Correlations (Rs) between Niño indicators and flow (Q) of RdlP, Uruguay and Parana rivers.
Climatic, hydrological, and oceanographic conditions were determined during the 2009–2010 El Nino/Southern Oscillation (ENSO) on the north coast of the Río de la Plata (RdlP) estuary. The maximum monthly rainfall was observed in the middle and upper La Plata basin during September 2009 and February 2010 (“El Niño” phase, (EN)). The monthly flow of RdlP showed an increase with rainfall and significant differences between ENSO phases. The wind stress showed fluctuations in both phases, being less intense during EN, during which time maximum flow of RdlP was observed. During the EN phase, increased precipitation contributed to variations in salinity and absence of water column stratification in the north coast of RdlP. This was also associated with variations in Secchi depth, oxygen saturation, and nutrient concentrations. The spatial location of the turbidity front was associated with the flow of the RdlP and wind stress, thus conditioning the physicochemical characteristics of the water column, mainly during EN phase.
- estuarine dynamics
- river flow
- physicochemical properties
- Río de la Plata
“El Niño/Southern Oscillation” (ENSO) is a global phenomenon related to the ocean-atmosphere interactions in the Equatorial Pacific, with cycles consisting in a warm phase (“El Niño”), periods with neutral years (“Neutral”), and a cold phase (“La Niña”), with a 3–7 years periodicity [1, 2]. This event activates changes in the general atmospheric circulation, causing climate and hydrologic changes in continental areas, coastal areas, and tropical and extra-tropical zones of the Pacific or Atlantic Oceans [3, 4]. These modifications cause changes in the systems physicochemical conditions at different organizational levels, impacting on the composition and abundance of benthic, planktonic, and nektonic communities [5, 6, 7].
In the La Plata basin, seasonality of the climatic elements determining the hydrologic cycles is ruled by the South Atlantic anticyclone, which is more intense in winter. The region has a warm season (October–April) with an average rainfall of 5.5 mm d−1 and maximum values near 9 mm d−1 and a cold season (May–September) with average rainfall less than 2 mm d−1 . The alteration of the rainfall regime is one of the strongest signals of the changes caused by the ENSO event for the southern region of South America. During the warm phase, there is an increase in the spring rains, while during the cold phase, there is a decrease in rainfall [9, 10]. On the other hand, flows of the hydrological systems of the La Plata basin, including those of the main tributary rivers of the RdlP estuary (Parana and Uruguay rivers), are highly sensitive to modifications in the rainfall regime, with inter-annual and inter-decadal variability, being affected by ENSO events in their warm phase [11, 12].
Surface ocean temperature is the most analyzed oceanographic variable regarding the “El Niño” phase and its ecosystem effects in oceanic and coastal marine zones . Nevertheless, estuarine zones are expected to experience a greater impact of hydrologic effects (fresh water or ocean water inputs) . ENSO impacts in coastal zones include increased rainfall, hydrological changes in basins, modifications in river flow , and generation of inter-annual variations in fresh water discharges for estuarine systems [16, 17].
ENSO is considered in the RdlP as a large-scale atmospheric forcing effect on the discharges of the tributary rivers [15, 17], changes in salinity and nutrients , as well as in the location and location of turbidity or salinity fronts [19, 20]. Several studies have described the relationship between ENSO events, RdlP flow, and salinity of the estuarine system [18, 19, 21]. According to , surface sediments on the north coast of RdlP showed a variability in the trophic state related to the ENSO event, with differences between the phases (“El Niño” and “La Niña-Neutral”).
The aim of this study was to identify the main environmental drivers that promote modifications in the oceanographic conditions on the north coast of the RdlP, during ENSO 2009–2010. We measured the meteorological (rainfall and wind), hydrological (principal rivers flow), and oceanographic (temperature, salinity, oxygen saturation, Secchi depth, chlorophyll
2.1. Study area
The La Plata basin is the second largest river basin in the South America, covering an area of 3.1 × 106 km2; it includes five countries (Argentina, Brazil, Bolivia, Paraguay and Uruguay) with some of the most populated cities in South America including Sao Paulo, Buenos Aires, and Montevideo. The main rivers draining into the estuary of RdlP are the Parana and Uruguay rivers (Figure 1A).
