Po River Discharge Research Articles

Dissolved Trace Metals and Organic Matter Distribution in the Northern Adriatic, an Increasingly Oligotrophic Shallow Sea

We report a monthly distribution of Zn, Cd, Pb, Cu, Ni and Co, biologically relevant trace metals (TMs), within one year, in the productive surface layer at two stations with different trophic characters in the northern Adriatic (NA). The TM data was accompanied by a multivariable dataset, including dissolved organic carbon (DOC), surface-active organic substances (SAS), nitrogen-containing polymeric organic material (N-POM), nutrients, pH, dissolved O2, chlorophyll a (Chl a), seawater temperature, and the Po River discharge rate. At the eu- to mesotrophic station 108, the concentrations of dissolved TMs were 5–116 nM for Zn, 0.04–0.18 nM for Cd, 0.05–0.63 nM for Pb, 3–17 nM for Cu, 4–11 nM for Ni, and 0.2–1.2 nM for Co, while at the oligotrophic station 107, they were 6–224 nM for Zn, 0.03–0.16 nM for Cd, 0.05–1.25 nM for Pb, 3–17 nM for Cu, 4–19 nM for Ni, and 0.1–0.7 nM for Co. The characterization of organic matter (OM) in conjunction with the analysis of correlations with TMs indicated that the OM–TMs interactions differed between the two stations; namely, the freshly produced OM detected at station 108 was probably involved in the complexation of Cu, Co, Cd, and Ni, whereas at station 107, such complexation processes were not present. Accumulation of DOC was observed during the summer months at both stations. Our results present a significant contribution to biogeochemical studies in the NA by focusing on the complexity of TM–OM interactions, which is a prerequisite for interpretation of their responses to local and global changes.

Ecological Importance of Alkaline Phosphatase Activity in Changing Marine Environmental Conditions

The effects of changing hydrological and climatic conditions on the dynamics of phosphorus (P) were studied in the Northern Adriatic Sea (NA), a coastal system currently affected by these changes. P limitation is one of the key stresses in the NA and it is a globally important phenomenon in aquatic ecosystems. Therefore, the response to P stress by inducing alkaline phosphatase activity (APA) was studied in characteristic water types in the NA, formed by the changing freshwater input in different thermic conditions. APA was important in providing P for microbial growth in upper waters dominated by assimilation during the warmer part of the year in stratified conditions. Contrarily, APA was not important during mixing in the colder part of the year, as well as in waters dominated by regeneration. In waters influenced by freshwater, temperature had no effect on APA, while in high-salinity waters, temperature was an important factor for APA increase. The highest APA occurred during riverine nutrients supply, indicating that the alteration of anthropogenic nutrient loads might strongly change P status in this coastal system. Furthermore, predicted increases in winter sea temperature and summer Po River discharge could delay the water column mixing, prolonging periods of P limitation.

Near-Surface Transport Properties and Lagrangian Statistics during Two Contrasting Years in the Adriatic Sea

This paper describes the near-surface transport properties and Lagrangian statistics in the Adriatic semi-enclosed basin using synthetic drifters. Lagrangian transport models were used to simulate synthetic trajectories from the mean flow fields obtained by the Massachusetts Institute of Technology general circulation model (MITgcm), implemented in the Adriatic from October 2006 until December 2008. In particular, the surface circulation properties in two contrasting years (2007 had a mild winter and cold fall, while 2008 had a normal winter and hot summer) are compared here. In addition, the Lagrangian statistics for the entire Adriatic Basin after removing the Eulerian mean circulation for numerical particles were calculated. The results indicate that the numerical particles were slower in this simulation when compared with the real drifters. This is because of the reduced energetic flow field generated by the MIT general circulation model during the selected years. The numerical results showed that the balanced effects of the wind-driven recirculation in the northernmost area(which would be a sea response to the Bora wind field) and the Po River discharge cause the residence times to be similar during the two selected years (182 and 185 days in 2007 and 2008, respectively). Furthermore, the mean angular momentum, diffusivity, and Lagrangian velocity covariance values are smaller than in the real drifter observations, while the maximum Lagrangian integral time scale is the same.

