it has been previously reported that a number of copepod species and life-stages avoid hypoxic waters (Castro et al., 1993; Escribano et al., 2009), as well as for most gelatinous zooplankton groups (Pages et al., 2001; Giesecke and Gonzalez, 2005; Escribano et al., 2009). In the same OMZ region, but further north in Peruvian waters, two diurnal scattering layers were observed, one over the OMZ and another, mainly composed of adult euphausiids, in the core of the OMZ (Ballón et al., 2011). Euphausiids, salps and myctophid fish were also observed in the core of the eastern tropical North Pacific OMZ (Mass et al., 2014). Seibel et al. (2016) reported Euphausia eximia and Nematoscelis gracilis tolerance to hypoxic water and suggest this tolerance would enable these species to reduce their energy expenditure by at least 50 % during their daytime migration.

The highest Sv values observed in Puyuhuapi Fjord occurred at DO concentrations between 2 and 5 mL L-1, while in Jacaf Channel between 3 to 6 mL L-1. DO values of 3.5 and 4.5 mL L-1 seemed to represent appropriate conditions for most macrozooplankton species in Puyuhuapi Fjord and Jacaf Channel, respectively, which are similar to the values indicated by Ekau et al. (2010) for zooplankton. Our results also showed that macrozooplankton preferred oceanic waters with salinity values >31 g kg-1, and temperatures between 8 and 10 ∘C (Figs. 4 and 9). Nonetheless, it must be considered that these preference values were estimated from observational data and limited sampling rather than from controlled experiments.

Vertical overlapping observed between fish and macrozooplankton abundances suggests that prey–predator interactions might be enhanced under hypoxic conditions. Pollution and climate change are continually expanding the extent of hypoxic waters around the world, both in coastal waters and open oceans (Breitburg et al., 2018). While the links between recent anthropogenic perturbations, such as the salmon aquaculture expansion and hypoxia in Patagonian fjords, are still under debate, it is important to keep this potential impact upon habitat reductions and enhanced prey–predator interactions under consideration as it might cause changes in zooplankton groups' distributions and abundance, particularly those that do not tolerate low DO concentrations.

The fact that some biological backscattering occurred within the hypoxic layer in our study indicates that hypoxia does not affect all macrozooplankton species equally and that some of them can inhabit this deeper layer, e.g., euphausiid species (Mass et al., 2014; Seibel et al., 2016). Hypoxia-tolerant species residing below and within minimum DO layers have been reported, for example, further north along the Chilean coast during the upwelling season, leading support to the hypotheses on predation evasion and horizontal transport aiming to explain such behavior (Castro et al., 2007). Within this context, Euphausia pacifica has been reported to exhibit the highest abundance of zooplankton species present in hypoxic waters in Hood Canal, USA (Sato et al., 2016). Other euphausiids have also been reported to be present in other hypoxic systems in Chile (Escribano et al., 2009; Gonzalez et al., 2016). It has been shown that Euphasia vallentini is a dominant euphausiid species known to carry out extensive vertical migrations in Patagonian fjords, hence we speculate it might be one of the species occurring in the less oxygenated waters of our study. Unfortunately, due to sampling gear restrictions, we were unable to sample the hypoxic layer, nor to firmly identify the species occurring at this depth. Therefore, future research will be necessary to understand the relationship of the deep, yet scarce, macrozooplankton within the hypoxic waters in Puyuhuapi Fjord. As vertical mixing is a mechanism that could reduce the presence of hypoxic zones in fjords, values of turbulent kinetic energy dissipation were compared to the depth strata of macrozooplankton.

Patagonian fjords and channels cover an area of ∼240000 km2 and feature a complex marine topography, including submarine sills and channel constrictions (Iriarte et al., 2014; Inall and Gillibrand, 2010). Bernoulli aspiration, internal hydraulic jumps and intense tidal mixing are all processes that can be found near a fjord sill (Farmer and Freeland, 1983; Klymark and Gregg, 2003; Inall and Gillibrand, 2010; Whitney et al., 2014). Our data showed elevated values of turbulent kinetic energy dissipation in Jacaf Channel (ε=10-5 W kg-1 and Kρ=10-3 m2 s-1) near the sill from 0 to 60 m depth. These values are similar to those observed at the sill of Knight Inlet in Canada (Klymark and Gregg, 2003). Lower ε values were found in Puyuhuapi Fjord (Fig. 10). The elevated vertical mixing (high Kρ) in Jacaf Channel is probably due to the barotropic tide interacting with the submarine sill (Schneider et al., 2014; Figs. 10, 12 and Table 2). This was also observed in Martinez Channel (Pérez-Santos et al., 2014), central Patagonia, where semidiurnal internal tides were found to dominate the estuarine dynamics (Ross et al., 2014). This region is highly influenced by the Baker River, whose discharge enhances stratification and introduces suspended solids that subsequently limit productivity in the water column (González et al., 2010, 2013; Daneri et al., 2012).

The evident aggregation of macrozooplankton and fish found near Jacaf sill (within ∼1 km) matches the area exhibiting the highest ε values (∼10-5 W kg-1; Fig. 12). Thin (2–5 m) and thick (10–50 m) regions of enhanced vertical shear measured directly with the VMP-250 microstructure profiler contribute to vertical mixing. Subsequently this enhances the exchange between the subsurface, rich nutrient layer (Fig. 2) and the photic layer, leading to increased phytoplankton productivity (Montero et al., 2017a, b), as shown in the conceptual model of Fig. 13. Thus, the acoustic and turbulence measurements collected near Jacaf sill promote the importance of a sill in influencing the vertical distribution of oxygen, macrozooplankton and fish on both sides of the sill.

A summary of the processes that can contribute to macrozooplankton vertical distribution and aggregation in Puyuhuapi Fjord and Jacaf Channel are presented in a Fig. 13. In Puyuhuapi Fjord, at 100 m depth a high nutrient and high production layer (Daneri et al., 2012; Montero et al., 2017a, b) is separated from a hypoxic layer below, which limits species distribution and lacks significant aggregations of zooplankton. Above the hypoxic waters, turbulent mixing enhances contact between macrozooplankton predators and their prey (Visser et al., 2009). In Jacaf Channel, the hypoxic layer occurs deeper in the water column than in Puyuhuapi Fjord, which stretches the vertical distribution of macrozooplankton to a deeper range. Turbulent mixing also increases primary and secondary production, through enhanced nutrient availability and favors encounters of macrozooplankton with potential prey, increasing growth and survival rates (Visser and Stips, 2002; MacCready et al., 2002; Klymak and Gregg, 2004; Lee et al., 2005; Visser et al., 2009; Whitney et al., 2014).

Results showed similar groups of macrozooplankton (>5 mm) in Puyuhuapi Fjord and Jacaf Channel: euphausiids, chaetognaths, medusae and siphonophores during summer (January 2014) and winter (winter 2014). However, euphausiids were not observed in fall 2013, which was an unexpected result which deserves further confirmation and analysis. In contrast, fall 2013 sampling presented the highest acoustic abundances within the time series (Fig. 3). The elevated accumulation of macrozooplankton species around the sill may impose a significant modification in the amount and quality of carbon exported to deeper waters in particular zones of the fjords. Future studies on carbon flux quantification in fjords should incorporate sill regions to test this hypothesis, in order to improve ocean pumping assessments in the context of climate change and variability.