The RdlP is an estuarine system characterized by the presence of a salt wedge, low river discharge seasonality, low tidal amplitude (< 1 m), an extensive and permanent connection to the sea, and high susceptibility to atmospheric drivers because of its large size and its shallow depth. The mean annual RdlP flow governs the salinity and has monthly to inter-annual variations of 25,000 m3s−1 [22, 23, 24]. In this system, several authors report biological processes , impacts on water quality, or presence of invasive alien species  associated with the spatiotemporal fluctuation of its front area .
The montevideo coastal zone (MCZ) is located on the north coast and middle zone of the RdlP between the mouths of the Santa Lucia River and Carrasco stream with an approximate extension of 50 km (Figure 1B). The largest city in Uruguay, Montevideo, covers ≈ 49% of the coastal zone . The MCZ is characterized by high levels of industrial and harbor activities [26, 27] but also with urban spaces for recreation (sandy beaches and rocky shores), diving, artisanal or sport fishing areas, and conservation areas (protected area Punta Yeguas and Santa Lucía Wetlands). Several studies in the MCZ identified a contamination gradient from the innermost Montevideo Bay area to the outer and adjacent coastal zone. Three contamination areas have been identified within the region: high (internal Montevideo Bay and Montevideo Harbor), medium (external Montevideo Bay and adjacent coast), and low impact (adjacent coastal zone, includes Punta Brava and Punta Yeguas) regions . Montevideo Bay is 10 km2 and has a mean depth of ≈ 5m. In this system, nutrients (nitrogen, phosphorus), heavy metals, and hydrocarbons pollution have been associated with significant degradation of ecosystem [27, 28, 29, 30, 31]. These pollutants are derived from industrial activities, from “La Teja” refinery, harbor operations (i.e., navigation, bulk loading and dredging), or contributions from domestic and industrial effluents from Pantanoso, Miguelete, and Seco streams. In the East zone, 2000 m off the coastline (Punta Brava), the submarine outflow of the sanitation system of Montevideo is located. In addition, in this zone, there are recreation areas with presence of sandy beaches, alternating with rocky shores. In the West zone (Punta Yeguas), another submarine outflow with similar characteristics in Punta Brava will be installed soon.
2.2. Weather conditions
Variability and magnitude of ENSO were determined by the Ocean Niño Index (ONI) considering the monthly anomalies of ocean surface temperature (SST) in the Niño 3.4 region (2009–2011); values were obtained from http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml.
Total monthly precipitation to the upper zone of La Plata basin (22°00′–40°00′ S and 48°00′–64°00′ W) was obtained from the Global Precipitation Climate Center (GPCC; http://gpcc.dwd.de). The product of monitoring of precipitation on earth surface was analyzed (1.0° × 1.0°). Values are expressed as rainfall “isoareas” (mm month−1), using monthly values (March 2009 and August 2011).
Wind speed and direction were collected on an hourly basis at the Punta Brava meteorological station, RdlP (34°56′S and 56°09′W; March 2009 and August 2011). We calculated the average monthly speed (± SD), minimum, and maximum and are expressed in m s−1. We did not consider maximum wind speeds (streaks of wind) in these calculations. To determine the direction of the prevailing winds, 16 quadrants were considered, and monthly relative frequency was calculated. Wind stress (Eq. (1)
where βa = air density, 1.2 kg m−3, CD: drag coefficient: 0.0013, and V wind: wind speed.
2.3. Hydrological conditions
The daily river flow of the Uruguay and Parana rivers was obtained from the National Water Institute (www.ina.gov.ar) for the period 2009–2011. The river flow of the RdlP was obtained by adding the daily river flow of the Uruguay and Parana rivers. Monthly averages (± SD) were calculated and expressed as m3 s−1.
