Seasonal hydrography and surface outflow in a fjord with a deep sill: the Reloncaví fjord, Chile

Abstract

Seasonal data on temperature, salinity, dissolved oxygen (DO) and chlorophyll, combined with meteorological and river discharge time series, were used to describe the oceanographic conditions of the Reloncaví fjord (41°35'-S, 72°20'-W). The winds in the fjord valley mainly blow down-fjord during the winter, reinforcing the upper layer outflow, whereas the winds blow predominantly up-fjord during the spring and summer, contrary to the upper layer outflow. The fjord, with a deep sill at the mouth, was well stratified year-round and featured a thin surface layer of brackish water with mean salinities between 10.4-±-1.4 (spring) and 13.2-±-2.5 (autumn). The depth of the upper layer changed slightly among the different studied seasons but remained at 4.5-m near the mouth. This upper layer presented a mean outflow (<i>Q</i>₁) of 3185-±-223-m³-s^-1, which implies a flushing time of 3 days for this layer. The vertical salt flux was -37 tons of salt per second, similar to the horizontal salt flux observed in the upper layer. These estimates will contribute to better management of the aquaculture in this region.

Ocean Sci., 12, 533544, 2016 www.ocean-sci.net/12/533/2016/ doi:10.5194/os-12-533-2016 Author(s) 2016. CC Attribution 3.0 License.

Manuel I. Castillo1,2, Ursula Cifuentes2,6, Oscar Pizarro2,3,4, Leif Djurfeldt5, and Mario Caceres1

1Facultad de Ciencias del Mar y de Recursos Naturales, Universidad de Valparaso, Via del Mar, Chile

2Center for Oceanographic Research in the Eastern South Pacic (COPAS)-Sur Austral, Universidad de Concepcin, Concepcin, Chile

3Departamento de Geofsica, Universidad de Concepcin, Concepcin, Chile

4Instituto Milenio de Oceanografa, Universidad de Concepcin, Concepcin, Chile

5Department of Oceanography, Earth Sciences Center, Gothenburg University, Gothenburg, Sweden

6Departamento de Oceanografa y Medio Ambiente, Instituto de Fomento Pesquero, Valparaso, Chile

Correspondence to: Manuel I. Castillo ([email protected])

Received: 19 September 2015 Published in Ocean Sci. Discuss.: 29 October 2015 Revised: 9 March 2016 Accepted: 29 March 2016 Published: 12 April 2016

Abstract. Seasonal data on temperature, salinity, dissolved oxygen (DO) and chlorophyll, combined with meteorological and river discharge time series, were used to describe the oceanographic conditions of the Reloncav fjord (41 35[prime] S, 72 20[prime] W). The winds in the fjord valley mainly blow down-fjord during the winter, reinforcing the upper layer outow, whereas the winds blow predominantly up-fjord during the spring and summer, contrary to the upper layer outow. The fjord, with a deep sill at the mouth, was well stratied year-round and featured a thin surface layer of brackish water with mean salinities between 10.4 [notdef] 1.4 (spring) and 13.2 [notdef] 2.5

(autumn). The depth of the upper layer changed slightly among the different studied seasons but remained at 4.5 m near the mouth. This upper layer presented a mean outow (Q1) of 3185 [notdef] 223 m3 s1, which implies a ushing time of

3 days for this layer. The vertical salt ux was 37 tons of

salt per second, similar to the horizontal salt ux observed in the upper layer. These estimates will contribute to better management of the aquaculture in this region.

1 Introduction

Fjords are narrow, generally deep coastal inlets associated with the advance and retreat of glaciers (Stigebrandt, 2012). Studies of these areas have been widely reported for Scandinavian and northeast Pacic fjords (Farmer and Freeland,

Seasonal hydrography and surface outow in a fjord with a deep sill: the Reloncav fjord, Chile

1983; Inall and Gillibrand, 2010), but little is known about the physical dynamics of one of worlds most extensive fjord region: the austral Chilean fjords (Silva and Palma, 2008; Pantoja et al., 2011; Iriarte et al., 2014).

The austral Chilean fjord area extends from 41.5 to55.9 S, a length of 1700 km ( 40 % of the total length of

Chile) and an area of 2.4 [notdef] 105 km2 (Silva et al., 2011). Since

early eighties, the region from 41.5 to 42 S has been intensively used for sh, shellsh and seaweed production. Recently, the southern limit of the aquaculture is 46 S, and there are plans to expand to 55 S in 2015 (http://www.subpesca.cl

Web End =http://www.

http://www.subpesca.cl

Web End =subpesca.cl ). Most of the Chilean aquaculture production comes from salmon farms, which have become the fourth largest economic activity in Chile (Buschman et al., 2009). Despite the high utilization of fjords, knowledge of the physical dynamics remains limited. In fact, in the Chilean fjord region, only limited environmental data are available in both space and time (e.g., Silva and Palma, 2008). As an example, there are only preliminary studies (e.g., Dvila et al., 2002) on the impact of the freshwater supply on Chilean Patagonia circulation in regions with high river discharge (Niemeyer and Cereceda, 1984).

One of the rst fjords used for salmon aquaculture in Chile was the Reloncav fjord (centered at 41.5 S, 72.5 W). Although this is one of the most studied fjords in southern

Chile, oceanographic information is relatively scarce, and several questions regarding its natural and anthropogenic

Published by Copernicus Publications on behalf of the European Geosciences Union.

