Po River Prodelta Research Articles

Hotspots of microplastic accumulation at the land-sea transition and their spatial heterogeneity: The Po River prodelta (Adriatic Sea)

Deltas are the locus of river-borne sediment accumulation, however, their role in sequestering plastic pollutants is still overlooked. By combining geomorphological, sedimentological, and geochemical analyses, which include time-lapse multibeam bathymetry, sediment provenance, and μFT-IR analyses, we investigate the fate of plastic particles after a river flood event providing an unprecedented documentation of the spatial distribution of sediment as well as of microplastics (MPs), including particles fibers, and phthalates (PAEs) abundances in the subaqueous delta. Overall sediments are characterized by an average of 139.7 ± 80 MPs/kg d.w., but display spatial heterogeneity of sediment and MPs accumulation: MPs are absent within the active sandy delta lobe, reflecting dilution by clastic sediment (ca. 1.3 Mm3) and sediment bypass. The highest MP concentration (625 MPs/kg d.w.) occurs in the distal reaches of the active lobe where flow energy dissipates. In addition to MPs, cellulosic fibers are relevant (of up to 3800 fibers/kg d.w.) in all the analyzed sediment samples, and dominate (94 %) with respect to synthetic polymers.Statistically significant differences in the relative concentration of fiber fragments ≤0.5 mm in size were highlighted between the active delta lobe and the migrating bedforms in the prodelta. Fibers were found to slightly follow a power law size distribution coherent with a one-dimensional fragmentation model and thus indicating the absence of a size dependent selection mechanism during burial. Multivariate statistical analysis suggests traveling distance and bottom-transport regime as the most relevant factors controlling particle distribution. Our findings suggest that subaqueous prodelta should be considered hot spots for the accumulation of MPs and associated pollutants, albeit the strong lateral heterogeneity in their abundances reflects changes in the relative influence of fluvial and marine processes.

The last 50 years of organic contamination of a highly anthropized tributary of the Po River (Italy)

We examined the temporal profiles of many organic micropollutants analysed in a sediment core sampled from a highly anthropized tributary of the Po River, the Lambro River. Analysed for extractable organic halogens (EOX), total petroleum hydrocarbons (C10–C40TPH), polycyclic aromatic hydrocarbons (PAHs), common legacy pollutants (DDTs, PCBs), halogenated flame retardants (PBDEs, DBDPE, TBBPA-bis, TCBPA, TBBPA, HBCDs), organotins (TBT, TPhT), antimicrobials (TCS, TCC), fragrances (AHTN, HHCB) and phthalates (DMP, DEP, DnBP, BBP, DEHP, DnOP), the dated sediment core revealed the historical record of 50 years of chemical contamination discharged into the Lambro and thereby the Po River. In this regard, the peak levels of PCBs and DDTs found in Lambro sediments were also identified in other sediment cores collected from the Po River prodelta in the Adriatic Sea, thus hundreds of kilometres downstream (Combi et al., 2020). The highest risk to aquatic organisms was associated with decades of high levels of C10–C40 TPH, PBDEs, PCBs, PAHs, DDTs, EOX, TCC, AHTN and DEHP, which in different periods of the contamination history, showed exceedances of guideline/threshold values. C10–C40 TPH and TCC, for example, were very high in the 1960s, whereas PCBs, DDTs, and PBDEs, peaked from the 1980s onward. The corresponding sums of PEC quotients ranged between 0.48 and 28.63, with a mean value (±SD) for the entire recording period of 10.62 ± 9.83. Environmental legislations and improved wastewater treatments were the main drivers of the recent downward trends observed for most of the chemicals investigated. Floods in turn resulted in macroscopic yet temporary improvements in the chemical quality of the tributary, conveying contaminated sediments into the Po River.

Trace metals accumulation on modern sediments from Po river prodelta, North Adriatic Sea

Trace Metals (TMs: Pb, Zn, Cu, Cr, Cd, and Ni), major elements (Al and Fe) and radionuclides (210Pb/137Cs) investigations on EL-C01 sediment core provides new information on sedimentary regime and anthropogenic impacts during the last 175 years in the Po river prodelta area. The results allow to identify some TMs/Al peak in the upper part of the core, likely related to the major flood events of the Po river during the XXth century. Sediments deposited after the year 1900 exhibit a gradual increase of some TMs concentrations compared to pre-industrial era values. In particular, Pb, Zn, and Cu high contents are detected after the World War II and during the “Italian Economic Miracle” period. The decrease of heavy metal (Zn and Pb) contents from the second half of the 1980s is probably the effect of the Italian Law 319/76 and anti-pollution environmental policies concerning industrial and urban emitters.