2.4. Oceanographic conditions
Eighteen oceanographic surveys comprising 25 stations (located at 2000 m of coastline, depth 3–10 m; Figure 1B) were conducted in the MCZ between March 2009 and July 2011. The study area was divided into the Eastern zones (E1, E2, E9, PC, CN, CE, CS, CW stations), Western zones (W3 to W8, PY, YN, YE, YS, YW stations), and Montevideo Bay (MB) (B1 to B5 stations). Water temperature and salinity were determined
2.5. Laboratory analysis
2.6. Data analysis
For the analysis of the temporal variation associated with ENSO, the EN and “La Niña-Neutral” months were defined according to Oceanic Niño Index (ONI). Nonparametric analyses (U Mann-Whitney) were performed to assess temporal differences (ENSO phases) in hydrological variables (Uruguay, Parana, and RdlP flow), and spatiotemporal differences in oceanographic parameters (temperature and salinity). Nonparametric correlations (Rs, Spearman) were performed between climatic (wind stress, Niño indexes), physicochemical parameters, and river flow (RdlP, Uruguay, and Paraná). Linear relationships between river flows and average salinity were performed; both variables were transformed by log (x + 1) to fit the assumption of normality. A principal component analysis (PCA) was performed with all the physicochemical parameters (temperature, salinity, oxygen saturation, Secchi depth, Chl
3. Results and discussion
3.1. Weather conditions
The ONI showed the minimum values during October–December 2010 (<−1.5°C) and maximum values in December 2009 and January 2010 (<1.5°C; Figure 2). The 2009–2010 ONI values allowed for the identification and development of an ENSO event with an “El Niño” (EN) phase (July 2009–April 2010), a subsequent “La Niña” (LN) cold phase (June 2010–April 2011; September–December 2011), and neutral conditions (April–June 2009, April–May 2010, and May–August 2011) [2, 36].
The 2009–2010 ENSO event was classified as “moderate to strong” . When comparing the maximum 2009–2010 value of this indicator (October 2009–January 2010: 1.2) with those of recent ENSO events, we found that they were smaller than those corresponding to strong events (October–January: 1982–1983: 2.1; 1997–1998: 2.3; 2015–2016: 2.2), although they were similar to those of moderate events (1986–1987 and 1991–1992) . Although in the range of previous moderate events, the 2009–2010 ENSO had characteristics of its own, including high SST anomalies in the Central Pacific and the fastest reported transition to the LN phase [2, 36]. The warm phase of the 2009–2010 was classified into the “WP” (warm pool) type and differs from the “ Eastern Pacific” (EP) type due to the fact that SST anomalies happen in the central and not in the western zone of the Pacific Ocean . It is also known as CP or WP, dateline ENSO or “El Niño Modoki” , and has different teleconnections and differential climatic impacts compared to the “EP” . The analysis of the effects in the MCZ of the 2009–2010 ENSO, with “El Niño Modoki” characteristics, will permit the identification of variations in the behavior of meteorological variables and their impacts in the hydrologic behavior of the main hydrological systems of the La Plata basin, as well as over the RdlP estuary and its oceanographic conditions.
Total monthly precipitation for the La Plata basin during the period March 2009–August 2011 showed minimum values in April 2009 (0–10 mm month−1) and maximum during September 2009 and February 2010 (Figure 3). The highest rainfall for the upper basin was reported in November 2009 (600 mm month−1), while the values in February 2010 exceeded 550 mm month−1. The teleconnections analyzed for the ENSO event in South America (Atlantic coast), specifically for the La Plata basin, are mainly related to their effect on rainfall variability . This variability caused anomalies in surface air temperature in Argentina and southern Brazil during the “El Niño Modoki” (June–September 1979–2004), as well as higher-than-average rainfall between the months of December–February (1979–2004 period) . In addition,  found an increase in spring rainfall during the EN warm phase. In this study, we found that the maximum rainfall values in the middle and upper basin of the Uruguay River were recorded in November 2009 and December 2010, corresponding to the warm ENSO phase (EN: July 2009–April 2010) and to the spring months of the calendar year (September–December) and minimum rainfall values (April 2009 and October 2010) which corresponded to those months of Neutral and LN phase.
