The Role of Nitrogen and Phosphorus in Eutrophication of the Northern Adriatic Sea: History and Future Scenarios

Abstract

In the last two decades of the 21st century, a gradual decrease in nitrogen and phosphorus has been observed along the coastal area of the Northern Adriatic Sea. This depletion is attributed to reduced river flows. Studies conducted over the past four decades have indicated that the N/P ratio in the open sea is unlikely to undergo significant change. In fact, it tends to increase due to the unique characteristics of the Northern Adriatic Sea, which experiences slow water turnover and is influenced by strong winds. Additionally, the Northern Adriatic Sea receives a substantial amount of freshwater from rivers, accounting for about one-third of the total freshwater flow into the Mediterranean. These rivers carry nutrient loads that contribute to the high productivity and abundance of fish in this sea, making it one of the most productive areas in the Mediterranean. It has been observed that the cessation of anthropogenic phosphorus input, which has been regulated since the late 1980s with legislation limiting its use in detergents, has significantly affected the trophic chain. The aim of this review is to provide an overview of the eutrophication trend in the Northern Adriatic Sea, highlighting the importance long-term data series.

Evolution of Freshwater Runoff in the Western Adriatic Sea over the Last Century

“…Overall, in the period from 1917 to 2008, the runoff of the Po ranged from 20.54 to 82.49 km 3 yr −1 , with an average value of 47.17 km 3 yr −1 [40]. This decreasing trend continued until October 2023 (Figure 3), as also reported in Marini and Grilli, 2023 [23]. However, regarding the Adige, Brenta, The most affected areas by river runoff in the Adriatic Sea during all seasons are the northern and western regions, where a strong lateral temperature and nutrient gradient is experienced in spring and autumn [19,41].…”

“…Due to the predominant cyclonic circulation of the Adriatic Sea, the Po River and other alpine rivers located in the northernmost part of the basin discharge nutrients into its waters along the western side. These rivers are responsible for significant eutrophication processes that have been periodically observed along the Italian coasts [20][21][22][23][24], at least since the 1960s, causing hypoxia phenomena and dinoflagellate blooms [25,26]. Since the 1950s, the ever-increasing use of fertilizers, prompted by intensive farming along the Po River basin, has had a significant impact on the inflow of nitrogen (N) and phosphorus (P) into the Northern Adriatic Sea [27,28].…”

“…Since the 1950s, the ever-increasing use of fertilizers, prompted by intensive farming along the Po River basin, has had a significant impact on the inflow of nitrogen (N) and phosphorus (P) into the Northern Adriatic Sea [27,28]. The release of these nutrients in the sea creates the perfect storm for the eutrophication process, where an excessive growth of phytoplankton and macroalgae creates a biomass cover that prevents sunlight and oxygen from penetrating the water column, potentially depleting oxygen levels and ceasing primary production within the ecosystem [23,29].…”

Projected climate oligotrophication of the Adriatic marine ecosystems

“…This decrease amounts to about 35% on the year for the rivers with discharge location in the Northern Estuarine area, rising to about 40% if the sole Po Plain is considered (Figure 6, histograms NE and NE PV ). In this regard it is worth mentioning that an early signal of a regime shift toward oligotrophication was revealed at the beginning of the 1980s by long time-series of water column nutrients, chlorophyll a, and phytoplankton biomass (Marić et al, 2012;Marini and Grilli, 2023), and became more evident at the turn of the 2000s in the Gulf of Trieste (Cozzi et al, 2020) and in the North Eastern Adriatic (Djakovac et al, 2012), evidencing main the role of the decline in Po River discharge (Cozzi et al, 2018) in the North Adriatic basin, and that of minor rivers in the coastal areas of the Central Adriatic (Ricci et al, 2022).…”

Interannual Variation in the Zooplankton Community of the North Adriatic Sea under Short-Term Climatic Anomalies

“…This anomaly has negative returns for the hydrological conditions of the Po River, which faces more frequent and more alarming droughts [16,22]. The Po River is the main contributor to the Adriatic's freshwater input; therefore, a severe reduction in its flow is expected to greatly affect the oceanographic conditions of the whole basin [16,[23][24][25].…”