CLIMATE RESEARCHClim Res

Vol. 31: 151–165, 2006 Published July 27

1. INTRODUCTION

To better understand how the spreading mechanismsof Po River waters and their particulate load affect theprimary production dynamics in the northern AdriaticSea, it is important to improve our knowledge of disper-sion mechanisms of Po River inputs into the early im-pact zones of the North Adriatic basin in various dis-charge regimes and under various seasonal conditions.

One of the main obstacles encountered when inves-tigating the northwestern Adriatic Sea is the difficultyof identifying, with in situ observations, the impactzones and maximum expansion processes of Po River

inputs towards the centre of the basin during excep-tional floods in different seasons. Usually such extremeevents cannot be foreseen, and thus marine surveyscannot be planned a priori. For the same reasons it isdifficult to investigate how and where algal bloomsdevelop when such exceptional events occur, and withwhat mechanisms and at what depths the settlingautochthonous organic matter degrades and consumesoxygen in the water column, which eventually affectsearly diagenesis on the bottom sediments (Berner1980, Barbanti et al. 1992). Actually, the autochthonousorganic matter is particularly reactive and sensitive toa partial mineralization already present in the water

© Inter-Research 2006 · www.int-res.com*Corresponding author. Email: federico.spagnoli@fg.ismar.cnr.it

Anomalous Po River flood event effects onsediments and the water column of the northwestern

Adriatic Sea

F. Frascari1, F. Spagnoli2,*, M. Marcaccio3, P. Giordano1

1CNR-ISMAR, Sezione di Geologia Marina, Via Gobetti 101, 40129 Bologna, Italy2CNR-ISMAR, Sede di Ancona, Largo Fiera della Pesca 2, 60125 Ancona, Italy

3ARPA Emilia Romagna, Via Triachini 17, 40138 Bologna, Italy

ABSTRACT: The aim of this study was to investigate the spreading pattern and trophic impact of theanomalous Po River flood in the fall of 1994 in the marine area offshore of the Po River delta. Thestudy was carried out by surveying the areal distribution of physical and biogeochemical parameters(pH, redox potential [Eh], water content, grain-size, mineralogy, organic and inorganic C, total N,total organic and inorganic P) in surficial and sub-surficial sediments, and physical-chemical proper-ties (salinity, temperature and dissolved O2) of the water column. Water column investigations high-lighted a fresh, turbid and relatively cold Po River water wedge on the basin surface that extendedmore than 40 km offshore, well above the usual limits of the Po River plume influence, which was aresult of slow geostrophic currents and no wave motion in the basin. The distribution of minerals insediments (such as serpentine and dolomite), derived from river catchments, provided cues on themaximum extension of Po and northern river influences. Biogeochemical tracers such as organic Cand organic P, together with C:N ratios, provided indirect evidence on the development of algalblooms owing to the Po River flood run-off. Indeed, they indicated areas where there was the depo-sition of autochthonous reactive organic matter, derived from fresh plankton biomass. Their locationsuggests that the maximum development of plankton blooms occurred along the external edge of theplume, where warm basin waters mixed with colder nutrient-rich waters from the river. The data alsosuggest that significant amounts of newly formed and degrading organic matter reached bottomwaters at the centre of the basin, thus increasing the degree of hypoxia.

KEY WORDS: Adriatic Sea · Po River · Organic matter · Biogeochemical tracer · Algal bloom · Flood

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Clim Res 31: 151–165, 2006

column, where it consumes dissolved oxygen. Further-more, the organic matter, which is deposited togetherwith fine inorganic particles on the bottom, feeds thebacterial degradation processes that are first oxicand later anoxic (i.e. when degradation becomes moreintense or takes place at greater depths in the sedi-ments) (Froelich et al. 1979, Berner 1980, Orel et al.1993). In this manner, degradation processes consumedissolved oxygen in bottom waters and cause the slowrelease of nutrients. Indirect indications of depositionand degradation of organic matter, relative to a singleevent or averaged over seasonal, annual or multi-annual periods, may be obtained frombottom sediment studies. Bottom sedi-ments record both the offshore dis-persion of suspended river organicand inorganic particulate and the de-position of autochthonous organicmaterials. However, in order to beable to discriminate single sedimen-tation or reworking processes, sea-bottom sediment investigations arenot enough; we need observationsthat are less averaged out over time,namely those that take place in thewater column.