The average monthly wind speed between March 2009 and August 2011 was 5.8 m s−1 ± 2.9 (
3.2. Hydrological conditions
The average monthly flow of the RdlP, Uruguay, and Parana rivers showed an oscillation between minimum values during January and June 2009 and maximum between December 2009 and March 2010. Differences in the months of maximums discharges of the Uruguay and RdlP (December 2009) with the Parana River (March 2010) were observed (Figure 6).
The great rivers of the La Plata basin and tributary rivers of the RdlP are highly sensitive to rainfall variations, their flows being impacted during EN phases [11, 12]. In this study, we found strong correlations in the hydrological behavior of the Uruguay and Parana rivers in response to the increase of rainfall during the 2009 spring, coinciding with the EN warm phase. The maximum flow of the Uruguay and Parana rivers was recorded during December 2009–March 2010, and the minimum flow corresponded to those months prior to the development of the warm phase (January and July 2009). The RdlP flow had two peaks: the first in December 2009, coinciding with the maximum observed flow for the Uruguay River, and a second during the months of February–March 2010, coinciding with the maximum flow of the Parana River, previously indicated for this system by .
Monthly average flow of RdlP, Uruguay, and Parana rivers showed significant differences between the two ENSO phases (Mann-Whitney U: RdlP: Z = −12.17; Uruguay River: Z = −11.09; Parana River: Z = −8.15; P < 0.01). Significant associations between the flow rates of the three water systems were observed during both phases: EN: RdlP-Uruguay River (Rs (0.025, 10) = 0.697) and RdlP-Parana River (Rs (0.04, 10) = 0.818), “La Niña-Neutral”: RdlP-Uruguay River (Rs (0.00, 21) = 0.823) and RdlP-Parana River (Rs (0.00, 21) = 0.970). For the period December 2009 to March 2010, the RdlP flow exceeded 40,000 m3 s−1, the greatest average flow registered in 16 years (1999–2014). Nevertheless, average values of the RdlP flow during the 2009–2010 ENSO event (24,806 m3 s−1) and its different phases (EN: 32,933 m3 s−1; “La Niña-Neutral,” LNN: 22,039 m3 s−1) were similar to other ENSO events previously registered for the RdlP (1961, 2008: 24,700 m3 s−1 and 1998, 2008: 24,000 m3 s−1; [19, 20]). Average flow of the Uruguay River during low (3000–4000 m3 s−1) or high (< 7000 m3 s−1) discharge periods is lower than the ones found for LNN (Q Uruguay River = 5238 m3 s−1) and EN (Q Uruguay River = 9970 m3 s−1) in this study, although they fall into the range reported for ENSO events, characterized by their high variability (1000–20000 m3 s−1; [19, 20]). On the other hand, the average discharge of the Parana River for the 1884–1975 period (17,000 m3 s−1) ranged between 8000 and 22,000 m3 s−1 , falling within the range of the flow found in this study.
3.3. Oceanographic conditions
The average water temperature (surface-bottom) displayed a seasonal pattern with minimum temperatures in winter (July 2010: 10.4 ± 0.2°C and July 2011: 10.6 ± 0.6°C) and maximum in summer (January 2010: 25.9 ± 1.1°C; Figure 7). No significant differences between surface and bottom or between areas of the study were detected (P > 0.05). The average salinity ranged between minimum values during July 2010 (0.6 ± 0.5) and maximum values in March 2009 and January 2011 (21.2 ± 6.1, 22.3 ± 8.8, respectively). Salinity values in surface and bottom waters showed no significant differences for most of the study; however, differences were found at the beginning (March and July 2009, Mann-Whitney U: Z = −3.46 and Z = −2.57; P < 0.01) and at the end of the study period (June and July 2011; Mann-Whitney U: Z = −2.95 and Z = −3.06; P < 0.01). During the warm phase, the MCZ comprised brackish water with oligo to mesohaline conditions (maximum salinity: 15) and a temperature range between 15 and 28°C; in the cold-neutral phase, the water column salinity was highly variable (rank salinity: 0.1 to 33), with temperatures between 10 and 29°C. In July 2009, salinities were higher (15–30 range), although average water temperatures were lower than that during the EN phase (Figure 7).