Robust Decadal Hydroclimate Predictions for Northern Italy Based on a Twofold Statistical Approach

The Mediterranean area belongs to the regions most exposed to hydroclimatic changes, with a likely increase in frequency and duration of droughts in the last decades. However, many climate records like, e.g., North Italian precipitation and river discharge records, indicate that significant decadal variability is often superposed or even dominates long-term hydrological trends. The capability to accurately predict such decadal changes is, therefore, of utmost environmental and social importance. Here, we present a twofold decadal forecast of Po River (Northern Italy) discharge obtained with a statistical approach consisting of the separate application and cross-validation of autoregressive models and neural networks. Both methods are applied to each significant variability component extracted from the raw discharge time series using Singular Spectrum Analysis, and the final forecast is obtained by merging the predictions of the individual components. The obtained 25-year forecasts robustly indicate a prominent dry period in the late 2020s/early 2030s. Our prediction provides information of great value for hydrological management, and a target for current and future near-term numerical hydrological predictions.

Dissolved organic carbon and surface active substances in the northern Adriatic Sea: Long-term trends, variability and drivers

The paper presents a unique time series of organic matter content (dissolved organic carbon, DOC, and its surface active substances fraction, SAS) collected in the northern Adriatic along the Po-Rovinj transect between 1998 and 2017. The data were collected on a monthly or bimonthly basis. Seasonal variance of organic matter content does not exceed 30% of its total variance, while the DOC and the SAS trends are significantly negative and positive, respectively, over the whole transect. The organic matter content, however, exhibits pronounced interannual and decadal changes, with periods of high and low carbon content and evident changes in composition of the SAS content. The changes indicate altering episodes between eutrophication and oligotrophication, embedded in the overall oligotrophication trend in the considered period. Both series were correlated with the potential local and regional yearly-averaged drivers in both atmosphere and sea. DOC is most strongly (significant at 99%) correlated with the Po River discharges, at the phase lag of −1 to −2 years. For the SAS, the largest correlations (significant at 99%) are obtained with the Adriatic-Ionian Bimodal Oscillating System index (BiOS index), at the phase lag of −3 to −4 years. Correlations between the organic matter content and the hemispheric or the regional climate indices (North Atlantic Oscillation, East Atlantic/West Russia, East Atlantic, Scandinavian, and Mediterranean Oscillation) are much lower and only sparsely significant at 95% at some phase lags. The same was found for the other local drivers (precipitation and net heat flux). Our analysis highlights the importance of remote processes, like the BiOS, that weren't previously considered to shape the biogeochemical properties of such shallow coastal region impacted by freshwater load. To properly assess such impacts, long-term ecological monitoring and homogenized data series are required.

In-situ trace metal (Cd, Pb, Cu) speciation along the Po River plume (Northern Adriatic Sea) using submersible systems

Information on the distribution and speciation of trace metals is of critical importance for our ability to interpret the links between the bioavailability and uptake of an element, and its biogeochemical cycle in coastal environments. Within the framework of the European Project “In-situ automated Monitoring of Trace metal speciation in Estuaries and Coastal zones in relation with the biogeochemical processes (IMTEC)”, the chemical speciation of Cd, Pb and Cu was carried out along the Po River plume in the period 27 October – 2 November 2002. During the cruise, five Voltammetric In-situ Profiling systems and one Multi Physical Chemical Profiler, as well as conventional voltammetric instruments, were successfully applied in order to evaluate the distribution of Cd, Pb and Cu between different fractions (free ion, dynamic, colloidal, dissolved and particulate fractions) and to assess the evolution of these fractions during estuarine mixing and in the water column. Dynamic concentrations were 0.05–0.2 nmol L−1 Cd, 0.02–0.2 nmol L−1 Pb, and 0.15–4.0 nmol L−1Cu. Cd was mainly present as dynamic fraction (40–100% of the dissolved Cd). High proportions of Pb (~70%) and Cu (~80%) were present as colloids probably of biogenic origin. Principal components analysis reveals a strong influence of the Po River discharge on the spatial and vertical distributions of metal species. Almost all the metal fractions globally decreased following the salinity gradient. Metal concentrations are far below (at least one order of magnitude lower) the Environmental Quality Standard established by the Italian law. However, the Cu dynamic fraction showed concentrations likely to be toxic to sensitive phytoplankton community and to have negative effects on larva development of coastal macroinvertebrate species (toxicity data extracted from literature).