534 M. I. Castillo et al.: Seasonal hydrography and surface outow in a fjord with a deep sill

variability remain unanswered. Soto and Norambuena (2004) noted the concern about the impact of the aquaculture on the system. As an example, Valle-Levinson et al. (2007) found lower (but still above critical levels) dissolved oxygen (DO) concentrations (> 2 mL L1) near the head of the fjord, but its variability and impact on the biology in different seasons remain unknown. In addition, in this region Len-Muoz et al. (2013) indicated the existence a signicant association between the increase of surface salinity and low DO concentrations, but the variability and relationship between these parameters below 2 m depth remain unknown. Montero et al. (2011) made along-fjord observations that focused on seasonal variability of primary production. They did not observe DO as low as Valle-Levinson et al. (2007); thus, a detailed DO description is needed.

The mean circulation in the Reloncav fjord suggests that the along-fjord currents have a three-layer vertical pattern: a thin (< 5 m) outow upper layer, a thick intermediate inow layer (> 5 and < 100 m) and a weak deep (> 100 m) outow layer (Valle-Levinson et al., 2007; Castillo et al., 2012). This three-layer pattern could be an important structure but has only been sporadically observed because it can be masked by wind forcing, remote forcing and freshwater pulses (Valle-Levinson et al., 2014).

Despite the diverse studies made in the Reloncav fjord, many questions remain unanswered regarding its hydro-graphic conditions and circulation, such as the seasonal variability of the salinity and the exchanges with the area outside the fjord. Here, we present a study of the hydrographic seasonality and salinity uxes using an extensive and high-quality data set.

2 Study area

The Reloncav fjord has an overall length is 55 km and the averaged width of 2.8 km (Table 1). It connects directly to Reloncav sound and indirectly to Ancud gulf, which is connected to Pacic Ocean through the Chacao channel (to the north of Chilo island) and by the Corcovado gulf (Fig. 1).There is a deep sill ( 200 m depth) located 15 km from the

mouth, but this structure does not seem to be a barrier to the exchange of properties with external waters. The fjord has four sub-basins: (I) mouthMarimeli, (II) MarimeliPuelo, (III) PueloCocham and (IV) CochamPetrohue.The mean depths of the sub-basins are 440, 250, 200 and 82 m, respectively (Fig. 1).

The main fresh water input to the fjord is through the Puelo river, which enters at the center of the fjord and delivers an annual mean discharge of 650 m3 s1. Another important freshwater supply (annual mean discharge of 255 m3 s1) is the Petrohue river (located at the head). Minor freshwater inputs are associated with the Cocham river (annual mean of 20 m3 s1) (Niemeyer and Cereceda, 1984) and the Canutillar hydroelectric plant (75.5 m3 s1 annual mean) (Fig. 1).

Table 1. Topographic features of the sub-basins in the Reloncav fjord (RF). Here, b is the width, L is the length, and z is the depth.

Sub-basin b L z (km) (km) (m)

I 2.24.5 14 400460 II 2.34.2 15 140280 III 3 16 180200 IV 1.11.6 10 35110

RF mean 2.8 55 250

The fresh water input due to direct precipitation on the fjord represents only 2 % of the river discharge (Len-Muoz et al., 2013), and for the water and salt balances made in this study, its contribution was considered to be balanced by evaporation.

Winds in the region exhibited large seasonal variability.

North and northwest winds predominate during autumn and winter, while south and southwest winds predominate during spring and summer (Saavedra et al., 2010). The seasonal changes in the wind pattern were associated with an abrupt austral winterspring transition observed in the temperature of the surface layer in the Reloncav fjord (Montero et al., 2011). During winter, the along-fjord wind stress (y) is mainly directed out of the fjord, with intensities of < 0.2 N m2. In summer, y is directed into the fjord, opposing the surface outow, with intensities between 0.1 and0.3 N m2. Additionally, during this season y had a clear diurnal cycle (Montero et al., 2011) probably related to the radiational tide effect (Farmer and Freeland, 1983; Rabinovich and Medvedev, 2015).

The currents near the mouth have a three-layer pattern. The thin upper outow was relatively fast, reaching 30 cm s1 near the surface. Below the upper layer, the intermediate inow never exceeds 10 cm s1. The deep layer is thick and weak ( 1 cm s1). This third layer has been sug

gested to be a consequence of tidal rectication of the ow (Valle-Levinson et al., 2007) and recently has been studied in detail in different fjords in southern Chile (Valle-Levinson et al., 2014). This pattern could change seasonally between a two-layered structure during winter and a three-layered structure during spring and summer.

Additionally, there is evidence of an internal oscillation with a period of 3 days (Castillo et al., 2012). One of the most recent studies on this region (Len-Muoz et al., 2013) found a signicant association between the temporal increase in near-surface (1.5 m depth) salinity with lower surface DO concentrations; however, their observations did not describe the vertical structure or distribution of each parameters within the fjord. The objectives of this study were to examine and describe the seasonality of the hydrography of the Reloncav fjord and to estimate the upper ow to obtain reliable ushing time estimations.

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M. I. Castillo et al.: Seasonal hydrography and surface outow in a fjord with a deep sill 535

Figure 1. (a) The Reloncav fjord region and location of the instruments. The upper left insert shows the general region. The ADCP moorings are near the mouth (ADCP). On the right, the insets show the (b) Cocham and (c) mouth regions. The lower inset (d) shows the along-fjord bathymetry, in which the segmented lines indicate the sub-basin limits: mouthMarimeli (I), MarimeliPuelo (II), PueloCocham (III) and CochamPetrohue (IV). The diamonds represent the location and depths of the ADCP mooring shown in panel (c).

