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Quantification of the surface brackish water layer and frontal zones in southern Chilean fjords between Boca del Guafo (43°30′S) and Estero Elefantes (46°30′S)
Continental Shelf Research 31 (2011) 162–171
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Research papers
Quantification of the surface brackish water layer and frontal zones in southern Chilean fjords between Boca del Guafo (431300 S) and Estero Elefantes (461300 S) Carolina Calvete a,n, Marcus Sobarzo b a b
Hydrographic and Oceanographic Service of the Chilean Navy, Playa Ancha, Valparaı´so, Chile ´n, Chile Department of Oceanography and COPAS-Sur Austral, University of Concepcion, Casilla 160-C, Concepcio
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2009 Received in revised form 22 September 2010 Accepted 24 September 2010 Available online 15 December 2010
The southern Chilean region between the Boca del Guafo passage and Estero Elefantes contains one of the estuarine zones with the greatest freshwater influence on the planet. At the surface, plumes of freshwater from the fjord heads to their mouths, emptying into the Moraleda–Costa–Elefantes channel system and then the coastal ocean. The influence of this freshwater on the region’s estuarine dynamics, coastal ecology, and biogeochemical processes has only recently begun to be elucidated. Using hydrographic data from the CIMAR-Fiordos cruises (1998–2001), this study quantifies the equivalent height of freshwater, emphasizing the role it plays in the potential energy anomaly and front locations, as well as its relationship with river discharges. Using a criterion of equivalent height of freshwater 4 15% (density o 1021 kg/m3 and salinity o 28), the brackish layer was found to be 1–15 m thick (except in Estero Elefantes), with horizontal extensions on the order of 100 km. The limits of this layer tended to coincide with frontal zones having potential energy anomaly gradients 40.005 J/m4. The frontal zones were located in the extreme southeast of Jacaf Channel, at the head of Ventisquero Sound, in the central part of the Puyuguapi and Moraleda channels, and at the head and mouth of Ayse´n Fjord. The equivalent height of freshwater and potential energy anomaly showed a good correlation with the accumulated (5-day) river discharges (r2 ¼ 0.87), which were greatest toward the fjord heads in spring. The brackish surface water had short residence times (3.5 days) in Aysen Fjord, unlike the deep layer, which other authors report to have a longer residence time (near 1 year). & 2010 Elsevier Ltd. All rights reserved.
Keywords: Fjords Brackish layer Fronts Equivalent height of freshwater Potential energy anomaly
1. Introduction One of the largest estuarine zones on Earth is found in southern Chile between the Boca del Guafo passage (431300 S) and Estero Elefantes (461300 S). The numerous fjords distributed along this eastern coast receive freshwater from terrestrial drainage, rivers, rains, and thawing (Pickard, 1971; Chuecas and Ahumada, 1980; Silva et al., 1995; 1998; Da´vila et al., 2002; Sobarzo, 2009). These fjords exchange water with Moraleda Channel, which is meridional in orientation and finally communicates with the coastal ocean through the Boca del Guafo passage. This scenario creates a gradient of brackish water from the fjords toward Boca del Guafo, giving rise to estuarine circulation dominated by a surface outflow of low-salinity water and a bottom inflow of saltier water that is restricted in some fjords due to their shallow depths (Pickard, 1971; Silva et al., 1995, 1997, 1998). A series of zonally oriented channels
n
Corresponding author. E-mail address: [email protected] (C. Calvete).
0278-4343/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2010.09.013
located on the western side of this estuarine area communicate Moraleda Channel with the adjacent coastal ocean. The freshwater of the inner fjords tends to disperse over the subsurface salt water, resulting in a brackish layer that induces strong vertical stratification and density fronts; these features determine the physical and biological characteristics of the fjord systems (Chapman and Lentz, 1994; Hill, 1998). The water column stratification in these regions is due to the balance between buoyancy and mixing forces. Whereas buoyancy tends to provide vertical stability through the heating and cooling relationship as well as that of evaporation, precipitation, and terrestrial drainage, stratification tends to be disrupted by internal waves and the frictional effects of the wind and bottom. The extension and structure of the buoyant plumes are influenced by tides, winds, density differences, river discharges, bathymetry, and in some cases the Earth’s rotation (Garvine, 1974; Mestres et al., 2003). The frontal limits of the plumes may be gradual or abrupt, causing significant horizontal gradients in the hydrography and biological variables (Simpson and James, 1986; Garvine, 1987; Largier, 1993; Lewis, 1997; Yankovsky and Chapman, 1997).
C. Calvete, M. Sobarzo / Continental Shelf Research 31 (2011) 162–171
Besides the relevance of the brackish layer on the physical processes of transport and mixing, other studies have reported high concentrations of dissolved silicon in this layer, highlighting its importance for primary production rates and phytoplankton biomass (Pizarro et al., 2005; Iriarte et al., 2007). The importance of brackish layers in this area is unknown, as virtually no studies that quantify their distribution. Thus, this study focuses on determining the equivalent height of the freshwater between the Boca del Guafo passage, Estero Elefantes and the associated frontal zones. The main goal of this work is to establish simple relationships between freshwater distributions and the discharges of main rivers in order to quantify the equivalent height of freshwater, thereby emphasizing its role in the potential energy anomaly and front locations.
2. Materials and methods The hydrographical data used was gathered during four CIMAR-fiordos cruises (CF4-P1, CF4-P2, CF7-P1, CF7-P2) carried out in spring, summer, winter and spring, respectively, in the study zone (Fig. 1). These data were collected with two CTDs (Sea-Bird 19 and Sea-Bird 25; Table 1) and were calibrated by standard methods. We only used data collected during the descent of the CTDO and from 2 m depth down. The data were also averaged for every 1 m. These cruises were coordinated by the National Oceanographic Committee (CONA) and were carried out on board the Chilean Navy vessel Agor-60 Vidal Gormaz (Table 1). River discharges and precipitation data for 1990–2003 were obtained from the monitoring stations maintained by Chile’s General Water Administration. In these series, missing months ranged between 1 and 96, and the time-series available for the Aysen and Cisnes rivers were shorter (1996–2003 and 2001–2003). Wind data (velocity and direction, taken every 15 min) were obtained from the vessel during each of the four cruises. However, these winds were highly variable in direction, and magnitude, influenced by the local topography of each fjord and channel. No relationship was found when comparing wind direction and intensity with the distribution of
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Table 1 Oceanographic data used in this study. CIMAR-Fjords cruises 1998–2001. Cruise
Date (mm/dd/yyyy)
CF4-P1 CF4-P2 CF7-P1 CF7-P2
Start
End
09/28/1998 02/26/1999 07/07/2001 11/12/2001
10/09/1998 03/05/1999 07/21/2001 11/27/2001
No. stations
Variables
48 37 42 42
T, T, T, T,
S S S S
and and and and
r r r r
Instruments
CTD-SBE19 CTD-SBE19 CTD-SBE25 CTD-SBE25
the brackish layer, indicating that the wind was not decisive in the distribution of the brackish layer during these four cruises. Nor did we find relationships between the wind and the deepening of the brackish layer. The wind did not exceed 6.3 m/s during any of the four cruises studied and, thus was not considered. The equivalent height of freshwater (in meters) was calculated following Blanton and Atkinson (1983): Z ðSref SðzÞ Þ Kf ¼ dz ð1Þ ðSref Þ D where Sref is the reference salinity, S(z) is the salinity at depth z, and D is the reference depth of integration. The choice of the reference depth (D) is important. If it is too deep, the Kf will include freshwater imported from the intermediate layer in addition to the locally supplied freshwater. Conversely, if the reference depth is too shallow, a the Kf will not include all of the locally supplied freshwater (Green et al., 2004). Since the halocline in this zone is located between 10 and 15 m depth (Silva et al., 1998), Kf was integrated down to 20 m to include all of the brackish water in the water column. In this study, we used a reference salinity of 34 since this value was observed below 150 m depth and only near Boca del Guafo (Silva et al., 1998). In general, Kf does not represent the thickness of the surface brackish layer but should be interpreted as m3/m2 of freshwater. Also, the percentages of Kf were obtained for each meter of depth in order to recognize the vertical extension of the brackish surface layer. This percentage was obtained by Sref SðzÞ % Kf ¼ x100 ð2Þ Sref The geographic location of the frontal zones was obtained by calculating the spatial gradient of the potential energy anomaly (PEA). This anomaly was calculated according to Simpson et al. (1990) Z g 2 PEA ¼ f ¼ ðr rÞzdz ð3Þ h h m where g is the acceleration of gravity, h the water column depth (20 m), rm the mean water column density, and r the density at depth z. The horizontal gradient of the PEA for each pair of stations considered was obtained by fstation1 fstation2 rPEA ¼ ð4Þ Distanceðstation1station2Þ After evaluating this variable, the maximum values (40.005 J/m4) were used to locate the geographic position of the fronts existing in the study zone. For the case of Aysen Fjord and Ventisquero Sound, transport was calculated using Knudsen’s equations (Lewis, 1997): S2 Qout ¼ Qf ð5Þ S2 S1
Fig. 1 Maps showing the locations of the hydrographic stations during the cruises CF4-P1 (spring 1998), CF4-P2 (summer 1999), CF7-P1 (winter 2001), and CF7-P2 (spring 2001).