Hydrodynamics of the Po River‐Delta‐Sea System

AbstractIn this study we investigated the hydrodynamics and the interaction between riverine and marine waters in the Po River‐Delta‐Sea (RDS) system (Italy). Through the application of an unstructured 3D numerical model to a domain comprising the Po river branches, seven coastal lagoons and the shelf sea, many of the hydrodispersive phenomena evolving from the interaction of the different water bodies of the RDS system were described. The model was successfully calibrated and validated for water levels, fluxes at the inlets and at the river branches, and water temperature and salinity observations. The use of such a comprehensive approach allows the characterization of the general hydrodynamics of all the components of the Po RDS system, of the water exchange between different water bodies as well as the Po river discharge distribution among all branches. The analysis of calculated coastal current patterns in the prodelta of two different years and during a flood event confirms that the Po River is the main driver of the baroclinic coastal sea circulation. However, when strong winds occur (Sirocco and Bora), the surficial circulation of the shelf area is significantly modified. Generally, the lagoons of the Po RDS system can be considered hydrodynamically active since their tidal prisms represent a relevant fraction of each lagoon's volume. The natural consequence is that the lagoons show an active flushing with water renewal times in the order of few days. The hydrodynamics is mainly forced by the tide and is locally enhanced by riverine inputs and wind. Where freshwater inputs are significant, they are responsible of the lagoons salinity patterns that show high spatial gradients (not all the lagoons are well mixed) and temporal variability. The variability of the freshwater discharges and the complex morphology of the Po RDS allows the presence of both marine and riverine environments.

Wavelet analysis of remote sensing and discharge data for understanding vertical ground movements in sandy and clayey terrains of the Po Delta area (Northern Italy)

Subsidence phenomena change the proneness of urban and coastal areas to huge flooding, particularly, in river delta regions. Many natural and/or anthropic processes can induce vertical ground movements; identifying the causes of the observed phenomena is a useful, even if not easy, task for flood forecasting. In order to improve the knowledge of natural versus anthropic contributions to vertical ground movements, in the present work data analysis techniques (polynomial regression analysis, frequency analysis and cross wavelet and wavelet coherence analysis) are used to analyze both the long- and short-time scale dependencies between Continuous Global Positioning System (CGPS) and variations of meteorological, hydrogeological and hydrological data collected in the Po River Delta area (Northern Italy) from January 2009 to December 2015. The main findings are as follows: (i) occurrence of positive correlations and periodic oscillations of about 2–3years for all the analyzed meteorological, hydrogeological and hydrological records; (ii) a general decrease of the ground level with a faster soil lowering in clayey terrain with respect to sandy terrain; and, (iii) negative correlations between CGPS and Po River discharge records at the intra-annual scale (2–6months), which are attributed to an isostatic rebound induced by the change of the river discharge due to precipitation changes over northern Italy.

Dissolved organic carbon as potential indicator of global change: A long-term investigation in the northern Adriatic

Dissolved organic carbon (DOC) is an essential component of the biogeochemical marine system, effecting biological and chemical reactions that take place in the sea. DOC represents a dynamic component of the global carbon cycle.This paper reports 25years of measurements of DOC content and distribution at seven stations along the transect Po River delta – Rovinj in the northern Adriatic (NA). The results show strong temporal and spatial variability: (1) The highest average DOC concentrations were observed in 1998 and 2002 (143μmol/L and 137μmol/L, respectively); (2) The minimum average DOC was recorded in 2006 (88μmol/L) and (3) The short-term DOC accumulation (up to 203, average 102μmol/L) for the years 2009 to 2012, was observed during the summer and autumn months followed by unusually low DOC concentrations during the winter and spring. The DOC results from the more recent monitoring at the same stations indicate primarily oligotrophic characteristics of the NA seawater (88μmol/L).The results of DOC variability and distribution in the NA appears to be strongly influenced by complex circulation patterns. This paper provides a “link” between the Ionian circulation and the NA ecosystem as a part of the recently identified Adriatic-Ionian Bimodal Oscillating System (BiOS). A good agreement between the BiOS oscillation and other variables related to the DOC concentration, like the NA A and B winter types, the Po River discharge, salinity, chlorophyll a, occurrence of hypoxic-anoxic conditions, eutrophication and oligotrophication, suggests that DOC might be a good tool and indicator of global change.