3 Data and methods

Discharge, meteorological, hydrographic (CTD) and current (ADCP) measurements

Except for the acoustic Doppler current meter proler (ADCP) current time series, most data were registered in all seasons. The representative months for each season used in this study were September to November for spring, December to February for summer, March to May for autumn and June to August for winter. A right-handed coordinate system was used for currents and surface wind stress vectors, where z is positive upward and the along-fjord y component was positive toward the fjord head. Consequently, the cross-fjord x component was positive toward the east at the head and toward the south at the mouth.

The Puelo river discharge data were provided by Direccin General de Aguas, Chile (Direccin General de Aguas, 2015; http://www.dga.cl

Web End =www.dga.cl ). The data are regularly collected at a station located 12 km up-stream of the mouth of the Puelo river (Fig. 1) and extended from January 2003 to December 2011. In this data set, gaps represented 2 % of the total. Although

the discharge of the Petrohue river (RPt) was not directly measured, an estimate of its runoff was obtained using the Puelo river (RP) discharge via a linear regression between both annual cycles. The annual cycle of the RP was estimated

with data from 1975 to 1981, and the annual cycle of the

RPt was estimated with data from 1941 to 1982 (Niemeyer and Cereceda, 1984). Both annual cycles were highly correlated (R2 = 0.88), and RPt = 0.519 [notdef] RP 68.173. Due to

the lack of data during the study period, the discharges of the

Cocham river (20 m3 s1) and the Canutillar hydroelectric(75.5 m3 s1) were considered to be constant (Niemeyer and

Cereceda, 1984; http://www.cdec-sic.cl

Web End =www.cdec-sic.cl http://www.cdec-sic.cl

Web End = ).

A meteorological station was installed near the Puelo river mouth (see Fig. 1). The station included sensors for wind direction and magnitude (here, wind directions are referred to by the direction from which the wind comes according to meteorological convention), solar radiation, rain and air temperature. The wind magnitude and direction sensors were installed 10 m a.s.l. and were set to collect data every 10 min from 12 June 2008 to 30 March 2011. In this data set, gaps represented only 0.04 %. Wind stress () was calculated using a drag coefcient that is dependent on the magnitude (see Large and Pond, 1981) and a constant air density of1.2 kg m3.

The hydrographic data were collected using a

SeaBird 25 CTD (conductivity, temperature, pressure) equipped with a SeaBird 43 dissolved oxygen sensor and a Wet-Lab/Wet-Star uorometer (ECO-AFL). The concentration of chlorophyll a (mg m3) from uorescence was estimated according to the relationship provided by

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536 M. I. Castillo et al.: Seasonal hydrography and surface outow in a fjord with a deep sill

the CTD manufacturer. The CTD Oxygen/Fluorometer (CTDOF) measurements were conducted at 19 along-fjord stations (Fig. 1). The CTD measurements were conducted in transects that took between 12 and 18 h on 7 August 2008 (winter), 9 November 2008 (spring), 6 February 2009 (summer) and 9 June 2009 (autumn). The winter measurements only reached a depth of 50 m due to problems with the

oceanographic winch. During those casts, the CTD was not equipped with oxygen sensor.

Current measurements were made using ADCPs. Near the mouth of the fjord, a mooring with two ADCPs was installed. The mooring included a 75 kHz ADCP located near the bottom (450 m depth) and a 300 kHz ADCP located at 10 m depth. Another mooring with a 300 kHz at 15 m depth was installed near Cocham. The objective for installing the 300 kHz ADCP at 10 m depth was to obtain good veloc

ity measurements near the surface. The instruments in both systems were programmed to measure every 10 min in depth cells of 1 m. The reference depth for the velocity proles was the surface (all data sets are available at the site of the COPAS-SUR Austral, Universidad de Concepcin, 2012).Currents were decomposed into along-fjord (v) and cross-fjord (u) components using the right-handed coordinate system mentioned above. To focus on the subtidal and sub-inertial variability, the along-fjord wind stress (y) and currents (u, v) were ltered using a cosine-Lanczos low-pass lter with a half-amplitude of 40 h.

The upper volume ux (Q1) was estimated using the velocity proles at the mouth and Cocham (Fig. 1). The Q1 was estimated according to the relationship

Q1 = b

z=v0

[integraldisplay]

z=0

vdz, (1)

where b is the fjord width near the surface at the mooring location (b was considered constant, despite changes in sea level of approximately 6 m during spring tides) and v is the along-fjord velocity, which changes with depth z. The integration was made between the surface (z = 0) and the depth

at which v is zero (z = v0). The use of up-looking ADCPs

implies a lack of approximately 6 % (1 m for both ADCPs) of range due to side lobe effect. To estimate v up to the surface, two methods of extrapolation were used: a linear method and a nearest method, similar to that used by Kirincich et al. (2005). Note that negative (positive) values of y and v indicate directions out of (into) the fjord. Similar interpretations must be performed for Q1.

Based on the estimation of Q1, it is possible to obtain the ushing time of the upper layer (Ft1) if the total volume of the upper layer (V1) is introduced. Thus, Ft1 = V1Q11. Ad

ditionally, if the upper mean salinity (S1) is considered, it is possible to estimate the horizontal salt ux: Fsh = Q1S1.

The Fsh was compared with the total vertical salt ux (FsT) at the base of the surface layer. To obtain FsT, it is necessary

Figure 2. Seasonal variability of the wind vector in the Reloncav fjord during the period June 2008 to March 2011. Frequency histograms of direction and magnitude for each season: (a) spring,(b) summer, (c) autumn and (d) winter. The annual cycle of the discharge into the Reloncav fjord is shown in panel (e). Notice that the Cocham discharge is included but is low (20 m3 s1) compared to the other sources.

obtain the vertical salt ux (Fsv), which was estimated using Fsv = z@S/@z. Here, the eddy diffusivity ( z) was estimated

using the eddy viscosity (Az) based on the relation suggested by Pacanowski and Philander (1981), where Az = 0.01(1 +

5Ri)2 + 104 and z = Az(1 + 5Ri)1 + 105. Here, Ri =

N2/(@v/@z)2 is the Richardson number that was obtained from direct measurements of the buoyancy frequency (N2)

and the vertical shear of the along-fjord currents (@v/@z).