Natural Versus Anthropic Influence on North Adriatic Coast Detected by Geochemical Analyses

This study focused on the geochemical and sedimentological characterization of recent sediments from two marine sites (S1 and E1) located in the North Adriatic Sea, between the Po River prodelta and the Rimini coast. Major and trace metal concentrations reflect the drainage area of the Po River and its tributaries, considered one of the most polluted areas in Europe. Sediment geochemistry of the two investigated sites denote distinct catchment areas. High values of Cr, Ni, Pb and Zn detected in sediments collected in the Po River prodelta (S1 site) suggest the Po River supply, while lower levels of these elements characterize sediments collected in front of the Rimini coast (E1 site), an indication of Northern Apennines provenance. Historical trends of Pb and Zn reconstructed from the sedimentary record around the E1 site document several changes that can be correlated with the industrialization subsequent to World War II, the implementation of the environmental policy in 1976 and the effects of the Comacchio dumping at the end of 1980. At the S1 site, the down core distributions of trace elements indicate a reduction of contaminants due to the introduction of the Italian Law 319/76 and the implementation of anti-pollution policies on automotive Pb (unleaded fuels) in the second half of the 1980s.

Structural and functional response of coastal macrofaunal community to terrigenous input from the Po River (northern Adriatic Sea)

To assess the influence of river-derived material on the benthic ecosystem in the Po River prodelta area, we investigated the structural and functional features of macrofaunal invertebrates in 14 sites. Biodiversity, species composition, response and effect traits of the community were determined and related to the main sediment physical-chemical and nutritional features (biopolymeric carbon, BPC; protein to carbohydrate ratio, Prt/Cho ratio; phaeopigment to chlorophyll-a ratio, Phaeo/Chl-a), as well as contaminants. We found significant spatial differences in community structure and its functional expression between northern and southern stations, and between nearshore and offshore ones. The shallower stations located nearby the main river mouth displayed higher densities, diversity and functional redundancy compared to southern stations. The main discriminating factors for the community were the higher percentage of sand and Prt/Cho nearby the river mouth, and fine grain-size and high Phaeo/Chl-a at the southernmost and deeper stations. Contaminant concentrations did not seem to severely affect the macrofaunal community. Changes in community structure were due to the dominance of few species, which occurrence varied according to grain-size and organic matter quality that in turn were linked to the different magnitude of depositional loads in the investigated area.Focusing on response traits, we observed a clear influence of high loads nearby the main river mouth, through the dominance of sessile and tube-builder trait-categories at stations with a higher sand content. In contrast, we detected a major expression of burrowers and interface crawlers, and swimmers at southern and principally offshore muddy stations. Effect traits revealed that in the northern part of the prodelta prevailed suspension, surface deposit feeders and conveyor invertebrates that are, respectively, fundamental players in the benthic-pelagic coupling and elemental cycling within benthos. In the southern part, subsurface deposit feeder and biodiffuser organisms dominated. The latter are able to rework huge amounts of sediments also in the deepest layers, amplifying the elemental cycling and decomposition. In highly dynamic systems, as delta areas, the presence of invertebrates able to exploit different resources through bioturbation activities may enhance the functions of riverine costal ecosystems.

Historical sedimentary deposition and flux of PAHs, PCBs and DDTs in sediment cores from the western Adriatic Sea.

The sources and depositional history of polycyclic aromatic hydrocarbons (PAHs) and organochlorine compounds (OCs) over the last century were investigated in sediment cores from the North Adriatic Sea (Po River prodelta) and the South-Western Adriatic Margin (SWAM). Contaminant concentrations were higher in the Po River prodelta. ∑16PAHs ranged from 193 to 533 ng g-1, ∑5PCBs ranged from 0.9 to 5.2 ng g-1 and ∑DDTs (p,p'-DDD+p,p'-DDE) ranged from 0.1 to 2.5 ng g-1. In the SWAM, ∑PAHs ranged from 11 to 74 ng g-1 while ∑PCB and ∑DDT concentrations were close to the MQL. Accordingly, contaminant fluxes were much higher in the northern (mean values of 152 ± 31 ng cm2 y-1 and 0.70 ± 0.35 ng cm2 y-1 for PAHs and OCs, respectively) than in the southern Adriatic (2.62 ± 0.9 ng cm2 y-1 and 0.03 ± 0.02 ng cm2 y-1 for PAHs and OCs, respectively). The historical deposition of PAHs seemed to be influenced by the historical socioeconomic development and by changes in the composition of fossil fuel consumption (from petroleum derivatives to natural gas) in Italy from the end of the 19th century to the present. Similarly, vertical variations in DDT concentrations matched its historical use and consumption in Italy, which started around in the mid-late 1940s to fight typhus during the II World War. Contaminant concentrations detected in sediments does not seem to pose ecotoxicological risk for marine organisms in the Adriatic Sea.