Qin ¼ Qf
S1 S2 S1
ð6Þ
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where Qout is the surface outflow, Qin is the bottom inflow, Qf represents river discharge, and S1 (S2) is the surface (bottom) salinity. The separation between the bottom and surface layers was considered to be located in the lower limit of the halocline. S1 (S2) is the mean value of the all transect stations for each channel. Ayse´n Fjord¼Stations 15-16-17-17a-18-19-20-21-21a. Puyuguapi Sound¼Stations 42-41-40-39-38-37-36-35.
3. Results 3.1. Fluctuations of precipitation and river discharges The monthly average precipitation recorded at four locations between Boca del Guafo and Aysen Fjord (Marin Balmaceda,
Puyuhuapi, Puerto Cisnes, Puerto Chacabuco) showed a defined annual signal, with higher rainfall in winter ( Z200 mm) as compared to spring (Fig. 2, left). In summer, rainfall was close to or less than 200 mm. The highest annual mean rainfall was recorded in Puyuhuapi (252 mm) and the lowest in Marin Balmaceda (153 mm). On the other hand, the four rivers studied, which discharged into fjords Puyuhuapi (Ventisqueros and Cisnes rivers) and Aysen (Aysen and Lagunillas rivers), showed a different seasonal pattern in relation to rainfall, with higher discharges in spring (Fig. 2, right). Most likely, this increased river discharge in spring was related to melting ice. The Aysen and Cisnes rivers provided the highest annual mean river flows with 521 and 218 m3/s, respectively. The daily discharges of the rivers Ventisqueros, Aysen, and Lagunillas showed a similar pattern during the years of the hydrographic cruises (1998, 1999, 2001). Although clearly the
Fig. 2. Distribution of the mean (—) and standard deviation (- - - -) of historical rainfall (a, b, c, d) and river discharges (e, f, g, h) (1993–2003). Data from the General Water Administration (DGA), Chile.
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Aysen River discharge is clearly greater than that of Ventisqueros and Lagunillas, these three time-series tended to be coherent, showing peaks of freshwater every one or two months (Fig. 3). According to this figure, none of the four cruises took place during maximum freshwater discharges. In addition, although the CF4-P1 cruise was carried out in spring, the freshwater input was exceptionally low in September 1998. 3.2. Equivalent height of freshwater (Kf) Concordant with the heterogeneous distribution and magnitude of the freshwater input, the Kf values were greatest (close to 8 m) toward the head of the fjords and decreased toward Moraleda Channel and Boca del Guafo (Fig. 4). Peaks in Kf (between 7 and 8 m) were found at the head of Aysen Fjord (St. 21a) and in the far south of Estero Elefantes (St. 27). These maxima can be explained by the high discharges of the Aysen River and by the location of the glaciers and lagoons Gualas (461290 S; 731450 W) and San Rafael (461390 S; 731560 W) at the south end of Estero Elefantes. For cruises CF4-P2 (summer) and CF7-P2 (spring), the peaks close to 8.0 m in the inner
165
portion of Aysen Fjord (Fig. 4b and d) were coherent with the major discharges shown in Fig. 3b. Moraleda Channel received most of the brackish water coming from each of the five fjords located in this zone (Jacaf, Puyuguapi, Aysen, Quitralco, Cupquelan), showing a notable increase in Kf from 451S to the south (near the mouth of Puyuguapi Fjord). North of 451S, the Kf diminished, reaching values between 0 and 2 m. The vertical structure of Kf (in %) for Aysen Fjord is shown in Fig. 5. During CF7-P2 (spring 2001), this reached 80% to 90% near the surface (to 7 m depth) and toward the head of this fjord, consistent with Fig. 4d. During CF4-P1 (spring 1998), these values reached just up to 60%, showing interannual variations. Whereas the cline of brackish water was observed at depths over 5 m at all stations in CF7-P1, during CF4-P1 and CF4-P2, some stations showed these haloclines near 10 m depth. In general, when the cline of brackish water was deeper, the percentage of Kf in the surface layer was lower, indicating different vertical mixing conditions in the surface layer (for example, CF4-P1 and CF7-P1). Due to the reference salinity used (34), a Kf close to 10% at depths
Fig. 3. Distribution of the daily discharges of the rivers (a) Ventisqueros, (b) Aysen, and (c) Lagunillas.
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Fig. 4. Spatial distribution of the Kf during the cruises (a) CF4-P1, (b) CF4-P2, (c) CF7-P1, and (d) CF7-P2.
greater than 20 m did not signify the occurrence of brackish waters at those depths. Thus, we chose an arbitrary Kf value of 15% as the lower limit of the occurrence of brackish water in the water column for this study (see the vertical red line in Fig. 5). The vertical distribution of the isoline of Kf ¼15% was compared with the vertical density fields in different fjords. This isoline
coincided approximately with the depth of the 21 kg/m3 isopycnal. In Fig. 6, these two criteria were used to determine the horizontal extension and depth of the brackish layer in each fjord and during different cruises. In the case of the Jacaf Channel, the brackish layer fluctuated between a plume trapped toward Ventisquero Sound (CF7-P1) and
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167
Fig. 5. Vertical distribution of sigma-t and isoline of Kf (15% isoline in cyan) for the fjords (a) Jacaf, (b) Puyuguapi, (c) Ayse´n, and (d) Boca del Guafo–Estero Elefantes.
a layer around 12 m thick that moved approximately 30 km toward the mouth of the channel (CF7-P2; Fig. 6a). In Puyuguapy Fjord–Ventisquero Sound, the brackish layer was more extended, separating into two plumes during CF7-P1 (Fig. 6b). Both plumes were associated with two different rivers: the Cisnes River around the middle of Puyuguapi Fjord (near St. 40) and the Ventisquero River at the head of Ventisquero Sound (near St. 35). During CF7-P1, the brackish layer did not exceed 4 and 8 m depth at St. 40 and 35, respectively. On the other hand, during the cruise CF7-P2, a single layer of brackish water reached around 10 m depth and spanned nearly 100 km of horizontal extension. In Aysen Fjord, the brackish layer extended approximately 40–80 km toward Moraleda Channel; its thickness never exceeded 15 m depth and tended to decrease toward the east (Fig. 6c). Because St. 21a was not located close to the mouth of the Aysen River, during periods of low river discharges (Fig. 6c; CF7-P1), it failed to show a surface brackish layer. The extent of the brackish layer ranged according to discharges of the Aysen River (Fig. 3b). In Estero Elefantes (at the southern end of Moraleda Channel), the brackish layer occupied at least the first 40 m depth due to rivers and glacier melting associated with Laguna San Rafael and Ensenada Gualas. The 21 kg/m3 isopycnal was located between St. 24 and 26, except during CF7-P1, when it reached St. 15 near the mouth of the Aysen River and approximately 150 km from Estero Elefantes (Fig. 6d). In addition, considering only the 50-m surface layer, the Paso del Medio sector (St. 15) defined the south boundary of the subsurface 24 kg/m3 isopycnal (31.64 isohaline). This is the reason for the occurrence of the 24 kg/m3 isopycnal in the Jacaf and Puyuguapi–Ventisquero fjords (Fig. 6a and b) as well as its disappearance in the first 50 m of the Aysen (Fig. 6c) and Quitralco fjords (the latter is not shown). Unlike the other fjords, in Quitralco Fjord, the isoline of Kf ¼15% did not coincide with the 21 kg/m3 isopycnal. This difference was larger during CF7-P1 than CF7-P2 (figure not shown).