Water temperature demonstrated greater temporal than spatial variation, with the lack of any differences between different depths or sampling study zones. Minimum and maximum values corresponded to winter and summer, respectively, and were associated with environmental factors. Similar results were found in long time series in MCZ, and the seasonal patterns are consistent with previous studies for the middle RdlP [22, 24, 25, 39]. Reduced variation coefficients were found in summer (February 2010: 1%), which increased in July 2009 (16%). During this investigation, the average water temperature in July 2009 was 12.4 ± 2.1°C higher (10.7 ± 0.5°C) than during the other winter months study period (July 10, June and July 11). These results suggest an increase in temperature in the months prior to the development of EN over the MCZ. In this regard,  found a variability, related with years of pre- ENSO events in winter (April–October), in the anomalies of ocean surface air temperature (SST) in eight points of the South-West Atlantic region. The EN phase is characterized by negative anomalies in the SST in the Brazil Current, while during LN phase, cold anomalies were recorded in the Brazil Current and warm ones in the Malvinas Current. In the RdlP estuary, the influence of ocean bodies over adjacent coastal waters can be observed: sub-Antarctic cold waters between autumn and late spring and subtropical waters between late spring and autumn . The anomalous records of July 2009 could be related to the LN phase prior to the effect of the EN event in the coastal zone of the RdlP.
Salinity is the main physicochemical variable of the water column, or “master parameter” of the RdlP, operating as regulator of biogeochemical, ecological, and sedimentological processes [18, 20, 22, 24, 41]. During most of the months of the EN (October 2009–July 2010) and LNN (September 2010–February 2011), we observed a mixed water column with no salinity stratification. Nevertheless, in the months prior to the EN (March and July 2009) and in LNN months (June and July 2011), the water column within MCZ was stratified. Vertical mixing of the water column in the study area may be generated by the predominant winds (speed and direction)  or by the fresh water inputs from the Uruguay and Parana rivers [18, 24, 40]. We identified the flow as the predominant driver, over wind stress, of the extension of the discharge plume of the RdlP during the EN months; this effect may also explain the lack of stratification along coast north of the RdlP during EN phase.
According to studies performed with long time series (1935–1975 Montevideo Bay; 1971–1991 Punta Brava), salinity in MCZ shows annual variations, with minimum monthly average values in autumn-winter and maximum ones in summer; minimum salinity values may occur throughout all the year, particularly in February, May, June, August, October, and December, although they have not been registered in January and rarely in July . The minimum average salinity values were found during July 2010; however, according to , this month rarely displays such minimum salinity values. In addition, the mean salinity was 5.0 ± 4.6 higher during EN phase and even higher (12.7 ± 9.8) in LNN months. According to the inverse relationship between salinity and RdlP flow, the anomalous values are related to the ENSO 2009–2010 event, to the minimum salinity values during the EN months·(October 2009–July 2010) and to the maximum values during LNN, respectively, which are associated with maximum and minimum values of the RdlP flow.
The first two components of PCA accounted for 67.5% of the total variance of the spatiotemporal variability. Principal component 1 (38.6%) showed a positive correlation with salinity (0.736), Secchi depth (0.798), and oxygen saturation (0.589), interpreted as gradient of hydrological variability. Principal component 2 (28.9%) showed a positive correlation with temperature (0.882) and Chl
The location and presence of the turbidity front is one of the environmental features regulating ecological processes in the RdlP [19, 24, 25]. The influence of ENSO events on this feature have been poorly studied . Our results agree with those of , who found that the turbidity front was related to the salinity and discharge flow of the RdlP. In this study, we showed the relationship between the RdlP flow and the spatial location of the turbidity front that conditioned the physicochemical characteristics of the water column of the MCZ, mainly during the EN phase. In Figure 9, we show satellite images (MODIS) of the RdlP where different locations of the turbidity front are depicted. Before the development of the EN phase (February 2009), the turbidity front was located west of the MCZ. During the EN phase (July 2009–April 2010), the turbidity front was located adjacent to the MCZ, while during the LNN (October 2010–January 2011), the front was again west of the MCZ, with less turbid waters in the MCZ. The monthly average Secchi depth was directly correlated with salinity, thus reflecting the occurrence of low turbidity waters in the MCZ during the low discharge flows of the RdlP. In addition, the PCA showed an arrangement of the sampling stations according to the hydrological behavior and seasonal variation. Coastal zones influenced by river discharges show gradients in nutrient concentrations related with changes in salinity . Both TN and TP concentrations were positively associated with the RdlP and Parana flows and negatively associated with salinity and Secchi depth. In addition, the stations with the highest nutrient concentrations were associated to the axis 1 of the PCA, which represents the hydrological variability of the system. These results suggest that an increase in the RdlP flow promotes an increase of nutrient availability in the MCZ, particularly during the EN phase.
