Flow Regime and Nutrient-Loading Trends from the Largest South European Watersheds: Implications for the Productivity of Mediterranean and Black Sea’s Coastal Areas
Abstract
:1. Introduction
2. Materials and Methods
2.1. Geographical Settings of River Basins
2.2. River Flow Data
2.3. Chemical Parameters
2.4. Data Analysis
3. Results
3.1. Daily to Multi-Decadal Variability of River Flows
3.2. Nutrients and Organic Matter in River Waters
3.3. River Discharges of Biogenic Elements and Eutrophication Potential in Coastal Zones
4. Discussion
4.1. Seasonal to Decadal Trends of River Flows and Coastal Hydrology
4.2. Loading of Biogenic Elements and Impacts on Fluvial and Coastal Systems
5. Conclusions
- Flow dynamics of Ebro, Rhône, Po and Danube rivers exhibits a different incidence of freshets and droughts and distinct annual cycles. This feature suggests the importance of regional climatic factors in these drainage basins, despite the widespread presence of flow regulation systems.
- Annual water discharges of the Ebro significantly decreased during the last century, whereas those Rhône, Po and Danube showed multi-decadal oscillations. For the Ebro, this difference is consistent with the rise of anthropogenic usage of freshwater in the drainage basins and with regional climate changes. For the other rivers, interannual variability of water discharge is still prevailing on long-term trends.
- The decrease of water discharge of the Ebro was concomitant to a reduction of flow variability in all the seasons. For the Rhône, Po and Danube, the decrease of discharges occurred mostly in summer, with a concomitant increase of flow variability that suggests a greater instability of climatic conditions in their regions.
- The concentrations of inorganic nutrients, TN and TP in the waters of Ebro and Po are about 50% higher than in those of Rhône and Danube. This finding suggests that the former two watersheds might be the most impacted ones by nutrient pollution, in a future scenario of reduced runoff, even in the presence of constant inputs due to agricultural, urban and industrial activities.
- The concentrations of DIN and SiO2 show a clear annual cycle in these rivers, with the lowest levels (−50%) being reached in spring and summer. This cycle changes seasonally quantity and composition of the nutrient pool delivered into the receiving coastal water bodies.
- The analysis of nutrient budgets indicated that these rivers have changed from a past condition characterized by large discharges of nutrients, with a rather balanced N:Si:P ratio, to overloads of DIN and SiO2 with respect to PO43−. This process has reduced the eutrophication of rivers, estuaries and coastal marine environments inducing, however, changes of ecological conditions that have to be further assessed.
- Phosphorus scarcity is a common feature of these river and coastal ecosystems, but its potential ability to limit primary production significantly reduces if organic phosphorus is considered at least partially available for the growth of phytoplankton. For this reason, the real bioavailability of riverine organic phosphorus for auto- and hetero-trophs should be better investigated as it could play a key role in the regulation of the productivity and structure of plankton communities.
- In the current post-eutrophic phase, the discharge of riverine TOC to the coastal zone is not negligible with respect to the eutrophication potential of river nutrients.