3.3. Stratification and determination of frontal zones The PEA quantifies the deficit in potential energy due to stratification, on average over 20 m depth, compared with that of the fully mixed water column. Waters that are vertically more homogeneous (smaller f values) are located in the Estero Elefantes, Moraleda Channel and Boca del Guafo (0–2 J/m3). In the inner fjords, f increases, indicating a more stratified water column (Fig. 7). This highlights an important difference between the Estero Elefantes and the fjords. In Estero Elefantes, the water column is vertically homogeneous with a brackish layer exceeding 20 m depth in thickness due to the huge input of freshwater
received from glaciers and rivers. The fjords, however, are highly stratified in the first 20 m depth due to a thin freshwater layer. Using these results, we calculated the horizontal gradients of PEA for each pair of stations, defining a gradient greater than or equal to 0.005 J/m4 as an indicator of a frontal zone. In general, this value coincided with the rise or almost vertical position of the isopycnals. In Jacaf Channel, the maximum spatial gradients of PEA were observed toward the center of the channel (CF7-P2) and in connection with Ventisquero Sound (CF7-P1) (Fig. 7c and d). In the Puyuguapi–Ventisquero Fjord, the greatest PEA gradients were observed toward Ventisquero Sound (CF7-P1) and between St. 41 and 40 (CF7-P2). In Estero Quitralco, the PEA gradients did not exceed 0.005 J/m4 (Fig. 7c and d). On the other hand, Aysen Fjord showed high values of PEA along its principal axis, exceeding 0.005 J/m4 and having peaks on the order of 0.0144 J/m4 (St. 17a-17; CF7-P1) and 0.0140 J/m4 (St. 19– 18; CF7-P2; Fig. 7). In the case of Moraleda Channel, PEA gradients did not exceed 0.005 J/m4 except between St. 14 and 15 (NE of Estero Elefantes), which is consistent with that observed in Fig. 6d. In a first approximation, the horizontal extension of the brackish layer and the location of fronts are dependent on the freshwater input into the fjords.
3.4. Relationships between river discharge, Kf, and Knudsen transport The relationship between Kf and river discharges was analyzed only for the cases of the Aysen and Puyuguapi–Vestiquero fjords. In the case of Aysen Fjord, the most important discharges come from the rivers Aysen, Lagunillas, and Cuervo. Of these three rivers, we used only the Aysen, which has the highest discharge, collecting ˜ iguales, Simpson, and Claro. For the water from the tributaries Man Puyuguapi–Ventisquero Fjord, information was only available from the Ventisquero (at the fjord head) and Cisnes rivers. The accumulated river discharge values determined during the five days prior to each cruise provided the best simple linear regression between Kf and river discharges calculated for each cruise at the stations closest to the river mouths (Fig. 8). Other relationships (10, 15, and 30-day accumulated river discharges) were not significant. This regression showed that 87% of the variability of the Kf responded to riverine input at the heads of these fjords (a ¼0.05; p ¼0.000024; Fig. 8). The Cisnes River, located near the middle of Aysen Fjord, was not included in this regression because its plume can move in two possible directions (toward the head or mouth of the fjord) depending on winds and tides. Thus, there is probably no relationship between the Cisnes River flow and the station near the fjord head since the plume of
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Fig. 6. Vertical distribution of sigma-t and isoline of Kf (15% isoline in cyan) for the fjords (a) Jacaf, (b) Puyuguapi, (c) Ayse´n, and (d) Boca del Guafo–Estero Elefantes.
this river is mostly directed towards the mouth of the fjord. The other rivers (Aysen and Venstisqueros) are located at the heads of the fjords with more limited advection options. Using the Knudsen equations (Lewis, 1997) and 5-day mean river discharges prior to the samplings, we obtained a surface
outflow of water from Aysen Fjord that fluctuated between 860 and 1471 m3/s. Consistent with periods of greater discharges, the freshwater outflow from this fjord (Qout) tended to be higher during the cruises CF4-P2 (summer 1999) and CF7-P2 (spring 2001). The inflow of oceanic water fluctuated between 629 and
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169
Fig. 7. Horizontal distribution of potential energy anomaly (PEA) and location of frontal zone (—) during the cruises (a) CF4-P1, (b) CF4-P2, (c) CF7-P1, and (d) CF7-P2.
1075 m3/s. For the case of the Puyuguapi–Ventisquero Fjord, Qout fluctuated between 876 and 1075 m3/s and Qin between 854 and 780 m3/s. Based on this transport, we calculated the velocity of vertical entrainment by means of the following equation (Lewis, 1997):
$¼
Qf S 1 Q ¼ in Ah S2 S1 Ah
ð7Þ
where Qf is the river discharge, S1(S2) the mean salinity of the upper (lower) layer, and Ah the surface area of the fjord (Aysen¼288 106 m2, Puyuguapi¼456 106 m2). This velocity does not imply diffusive flows through the density interface; rather, it approximates a rate of vertical exchange by advective mixing (Lewis, 1997). On average, the entrainment velocity was 0.27 m/day and 0.15 m/day for the Aysen and Puyuguapi fjords, respectively (Table 2). These equations describe steady state
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processes based on an assumption of no overly rapid changes in sea level within these fjords. In this regard, Fierro et al. (1999) have reported M2 tidal amplitudes of 76.33 and 81.67 cm at two locations near the mouth of Aysen Fjord and tidal currents in the range of 0.1–0.2 m/s (Ca´ceres et al., 2002). Thus, changes in sea level and associated tidal currents are not too great, making the application of these equations plausible. Finally, the volume of brackish water (Vf) and the residence time (tf) of the brackish layer within the fjord were calculated with Eqs. (8) and (9). S2 S1 H1 S2
ð8Þ
Vf Qf
ð9Þ
Vf ¼ A
tf ¼
According to these calculations, the average volume of brackish water in Aysen Fjord during the cruises studied was on the order of 94 million m3 (with a minimum of 72 and a maximum of 117 million m3). Thus, the input of freshwater to this fjord (Qf) has a mean residence time (tr) of 3.5 days and is subject to entrainment. These results are consistent with the significant linear regressions found between the Kf and the accumulated 5-day river discharges and not with accumulated discharges greater than one week.
Fig. 8. Lineal regression between Kf versus the accumulated (5-day) river discharges. Ayse´n and Puyuguapi–Ventisquero fjords. Cisnes River (gray squares) was not included in the regression.