Links between microbial processing of organic matter and the thermohaline and productivity features of a temperate river-influenced Mediterranean coastal area

Regions of freshwater influence are affected by a constant interplay of different environmental factors that rule their seasonal and interannual highly variable dynamics. Riverine freshwater discharges alter the buoyancy of the local water masses, generating dynamic fronts and increasing water column stratification and nutrient availability. In such a heterogeneous context the paradigms for biological-physical interactions driving biogeochemical dynamics in the open seas do not always apply. We present here data from 12 surveys spanning 12 years (from 2004 to 2016) in the Po River prodelta area, which is highly representative of the above-described complexity, as it conveys the second largest freshwater discharge to the Mediterranean Sea. In particular, we focused on the microbially-mediated organic matter processing by estimating the numerical abundances of non-pigmented (‘heterotrophic’) and photoautotrophic (Synechococcus) prokaryotes, heterotrophic production and the activity of the exoenzymes beta-glucosidase, alkaline phosphatase and leucine aminopeptidase. The abundance of Synechococcus and heterotrophic prokaryotes as well as microbial activities were positively correlated to temperature and chlorophyll a concentration. Concomitantly, each variable was negatively correlated to salinity. In high-temperature, high salinity and low chlorophyll a samples, the heterotrophic production was limited and deviated from the typical linearity of the Arrhenius' law, suggesting resource limitation rather than temperature control. A multivariate analysis performed to analyze the pattern of organic matter degradation/utilization highlighted 11 distinct clusters of functional diversity. Temperature, salinity and chlorophyll a concentration data alone were able to explain most of the functional cluster separation as evidenced by a regression tree analysis. For freshwater-influenced samples, salinity was the only variable affecting metabolic patterns, whereas in typically marine waters, at salinity ≥37, metabolic profiles were determined by specific combinations of all 3 parameters.

Role of depositional dynamics and riverine input in shaping microbial benthic community structure of Po prodelta system (NW Adriatic, Italy)

To understand to what extent depositional dynamics and riverine input contribute to shape the microbial benthic community structure in coastal systems, surface sediments were collected along one nearshore and two land-sea transects in the Po River prodelta, and analyzed for lithology, sediment structures, grain-size, short-lived radioisotopes (7Be and 137Cs) and organic matter (OM) quantity and origin (C, N, stable isotopes). The most recent surface supply (i.e. 5–6 mo = SUP layer) and the underneath, older sub-surface deposits (i.e. >6 mo = SUB layer) were analyzed at six selected sampling sites and checked for possible differences in the microbial community abundance and structure. According to different depositional dynamics, three distinct groups of sites were detected: nearshore muddy and sandy sites, and offshore sites. Sedimentary records reflected the interplay of different elements and/or processes acting in this complex and highly dynamic system. River-derived sediments, as depicted by high contribution of silty particles and recent riverine OM inputs, were confined to nearshore locations in front of the main river mouth. They resulted highly correlated to bacterial distribution and prokaryotic abundances in SUP layer. Nevertheless, in subsurface sediments affected by the delivery of high amount of mud, potential burial phenomena and/or the development of anoxic conditions occurred, leading to a change in the bacterial community structure. We speculated that the decrease of Gamma- and increase of Delta-Proteobacteria in the SUB layer could be related to their sulfur-oxidizing and sulfate-reducing activities, respectively. On the other hand, at offshore sites, the presence of a low, and regular sedimentary input and less intense hydrodynamic conditions favored a more stable microbial structure across surface and subsurface sediments. Four discriminating proxies, such as grain-size representing the system energy, Total N as an indicator of land-derived nutrient inputs, 7Be as a tracer of the freshness of riverine discharge, and stable C isotope as an indicator of the type and age of OM, mainly summarize the complex relationships between sedimentary dynamics and bacterial community structure in the Po River prodelta area.