Extremely dry and warm conditions in northern Italy during the year 2015: effects on the Po river water

The presented research highlights relationships between the climatic anomalies that occurred in northern Italy in 2015 and the water system of Po river. We investigated the effect of anomalous high temperature and paucity of meteoric precipitation on the Po river discharge and water geochemistry. The new geochemical data, carried out on river water sampled at Pontelagoscuro (close to the city of Ferrara) and in the delta, have been compared with an extended dataset collected since 2009. The comparison emphasizes that water samples of 2015 were characterized by a high electrical conductivity due to high concentrations of conservative ionic species (e.g. Na, Cl, SO4) and nutrients such as nitrate. Oxygen and hydrogen isotopes, particularly sensitive to the observed climatic changes, reveal in a δ18O-δD diagram evaporative trends (highlighted by displacement from the Meteoric Water Lines) with a magnitude that was not recorded in the last years. The monitoring is currently in progress to develop functions that relate geochemical parameters to the evolving meteo-hydrological conditions.

Influence of the Po River runoff on the bacterioplankton community along trophic and salinity gradients in the Northern Adriatic Sea

AbstractThe Po River runoff strongly affects the oceanographic and ecological characteristics of the Northern Adriatic Sea. Catalysed reported deposition ‐ fluorescence in situ hybridization (CARD‐FISH) analysis was employed to assess how the composition of the coastal bacterioplankton community is influenced by the river runoff in two different seasons (spring and autumn). Samples were collected from the water column along a coastal–offshore transect in the Northern Adriatic Sea at different depths. Four clone libraries were then constructed from coastal (0 m) and offshore waters (−65 m). Higher abundances of bacteria were recorded in coastal waters as compared with the offshore samples. This result was mainly due to the trophic state of the water column, and it was related to salinity. Particularly, Actinobacteria and Gammaproteobacteria were affected by the riverine inputs, whereas Bacteroidetes and Alphaproteobacteria showed only minor responses. This was particularly clear in the autumn sample, in which a clear difference between the coastal and the offshore samples was found due to a strong influence of the less saline river water with high nutrient concentrations. Analysis of 205 partial length 16S rRNA gene sequences indicated a high diversity with the dominance of Alphaproteobacteria, Bacteroidetes and Gammaproteobacteria mainly affiliated to coastal and marine bacterioplankton clades. Actinobacteria were also detected and, together with Gammaproteobacteria, strongly follow the freshwater intrusion. Overall, our results indicate that the seasonal difference in the Po River discharge greatly affects the bacterioplankton community. In spring we observed a smooth transition from coastal to open‐sea conditions owing to the more superficial freshwater plume. In autumn the deeper freshwater intrusion in the coastal site and thermal stratification offshore resulted in a marked difference between the coastal and offshore microbial communities.

Factors favouring phytoplankton blooms in the northern Adriatic: towards the northern Adriatic empirical ecological model

Abstract. Influenced by one of the largest Mediterranean rivers, Po, the northern Adriatic (NA) production is highly variable seasonally and interannually. The changes are especially pronounced between winters and seemingly reflect on total Adriatic bioproduction of certain species (anchovy). We analysed the long-term changes in the phytoplankton production in the region, as derived from monthly oceanographic cruises, in relation to concomitant geostrophic currents distribution in the area and to Po River discharge rates in days preceding the cruises. In winter and early spring the phytoplankton abundances depended on existing circulation fields, in summer and autumn they were related to Po River discharge rates 1–15 days earlier and on concomitant circulation fields, while in late spring phytoplankton abundances increased 1–3 days after high Po River discharge rates regardless of the circulation fields. During the entire year the phytoplankton abundances were dependent on forcing of the previous 1–12 months of surface fluxes and/or Po River rates. The role of wind was uncertain but that was partly due to unmatched sampling time frames between meteorological and sea data. Low evaporation rates in November reflected significantly on the next February circulation pattern and, although with somewhat lower significance, on large phytoplankton blooms in the same month. We showed that the role of wind in evaporative flux enhancements is not straightforward as evaporative fluxes are highly dependent on other factors, e.g. air–sea temperature difference. Wind-induced vertical mixing was only sporadically related to phytoplankton abundances. From 1990 to 2004 a shift towards large winter bioproduction induced by circulation changes appeared. The investigations performed represent the preliminary actions in the construction of an empirical ecological model of the NA which can be used in the sustainable economy of the region, as well as for validation of the numerical ecological model of the region, which is currently being developed.