FsT was estimated by introducing the surface horizontal area (Ah) at the mean depth of the upper layer: FsT = FsvAh.

4 Results

4.1 Meteorological conditions and fresh water supply

The winds were dominantly (up to 20 %) from the southeast and south during spring, summer and autumn. In contrast, northerly winds were dominant (ca. 30 %) during winter. The strongest winds (> 10 m s1) were southerly and southeast-erly during spring and summer (Fig. 2).

The seasonal variations in the daily cycle (in local time, LT) of the air temperature ( C), solar radiation (W m2), wind stress (N m2) and wind vector (m s1) were also analyzed (Fig. 3). The amplitudes of the daily cycles for all the variables were smaller during the winter (JuneSeptember) and larger during the spring and summer (NovemberFebruary). The air temperature exhibited a narrower range (between 6 and 8 C) in winter compared with summer (12

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M. I. Castillo et al.: Seasonal hydrography and surface outow in a fjord with a deep sill 537

(winter)

November 4.79 [notdef] 0.53 1007.63 [notdef] 5.32 1024.78 [notdef] 0.62

(spring)

February 4.68 [notdef] 0.26 1008.77 [notdef] 3.26 1024.78 [notdef] 0.63

(summer)

June 4.05 [notdef] 0.41 1009.90 [notdef] 3.92 1024.95 [notdef] 0.48

(autumn)

Figure 3. Seasonal variability in the daily cycle of the meteorological variables: (a) air temperature, (b) solar radiation, (c) wind stress magnitude and (d) wind velocity (meteorological convention). The y axis is the local hour to be consistent with the daylight hours.

and 18 C). The solar radiation was clearly related to the variations in daylight (longer in summer than in winter). Similar patterns were observed for air temperature and wind stress () magnitude. In winter, the amplitude of the daily cycle of was nearly zero, while during spring and summer, the maximum values of were observed between 15:00 and 18:00 LT.

The freshwater supply due to river discharges in the Reloncav fjord peaked during June (winter), when the mean discharge was 1400 [notdef] 400 m3 s1 (hereafter, the symbol [notdef]

indicates the standard deviation). In this region, rivers typically have a secondary discharge peak associated with springsummer snowmelt, which was observed in November (1300 [notdef] 300 m3 s1). Lower river discharges were observed

during late summer (FebruaryMarch) and were lower than the annual mean of the Puelo river (< 650 m3 s1).

4.2 Seasonal hydrography: along-fjord CTD measurements

Based on the depth of the 24 and 31 isohalines, three layers were dened to describe the hydrographic conditions in the Reloncav fjord. The upper layer was dened between the surface and the depth of the 24 isohaline (ih24), which coincides with the depth of the maximum gradient in along-fjord salinity. The rate of increasing density with depth throughout this upper layer was rather constant, and the upper layer lacked a clear mixing layer. The mean temperature in this layer was 8.68 [notdef] 0.32 C during winter

and 17.79 [notdef] 0.37 C during summer. Furthermore, the mean

salinity was 10.43 [notdef] 1.36 during spring and 13.18 [notdef] 2.47

Table 2. Seasonal statistics of the mean depth of the upper layer and the densities of the upper and deep layers.

h1 1 2(m) (kg m3) (kg m3)

August 4.60 [notdef] 0.60 1009.72 [notdef] 4.32 1024.62 [notdef] 0.74

during autumn. Additionally, the pycnocline depth at the mouth of the fjord was observed at 1.7 m during winter and at 2.9 m during summer. Near the head of the fjord, the pycnocline reached a maximum depth of 8 m during winter. Seasonal changes in the mean depth of the pycnocline for the entire fjord were small: it changed from 4.05 [notdef] 0.41 m in au

tumn to 4.79 [notdef] 0.53 m during spring (Table 2). This suggests

that the fjord maintains upper-layer stratication throughout the different seasons, even with signicant changes in the river discharges and winds (Figs. 4 and 6).

The 31 isohaline (ih31) represents the upper limit for the Modied Subantarctic Water (MSAAW) located in the inland sea outside the Reloncav fjord (Silva and Palma, 2008).The intermediate layer (at depths between the ih24 and ih31) had mean temperatures ranging from 10.22 [notdef] 0.14 C in win

ter to 15.29 [notdef] 0.48 C in summer, which are consistent with

the high degree of radiation in summer. The mean salinities ranged from 28.98 [notdef] 0.46 in autumn to 29.61 [notdef] 0.37 in win

ter. In addition, the mean depth of the ih31 shoaled from10.97 [notdef] 2.49 m in spring to 7.96 [notdef] 0.84 m in autumn, sug

gesting that the water was more saline in autumn than in the other seasons (Fig. 4).

Slight changes in both temperature and salinity were observed in the deep layer (at depths > ih32). The ob-served temperatures ranged from 10.61 [notdef] 0.05 C (winter)

to 10.96 [notdef] 0.12 C (autumn), and the salinities ranged from

32.27 [notdef] 0.16 (winter) to 32.68 [notdef] 0.16 (autumn). This pattern

is consistent with the presence of more saline waters during autumn.