4. Discussions and conclusions In this study, we analyzed hydrographic information from four CIMAR-Fjords cruises carried out between 1998 and 2001, focusing on the quantification of Kf in the fjord zone between Boca del Guafo and Estero Elefantes. Previous works have considered the freshwater fraction in the coastal ocean adjacent to the fjords (Da´vila et al., 2002). In general, the contribution made by rivers and rainfall into the fjords and channels of this zone has been poorly studied. Consequently, the influence of brackish layer on biogeochemical processes, exchanges of gases between the ocean and atmosphere, biological productivity, and the transport of larvae and pollutants, among others, are still not well understood. The surface brackish water in this study was defined as water with a %Kf 415% and a density (salinity) o1021 kg/m3 (o28). According to Sievers and Prado (1994), these values correspond to estuarinesalty water, with salinities between 21 and 31. The horizontal limit of this brackish layer gave rise to a density front determined by the PEA. Studies carried out in the Celtic Sea (English Channel) have shown values on the order of 10 J/m3 in well-mixed regions and close to 180 J/m3 in regions with very well-defined horizontal changes in density (Lewis, 1997). This last value coincided with an almost vertical position of the isopycnals. Studies carried out in the Sea of Clyde (Scotland), a semi-enclosed basin with a similar circulation regime as the fjords in our case study, showed surface salinity gradients on the order of 5 10 5 1/m; these are responsible for maintaining high-density gradients on the order of 10 4 kg/m4 (Kasai et al., 1999). In most of the fjords of our study, the maximum surface salinity gradients (10 3–10 4 1/m) and density (10 3–10 4 kg/m4) gradients were on the order of or exceeded the values defined by Kasai et al. (1999), corresponding to horizontal gradients of PEA equal to or greater than 0.005 J/m4. Since the PEA is a calculation that considers a 20-m layer, it is more robust for the detection of fronts. The most intense and recurrent gradients were located in the extreme southeast of Jacaf Fjord and Ventisquero Sound, in the intermediate zone of Puyuguapi Fjord, at the head and mouth of Aysen Fjord, and in the intermediate area of Moraleda Channel. The greatest values (on the order of 0.01 J/m4) were found in Aysen Fjord. Likewise, Ca´ceres (2004), utilizing satellite images, also observed a significant frontal zone in the extreme southeast of the mouth of Aysen Fjord. Short freshwater residence times have been reported in other places (Monbet, 1992; Wang et al., 2004; Liu et al., 2008). In the case of Aysen Fjord this value was 3.5 days. Elsewhere (Danshuei Estuary, Taiwan), such relatively short time scales have been indicated to be one of the limiting factors resulting in low phytoplankton biomass despite extremely high nutrient concentrations. This short residence time has also been proposed as the reason many pollutants exerts their effects beyond the coastal waters (Wang et al., 2004). On the other hand, this quick renewal of the brackish surface water within the fjords contrasts with the notably greater residence times (close to 1 year) found for the deep layer (Salinas and Hormazabal, 2004).
Table 2 Knudsen transport and entrainment velocities for the Ayse´n and Puyuguapi fjords during the cruises CF4-P1, CF4-P2, CF7-P1, and CF7-P2. Fjords
Cruise
Mean flow (5 days before)
Mean surface salinity
Mean bottom salinity
Qout (m3/s)
Qin (m3/s)
w (m/day)
Ayse´n Ayse´n Ayse´n Ayse´n Average Puyuguapi Puyuguapi Average
CF4-P1 CF4-P2 CF7-P1 CF7-P2
210 437 231 396 318.5 193 295 244
25.30 21.16 22.51 22.55 22.88 26.64 23.80 25.22
30.91 31.43 30.78 30.86 30.99 32.66 32.80 32.73
1157 1337 860 1471 1206.3 876 1075 975.5
947 900 629 1075 887.8 854 780 817
0.28 0.27 0.19 0.32 0.27 0.16 0.15 0.15
CF7-P1 CF7-P2
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The hydrodynamics and ecology of the surface layers in the fjords of southern Chile are sensitive to variability in the freshwater input. Climate change or future human interventions in these rivers (for example, hydroelectric dams) are likely to modify their total freshwater discharges, having some impact on the fjords (Green et al., 2004; Liu et al., 2008). New studies using direct current measurements and greater spatial and temporal resolution of hydrographic stations combined with numerical modeling will be necessary to corroborate some of these approximations. The main conclusions of this study are: 1. The study area showed a seasonal rainfall regime with higher values between May and August. 2. The flow of the four rivers studied showed a nival regime with higher discharges in spring. The Ventisqueros River showed a glacial regimen, due the proximity of glaciers in that zone. The Aysen River provided the largest freshwater discharge and all series were consistent showing peaks every one or two months. 3. The greatest values of Kf (close to 8 m) were found toward the head of the fjords and in the far south of Estero Elefantes decreasing toward Moraleda Channel and Boca del Guafo during the cruises CF4-P2 (summer 1999) and CF7-P2 (spring 2001). 4. The brackish layer in Estero Elefantes was more than 20 m thick due to the huge input of freshwater coming from glaciers and rivers in that place. The fjords, however, were highly stratified between approximately 5 and 15 m depth due to a thin freshwater layer. 5. The accumulated river discharge during the five days prior to each cruise provided the best simple linear regression between Kf and river discharges calculated for each cruise at the stations closest to the river mouths (r2 ¼ 0.87). 6. On average, the entrainment velocity was 0.27 and 0.15 m/day for the Aysen and Puyuguapi Fjords, respectively. 7. The mean residence time (tr) of Aysen Fjord was 3.5 days. This result was consistent with the significant linear regressions found between the Kf and the accumulated 5-day river flow. Acknowledgements The authors wish to acknowledge the Hydrographic and Oceanographic Service of the Chilean Navy for the availability of data and support given to the first author, who is in the Master’s program of the Universidad de Concepcio´n. In addition, we wish to acknowledge Mr. Nelson Silva for his exhaustive review and constructive suggestions. The co-author was partially financed by the project FONDECYT no. 1050487 and by the Basal Financing Program COPAS SUR AUSTRAL (CONICYT. PFB-31/2007). References Blanton, J., Atkinson, L., 1983. Transport and fate of river discharge on the continental shelf of the southeastern United States. J. Geophys. Res 88 4730-4718. Ca´ceres, M., Valle-Levinson, A., Sepu´lveda, H., Holderied, K., 2002. Transverse variability of flow and density in a Chilean fjord. Cont. Shelf Res. 22, 1683–1698.
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Ca´ceres, M., 2004. Surface features of the circulation in fjords of southern Chile observed in ERS and LANDSAT images. Gayana 2 (2), 71–76. Chapman, D., Lentz, S., 1994. Trapping of a coastal density front by the bottom boundary layer. J. Phys. Oceanogr. 24, 1464–1479. Chuecas, L., Ahumada, R., 1980. Contribucio´n al conocimiento de las condiciones hidrogra´ficas de los fiordos de la regio´n Magalla´nica–Chile. Boletı´n Instituto Oceanogra´fico de Sao Paulo 29 (2), 95–100. Da´vila, P., Figueroa, D., Muller, E., 2002. Freshwater input into the coastal ocean and its relation with the salinity distribution off austral Chile (35–551S). Cont. Shelf Res 22, 521–534. Fierro, J.,Belmar, J., Rosales, S., Blanco, J., 1999. Caracterizacio´n del re´gimen de mareas en canales interiores en las vecindades de la constriccio´n Meninea. Resu´menes Ampliados Crucero CIMAR Fiordo 4. Comite´ Oceanogra´fico Nacional, Chile, p. 76. Garvine, R., 1974. Physical features of the Connecticut River outflow during high discharge. J. Geophys. Res 79 (6), 831–846. Garvine, R., 1987. Estuarine plumes and fronts in shelf waters: a layer model. J. Phys. Oceanogr. 