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Appendix A
Month | Ebro | Rhône | Po | Danube |
---|---|---|---|---|
km3 month−1 yr−1 | ||||
Jan. | −0.013 *** | +0.011 | +0.001 | +0.045 |
Feb. | −0.014 *** | +0.008 | +0.001 | +0.041 * |
Mar. | −0.020 *** | −0.001 | −0.009 + | −0.005 |
Apr. | −0.019 *** | −0.002 | −0.007 | +0.025 |
May | −0.015 *** | −0.010 + | −0.001 | −0.015 |
Jun. | −0.014 *** | −0.010 + | −0.015 * | −0.052 + |
Jul. | −0.005 *** | −0.009 * | −0.016 ** | −0.040 |
Aug. | −0.002 * | −0.007 ** | −0.008 * | −0.014 |
Sep. | −0.003 *** | −0.004 | +0.001 | +0.011 |
Oct. | −0.005 *** | −0.001 | −0.004 | +0.020 |
Nov. | −0.010 *** | −0.001 | −0.005 | +0.006 |
Dec. | −0.013 *** | +0.012 + | 0.000 | +0.007 |
Month | Ebro | Rhône | Po | Danube |
---|---|---|---|---|
% yr−1 | ||||
Jan. | −0.116 + | +0.065 | −0.065 | −0.123 * |
Feb. | −0.211 *** | −0.002 | −0.054 | −0.135 |
Mar. | −0.060 | +0.019 | −0.106 | +0.028 |
Apr. | −0.019 | +0.082 | −0.021 | +0.074 + |
May | +0.063 | +0.047 | +0.007 | +0.201 *** |
Jun. | −0.040 | +0.105 + | +0.119 * | +0.123 * |
Jul. | −0.420 *** | +0.279 *** | +0.134 ** | +0.127 * |
Aug. | −0.417 *** | +0.128 * | −0.012 | +0.091 + |
Sep. | −0.481 *** | +0.020 | −0.086 | +0.072 |
Oct. | −0.251** | +0.006 | −0.042 | +0.004 |
Nov. | −0.222 *** | +0.062 | −0.009 | −0.012 |
Dec. | −0.165 * | +0.017 | +0.038 | +0.034 |
Year | F-TSM kt yr−1 | F-NO3− kt-N yr−1 | F-NH4+ kt-N yr−1 | F-NO2− kt-N yr−1 | F-PO43− kt-P yr−1 | F-SiO2 kt-Si yr−1 | F-TOC kt-C yr−1 | F-TN kt-N yr−1 | F-TP kt-P yr−1 |
---|---|---|---|---|---|---|---|---|---|
Ebro | |||||||||
Median | 70.91 | 19.17 | 0.50 | 0.19 | 0.50 | 30.37 | 29.28 | 26.12 | 0.81 |
1th Quartile | 51.70 | 15.74 | 0.27 | 0.15 | 0.44 | 21.64 | 25.70 | 21.85 | 0.51 |
3rd Quartile | 98.91 | 24.64 | 0.69 | 0.28 | 1.08 | 50.20 | 43.21 | 34.11 | 1.14 |
Min. | 21.03 | 21.03 | 0.18 | 0.09 | 0.20 | 16.16 | 12.83 | 13.68 | 0.42 |
Max. | 224.10 | 38.15 | 1.48 | 0.43 | 1.95 | 62.14 | 49.33 | 47.24 | 1.59 |
Rhône | |||||||||
Median | 2030.56 | 79.98 | 2.91 | 1.27 | 2.33 | 105.05 | 155.20 | 101.91 | 4.85 |
1th Quartile | 1262.63 | 65.71 | 2.39 | 0.98 | 2.11 | 78.11 | 143.92 | 89.27 | 4.08 |
3rd Quartile | 3972.30 | 83.93 | 3.43 | 1.53 | 3.28 | 116.62 | 229.02 | 109.66 | 5.39 |
Min. | 405.14 | 49.67 | 1.16 | 0.44 | 0.79 | 58.02 | 124.25 | 62.79 | 2.09 |
Max. | 7998.60 | 99.54 | 4.59 | 2.52 | 6.24 | 156.88 | 279.43 | 115.80 | 7.13 |
Po | |||||||||
Median | 5546.90 | 102.11 | 3.99 | 1.44 | 2.88 | 137.49 | 246.42 | 156.80 | 8.53 |
1th Quartile | 3434.51 | 85.43 | 3.05 | 1.16 | 2.28 | 118.75 | 229.18 | 111.37 | 6.27 |
3rd Quartile | 7771.78 | 126.30 | 6.08 | 1.85 | 3.30 | 183.28 | 299.69 | 227.11 | 10.25 |
Min. | 1030.47 | 51.42 | 1.60 | 0.54 | 1.67 | 64.39 | 225.14 | 94.19 | 3.83 |
Max. | 16,292.57 | 179.19 | 9.69 | 2.53 | 4.00 | 243.46 | 366.53 | 295.16 | 18.71 |
Danube | |||||||||
Median | 6565.15 | 338.77 | 62.72 | 7.95 | 8.28 | 360.13 | 1139.75 | 446.30 | 18.96 |