In general, surface waters in the Reloncav fjord are over-saturated with respect to oxygen (DO > 6 mL L1) during spring and summer but feature lower DO values in autumn and winter. Oversaturated waters were observed between 1 m and 15 m depth in spring and between 2 and 10 m depth during summer. The DO values were as high as 10 mL L1 in the sub-basins III and IV near of the head (Fig. 5). In addition, waters with DO values of < 3 mL L1 (50 % saturation, es

timated from in situ measurements) were observed near the bottom in sub-basin III during spring. These waters occupied a more extended area in the fjord basin during summer and

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538 M. I. Castillo et al.: Seasonal hydrography and surface outow in a fjord with a deep sill

Figure 4. Along-fjord seasonal distribution of temperature (above) and salinity (below) for winter, spring, summer and autumn. The gure includes the CTD station number in the top of each panel and the sub-basins numbers below.

Figure 5. Along-fjord seasonal distribution of dissolved oxygen (above) and chlorophyll (below) for winter, spring, summer and autumn. The gure includes the CTD station number in the top of each panel and the sub-basins numbers below. No DO measurements were obtained during winter.

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M. I. Castillo et al.: Seasonal hydrography and surface outow in a fjord with a deep sill 539

Figure 6. Low-frequency time series of the Puelo river (a), along-fjord wind stress (b), and the volume ux of the upper layer at the mouth (c)

and at Cocham (d). Note the use of a different scale for the volume ux at each location. The segmented line indicates the seasonal shift in the pattern of winds between late winter and early spring.

autumn. Waters with DO values of < 2.5 mL L1 were ob-served near the bottom of sub-basins III and IV during summer and autumn.

The surface concentration of chlorophyll a (Chl a) was extremely low during winter (slightly greater than 0 mg m3), and no major changes occur among the seasons. In general, water in the fjord yielded Chl a values of < 6 mg m3 during winter, spring and autumn, with especially low Chl a values ( 1 mg m3) during winter. The exception was ob-

served during summer in water at depths between 3 and 12 m, where Chl a was as high as 25 mg m3 along the entire fjord.

An interesting feature was observed at the entrance of sub-basin IV: this high concentration was disrupted, likely due to changes in depth and width of the fjord in this region (Fig. 5).

4.3 Variability of the upper ow

In the period between 8 August and 9 November 2008, the ltered time series of Puelo river discharge, along-fjord wind stress (y) and upper ows were compared (Fig. 6).

The Puelo river discharge had two contrasting periods. The rst occurred at the end of August (winter) and featured high discharges (> 103 m3 s1). This pattern changed in the second week of September (spring), when the discharge was between 500 and 650 m3 s1 (Fig. 6a). Similarly, the y pattern changed from negative during winter to positive during spring. This is a seasonal change from winter to spring conditions, which are then maintained during summer (see Castillo

et al., 2012). In general, [notdef]y[notdef] was < 3 [notdef] 102 N m2. There

were three events during which this intensity was exceeded:11 August, 15 August and 16 September. In all three of these cases, y was oriented towards the head of the fjord (Fig. 6b).

Using the subtidal current proles, the upper-layer ow was estimated based on a width (b) of 2.9 km at the mouth and 1.3 km at Cocham. The time series of volume ux (Q1)

estimated with a nearest extrapolation sub-estimate in approximately 8 % compared the linear extrapolation. All the results and discussion are based on the linear extrapolation.

At Cocham, Q1 tended to be higher during the end of winter than during early spring. The inows were observed only during spring. In those cases, Q1 < 103 m3 s1, and the average Q1 was 583.31 [notdef] 446.43 m3 s1. Additionally,

Q1 had oscillations of approximately 23 days, which are not present in the river discharge or wind-stress time series (Fig. 6d).

During the end of winter, the outow reached peaks greater than 7.5 [notdef] 103 m3 s1 at the mouth. Q1 tended to

decrease toward spring and rarely exceeded 5 [notdef] 103 m3 s1.

There were intense inow events ( 2.5 [notdef] 103 m3 s1) that

were also highly correlated with wind events (in the same direction) with intensities of approximately 2 [notdef] 102 N m2. A

cross-correlation analysis between y and Q1 at the mouth indicated a maximum coefcient of correlation of 0.7 with a 4 h lag, which implies a signicant relationship between the wind stress and the upper ow. Similarly to Cocham, the

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540 M. I. Castillo et al.: Seasonal hydrography and surface outow in a fjord with a deep sill

Q1 time series had 3-day oscillations, and these waves seem to be more evident during the early spring (Fig. 6c).

It is interesting to compare the winter and spring conditions using the mean velocity proles and ows for each period. During the end of winter, winds were out of fjord (mean wind stress of 0.3 [notdef] 7 [notdef] 102 N m2) in the same

direction as the upper current with intensities larger than

50 cm s1. Under these conditions, Q1 had a mean depth of 5.31 m. During the winter, the mean Q1 was as high as

4045 [notdef] 283 m3 s1 (outow), which was 3 times larger

than the input of fresh water (R) into the fjord (Fig. 7a).

In early spring, y was oriented in an opposite (on average) direction to the upper currents (i.e., into the fjord) with a mean intensity of 1.1 [notdef] 5 [notdef] 102 N m2, which was approx

imately 4 times greater than in winter. These opposing winds likely reduced the surface outow, which never exceeded

30 cm s1 during this period. In addition, during spring, the outow was approximately half (2050 [notdef] 143 m3 s1)

the outow observed in winter and nearly twice as large as R (Fig. 7b).

Combining the observed Q1 and typical salinity of the upper layer during winter and spring, it was possible to obtain the horizontal salt ux associated with the upper layer (Fsh).

In winter, Q1 was 4045 m3 s1, the mean salinity (S1) was12.9 kg of salt per cubic meter (kg salt m3), and a mean density (1) of 1009.7 kg m3 was assumed for the upper layer.