Dynamics of particles along the western margin of the Southern Adriatic: Processes involved in transferring particulate matter to the deep basin

The Southern Adriatic is an area of dense shelf water (DSW) cascading and open-ocean convection. The impact of DSW cascading events in transferring organic matter to the deep benthic community and in producing a wide range of bedforms along the continental margin has been highlighted in recent years. In order to improve our knowledge on the interannual variability of the DSW cascading with the ultimate goal to understand timing of DSW formation and transport as well as the cascading process, its variability in intensity and duration, and the ultimate impact of the dense water on the deep sea, in March 2009 we deployed an instrumented mooring in a field of sediment waves (860 m depth) located down current to the Bari canyon. In March 2010, a second mooring was installed in the northern channel of the canyon. Winters 2009, 2010 and 2011 were mild and particularly wet and the Po river discharge remained relatively high throughout the whole winter. Hence, we expected weak dense shelf water formation associated with a shallow shelf water overflowing off the Adriatic shelf. By contrast, in winter 2012, the North Adriatic experienced a severe cold wave with NE Bora winds and reduced fresh water input. These weather conditions allowed the formation of extremely dense shelf water. Three additional moorings were quickly deployed during this cold event in the Southern Adriatic Sea to investigate the spatial variability of particle and organic matter fluxes exiting from the Adriatic continental shelf under the influence of a strong DSW cascading event. Slow near-bottom currents, never exceeding 40 cm s− 1, were recorded between March 2009 and February 2012. Water temperatures depicted minor negative shifts. Total mass fluxes (TMFs) were low (annual avg., 1.7–2.8 g m− 2 d− 1 and 0.3–0.6 g m− 2 d− 1 in the canyon and in the sediment wave field, respectively; and peak values ≤ 6.8 g m− 2 d− 1), but showed significant seasonal and interannual variability. Fluxes in the canyon were higher than those measured in the sediment wave field at deeper water depth. Mass flux peaks during the 2112-DSW cascading were up to 5 times higher than the peaks of previous years (up to 18.70 g m− 2 d− 1), with a spatial variability mainly driven by the localized pathways of DSW cascading. In the canyon, near-bottom currents exceeded 70 cm s− 1 and temperature dropped to 12.2 °C while current speeds were high also at 1200 m depth (~ 60 cm s− 1) in the moat surrounding the Dauno seamount. Surprisingly, mass flux peaks occurred from the 16th February to the 1st of March, 3–4 weeks ahead of the usual DSW occurrence, suggesting an early arrival of the DSW. The deep DSW cascading was the main process driving the particle transfer across the southern Adriatic margin during late winter–spring 2012. Mooring data showed a NW–SE gradient of temperature and kinetic energy from upslope to basin floor, indicating slope transverse flow modulated by local obstruction caused by the rugged seafloor topography. Bari canyon is one of the sites of DSW flow and in this area we have the possibility to extend observations back in time using continuous mooring data since 2009 and previous published materials; this approach allowed evaluation of additional mechanisms of particle transport (e.g., open-ocean convection, storm-driven downward transport, shallow dense water cascading) that are particularly relevant in years when the DSW formation is less vigorous and cascading processes are sluggish. The small amplitude of total mass flux peaks, the weak currents and the relatively high and constant temperatures recorded during 2009, 2010 and 2011 springs are consistent with an enhanced vertical particle rain from a mid-water nepheloid layer, triggered in turn by a shallow cascading of not-particularly dense shelf water detaching from the seafloor when reaches its neutral buoyancy. Thus, the intensity of DSW cascading (shallow vs. deep) plays a first order control on the particulate fluxes through the western margin of the Southern Adriatic, while storm-induced sediment transport can occasionally be relevant too.

Marine Sediments Remotely Unveil Long-Term Climatic Variability Over Northern Italy.