1th Quartile | 4820.07 | 297.08 | 36.09 | 5.67 | 6.19 | 298.29 | 968.46 | 374.27 | 13.17 |
3rd Quartile | 10,478.27 | 406.26 | 75.93 | 8.92 | 11.27 | 452.54 | 1311.04 | 587.20 | 21.95 |
Min. | 2645.97 | 188.40 | 26.08 | 2.14 | 4.13 | 189.11 | 797.17 | 270.00 | 10.39 |
Max. | 19,077.89 | 535.22 | 89.79 | 12.05 | 15.96 | 935.85 | 1482.33 | 714.05 | 40.47 |
Parameter | Ebro | Rhône | Po | Danube |
---|---|---|---|---|
kt yr−2 | ||||
F-TSM | −1.2 + | −10.6 | −171 | +190 |
F-NO3− | −0.23 | −0.11 | −0.35 | −8.31 |
F-NH4+ | +0.01 | −0.06 | −0.28 *** | −3.58 ** |
F-NO2− | −0.01 * | −0.04 *** | −0.06 *** | −0.18 |
F-DIN | −0.36 + | −0.35 | −0.81 | −11.1 |
F-PO43− | −0.06 *** | −0.22 *** | −0.06 ** | −0.18 |
F-SiO2 | +1.3 | +0.04 | −1.06 | −7.04 |
F-TOC | −1.36 + | - | - | - |
F-TN | −0.22 | −0.52 | −1.07 | −32.5 |
F-TP | −0.05 ** | −0.09 | −0.12 + | 0.00 |
F-ON | +0.01 | −1.13 * | +0.09 | −4.30 |
F-OP | −0.01 | −0.02 | −0.04 | +0.08 |
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River (Gauging Station) | Drainage Basin † (km2) | River Length ‡ (km) | Inhabitants (n · 106) | Water Flow Data Series (yr) | Annual Water Flow (km3 yr−1) Median 1th–3rd Quartile Range | ||
---|---|---|---|---|---|---|---|
Ebro (Tortosa) | 86,800 | 930 | 3 | 1914–1931, 1951–2012 | 13.0 | 8.7–17.1 | 3.8–34.1 |
Rhône (Beaucaire) | 157,950 | 813 | 18 | 1920–2012 | 53.9 | 45.8–61.7 | 22.8–78.0 |
Po (Pontelagoscuro) | 74,000 | 682 | 16 | 1914–2012 | 45.5 | 38.8–55.7 | 26.2–82.7 |
Danube (Ceatal Izmail) | 801,500 | 2857 | 82 | 1931–2012 | 201.2 | 178.6–221.0 | 134.2–300.2 |
Total | 1,120,250 | 5282 | 120 | 1951–2012 | 316.8 | 297.9–344.9 | 208.6–424.0 |
River | Period | Mean Flow km3 yr−1 (m3 s−1) | Duration yr | Year of Change yr | Δ-Flow km3 yr−1 | Confidence α |
---|---|---|---|---|---|---|
1914–1979 | 16.5 (539) | 66 | ||||
Ebro | 1980–2010 | 9.1 (292) | 31 | 1980 | −7.4 | 0.0001 |
2011–2012 | 4.9 (154) | 2 | 2011 | −4.2 | 0.0209 | |
Rhône | 1920–1941 | 55.9 (1772) | 22 | |||
1942–1976 | 50.9 (1614) | 35 | 1942 | −5.0 | 0.1292 | |
1977–2002 | 56.7 (1797) | 26 | 1977 | +5.8 | 0.0284 | |
2003–2012 | 46.6 (1478) | 10 | 2003 | −10.1 | 0.0018 | |
Po | 1914–1941 | 51.3 (1593) | 28 | |||
1942–1974 | 43.7 (1390) | 33 | 1942 | −7.6 | 0.0163 | |
1975–2002 | 50.4 (1626) | 28 | 1975 | +6.7 | 0.0183 | |
2003–2008 | 32.6 (1058) | 6 | 2003 | −17.8 | 0.0006 | |
2009–2012 | 48.7 (1551) | 4 | 2009 | +16.1 | 0.0548 | |
Danube | 1931–1945 | 212.4 (6729) | 15 | |||
1946–1964 | 189.0 (5990) | 19 | 1946 | −23.4 | 0.0910 | |
1965–1982 | 225.3 (7141) | 18 | 1965 | +36.3 | 0.0016 | |
1983–2010 | 201.5 (6381) | 28 | 1983 | −23.8 | 0.0208 | |
2011–2012 | 164.6 (5213) | 2 | 2011 | −36.9 | 0.0003 |
River (Station) | Ebro (Tortosa) | Rhône (Arles) | Po (Polesella, Serravalle, Pontelagoscuro) | Danube (Reni, Sulina) |
---|---|---|---|---|
TSM (mg L−1) | 7.5 (5.0–11.0) | 12.3 (7.4–29) | 46.8 (28.4–110) | 26.5 (15.0–44) |
NO3− (µmol L−1) | 165 (142–194) | 97 (76–119) | 161 (124–202) | 111 (82–141) |