Thus, the total supply of salt associated with the upper layer during this season was Fsh = 52.3 tons of salt per second.

During spring, the upper layer salinity was 10.5 kg salt m3 (1 = 1007.6 kg m3), and Q1 was 2050 m3 s1, which im

plies a total salt supply of Fsh = 21.5 tons of salt per second

during this season. The relatively minor Fsh during the spring (compared with winter) was related to the high outow and discharge differences between the seasons. A representative mean of Fsh for the entire period can be obtained from the

Fsh average for winter and spring: 36.9 tons of salt per second.

To estimate the vertical salt ux (Fsv), the maximum N2 and maximum @v/@z were considered. In winter, Ri was 4.0 ( z = 1.6 [notdef] 105 m2 s1), whereas in spring, Ri

was 36.2 ( z = 1.1 [notdef] 105 m2 s1). In addition, the max

imum @S/@z values were 17.4 kg of salt m4 in winter and 18.2 kg of salt m4 in spring. The vertical salt ux (Fsv) was 2.8 [notdef] 104 kg of salt m2 s1 during winter and

1.9 [notdef] 104 kg of salt m2 s1 during spring. Thus, the aver

age salt ux is 2.3 [notdef] 104 kg of salt m2 s1. The total salt

ux (FsT) to the upper layer could be estimated assuming that Fsv is maintained over the horizontal area (Ah) at 5 m depth (which is the deeper limit for the outow; see Fig. 7).Here, Ah = 1.59 [notdef] 108 m2; thus, FsT = 3.7 [notdef] 104 kg of salt

per second, or 37 tons of salt per second.

Figure 7. Mean proles of along-fjord currents (v) at the mouth for the periods of winter (a), spring (b) and for the entire period of measurement shown in Fig. 6. The blue line indicates the observed mean, which is lacking near the surface. The red and black lines indicate two different extrapolations to the surface: linear (red) and nearest (blue). The mean volume uxes (Q1) obtained using the two extrapolations are included. Additionally, the averages of y for each period and the discharge (R) have been included.

5 Discussion

A particular feature of the Reloncav fjord is the deep sill located at 3 km from the mouth (Fig. 1d). Usually, in fjords with no or deep sills, the interior density distribution and variability is closely related to the external stratication (e.g., Pedersen, 1978). The earliest efforts to describe the Reloncav fjord were summarized by Bastn and Clement (1999), but their results are based on relatively few and sparse observations that preclude an adequate description of the seasonal variability.

5.1 Seasonality of the hydrography and freshwater inputs

To describe the seasonal conditions observed in the Reloncav fjord, it is necessary describe the external conditions in the region. In the Pacic Ocean in front of Chilo island ( 42.5 S, 74 W), the water mass distribution indicates the

presence of Subantarctic Water (SAAW) in the upper 100 m (salinity > 33) at the coast and farther offshore (2000 km).Below the SAAW and near the shore (> 10 km), the Equatorial Subsurface Water (ESSW) is perceptible to a depth of 350 m (Silva et al., 2009). Only these water masses could penetrate the Guafo mouth and occupy the inland sea of Chilo (Fig. 1). Here, the presence of several islands, sills and constrictions between the Corcovado and Ancud gulfs enhance turbulent mixing in the region. The mixing between SAAW and freshwater produces a water mass with a salinity

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M. I. Castillo et al.: Seasonal hydrography and surface outow in a fjord with a deep sill 541

between 31 and 33 and is known as the Modied Subantarctic Water (MSAAW) (Silva and Palma, 2008). The MSAAW occupies most of the interior basins of the Chilean fjord region (Prez-Santos et al., 2014). In summer, when river discharge is limited, surface salinities greater than 33 are present off the Guafo channel (Palma et al., 2011). In winter, the coastal temperature and salinity in the Chilean fjord region appear to be controlled by the freshwater inputs (Dvila et al., 2002).

The seasonal variability in the wind in the Reloncav fjord valley was consistent with the regional pattern observed in the south-central Chilean coast, with southerly winds during spring and summer and northerly winds during autumn and winter (e.g., Saavedra et al., 2010). During spring and summer, the alongshore wind stress promotes upwelling near the coast (Strub et al., 1998; Sobarzo et al., 2007). This process allows saltier deep water to reach the upper layer, thereby changing the near-shore hydrography.

This is also true for the Reloncav fjord, which featured lower salinity values and temperatures during the winter (Fig. 4), when discharge presented a relatively long-term mean (8 years) of 1300 m3 s1 (Fig. 2). It is worth noting that the highest salinities (> 33) in the Reloncav fjord were observed during autumn at the bottom of sub-basin I. In addition, these waters present relatively high temperatures ( 11 C) and low DO (Figs. 4 and 5). These results suggest

that denser ocean waters may reach the Reloncav sound in fall. Nevertheless, based on the limited spatial and temporal distribution of the data used in this study, it is not possible to know if this is a typical feature of the seasonal cycle.

