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

In the last century, large watersheds in Southern Europe have been impacted by a combination of anthropogenic and climatic pressures, which have rapidly evolved to change the ecological status of freshwater and coastal systems. A comparative analysis was performed for Ebro, Rhône, Po and Danube rivers, to investigate if they exhibited differential dynamics in hydrology and water quality that can be linked to specific human and natural forces acting at sub-continental scales. Flow regime series were analyzed from daily to multi-decadal scales, considering frequency distributions, trends (Mann–Kendall and Sen tests) and discontinuities (SRSD Method). River loads of suspended matter, nutrients and organic matter and the eutrophication potential of river nutrients were estimated to assess the impact of river loads on adjacent coastal areas. The decline of freshwater resources largely impacted the Ebro watershed on annual (−0.139 km3 yr−1) and seasonal (−0.4% yr−1) scales. In the other rivers, only spring–summer showed significant decreases of the runoff coupled to an exacerbated flow variability (0.1–0.3% yr−1), which suggested the presence of an enhanced regional climatic instability. Discontinuities in annual runoff series (every 20–30 years) indicated a similar long-term evolution of Rhône and Po rivers, differently from Ebro and Danube. Higher nutrient concentrations in the Ebro and Po (+50%) compared to Rhône and Danube and distinct stoichiometric nutrient ratios may exert specific impacts on the growth of plankton biomass in coastal areas. The overall decline of inorganic phosphorus in the Rhône and Po (since the 1980s) and the Ebro and Danube (since the 1990s) mitigated the eutrophication in coastal ecosystems inducing, however, a phase in which the role of organic phosphorus loads (Po > Danube > Rhône > Ebro) on coastal productivity could be more relevant. Overall, the study showed that the largest South European watersheds are differently impacted by anthropogenic and climatic forces and that this will influence their vulnerability to future changes of flow regime and water quality.

Changes in the spawning habitat of two small pelagic fish in the Northwestern Mediterranean

“…A decrease in chlorophyll a concentration has also been documented from satellite measurements off the Rhone River mouth, likely due to a decrease in the riverine runoff waters, and elsewhere in the Gulf of Lions and the Catalan coast for the period 1998–2009; a result also found in the outflow of other Mediterranean rivers (Colella et al, : Figure ). In a recent paper, Cozzi et al () documented the reduction in river flow and main nutrients (nitrogen and phosphorus) of the Ebro and Rhone rivers over the last four decades. On the other hand, and at local scale, a decreasing trend in the abundance of most phytoplankton groups has been documented in the Catalan coastal waters (Blanes Bay), associated with a reduction in nutrient availability (Nunes, Latasa, Gasol, & Estrada, ).…”

“…the Rhone and the Ebro. The flow of the Rhone, for instance, was very high in the early 1980s, at its lowest in the first years of the 21st c. and increasing again in the current decade, while the Ebro River flow has TA B L E 1 Characteristics of the ichthyoplankton surveys: sampling dates, number of stations, and depth range been decreasing steadily for the past 4 decades (Cozzi, Ibáñez, Lazar, Raimbault, & Giani, 2019). The variations in river flow are accompanied in all cases by a decrease in the nutrient load (Cozzi et al, 2019), which might explain the observed decrease in chlorophyll a concentration in the Gulf of Lions and NE Spain in the last two decades (Colella, Falcini, Rinaldi, Sammartino, & Santoleri, 2016).…”

Mediterráneo: realidad y retos de nuestro entorno natural

“…A diminution of the river runoff waters, in particular from the Rhone and Ebro Rivers, and a decrease in chlorophyll-a concentration has also been documented (coLeLLa et al, 2016;cozzI et al, 2019). At local scale, a decreasing trend in the phytoplankton abundance has been documented in the Catalan coastal waters (Blanes Bay), associated with a reduction in nutrient availability (nunes et al, 2018).…”

Challenges for Sustained Observing and Forecasting Systems in the Mediterranean Sea

“…Regular and long-term monitoring in the MOOSE network has provided significant results that can be used to interpret the temporal variability of nutrients and the zooplankton community, which are sensitive to deep vertical mixing events (Donoso et al, 2017). Coastal observations showed that the long-term evolution of nutrient inputs here is driven by Rhône River water discharges, which respond differently depending on whether a climatic or an anthropogenic forcing occurs (Cozzi et al, 2018).…”