Spatio-temporal analysis of prodelta dynamics by means of new satellite generation: the case of Po river by Landsat-8 data

This paper describes a procedure to perform spatio-temporal analysis of river plume dispersion in prodelta areas by multi-temporal Landsat-8-derived products for identifying zones sensitive to water discharge and for providing geostatistical patterns of turbidity linked to different meteo-marine forcings. In particular, we characterized the temporal and spatial variability of turbidity and sea surface temperature (SST) in the Po River prodelta (Northern Adriatic Sea, Italy) during the period 2013–2016. To perform this analysis, a two-pronged processing methodology was implemented and the resulting outputs were analysed through a series of statistical tools. A pixel-based spatial correlation analysis was carried out by comparing temporal curves of turbidity and SST hypercubes with in situ time series of wind speed and water discharge, providing correlation coefficient maps. A geostatistical analysis was performed to determine the spatial dependency of the turbidity datasets per each satellite image, providing maps of correlation and variograms. The results show a linear correlation between water discharge and turbidity variations in the points more affected by the buoyant plumes and along the southern coast of Po River delta. Better inverse correlation was found between turbidity and SST during floods rather than other periods. The correlation maps of wind speed with turbidity show different spatial patterns depending on local or basin-scale wind effects. Variogram maps identify different spatial anisotropy structures of turbidity in response to ambient conditions (i.e. strong Bora or Scirocco winds, floods). Since the implemented processing methodology is based on open source software and free satellite data, it represents a promising tool for the monitoring of maritime ecosystems and to address water quality analyses and the investigations of sediment dynamics in estuarine and coastal waters.

Detecting long-term temporal trends in sediment-bound metals in the western Adriatic (Mediterranean Sea)

Major and trace metal concentrations were determined in western Adriatic sediment cores. Based on sediment chronology, the earliest anthropogenic influence appeared as a Zn and Pb increase in the Po River prodelta starting from ~1914. The increasing contamination signal of these trace metals propagated southward as far as 450km with a growing delay, taking ~10years to reach the south Adriatic Sea. Although greater inventories of excess trace metals in the northern sector pointed to the influence of the intense human activities in the Po River drainage basin and Venice lagoon system, we observed a reduction of excess trace metals from mid-1980s, related to the implementation of stricter environmental regulations on chemical wastewaters. In contrast, an increase in trace metal accumulation in surficial sediment from the 2000s in front of the cities of Ancona and Bari suggested a recent local input of trace metals, probably due to harbor activities.

Spatial patterns and temporal trends of trace metal mass budgets in the western Adriatic sediments (Mediterranean Sea)

Spatial patterns of major (Al, Fe and Ti) and trace metals (Cu, Cr, Mn, Ni, Pb and Zn) measured in surficial sediments collected within the Late-Holocene mud-wedge in the western Adriatic Sea were analyzed to elucidate their sources, transport and mass budgets. Distributions of sedimentary trace metals, their fluvial inputs and accumulation loads reveal along-shore transport towards the southern Adriatic. Pb and to a lower extent Zn accumulation loads over time decreased significantly since 1988 in the North Adriatic, consistently with the implementation of regulations in the Western Europe, whereas Zn accumulation in the Po River prodelta remained unchanged since 1995. The Po River fluvial inputs accounted for half of Cr, Ni, Pb and Zn of the fluvial inputs into the western Adriatic Sea, contributing for the delivery of important amounts of Cr and Ni into the sediments, probably related to the natural occurrence of ultramafic rocks in the North sector. Collectively, ~30% of trace metal fluvial inputs discharged into the North sector are exported to the Central and South sectors. The Po River acts as both a bypass and an accumulation zone. In contrast, trace metal accumulation in the Central sector far exceed trace metal fluvial inputs, which suggested that this area is a preferential sink for particle-reactive river-borne material from the North Adriatic. The North sector shows moderate enrichment of Zn and Pb mainly related to the Po River influence. The anthropogenic fraction of Pb shows a large drop of ~30% from the North sector southwards, whereas Zn proportions remain fairly the same up to the Central sector only decreasing in the South sector.