In terms of DO, the volume of near-hypoxic waters (2 3 mL L1) increased from spring to autumn. In autumn, more than one-third of the fjord volume exhibited near-hypoxic conditions, whereas in the spring, the fjord basin waters were oxygenated, with DO values of > 3 mL L1 (Fig. 5). In addition, these low-oxygen conditions increased toward the head of the fjord. In fact, sub-basins III and IV are dominated by waters with DO values of < 3 mL L1 during summer and autumn. The low-oxygen water near the head of the fjord is a condition observed in several continental fjords that are similar to the Reloncav fjord, and these conditions are produced by the respiration of autochthonous particulate matter (Silva and Vargas, 2014). This typical low-DO at the head of the fjord has not be taken into account as selection criteria for the location of the marine concessions in the region. In the upper layer (at depths of < 20 m), the high DO (> 6 mL L1) and

Chl a (> 16 mg m3) values in summer suggest in situ productivity, in contrast the high DO (> 6 mL L1) during spring were related with Chl a concentrations of 1 mg m3. This

pattern could be due to a difference in the phytoplankton communities, which are dominated by dinoagellates in the summer and diatoms in the spring (Montero et al., 2011).Another possibility is the advection of water with high DO values during spring, but it is not possible to address this hypothesis in this study. The relatively well-ventilated (greater

than hypoxic levels) deep water observed in the Reloncav fjord seems to be a characteristic of the southern Patagonian deep fjords of Chile (Silva and Vargas, 2014). Similar characteristics have been observed in Bradshaw and Doubtful sounds in New Zealand (Stanton and Pickard, 1981). In contrast, Scandinavian fjords commonly feature shallow sills at the mouths of the fjords, which tend to isolate the deep water and promote anoxia (Stigebrandt, 2012). As an example, the By fjord required forced oxygenation of the deep water to reduce the eutrophication of the waters (see Stigebrandt et al., 2014).

The waters in the Reloncav fjord are dominated throughout the seasons by estuarine waters (EW) in the upper layer and MSAAW in the deep layer (Fig. 4). Recent studies have reported the presence of MSAAW in the Puyuhuapi fjord (44.6 S, 72.8 W) (Schneider et al., 2014) and in the

Martinez channel (47.8 S, 73.7 W) in southern Patagonia (Prez-Santos et al., 2014).

In the Reloncav fjord, there is an unique connection (at the mouth) with the outer conditions and its deep sill (at 12 km from the mouth) does not seem to be a barrier for the intrusion of MSAAW waters, which is greatest during autumn (Figs. 4 and 5). These conditions also contribute to the propagation of remote low-frequency oscillations to the interior of the Reloncav fjord, which have been attributed to 15-day oscillations observed in deep, along-fjord currents (Castillo et al., 2012).

5.2 Reloncav fjord exchanges

One important parameter in estuarine environments is the renewal capacity of the system. Unfortunately, the ADCP measurements do not cover the entire depth range of the fjord basin, which would be necessary to obtain a complete prole of the exchanges at the mouth. However, using the shallower ADCPs, it was possible to obtain reliable estimates of the surface outow in this location (Fig. 6).

The local winds of the Reloncav fjord have been highly consistent with the regional pattern. The study of Montero et al. (2011) compares a pixel outside Chilo island with the same meteorological data used here and found a signicant correlation between the two data sets (r = 0.44, p < 0.001).

Furthermore, the seasonal pattern of the region (Saavedra et al., 2010) coincides with the local pattern reported in this study (Fig. 3). In addition, the wind stress was highly correlated (r2 = 0.7) with the outow at the mouth. During winter,

y was negative, i.e., oriented in the same direction as the upper ow. Thus, y may enhance the estuarine circulation.

The surface outow estuarine circulation seems to be sustained even during the spring, when y is directed against the upper ow. This differs from other estuarine system, such as the Juan de Fuca strait, where the estuarine ow tends to switch between estuarine and transient ows due to the local wind inuence (Thomson et al., 2007). In the Reloncav fjord, an along-fjord wind stress of 3 [notdef] 102 N m2 is able

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542 M. I. Castillo et al.: Seasonal hydrography and surface outow in a fjord with a deep sill

to balance the typical along-fjord pressure gradient (Castillo et al., 2012) and produce the observed inows in the upper layer (Fig. 6b, c).

The estimates of the volume uxes could help to obtain a rst approximation of the water exchanges in the Reloncav fjord. In addition, the estimation of the vertical salt ux maxima might be useful to obtain upper limits on the vertical exchange of salt along the fjord. At the mouth, the average volume ux (Q1) estimated from direct observations was 3185 [notdef] 223 m3 s1. One interesting (operational) parameter

is the ushing time of the upper layer (Ft1), which is determined by Ft1 = V1Q11, where V1 is the upper layer volume

(8.30 [notdef] 108 m3). The ushing time of the upper layer (Ft1)

was 3 days, which is highly consistent with the period of the oscillations observed in the time series at the mouth and Cocham (Fig. 6).

A period of 3 days is also consistent with the natural period of oscillation in the fjord (internal seiche oscillations) reported by Castillo et al. (2012). These oscillations are mainly dominated by the rst baroclinic mode (Castillo et al., 2016).The oscillations likely play a role in the internal mixing of the fjord, similar to the Gullmar fjord (Arneborg and Liljebladh, 2001), where 36 % of the mixing is caused by the internal seiche. Additionally, this ushing time is similar to the Ft estimated by Calvete and Sobarzo (2011) for the Aysn fjord (45 16[prime] S, 73 18[prime] W); however their results were based on the fresh water fraction and a thick upper layer of 20 m for all the calculations, contrary to this study in which the upper layer depth was determined by the 24 isohaline depth (ih24).These ushing times estimations contrast with the 100 days estimated by Valle-Levinson et al. (2007) for the Reloncav fjord basin, but those estimates were made based on cross-fjord transects (measured on 1 day) of a towed ADCP near of the Puelo river (in the center of the fjord). Here, time series of Q1 consider 2 months of velocity proles based on the rst reliable estimations of the upper ow in the Reloncav fjord. In any case, these estimates must be taken carefully, and to expand the results to the fjord basin, future modeling studies must be performed to obtain the residence times of any properties in the fjord. Additionally, to study the mixing variability, future studies might include along-fjord micro-proler measurements.