17, 1877–1896. Green, J.A., Molvaer, J., Stigebrandt, A., 2004. Hydrography response of Holandsfjord to changed freshwater runoff. J. Geophys. Res. 109, C07021. doi:10.1029/ 2004JC002295. Hill, A., 1998. Buoyancy effects in coastal and shelf seas. The Sea, vol. 10. Wiley & Sons, USA pp. 21–62. Iriarte, J.L., Gonza´lez, H.E., Liu, K.K., Rivas, C., Valenzuela, C., 2007. Spatial and temporal variability of chlorophyll and primary productivity in surface waters of southern Chile (41.51–431S). Estuarine, Coastal Shelf Sci. 74, 471–480. Kasai, A., Rippeth, T., Simpson, J., 1999. Density and flow structure in the Clyde sea front. Cont. Shelf Res. 19, 1833–1848. Largier, J., 1993. Estuarine fronts: how important are they? Estuaries 16 1–11. Lewis, R., 1997. Dispersion in Estuaries and Coastal Waters. Wiley, New York 312pp. Liu, W.C., Chen, W.B., Kuo, J.T., 2008. Modeling residence time response to freshwater discharge in a mesotidal estuary. Taiwan J. Mar. Sys. 74, 295–314. Mestres, M., Sierra, J., Sa´nchez-Arcilla, A., Gonza´lez, J., Wolf, T., Rodrı´guez, A., Ouillon, S., 2003. Modelling of the Ebro river plume. Validation with field observations. Sci. Mar. 67 (4), 379–391. Monbet, Y., 1992. Control of phytoplankton biomass in estuaries: a comparative analysis of microtidal and macrotidal estuaries. Estuaries 15 (4), 563–571. Pickard, G.L., 1971. Some physical oceanographic features of inlets of Chile. J. Fish. Res. Board Can 28, 1077–1106. ˜ oz, V., Chaco´n, V., Pacheco, H., Pizarro, G., Montecino, V., Guzma´n, L., Mun Frango´pulos, M., Retamal, L., Alarco´n, C., 2005. Patrones locales recurrentes del fitoplancton en fiordos y canales australes (461–561S) en primavera y verano. Ciencia y Tecnologı´a del Mar 28, 63–83. Salinas, S., Hormazabal, S., 2004. Capacidad de transporte de la constriccio´n de Meninea para un flujo de dos capas y el efecto de la corriente de marea. Ciencia y Tecnologı´a del Mar 27 (1), 5–16. Sievers, H., Prado, R., 1994. Contraste de las caraterı´sticas oceanogra´ficas del Seno Ayse´n, Chile, entre invierno y verano (Lat. 451200 S). Revista de Biologı´a Marina. Valparaı´so 29 (2), 167–209. Silva, N., Sievers, H., Prado, R., 1995. Descripcio´n oceanogra´fica de los canales australes de Chile. Zona Puerto Montt-Laguna San Rafael (Crucero Cimar-Fiordo 1). Revista de Biologı´a Marina. Valparaı´so 30 (2), 207–254. Silva, N., Calvete, C., Sievers, H., 1997. Caracterı´sticas oceanogra´ficas fı´sicas y quı´micas de canales australes chilenos entre Puerto Montt y Laguna San Rafael, (Crucero Cimar-Fiordo 1). Ciencia y Tecnologı´a del Mar 20, 23–106. Silva, N., Calvete, C., Sievers, H., 1998. Masas de agua y circulacio´n para algunos canales australes entre Puerto Montt y Laguna San Rafael, Chile (Crucero CimarFiordo 1). Ciencia y Tecnologı´a del Mar 21, 17–48. Simpson, J., James, I., 1986. Coastal and estuarine fronts. In: Mooers, C.N.K. (Ed.), Baroclinic processes on continental shelves, Coastal and Estuarine Series 3. American Geophysical Union, Washington D.C. Simpson, J., Brown, J., Matthews, J., Allen, G., 1990. Tidal straining, density currents, and stirring in the control of estuarine stratification. Estuaries 13 (2), 125–132. Sobarzo, M., 2009. The Fjord region of Southern Chile: oceanographic aspects, Marine Benthic Fauna of Chilean Patagonia. Nature in Focus, p. 1000. Wang, C.F., Hsu, M.H., Kuo, A.Y., 2004. Residence time of the Danshuei River estuary, Taiwan. Estuarine, Coastal Shelf Sci. 60, 381–393. Yankovsky, A., Chapman, D., 1997. A simple theory for the fate of buoyant coastal discharges. Journal of Physical Research 27, 1386–1401.
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Research papers
Quantification of the surface brackish water layer and frontal zones in southern Chilean fjords between Boca del Guafo (431300 S) and Estero Elefantes (461300 S) Carolina Calvete a,n, Marcus Sobarzo b a b
Hydrographic and Oceanographic Service of the Chilean Navy, Playa Ancha, Valparaı´so, Chile ´n, Chile Department of Oceanography and COPAS-Sur Austral, University of Concepcion, Casilla 160-C, Concepcio
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2009 Received in revised form 22 September 2010 Accepted 24 September 2010 Available online 15 December 2010
The southern Chilean region between the Boca del Guafo passage and Estero Elefantes contains one of the estuarine zones with the greatest freshwater influence on the planet. At the surface, plumes of freshwater from the fjord heads to their mouths, emptying into the Moraleda–Costa–Elefantes channel system and then the coastal ocean. The influence of this freshwater on the region’s estuarine dynamics, coastal ecology, and biogeochemical processes has only recently begun to be elucidated. Using hydrographic data from the CIMAR-Fiordos cruises (1998–2001), this study quantifies the equivalent height of freshwater, emphasizing the role it plays in the potential energy anomaly and front locations, as well as its relationship with river discharges. Using a criterion of equivalent height of freshwater 4 15% (density o 1021 kg/m3 and salinity o 28), the brackish layer was found to be 1–15 m thick (except in Estero Elefantes), with horizontal extensions on the order of 100 km. The limits of this layer tended to coincide with frontal zones having potential energy anomaly gradients 40.005 J/m4. The frontal zones were located in the extreme southeast of Jacaf Channel, at the head of Ventisquero Sound, in the central part of the Puyuguapi and Moraleda channels, and at the head and mouth of Ayse´n Fjord. The equivalent height of freshwater and potential energy anomaly showed a good correlation with the accumulated (5-day) river discharges (r2 ¼ 0.87), which were greatest toward the fjord heads in spring. The brackish surface water had short residence times (3.5 days) in Aysen Fjord, unlike the deep layer, which other authors report to have a longer residence time (near 1 year). & 2010 Elsevier Ltd. All rights reserved.
Keywords: Fjords Brackish layer Fronts Equivalent height of freshwater Potential energy anomaly
1. Introduction One of the largest estuarine zones on Earth is found in southern Chile between the Boca del Guafo passage (431300 S) and Estero Elefantes (461300 S). The numerous fjords distributed along this eastern coast receive freshwater from terrestrial drainage, rivers, rains, and thawing (Pickard, 1971; Chuecas and Ahumada, 1980; Silva et al., 1995; 1998; Da´vila et al., 2002; Sobarzo, 2009). These fjords exchange water with Moraleda Channel, which is meridional in orientation and finally communicates with the coastal ocean through the Boca del Guafo passage. This scenario creates a gradient of brackish water from the fjords toward Boca del Guafo, giving rise to estuarine circulation dominated by a surface outflow of low-salinity water and a bottom inflow of saltier water that is restricted in some fjords due to their shallow depths (Pickard, 1971; Silva et al., 1995, 1997, 1998). A series of zonally oriented channels
n
Corresponding author. E-mail address: [email protected] (C. Calvete).
0278-4343/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2010.09.013
located on the western side of this estuarine area communicate Moraleda Channel with the adjacent coastal ocean. The freshwater of the inner fjords tends to disperse over the subsurface salt water, resulting in a brackish layer that induces strong vertical stratification and density fronts; these features determine the physical and biological characteristics of the fjord systems (Chapman and Lentz, 1994; Hill, 1998). The water column stratification in these regions is due to the balance between buoyancy and mixing forces. Whereas buoyancy tends to provide vertical stability through the heating and cooling relationship as well as that of evaporation, precipitation, and terrestrial drainage, stratification tends to be disrupted by internal waves and the frictional effects of the wind and bottom. The extension and structure of the buoyant plumes are influenced by tides, winds, density differences, river discharges, bathymetry, and in some cases the Earth’s rotation (Garvine, 1974; Mestres et al., 2003). The frontal limits of the plumes may be gradual or abrupt, causing significant horizontal gradients in the hydrography and biological variables (Simpson and James, 1986; Garvine, 1987; Largier, 1993; Lewis, 1997; Yankovsky and Chapman, 1997).
C. Calvete, M. Sobarzo / Continental Shelf Research 31 (2011) 162–171
Besides the relevance of the brackish layer on the physical processes of transport and mixing, other studies have reported high concentrations of dissolved silicon in this layer, highlighting its importance for primary production rates and phytoplankton biomass (Pizarro et al., 2005; Iriarte et al., 2007). The importance of brackish layers in this area is unknown, as virtually no studies that quantify their distribution. Thus, this study focuses on determining the equivalent height of the freshwater between the Boca del Guafo passage, Estero Elefantes and the associated frontal zones. The main goal of this work is to establish simple relationships between freshwater distributions and the discharges of main rivers in order to quantify the equivalent height of freshwater, thereby emphasizing its role in the potential energy anomaly and front locations.