A deep understanding of natural decadal variability is pivotal to discuss recently observed climate trends. Paleoclimate proxies allow reconstructing natural variations before the instrumental period. Typically, regional-scale reconstructions depend on factors like dating, multi-proxy weighting and calibration, which may lead to non-robust reconstructions. Riverine records inherently integrate information about regional climate variability, partly overcoming the above mentioned limitation. The Po River provides major freshwater input to Eastern Mediterranean, as its catchment encompasses a large part of Northern Italy. Here, using historical discharge data and oceanographic measurements, we show that Po River discharge undergo robust decadal fluctuations that reach the Ionian Sea, ~1,000 km South of Po River delta, through propagating salinity anomalies. Based on this propagation, we use a high-resolution foraminiferal δ18O record from a sediment core in the Ionian Sea to reconstruct North Italian hydrological variability on millennial-scale for the first time. The reconstruction reveals highly significant decadal variability that persists over the last 2,000 years. Many reconstructed extremes correspond to documented catastrophic events. Our study provides the first millennial-scale reconstruction of the strength of decadal hydrological variability over Northern Italy. It paves the way to assess the persistence of large-scale circulation fingerprints on the North Italian climate.

Regulating mechanisms of calanoid copepods variability in the northern Adriatic Sea: Testing the roles of west-east salinity and phytoplankton gradients

West-east variability of abundance and population structure of calanoid copepods was studied in 2002–2006 period at two stations in the northern Adriatic (NA), influenced to a different degree by the freshwater discharge of the Po River and regional circulation pattern shaped by the Istrian Coastal Countercurrent (ICCC) and Eastern Adriatic Current (EAC). Species-specific calanoid distribution and abundances were related to concurrently obtained salinity, temperature, Po River discharge and phytoplankton data. Significant differences between stations were determined for salinity, phytoplankton biomass and abundance, total calanoid density, total number of species and Shannon–Wiener biodiversity. The paper is especially concentrated to extremes in Po discharge recorded during the investigated period, which are discussed with respect to the observed variability in plankton components at each investigated station. At the western station, the proximity of the freshwater sources and consequent nutrients load was strongly coupled to the phytoplankton cycle and more tightly reflected in the calanoid abundances, whereas at the eastern station this link was appreciably weaker. Analysis of the community structure revealed the lack of true discriminating species between stations, while fairly stable pattern of occurrence and relative abundances was recorded for six key calanoid species (Pseudocalanus elongatus, Paracalanus parvus, Acartia clausi, Ctenocalanus vanus, Temora longicornis and Centropages typicus) at both sites. The role of EAC was significant in transport of open middle Adriatic species into the NA, which were more frequently recorded at the eastern station and extended their areal westwards more often in winter due to strengthening of the EAC.

Hydrological simulation of Po River (North Italy) discharge under climate change scenarios using the RCM COSMO-CLM

The impacts of climate change on Po River discharges are investigated through a set of climate, hydrological, water-balance simulations continuous in space and time. Precipitation and 2m mean temperature fields from climate projections under two different representative concentration pathways, RCP4.5 and RCP8.5, have been used to drive the hydrological model. Climate projections are obtained nesting the regional climate model COSMO-CLM into the global climate model CMCC-CM. The bias in climate projections is corrected applying the distribution derived quantile mapping. The persistence of climate signal in precipitation and temperature after the bias correction is assessed in terms of climate anomaly for 2041–2070 and 2071–2100 periods versus 1982–2011. To account for the overall uncertainty of emission scenarios, climate models and bias correction, the hydrological/water balance simulations are carried out using both raw and bias corrected climate datasets. Results show that under both RCPs, either considering raw and bias corrected climate datasets, temperature is expected to increase on the whole Po River basin and in all the seasons; the most significant changes in precipitation and discharges occur in summer, when the reduction of precipitation leads to an increase in low flow duration and occurrence likelihood, and in autumn and winter where precipitation shows a positive variation increasing the high flows frequency.

Mechanisms of hypoxia frequency changes in the northern Adriatic Sea during the period 1972–2012

Hypoxia events frequently occurred in the bottom layer of the northern Adriatic Sea (NAd) from mid-summer to mid-autumn, when the water column is highly stratified, with highly variable spatial extent and duration. To determine the mechanisms of changes in hypoxia frequency and their relation to environmental conditions, 40yr-long time series of dissolved oxygen and of parameters that describe freshwater influence, stratification processes, and circulation patterns were analysed. It was shown that seasonal hypoxic events in the open water areas coincided with the formation of cyclonic or anticyclonic circulation cells, whose stability was estimated by the appearance of the Istrian Coastal Counter Current (ICCC). The oxygenation of bottom waters during the period August–November of the last two decades has increased, whereas a decreasing trend was observed in surface waters. The frequency of hypoxic events at a larger scale in the NAd decreased since 1992, concurrently with reduced ICCC occurrences. However, the frequency of events in the western area, which is under a direct influence of Po River discharges, did not change significantly, although their intensity recently were lower than during the 1970s and 1980s.