NH4+ (µmol L−1) | 2.8 (2.2–5.5) | 3.3 (1.5–5.6) | 5.3 (2.6–10.2) | 15.0 (9.1–25) |
NO2− (µmol L−1) | 1.5 (1.1–2.0) | 1.6 (1.1–2.0) | 2.0 (1.3–3.0) | 2.1 (1.4–3.4) |
DIN (µmol L−1) | 171 (146–201) | 102 (84–125) | 168 (302–216) | 132 (105–163) |
PO43− (µmol L−1) | 2.3 (1.5–4.5) | 1.5 (1.1–1.9) | 1.9 (1.6–2.6) | 1.3 (0.6–2.0) |
SiO2 (µmol L−1) | 130 (93–160) † | 64 (51–78) | 113 (85–134) | 61 (37–82) |
TN (µmol L−1) | 211 (180–234) † | 123 (101–148) * | 249 (197–304) + | 147 (121–187) °° |
TP (µmol L−1) | 2.9 (2.6–3.6) | 2.6 (2.2–3.2) * | 4.8 (4.2–6.1) | 2.8 (1.9–4.0) |
TOC (µmol L−1) | 270 (208–341) † | 241 (199–304) | 340 (268–460) ° | 398 (294–569) ** |
ON (µmol L−1) | 28.1 (22–46) † | 18.2 (12.7–29) * | 76 (41–145) | 25.3 (19.2–33) °° |
OP (µmol L−1) | 1.0 (0.4–1.7) | 1.2 (0.8–1.8) * | 2.9 (1.9–4.1) | 1.2 (0.6–2.1) |
DIN/PO43− (molar) | 72 (36–118) | 73 (52–100) | 85 (61–107) | 94 (62–228) |
Si/DIN (molar) | 0.7 (0.5–1.0) † | 0.6 (0.5–0.7) | 0.6 (0.5–0.8) | 0.5 (0.4–0.8) °° |
TN/TP (molar) | 58 (44–70) † | 45 (37–59) * | 49 (37–64) | 55 (38–82) °° |
ON/OP (molar) | 20 (11–31) | 15 (10–22) * | 25 (16–46) | 17 (9–41) °° |
ON/TN (%) | 15 (11–21) † | 16 (10–22) * | 34 (20–49) | 16 (13–22) °° |
OP/TP (%) | 32 (13–57) | 49 (35–63) * | 60 (47–72) | 49 (30–72) |
Descriptor | Ebro NE Shelf of Spain | Rhône Gulf of Lion | Po NW Adriatic Sea | Danube NW Black Sea | Environmental Impacts |
---|---|---|---|---|---|
Annual freshwater discharge | Long-term decrease. | Interannual to multi-decadal oscillations increased recently. | Interannual to multi-decadal oscillations increased recently. | Interannual to multi-decadal oscillations. | Marked regional differences of river discharges. Increased droughts in summer more marked in SW Europe than in SE Europe. Increased oscillations of summer runoff in SE Europe. |
Long-term trend of monthly flows | Decrease in all months. | Decrease in late spring and summer. | Decrease in summer. | Small decrease in summer. | |
Long-term trend of flow oscillations | Decrease in all months. | Increase in summer. | Increase in summer. | Increase in spring and summer. | |
Dry seasons | Summer. | Summer. | Winter and summer. | Winter and summer. | |
Flow regime | Low with a high incidence of freshets. | High with low incidence of freshets. | Intermediate with a high incidence of freshets. | Very high with low incidence of freshets. | |
Concentrations of TSM and nutrients | Low TSM, high nutrients. | Low TSM, low nutrients. | High TSM, high nutrients. | Medium TSM, medium nutrients. | Impact of the decrease of TSM transport on estuarine and coastal areas. Distinct impacts of seasonal changes of nutrients on riverine, estuarine and coastal ecosystems. |
Variability of nutrient concentrations | Seasonal cycle (except PO43−). | Seasonal cycle (for PO43− only since 2007). | Seasonal cycle. | Seasonal cycle | |