2. Materials and methods The hydrographical data used was gathered during four CIMAR-fiordos cruises (CF4-P1, CF4-P2, CF7-P1, CF7-P2) carried out in spring, summer, winter and spring, respectively, in the study zone (Fig. 1). These data were collected with two CTDs (Sea-Bird 19 and Sea-Bird 25; Table 1) and were calibrated by standard methods. We only used data collected during the descent of the CTDO and from 2 m depth down. The data were also averaged for every 1 m. These cruises were coordinated by the National Oceanographic Committee (CONA) and were carried out on board the Chilean Navy vessel Agor-60 Vidal Gormaz (Table 1). River discharges and precipitation data for 1990–2003 were obtained from the monitoring stations maintained by Chile’s General Water Administration. In these series, missing months ranged between 1 and 96, and the time-series available for the Aysen and Cisnes rivers were shorter (1996–2003 and 2001–2003). Wind data (velocity and direction, taken every 15 min) were obtained from the vessel during each of the four cruises. However, these winds were highly variable in direction, and magnitude, influenced by the local topography of each fjord and channel. No relationship was found when comparing wind direction and intensity with the distribution of
163
Table 1 Oceanographic data used in this study. CIMAR-Fjords cruises 1998–2001. Cruise
Date (mm/dd/yyyy)
CF4-P1 CF4-P2 CF7-P1 CF7-P2
Start
End
09/28/1998 02/26/1999 07/07/2001 11/12/2001
10/09/1998 03/05/1999 07/21/2001 11/27/2001
No. stations
Variables
48 37 42 42
T, T, T, T,
S S S S
and and and and
r r r r
Instruments
CTD-SBE19 CTD-SBE19 CTD-SBE25 CTD-SBE25
the brackish layer, indicating that the wind was not decisive in the distribution of the brackish layer during these four cruises. Nor did we find relationships between the wind and the deepening of the brackish layer. The wind did not exceed 6.3 m/s during any of the four cruises studied and, thus was not considered. The equivalent height of freshwater (in meters) was calculated following Blanton and Atkinson (1983): Z ðSref SðzÞ Þ Kf ¼ dz ð1Þ ðSref Þ D where Sref is the reference salinity, S(z) is the salinity at depth z, and D is the reference depth of integration. The choice of the reference depth (D) is important. If it is too deep, the Kf will include freshwater imported from the intermediate layer in addition to the locally supplied freshwater. Conversely, if the reference depth is too shallow, a the Kf will not include all of the locally supplied freshwater (Green et al., 2004). Since the halocline in this zone is located between 10 and 15 m depth (Silva et al., 1998), Kf was integrated down to 20 m to include all of the brackish water in the water column. In this study, we used a reference salinity of 34 since this value was observed below 150 m depth and only near Boca del Guafo (Silva et al., 1998). In general, Kf does not represent the thickness of the surface brackish layer but should be interpreted as m3/m2 of freshwater. Also, the percentages of Kf were obtained for each meter of depth in order to recognize the vertical extension of the brackish surface layer. This percentage was obtained by Sref SðzÞ % Kf ¼ x100 ð2Þ Sref The geographic location of the frontal zones was obtained by calculating the spatial gradient of the potential energy anomaly (PEA). This anomaly was calculated according to Simpson et al. (1990) Z g 2 PEA ¼ f ¼ ðr rÞzdz ð3Þ h h m where g is the acceleration of gravity, h the water column depth (20 m), rm the mean water column density, and r the density at depth z. The horizontal gradient of the PEA for each pair of stations considered was obtained by fstation1 fstation2 rPEA ¼ ð4Þ Distanceðstation1station2Þ After evaluating this variable, the maximum values (40.005 J/m4) were used to locate the geographic position of the fronts existing in the study zone. For the case of Aysen Fjord and Ventisquero Sound, transport was calculated using Knudsen’s equations (Lewis, 1997): S2 Qout ¼ Qf ð5Þ S2 S1
Fig. 1 Maps showing the locations of the hydrographic stations during the cruises CF4-P1 (spring 1998), CF4-P2 (summer 1999), CF7-P1 (winter 2001), and CF7-P2 (spring 2001).
Qin ¼ Qf
S1 S2 S1
ð6Þ
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where Qout is the surface outflow, Qin is the bottom inflow, Qf represents river discharge, and S1 (S2) is the surface (bottom) salinity. The separation between the bottom and surface layers was considered to be located in the lower limit of the halocline. S1 (S2) is the mean value of the all transect stations for each channel. Ayse´n Fjord¼Stations 15-16-17-17a-18-19-20-21-21a. Puyuguapi Sound¼Stations 42-41-40-39-38-37-36-35.
3. Results 3.1. Fluctuations of precipitation and river discharges The monthly average precipitation recorded at four locations between Boca del Guafo and Aysen Fjord (Marin Balmaceda,
Puyuhuapi, Puerto Cisnes, Puerto Chacabuco) showed a defined annual signal, with higher rainfall in winter ( Z200 mm) as compared to spring (Fig. 2, left). In summer, rainfall was close to or less than 200 mm. The highest annual mean rainfall was recorded in Puyuhuapi (252 mm) and the lowest in Marin Balmaceda (153 mm). On the other hand, the four rivers studied, which discharged into fjords Puyuhuapi (Ventisqueros and Cisnes rivers) and Aysen (Aysen and Lagunillas rivers), showed a different seasonal pattern in relation to rainfall, with higher discharges in spring (Fig. 2, right). Most likely, this increased river discharge in spring was related to melting ice. The Aysen and Cisnes rivers provided the highest annual mean river flows with 521 and 218 m3/s, respectively. The daily discharges of the rivers Ventisqueros, Aysen, and Lagunillas showed a similar pattern during the years of the hydrographic cruises (1998, 1999, 2001). Although clearly the
Fig. 2. Distribution of the mean (—) and standard deviation (- - - -) of historical rainfall (a, b, c, d) and river discharges (e, f, g, h) (1993–2003). Data from the General Water Administration (DGA), Chile.
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Aysen River discharge is clearly greater than that of Ventisqueros and Lagunillas, these three time-series tended to be coherent, showing peaks of freshwater every one or two months (Fig. 3). According to this figure, none of the four cruises took place during maximum freshwater discharges. In addition, although the CF4-P1 cruise was carried out in spring, the freshwater input was exceptionally low in September 1998. 3.2. Equivalent height of freshwater (Kf) Concordant with the heterogeneous distribution and magnitude of the freshwater input, the Kf values were greatest (close to 8 m) toward the head of the fjords and decreased toward Moraleda Channel and Boca del Guafo (Fig. 4). Peaks in Kf (between 7 and 8 m) were found at the head of Aysen Fjord (St. 21a) and in the far south of Estero Elefantes (St. 27). These maxima can be explained by the high discharges of the Aysen River and by the location of the glaciers and lagoons Gualas (461290 S; 731450 W) and San Rafael (461390 S; 731560 W) at the south end of Estero Elefantes. For cruises CF4-P2 (summer) and CF7-P2 (spring), the peaks close to 8.0 m in the inner
165
portion of Aysen Fjord (Fig. 4b and d) were coherent with the major discharges shown in Fig. 3b. Moraleda Channel received most of the brackish water coming from each of the five fjords located in this zone (Jacaf, Puyuguapi, Aysen, Quitralco, Cupquelan), showing a notable increase in Kf from 451S to the south (near the mouth of Puyuguapi Fjord). North of 451S, the Kf diminished, reaching values between 0 and 2 m. The vertical structure of Kf (in %) for Aysen Fjord is shown in Fig. 5. During CF7-P2 (spring 2001), this reached 80% to 90% near the surface (to 7 m depth) and toward the head of this fjord, consistent with Fig. 4d. During CF4-P1 (spring 1998), these values reached just up to 60%, showing interannual variations. Whereas the cline of brackish water was observed at depths over 5 m at all stations in CF7-P1, during CF4-P1 and CF4-P2, some stations showed these haloclines near 10 m depth. In general, when the cline of brackish water was deeper, the percentage of Kf in the surface layer was lower, indicating different vertical mixing conditions in the surface layer (for example, CF4-P1 and CF7-P1). Due to the reference salinity used (34), a Kf close to 10% at depths
Fig. 3. Distribution of the daily discharges of the rivers (a) Ventisqueros, (b) Aysen, and (c) Lagunillas.