On the other hand, high Chl
3.4. Weather, hydrological, and oceanographic interactions
Significant correlations between Niño indexes (Niño 3.4 > Niño 4 > Niño 1 + 2 > Niño 3) and flows of RdlP, Uruguay, and Parana rivers were only found during EN event (n = 10; Table 1).
|Q RdlP||Q Uruguay river||Q Paraná river|
|−0.685 (P = 0.029*)||NS||−0.842 (P = 0.002**)|
|NS||0.673 (P = 0.033*)||NS|
|0.888 (P = 0.001**)||0.912 (P = 0.000**)||0.578 (P = 0.080 MS)|
|0.806 (P = 0.005**)||0.782 (P = 0.008**)||0.648 (P = 0.043*)|
During the “EN” phase, we observed significant correlations between the RdlP and Uruguay River flow and the different Niño indexes (Niño 1 + 2, Niño 3, Niño 3.4, and Niño 4), where Niño 3.4 showed the highest correlation coefficients. Similar correlation coefficients (r = 0.51) were observed between Niño 3.4 and the Uruguay River flow . The associations found in this study with the RdlP and Uruguay River flow were higher than those of the above mentioned authors. During ENSO events, joint monitoring of the Niño 3.4 index and the RdlP and the Uruguay River flow would allow the assessment of the temporal variability in hydrology of the system, promoting the generation of early warnings of floods or maximum water levels in both systems.
The wind stress and flow of the RdlP showed in phase and out of phase fluctuations (
Flow (RdlP, Uruguay and Parana rivers) and average salinity showed inverse linear relationships with higher coefficients recorded during the EN phase than during “LNN” phase (Figure 11).
Wind stress and discharge levels are the main processes responsible for extending the estuarine discharge plume of the RdlP over the Atlantic continental shelf . In this system, the E-ESE-E wind direction promotes the inflow of ocean water into the estuary, while W-WNW winds promote discharge of affluent rivers (Parana and Uruguay rivers), increasing or reducing average salinity and the upriver or downriver displacement of the saline front [18, 19]. During high discharge periods (Q RdlP < 30,000 m3 s−1), which are largely associated to ENSO events (5 in 7 between 1959 and 1990), only the event in 1992 presented wind stress conditions favorable for the dispersion of the river plume into the internal zone of the RdlP. In the remaining events, the wind strength opposite to the penetration of the plume was negligible . During September 2009–May 2010 (EN phase), we observed “out of phase” fluctuations between the RdlP flow and wind stress, while during “LNN” fluctuations were mostly of the “in phase” type (
During the 2009–2010 ENSO event, in the EN phase, there is an increase in rainfall from spring 2009, promoting an increase in the flows of the main tributary rivers to the RdlP estuary. During EN phase, the RdlP flow and wind stress are the principal drivers of oceanographic condition at the north coast of RdlP.
We thank PEDECIBA, ANII (SNI), UdelaR (CSIC-DT), and Intendencia de Montevideo for their financial support. We also thank the SOHMA (Service Oceanography, Hydrography and Meteorology of the Navy, Uruguay) staff by the wind records, and Mario Bidegain by the flow data. B.S. in Biol. Pablo Mones for the English translation.