Next generation sequencing reveals distinct fecal pollution signatures in aquatic sediments across gradients of anthropogenic influence

<p>Aquatic sediments are the repository of a variety of anthropogenic pollutants, including bacteria of fecal origin, that reach the aquatic environment from a variety of sources. Although fecal bacteria can survive for long periods of time in aquatic sediments, the microbiological quality of sediments is almost entirely neglected when performing quality assessments of aquatic ecosystems. Here we investigated the relative abundance, patterns and diversity of fecal bacterial populations in two coastal areas in the Northern Adriatic Sea (Italy): the Po river prodelta (PRP, an estuarine area receiving significant contaminant discharge from one of the largest European rivers) and the Lagoon of Venice (LV, a transitional environment impacted by a multitude of anthropogenic stressors). From both areas, several indicators of fecal and sewage contamination were determined in the sediments using Next Generation Sequencing (NGS) of 16S rDNA amplicons. At both areas, fecal contamination was high, with fecal bacteria accounting for up to 3.96% and 1.12% of the sediment bacterial assemblages in PRP and LV, respectively. The magnitude of the fecal signature was highest in the PRP site, highlighting the major role of the Po river in spreading microbial contaminants into the adjacent coastal area. In the LV site, fecal pollution was highest in the urban area, and almost disappeared when moving to the open sea. Our analysis revealed a large number of fecal Operational Taxonomic Units (OTU, 960 and 181 in PRP and LV, respectively) and showed a different fecal signature in the two areas, suggesting a diverse contribution of human and non-human sources of contamination. These results highlight the potential of NGS techniques to gain insights into the origin and fate of different fecal bacteria populations in aquatic sediments.</p>

Mapping turbidity patterns in the Po river prodelta using multi-temporal Landsat 8 imagery

Thirty-meters resolution turbidity maps derived from Landsat 8 (L8) images were used to investigate spatial and temporal variations of suspended matter patterns and distribution in the area of Po River prodelta (Italy) in the period from April 2013 to October 2015. The main focus of the work was the study of small and sub-mesoscale structures, linking them to the main forcings that control the fate of suspended sediments in the northern Adriatic Sea. A number of hydrologic and meteorological events of different extent and duration was captured by L8 data, quantifying how river discharge and meteo-marine conditions modulate the distribution of turbidity on- and off-shore. At sub-mesoscale, peculiar patterns and smaller structures, as multiple plumes and sand bars, were identified thanks to the unprecedented spatial and radiometric resolution of L8 sensor. The use of these satellite-derived products provides interesting information, particularly on turbidity distribution among the different delta distributaries in specific fluvial regimes that fills the knowledge gap of traditional studies based only on in situ data. A novel approach using satellite data within model implementation is then suggested.

Polychlorinated biphenyls (PCBs) in sediments from the western Adriatic Sea: Sources, historical trends and inventories

Sources, historical trends and inventories of polychlorinated biphenyls (PCBs) were investigated in sediments collected in five transects along the north-south axis of the western Adriatic Sea. The concentration of total PCBs (∑28 PCBs) ranged from <LOD (limit of detection) to 9.0ngg−1 in the sediment cores and between 0.1 and 2.2ngg−1 in recent sediments. Chronological records of PCB concentrations displayed a common pattern with historical PCB production and use, with the maximum peak values detected between the 1960s and the 1980s. Sediments deposited within the last two decades presented a ~40% to ~80% PCB reduction in comparison to the peak levels, reflecting the ban on PCB production and use since the late 1970s. PCB levels along with the presence of high-chlorinated congeners decreased southwards, indicating the Po River as the major source of PCBs in the western Adriatic Sea. This is further corroborated by the estimated inventories of PCBs, which were ~4–7 times higher in the Po River prodelta (256ngcm−2) in comparison to the middle and southern Adriatic, respectively, and about 100 times higher than the in the deep Adriatic Sea.

Patterns of benthic bacterial diversity in coastal areas contaminated by heavy metals, polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs).