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Fig. 4. Spatial distribution of the Kf during the cruises (a) CF4-P1, (b) CF4-P2, (c) CF7-P1, and (d) CF7-P2.
greater than 20 m did not signify the occurrence of brackish waters at those depths. Thus, we chose an arbitrary Kf value of 15% as the lower limit of the occurrence of brackish water in the water column for this study (see the vertical red line in Fig. 5). The vertical distribution of the isoline of Kf ¼15% was compared with the vertical density fields in different fjords. This isoline
coincided approximately with the depth of the 21 kg/m3 isopycnal. In Fig. 6, these two criteria were used to determine the horizontal extension and depth of the brackish layer in each fjord and during different cruises. In the case of the Jacaf Channel, the brackish layer fluctuated between a plume trapped toward Ventisquero Sound (CF7-P1) and
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167
Fig. 5. Vertical distribution of sigma-t and isoline of Kf (15% isoline in cyan) for the fjords (a) Jacaf, (b) Puyuguapi, (c) Ayse´n, and (d) Boca del Guafo–Estero Elefantes.
a layer around 12 m thick that moved approximately 30 km toward the mouth of the channel (CF7-P2; Fig. 6a). In Puyuguapy Fjord–Ventisquero Sound, the brackish layer was more extended, separating into two plumes during CF7-P1 (Fig. 6b). Both plumes were associated with two different rivers: the Cisnes River around the middle of Puyuguapi Fjord (near St. 40) and the Ventisquero River at the head of Ventisquero Sound (near St. 35). During CF7-P1, the brackish layer did not exceed 4 and 8 m depth at St. 40 and 35, respectively. On the other hand, during the cruise CF7-P2, a single layer of brackish water reached around 10 m depth and spanned nearly 100 km of horizontal extension. In Aysen Fjord, the brackish layer extended approximately 40–80 km toward Moraleda Channel; its thickness never exceeded 15 m depth and tended to decrease toward the east (Fig. 6c). Because St. 21a was not located close to the mouth of the Aysen River, during periods of low river discharges (Fig. 6c; CF7-P1), it failed to show a surface brackish layer. The extent of the brackish layer ranged according to discharges of the Aysen River (Fig. 3b). In Estero Elefantes (at the southern end of Moraleda Channel), the brackish layer occupied at least the first 40 m depth due to rivers and glacier melting associated with Laguna San Rafael and Ensenada Gualas. The 21 kg/m3 isopycnal was located between St. 24 and 26, except during CF7-P1, when it reached St. 15 near the mouth of the Aysen River and approximately 150 km from Estero Elefantes (Fig. 6d). In addition, considering only the 50-m surface layer, the Paso del Medio sector (St. 15) defined the south boundary of the subsurface 24 kg/m3 isopycnal (31.64 isohaline). This is the reason for the occurrence of the 24 kg/m3 isopycnal in the Jacaf and Puyuguapi–Ventisquero fjords (Fig. 6a and b) as well as its disappearance in the first 50 m of the Aysen (Fig. 6c) and Quitralco fjords (the latter is not shown). Unlike the other fjords, in Quitralco Fjord, the isoline of Kf ¼15% did not coincide with the 21 kg/m3 isopycnal. This difference was larger during CF7-P1 than CF7-P2 (figure not shown).
3.3. Stratification and determination of frontal zones The PEA quantifies the deficit in potential energy due to stratification, on average over 20 m depth, compared with that of the fully mixed water column. Waters that are vertically more homogeneous (smaller f values) are located in the Estero Elefantes, Moraleda Channel and Boca del Guafo (0–2 J/m3). In the inner fjords, f increases, indicating a more stratified water column (Fig. 7). This highlights an important difference between the Estero Elefantes and the fjords. In Estero Elefantes, the water column is vertically homogeneous with a brackish layer exceeding 20 m depth in thickness due to the huge input of freshwater
received from glaciers and rivers. The fjords, however, are highly stratified in the first 20 m depth due to a thin freshwater layer. Using these results, we calculated the horizontal gradients of PEA for each pair of stations, defining a gradient greater than or equal to 0.005 J/m4 as an indicator of a frontal zone. In general, this value coincided with the rise or almost vertical position of the isopycnals. In Jacaf Channel, the maximum spatial gradients of PEA were observed toward the center of the channel (CF7-P2) and in connection with Ventisquero Sound (CF7-P1) (Fig. 7c and d). In the Puyuguapi–Ventisquero Fjord, the greatest PEA gradients were observed toward Ventisquero Sound (CF7-P1) and between St. 41 and 40 (CF7-P2). In Estero Quitralco, the PEA gradients did not exceed 0.005 J/m4 (Fig. 7c and d). On the other hand, Aysen Fjord showed high values of PEA along its principal axis, exceeding 0.005 J/m4 and having peaks on the order of 0.0144 J/m4 (St. 17a-17; CF7-P1) and 0.0140 J/m4 (St. 19– 18; CF7-P2; Fig. 7). In the case of Moraleda Channel, PEA gradients did not exceed 0.005 J/m4 except between St. 14 and 15 (NE of Estero Elefantes), which is consistent with that observed in Fig. 6d. In a first approximation, the horizontal extension of the brackish layer and the location of fronts are dependent on the freshwater input into the fjords.
3.4. Relationships between river discharge, Kf, and Knudsen transport The relationship between Kf and river discharges was analyzed only for the cases of the Aysen and Puyuguapi–Vestiquero fjords. In the case of Aysen Fjord, the most important discharges come from the rivers Aysen, Lagunillas, and Cuervo. Of these three rivers, we used only the Aysen, which has the highest discharge, collecting ˜ iguales, Simpson, and Claro. For the water from the tributaries Man Puyuguapi–Ventisquero Fjord, information was only available from the Ventisquero (at the fjord head) and Cisnes rivers. The accumulated river discharge values determined during the five days prior to each cruise provided the best simple linear regression between Kf and river discharges calculated for each cruise at the stations closest to the river mouths (Fig. 8). Other relationships (10, 15, and 30-day accumulated river discharges) were not significant. This regression showed that 87% of the variability of the Kf responded to riverine input at the heads of these fjords (a ¼0.05; p ¼0.000024; Fig. 8). The Cisnes River, located near the middle of Aysen Fjord, was not included in this regression because its plume can move in two possible directions (toward the head or mouth of the fjord) depending on winds and tides. Thus, there is probably no relationship between the Cisnes River flow and the station near the fjord head since the plume of
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Fig. 6. Vertical distribution of sigma-t and isoline of Kf (15% isoline in cyan) for the fjords (a) Jacaf, (b) Puyuguapi, (c) Ayse´n, and (d) Boca del Guafo–Estero Elefantes.
this river is mostly directed towards the mouth of the fjord. The other rivers (Aysen and Venstisqueros) are located at the heads of the fjords with more limited advection options. Using the Knudsen equations (Lewis, 1997) and 5-day mean river discharges prior to the samplings, we obtained a surface
outflow of water from Aysen Fjord that fluctuated between 860 and 1471 m3/s. Consistent with periods of greater discharges, the freshwater outflow from this fjord (Qout) tended to be higher during the cruises CF4-P2 (summer 1999) and CF7-P2 (spring 2001). The inflow of oceanic water fluctuated between 629 and
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Fig. 7. Horizontal distribution of potential energy anomaly (PEA) and location of frontal zone (—) during the cruises (a) CF4-P1, (b) CF4-P2, (c) CF7-P1, and (d) CF7-P2.