Flood-driven transport of sediment, particulate organic matter, and nutrients from the Po River watershed to the Mediterranean Sea

The Po River (Northern Italy) discharge represents a considerable input of the land-derived material entering the Mediterranean Sea. Most of its particulate and dissolved constituents are supplied to the sea in response to short-lived climate events. Although these floods exert first-order control on the transport of organic and inorganic elements, both composition and magnitude of the river material are poorly constrained during high discharge periods. In order to fill this knowledge gap, in this study we carried out an event response sampling in the Po River in November 2011. Beginning in early November, intense rainfall occurred in the Po watershed that resulted in a flood of ∼6000m3s−1 (2.5year return period). Water samples were collected from the river before and during the flood. Dissolved nitrate, nitrite, ammonia, and silicate were measured and the particulate material was analyzed for total suspended sediment, elemental composition, δ13C, δ15N, grain-size, and 137Cs activity.Our results showed a temporal decoupling between solid and water discharge implying that predicted sediment loads simply derived from sediment rating curves could potentially give rise to large errors, especially when calculations are used to understand the sediment export in response short-lived events. The suspended organic material during high flow was dominated by soil organic matter while high δ15N indicated the influence of an additional 15N-enriched source (e.g., manure, sewage, and algal biomass) during low discharge. Because the concentrations of nitrite and ammonia were positively correlated with the content of particulate material in suspension, we inferred that nitrite and ammonia concentrations were driven by either bacteria activity (ammonification–nitrification) or ionic exchange whose rates were proportional to concentration of the suspended material. In addition, due to the dilution with nitrate-poor rainfall, the concentration of nitrate decreased with increasing water discharge. High concentrations of nitrate were instead attributable to the influx of nitrate-rich water from groundwater that is chronically contaminated and constitutes most of the baseflow during low flow. Our results indicate that the event-dominated transport in the Po drainage basin is particularly important for the organic matter supply as flood events account for at least one-third of the particulate annual export (organic carbon and nitrogen). Finally, this study has demonstrated the utility of event-response sampling for understanding the importance of event-dominated transport in rivers.

Northern Adriatic phytoplankton response to short Po River discharge pulses during summer stratified conditions

AbstractA 275‐km‐long transversel Northern Adriatic profile from the mouth of the Po River (Italian Adriatic coast) to the Kvarner region (Croatian coastal island area) was investigated in three successive case studies in August 2008, 2009 and 2010. The short Po River pulses in August result in the surface advection of riverine water, nutrients and phytoplankton from the western to the eastern side of the Adriatic. This surface spreading exhibits inter‐annual variability depending on the riverine discharge in the preceding period. The Po River discharge pulse in August 2010 in particular resulted in an extraordinary tongue‐like advection of riverine water, nutrients, and phytoplankton towards the Eastern Adriatic coast. The phenomenon was detected using both satellite imagery and classical oceanographic measurements. In the advective water, toxic dinoflagellates were most abundant in August 2010, when the influence of the Po was greatest.

Data Analysis to Detect Inhomogeneity, Change Points, Trends in Observations: An Application to Po River Discharge Extremes

The aim of this report within GEMINA project is to outline a procedure to detect inhomogeneity, shift, and trends in observed time series. The analysis of hydrological extremes will help to gain insight in the effects at regional/local scale of climate conditions. The same approach will be used to analyze the discharge obtained from numerical simulations that will be carried on in GEMINA project. The main tests for detecting inhomogeneity, change points, and (linear) trend will be presented and the application of tests to detect change points (SNH, Pettitt, Buishand, and Von Neumann) and trend (Mann-Kendall and Spearman) to annual maximum and minimum Po river discharge at the section of Pontelagoscuro is presented. In addition, a Literature review of the applications of the same tests to climate and hydrological variables is provided.