Variability of DIN/PO43−, TN/TP and Si/DIN ratios | Seasonal oscillations (low N/P in summer, low Si/DIN in spring). | Seasonal oscillations (low N/P in summer, low Si/DIN in spring). | Seasonal oscillations (low N/P in summer, low Si/DIN in spring). | Rather constant DIN/PO43− through the year, high TN/TP in spring. | |
Incidence of organic nitrogen and phosphorus on TN and TP pools | ON: low OP: low | ON: low OP: medium | ON: high OP: high | ON: low OP: high | Potential growth of marine plankton species able to utilize riverine ON and OP. |
Recent trends of annual loads of TSM, nutrients and OM | Decreases of TSM, N, P and TOC transport since the 1990s. Oscillations of SiO2. | Decrease of PO43− and ON transport since the 1980s. Oscillations of SiO2 and DIN. | Decreases of NH4+, NO2− and PO43− and TP transport since the 1980s. Oscillations of SiO2. | Decreased transport of NH4+ since the 1990s. Oscillating N, P transports since the 2000s. | Reduction of PO43− transport leading to phytoplankton biomass reduction. Oscillation of nutrient loads linked to runoff variability. |
Marine region of freshwater influence | Small, limited by continental shelf orography. | Medium, limited by continental shelf orography. | Large, enhanced by continental shelf orography. | Very large, enhanced by continental shelf orography. | Larger impacts in the coastal zones of Po and Danube, even if river loads are reduced. |
Eutrophication potential of river nutrient loads in the receiving coastal zones | Until 1995, high and balanced nutrient loads. Afterwards, excesses of DIN and SiO2 over PO43−. Low weight of OP in TP. | Until 1990, high and balanced nutrient loads. Afterwards, excesses of DIN and SiO2, over PO43−. Medium weight of OP in TP. | Until 1990, high and balanced nutrient loads, with a surplus of OP. Afterwards, excesses of DIN and SiO2, over PO43−. Persistent high weight of OP in TP. | Since 2000s, excesses of DIN and SiO2, over PO43−. Medium weight of OP in TP. | Shift form eutrophic conditions to oligotrophic, but still degraded, conditions around the 1990s. PO43− scarcity sometime potentially compensated by OP bioavailability. |
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Cozzi, S.; Ibáñez, C.; Lazar, L.; Raimbault, P.; Giani, M. Flow Regime and Nutrient-Loading Trends from the Largest South European Watersheds: Implications for the Productivity of Mediterranean and Black Sea’s Coastal Areas. Water 2019, 11, 1. https://doi.org/10.3390/w11010001
Cozzi S, Ibáñez C, Lazar L, Raimbault P, Giani M. Flow Regime and Nutrient-Loading Trends from the Largest South European Watersheds: Implications for the Productivity of Mediterranean and Black Sea’s Coastal Areas. Water. 2019; 11(1):1. https://doi.org/10.3390/w11010001
Chicago/Turabian StyleCozzi, Stefano, Carles Ibáñez, Luminita Lazar, Patrick Raimbault, and Michele Giani. 2019. "Flow Regime and Nutrient-Loading Trends from the Largest South European Watersheds: Implications for the Productivity of Mediterranean and Black Sea’s Coastal Areas" Water 11, no. 1: 1. https://doi.org/10.3390/w11010001