Prokaryotes in coastal sediments are fundamental players in the ecosystem functioning and regulate processes relevant in the global biogeochemical cycles. Nevertheless, knowledge on benthic microbial diversity patterns across spatial scales, or as function to anthropogenic influence, is still limited. We investigated the microbial diversity in two of the most chemically polluted sites along the coast of Italy. One site is the Po River Prodelta (Northern Adriatic Sea), which receives contaminant discharge from one of the largest rivers in Europe. The other site, the Mar Piccolo of Taranto (Ionian Sea), is a chronically polluted area due to steel production plants, oil refineries, and intense maritime traffic. We collected sediments from 30 stations along gradients of contamination, and studied prokaryotic diversity using Illumina sequencing of amplicons of a 16S rDNA gene fragment. The main sediment variables and the concentration of eleven metals, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) were measured. Chemical analyses confirmed the high contamination in both sites, with concentrations of PCBs particularly high and often exceeding the sediment guidelines. The analysis of more than 3 millions 16S rDNA sequences showed that richness decreased with higher contamination levels. Multivariate analyses showed that contaminants significantly shaped community composition. Assemblages differed significantly between the two sites, but showed wide within-site variations related with spatial gradients in the chemical contamination, and the presence of a core set of OTUs shared by the two geographically distant sites. A larger importance of PCB-degrading taxa was observed in the Mar Piccolo, suggesting their potential selection in this historically polluted site. Our results indicate that sediment contamination by multiple contaminants significantly alter benthic prokaryotic diversity in coastal areas, and suggests considering the potential contribution of the resident microbes to contaminant bioremediation actions.

Modeling deposition by wave-supported gravity flows on the Po River prodelta: From seasonal floods to prograding clinoforms

A simple model for wave-supported gravity flows is applied to sediment deposition off the mouth of the Po River at time scales ranging from a single major flood to steady-state clinoform progradation. Wave-supported gravity flows are a newly appreciated class of turbidity currents, which rely on the velocity shear produced by waves near the seabed to keep sediment in suspension. The modeling approach used here, which limits the gravity flow's sediment load via a critical Richardson number, is applicable to fine sediment transport near river mouths wherever wave energy is available to move abundant sediment offshore during floods. Results suggest this phenomenon can account for the majority of the fall 2000 flood deposit mapped by EuroSTRATAFORM investigators in the vicinity of the Po River prodelta and also for the rate of prodelta progradation observed off the dominant Pila outlet of the Po over a century time-scale. Model results predict that convergence of down-slope sediment transport by wave-supported gravity flows increases with bed slope but decreases with slope gradient, such that greatest deposition occurs near where steep slopes first stop increasing with distance offshore. Thus on profiles which reach maximum steepness near shore, like those off Tolle–Gnocca–Goro mouths today or off the Pila mouth 150 y ago, gravity-driven deposition occurs in shallower water. Over time, if deposition overwhelms subsidence, the prodelta becomes less steep near shore and steeper offshore, and the locus of deposition moves progressively into deeper water. If the prodelta is prograding across a relatively flat shelf, the shape of the prodelta eventually reaches a stable form which progrades seaward as a unit. This has occurred off the Pila; but subsidence has likely overwhelmed deposition off the Tolle–Gnocca–Goro, keeping steepest slopes and maximum deposition in shallower water.

Content and isotopic composition of organic carbon within a flood layer in the Po River prodelta (Adriatic Sea)

Thirty-three surface sediment samples from cross-shelf transects on the northern Adriatic shelf were collected in December 2000, soon after a 100-yr flood of the Po River, in order to determine the distribution of organic carbon (OC) along the main sediment dispersal system. To evaluate the temporal variability, stations were re-occupied eight times at seasonal intervals until June 2003. Downcore sediment profiles from two sites characterized by high flood deposit thicknesses were also examined to assess the OC variability within the flood layer. In December 2000, the highest contents of OC (up to 1.24 wt%) were measured in front of the main distributary mouths (Pila, Tolle and Gnocca-Goro) where the greatest thicknesses of the flood deposit were recorded. However, the influence of the Po di Gnocca-Goro sediment supply on the OC surface distribution declined after ∼1.5 years from the fall-2000 river flood, probably because these mouths are less active when the water discharge is lower. The δ 13C of organic matter was used to trace the dispersal of fluvial OC on the continental shelf. The δ 13C values ranged from −25.9‰ to −23.1‰. The fraction of fluvially derived organic particles decreased with increasing water depth according to a radial dispersal pattern around the Po River delta. This pattern persisted in all cruises. δ 13C values increased progressively until April 2002, suggesting an increasing marine contribution to the OC content but decreased again following a second minor flood event in November 2002. The molar C/N ratio was on average 10.0±1.6, with slightly lower values in southern and central areas. Assuming contributions from three OC end-members (terrestrial, riverine and marine), a mixing model based on δ 13C and the ratio of N to C (statistically more robust than C/N; Goñi, M.A., Teixeir, M.J., Perkley, D.W., 2003. Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA). Estuarine, Coastal and Shelf Science 57, 1023–1048) was applied in order to quantitatively assess the OC sources for Po shelf sediments. δ 13C is significantly and positively correlated with the marine OC fraction. The terrestrial fraction is inversely correlated with N/C, while the riverine fraction is positively correlated with N/C. The terrestrial OC source was the most abundant end-member (>70%) showing only little temporal variability regardless of the Po River water discharge. Temporal and spatial changes in OC composition suggest that: (a) the Po River prodelta is always dominated from terrestrial OC input; (b) the Po della Pila supplies most terrestrial OC, whilst other tributaries (e.g., Po di Gnocca-Goro) are secondary sources. However, these mouths are as important as the Po della Pila in affecting the riverine OC signature; (c) offshore, biological primary production raises the marine OC contribution. At two sites on the Po River prodelta, the 2000-flood deposit shows slight but consistent compositional changes of organic matter (N/C and δ 13C) which can help to recognize other flood events in the sedimentary record. The OC budget for the 2000-flood deposit accounts for a terrestrial+riverine OC supply of 68–162 Gg (10 9 g) against an OC deposition of 106–162 Gg (excluding the marine fraction), which implies a rapid and efficient sedimentation of the flood material, and scarce or negligible export out the study area. Flood events may thus enhance terrestrial carbon burial, whereas marine carbon arrives more slowly and may be largely mineralized at the sediment–water interface.