1075 m3/s. For the case of the Puyuguapi–Ventisquero Fjord, Qout fluctuated between 876 and 1075 m3/s and Qin between 854 and 780 m3/s. Based on this transport, we calculated the velocity of vertical entrainment by means of the following equation (Lewis, 1997):
$¼
Qf S 1 Q ¼ in Ah S2 S1 Ah
ð7Þ
where Qf is the river discharge, S1(S2) the mean salinity of the upper (lower) layer, and Ah the surface area of the fjord (Aysen¼288 106 m2, Puyuguapi¼456 106 m2). This velocity does not imply diffusive flows through the density interface; rather, it approximates a rate of vertical exchange by advective mixing (Lewis, 1997). On average, the entrainment velocity was 0.27 m/day and 0.15 m/day for the Aysen and Puyuguapi fjords, respectively (Table 2). These equations describe steady state
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processes based on an assumption of no overly rapid changes in sea level within these fjords. In this regard, Fierro et al. (1999) have reported M2 tidal amplitudes of 76.33 and 81.67 cm at two locations near the mouth of Aysen Fjord and tidal currents in the range of 0.1–0.2 m/s (Ca´ceres et al., 2002). Thus, changes in sea level and associated tidal currents are not too great, making the application of these equations plausible. Finally, the volume of brackish water (Vf) and the residence time (tf) of the brackish layer within the fjord were calculated with Eqs. (8) and (9). S2 S1 H1 S2
ð8Þ
Vf Qf
ð9Þ
Vf ¼ A
tf ¼
According to these calculations, the average volume of brackish water in Aysen Fjord during the cruises studied was on the order of 94 million m3 (with a minimum of 72 and a maximum of 117 million m3). Thus, the input of freshwater to this fjord (Qf) has a mean residence time (tr) of 3.5 days and is subject to entrainment. These results are consistent with the significant linear regressions found between the Kf and the accumulated 5-day river discharges and not with accumulated discharges greater than one week.
Fig. 8. Lineal regression between Kf versus the accumulated (5-day) river discharges. Ayse´n and Puyuguapi–Ventisquero fjords. Cisnes River (gray squares) was not included in the regression.
4. Discussions and conclusions In this study, we analyzed hydrographic information from four CIMAR-Fjords cruises carried out between 1998 and 2001, focusing on the quantification of Kf in the fjord zone between Boca del Guafo and Estero Elefantes. Previous works have considered the freshwater fraction in the coastal ocean adjacent to the fjords (Da´vila et al., 2002). In general, the contribution made by rivers and rainfall into the fjords and channels of this zone has been poorly studied. Consequently, the influence of brackish layer on biogeochemical processes, exchanges of gases between the ocean and atmosphere, biological productivity, and the transport of larvae and pollutants, among others, are still not well understood. The surface brackish water in this study was defined as water with a %Kf 415% and a density (salinity) o1021 kg/m3 (o28). According to Sievers and Prado (1994), these values correspond to estuarinesalty water, with salinities between 21 and 31. The horizontal limit of this brackish layer gave rise to a density front determined by the PEA. Studies carried out in the Celtic Sea (English Channel) have shown values on the order of 10 J/m3 in well-mixed regions and close to 180 J/m3 in regions with very well-defined horizontal changes in density (Lewis, 1997). This last value coincided with an almost vertical position of the isopycnals. Studies carried out in the Sea of Clyde (Scotland), a semi-enclosed basin with a similar circulation regime as the fjords in our case study, showed surface salinity gradients on the order of 5 10 5 1/m; these are responsible for maintaining high-density gradients on the order of 10 4 kg/m4 (Kasai et al., 1999). In most of the fjords of our study, the maximum surface salinity gradients (10 3–10 4 1/m) and density (10 3–10 4 kg/m4) gradients were on the order of or exceeded the values defined by Kasai et al. (1999), corresponding to horizontal gradients of PEA equal to or greater than 0.005 J/m4. Since the PEA is a calculation that considers a 20-m layer, it is more robust for the detection of fronts. The most intense and recurrent gradients were located in the extreme southeast of Jacaf Fjord and Ventisquero Sound, in the intermediate zone of Puyuguapi Fjord, at the head and mouth of Aysen Fjord, and in the intermediate area of Moraleda Channel. The greatest values (on the order of 0.01 J/m4) were found in Aysen Fjord. Likewise, Ca´ceres (2004), utilizing satellite images, also observed a significant frontal zone in the extreme southeast of the mouth of Aysen Fjord. Short freshwater residence times have been reported in other places (Monbet, 1992; Wang et al., 2004; Liu et al., 2008). In the case of Aysen Fjord this value was 3.5 days. Elsewhere (Danshuei Estuary, Taiwan), such relatively short time scales have been indicated to be one of the limiting factors resulting in low phytoplankton biomass despite extremely high nutrient concentrations. This short residence time has also been proposed as the reason many pollutants exerts their effects beyond the coastal waters (Wang et al., 2004). On the other hand, this quick renewal of the brackish surface water within the fjords contrasts with the notably greater residence times (close to 1 year) found for the deep layer (Salinas and Hormazabal, 2004).
Table 2 Knudsen transport and entrainment velocities for the Ayse´n and Puyuguapi fjords during the cruises CF4-P1, CF4-P2, CF7-P1, and CF7-P2. Fjords
Cruise
Mean flow (5 days before)
Mean surface salinity
Mean bottom salinity
Qout (m3/s)
Qin (m3/s)
w (m/day)
Ayse´n Ayse´n Ayse´n Ayse´n Average Puyuguapi Puyuguapi Average
CF4-P1 CF4-P2 CF7-P1 CF7-P2
210 437 231 396 318.5 193 295 244
25.30 21.16 22.51 22.55 22.88 26.64 23.80 25.22
30.91 31.43 30.78 30.86 30.99 32.66 32.80 32.73
1157 1337 860 1471 1206.3 876 1075 975.5
947 900 629 1075 887.8 854 780 817
0.28 0.27 0.19 0.32 0.27 0.16 0.15 0.15
CF7-P1 CF7-P2
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The hydrodynamics and ecology of the surface layers in the fjords of southern Chile are sensitive to variability in the freshwater input. Climate change or future human interventions in these rivers (for example, hydroelectric dams) are likely to modify their total freshwater discharges, having some impact on the fjords (Green et al., 2004; Liu et al., 2008). New studies using direct current measurements and greater spatial and temporal resolution of hydrographic stations combined with numerical modeling will be necessary to corroborate some of these approximations. The main conclusions of this study are: 1. The study area showed a seasonal rainfall regime with higher values between May and August. 2. The flow of the four rivers studied showed a nival regime with higher discharges in spring. The Ventisqueros River showed a glacial regimen, due the proximity of glaciers in that zone. The Aysen River provided the largest freshwater discharge and all series were consistent showing peaks every one or two months. 3. The greatest values of Kf (close to 8 m) were found toward the head of the fjords and in the far south of Estero Elefantes decreasing toward Moraleda Channel and Boca del Guafo during the cruises CF4-P2 (summer 1999) and CF7-P2 (spring 2001). 4. The brackish layer in Estero Elefantes was more than 20 m thick due to the huge input of freshwater coming from glaciers and rivers in that place. The fjords, however, were highly stratified between approximately 5 and 15 m depth due to a thin freshwater layer. 5. The accumulated river discharge during the five days prior to each cruise provided the best simple linear regression between Kf and river discharges calculated for each cruise at the stations closest to the river mouths (r2 ¼ 0.87). 6. On average, the entrainment velocity was 0.27 and 0.15 m/day for the Aysen and Puyuguapi Fjords, respectively. 7. The mean residence time (tr) of Aysen Fjord was 3.5 days. This result was consistent with the significant linear regressions found between the Kf and the accumulated 5-day river flow. Acknowledgements The authors wish to acknowledge the Hydrographic and Oceanographic Service of the Chilean Navy for the availability of data and support given to the first author, who is in the Master’s program of the Universidad de Concepcio´n. In addition, we wish to acknowledge Mr. Nelson Silva for his exhaustive review and constructive suggestions. The co-author was partially financed by the project FONDECYT no. 1050487 and by the Basal Financing Program COPAS SUR AUSTRAL (CONICYT. PFB-31/2007). References Blanton, J., Atkinson, L., 1983. Transport and fate of river discharge on the continental shelf of the southeastern United States. J. Geophys. Res 88 4730-4718. Ca´ceres, M., Valle-Levinson, A., Sepu´lveda, H., Holderied, K., 2002. Transverse variability of flow and density in a Chilean fjord. Cont. Shelf Res. 22, 1683–1698.
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