The mean outow at the mouth (3185 m3 s1) was

6 times the mean outow at Cocham (583 m3 s1). The outow at Cocham represents the volume ux of sub-basin IV (near the head of the fjord), which is dominated by the Petrohue river (Fig. 1). The Petrohue river discharge is estimated to be 318 m3 s1. Thus, the ratio R/Q1 was 0.55, which implies that the outow at Cocham is nearly twice the freshwater input in this sub-basin.

Another way to obtain estimates of the exchanges is to use the Knudsen relation for a two-layered model. This method has been used to estimate exchange ows in Chilean fjords (e.g., Valle-Levinson et al., 2007; Calvete and So-

barzo, 2011). However, the use of this relation requires the salinity to be in a steady state, which is only valid for long timescales (Geyer, 2010). Therefore, the volume ux of the upper layer is dened by Q1 = R/f . Here, f = (S2 S1)/S2

is the fraction of freshwater (e.g., Dyer, 1997) and R is the freshwater input to the fjord. In winter, f was 0.6, and in spring, f was 0.68. The outow estimated using the Knudsen relation during winter (spring) at the mouth was 2293 m3 s1 (1403 m3 s1). Notice that in both seasons the outows were underestimated. These values were 2 times smaller than

the values obtained using the mean observed ow and imply longer ushing times than observed at the mouth. In contrary, at Cocham (sub-basin IV), the freshwater fraction (f = R/Q1) was 0.58, similar to the observed fraction

in sub-basin I.

These results suggest that the estimates of the water renewal of the upper layer using the Knudsen relation are only valid in sub-basin IV (upper part of the fjord) and are not valid for the entire fjord. This could have signicant implications for the management of aquaculture of salmonids in the region because the salmon cages generally occupy the upper 20 m of the water column (Oppedal et al., 2011).

An interesting result was obtained from the estimates of the horizontal and vertical salt uxes for the upper layer in the period between late winter and early spring (Fig. 6). The results indicate that 37 tons of salt per second ows out

from the upper layer and that the same amount of salt is supplied to the upper layer by the turbulent mixing (Fig. 7).These results suggest that the lower layer is able to sustain the output of salt from the upper layer, thereby maintaining a (nearly) steady state in terms of the amount of salt in the fjord. These results must be treated carefully and likely require more attention in future observational and numerical models studies on this region.

6 Conclusions

Winds in the region were consistent with the seasonal regional pattern. Northerlies dominated during winter, and southerlies dominated during summer. The strongest winds (> 10 m s1) were southerly and southeasterly in the afternoon of spring and summer. The freshwater supply had two peaks over the course of the year: the largest peak occurred in winter (1400 [notdef] 400 m3 s1) during the pluvial season, and

the secondary peak occurred in spring (1300 [notdef] 300 m3 s1)

due to snowmelt.

The pattern of the hydrography had marked seasonal changes. The water was colder during winter than summer. In the upper 10 m, temperatures were nearly 8 C in winter and 18 C in summer. The dissolved oxygen concentration (DO)

of the Reloncav fjord was higher than 2 mL L1 in all seasons. The lowest DO was present during spring and autumn in sub-basin IV near the head of the fjord.

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M. I. Castillo et al.: Seasonal hydrography and surface outow in a fjord with a deep sill 543

The upper layer salinities (S1) and densities (1) were lower during spring and higher during autumn. The change in the along-fjord pycnocline depth was minimal, which suggests that stratication was maintained throughout the seasons. The small increment of salinity of the deep layer was consistent with the intrusion of subantarctic waters modied by mixing processes that likely occurred outside the fjord.

The mean Q1 at the mouth was 3185 [notdef] 223 m3 s1, which

was 6 times the outow of Cocham (583 m3 s1). At the

mouth, the results showed large differences between the volume ux estimated using the Knudsen relation and the ob-served outow. In contrast, at Cocham, the Knudsen relation appropriately estimated the volume ux of sub-basin IV.

In the period between late winter and early spring, the upper layer had a ushing time of 3 days, which is highly consistent with the natural internal period of the fjord.

The horizontal and vertical salt uxes were highly consistent in the period between late winter and early spring. An amount of 37 tons of salt per second was supplied to the

upper layer, and this amount of salt was very similar to the output of salt by the upper layer.

Data availability

The installation of the moorings for measuring the current, temperature and sealevel in the region was approved by the Chilean Navy through permit DS711. No specic permits were required to install the meteorological station because the location is a publicly controlled site. This study also did not involve any endangerment to species in the region.

The authors indicated that all data are available to download from a COPAS-SUR Austral website (http://www.reloncavi.udec.cl/

Web End =http://www. http://www.reloncavi.udec.cl/

Web End =reloncavi.udec.cl/ , last access 8 April 2016). The discharge data from the rivers of Chile are available from the Direccin General del Aguas de Chile website (http://www.dga.cl

Web End =www.dga.cl , last access 8 April 2016). Also, all data sets can be request from the corresponding author (Manuel I. Castillo).

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Acknowledgements. The authors want to thank to several graduate students from the University of Gothenburg, Sweden, and undergraduate and graduate students from the University of Concepcin (UdeC), Chile, who collaborated in the eldwork. This study is part of the PFB/31 COPAS-Sur Austral program and Centro de Investigacin en Ecosistemas de la Patagonia by FIP2007-21 and Comit Oceanogrco Nacional CIMAR-CONA C17F 11-07. Manuel I. Castillo was supported by FONDECYT grant no. 3130639 and by CONICYT-PAI no. 791220005. Finally, we want to thank the two anonymous reviewers for their observations, which helped to improve the present manuscript.

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