The large-scale distribution and internal geometry of the fall 2000 Po River flood deposit: Evidence from digital X-radiography

Event-response coring on the Po River prodelta (northern Adriatic Sea) coupled with shipboard digital X-radiography, resistivity profiling, and grain-size analyses permitted documentation of the initial distribution and physical properties of the October 2000 flood deposit. The digital X-radiography system comprises a constant-potential X-ray source and an amorphous silicon imager with an active area of 29×42 cm and 12-bit depth resolution. Objective image segmentation algorithms based on bulk density (brightness), layer contacts (edge detection) and small-scale texture (fabric) were used to identify the flood deposit. Results indicate that the deposit formed in water depths of 6–29 m immediately adjacent to the three main distributary mouths of the Po (Pila, Tolle and Gnocca/Goro). Maximal thickness was 36 cm at a 20-m site off the main mouth (Pila), but many other sites had thicknesses >20 cm. The Po flood deposit has a complex internal stratigraphy, with multiple layers, a diverse suite of physical sedimentary structures (e.g., laminations, ripple cross bedding, lenticular bedding, soft-sediment deformation structures), and dramatic changes in grain size that imply rapid deposition and fluctuations in energy during emplacement. Based on the flood deposit volume and well-constrained measurements of deposit bulk density the mass of the flood deposit was estimated to be 16×10 9 kg, which is about two-thirds of the estimated suspended sediment load delivered by the river during the event. The locus of deposition, overall thickness, and stratigraphic complexity of the flood deposit can best be explained by the relatively long sediment throughput times of the Po River, whereby sediment is delivered to the ocean during a range of conditions (i.e., the storm responsible for the precipitation is long gone), the majority of which are reflective of the fair-weather condition. Sediment is therefore deposited proximal to the river mouths, where it can form thick, but stratigraphically complex deposits. In contrast, floods of small rivers such as the Eel (northern California) are coupled to storm conditions, which lead to high levels of sediment dispersion.

Floc fraction in the waters of the Po River prodelta

Three independent methods of estimating the proportion of suspended material packaged within flocs, termed the floc fraction (f), were employed using hydrographic and suspended sediment data, and core and tripod-based observation data collected from the waters and sediments of the Po River prodelta throughout 2001. Using a floc size versus settling velocity relationship established in this study, floc fraction estimates were derived as follows: (1) by calculation of floc concentration in in situ images versus total suspended concentration using knowledge of floc effective density via Stokes' approximation; (2) by parameterization of disaggregated inorganic grain size (DIGS) distributions of bottom sediments to infer floc fraction in suspension necessary to produce the observed flux to the seabed; and (3) by calculating a mean representative floc fraction estimate for the system derived from estimates of floc bulk density. Calculated estimates of floc fraction are near unity at the river mouth, with the bulk of floc deposition occurring by the 8m-isobath. Seaward and to the south of the river mouth, the suspension is not highly flocculated (f∼0.08). Subsequent transport and removal of deposited sediment from the prodelta, as suggested in previous study, is believed to occur within the bottom boundary layer.