From 8 to 21 November 1994, a sur-vey cruise (PR94) was conductedwithin the framework of the PRISMA1 National Programme (Frascari et al.1998). The initial purpose of the cruisewas the deployment of 3 automaticoceanographic measurement stations,but it happened to coincide with ananomalous Po River flood. As a conse-quence, we tried to obtain functionaldata to understand the processesrelated to the Po River flood in themarine area in front of the Po Riverdelta by using all available instru-ments on board. To this aim, we per-formed both water column chemical-physical measurements and bottomsediment samplings and analyses.

The scope of this work is to show thesediment and water column processesand their extension that took place inthe marine offshore area in front ofthe Po River delta during the Novem-ber 1994 flood event, as deduced fromthe chemical-physical parameters ofthe water column and from the bio-geochemical, mineralogical and sedi-mentological composition of bottomsediments.

2. STUDY AREA

The study area extends about 40 km offshore fromthe Po River delta; it is delimited to the north by theAdige River mouth, and to the south by the Reno Rivermouth (Fig. 1).

From a hydrodynamic point of view, the marine areain front of the Po River delta is characterised bysummer water column stratification, with weak geo-strophic currents that form both counter-clockwise andclockwise gyres, and an autumn-winter structure inwhich the water column stratification disappears and a

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Fig. 1. Study area. Numbered dots: multiparametric (temperature, salinity and dis-solved oxygen) water column profiles and box-corer stations. Solid lines: tracksof multiparametric transects discussed in text (Transects A, B and C). Bathymetric

contour interval: 10 m

Frascari et al.: Po River anomalous flood event

north-south geostrophic current, the stronger northAdriatic current (N-Ad), occurs (Franco et al. 1982,Malanotte-Rizzoli & Bergamasco 1983, Artegiani et al.1989, Orlic et al. 1992). In this way, on a seasonal basis,Po River inputs extend further offshore in summer,whilst during winter they tend to be more confinedalong the western coast of the basin. As a generalresult the Po River inputs are conveyed almost entirelysouthwards.

The bottom sediment grain-sizes in the investigatedarea (Brambati et al. 1983) are the result both of thegeneral hydrodynamics of the northern Adriatic Seaand of the geological evolution of the last 20 000 yr(Colantoni et al. 1979b, Trincardi et al. 1994). Alongthe coast there is a thin coastal belt of sandy sediments,up to a depth of about 7 to 8 m, as a result of inputs ofcoarse material from rivers and coastal wave rework-ing; this is followed offshore by pelitic sediments in abelt along a north-south axis, deriving from suspendedmaterial inputs both from the Po River (between 18 ×106 t yr–1 [Dal Cin 1983] and 26 × 106 t yr–1 [IRSA 1977]),and to a lesser extent from other minor rivers. Towardsthe centre of the basin there is a gradual change tomainly sandy relict sediments (Colantoni et al. 1979a).

Po particulate inputs are composed of rock frag-ments and minerals of heterogeneous origin with thepresence of mainly silico-clastic elements, whilst thesolid load transported by rivers north of the Po has alargely carbonate composition (Guerzoni et. al. 1984).

The phytoplankton biomass and primary productiondynamics in the northern Adriatic Sea are charac-terised by a permanent high production, especially inthe coastal belt along the western coast (Fonda Umaniet al. 1992). The area with the higher production isquantified off the Po River delta and related to thespreading of its plume. In this area, a marked west-to-east gradient of the standing crop and production isobserved (Smodlaka & Revelante 1983). The phyto-plankton standing crop in the northern Adriatic isrelated to thermohaline distribution: during periods ofreduced vertical mixing and conspicuous river inputs,the surface community presents a cell density that is 25times the bottom layers, dominated by microflagellatesand diatoms; in contrast, when the Po River outflow isparticularly reduced, phytoplankton density is rela-tively homogeneous throughout the water column andis always dominated by microflagellates and diatoms.The South Po River delta coast is characterised by a 6to 10 km belt of spring diatom blooms and dinoflagel-late summer monospecific blooms (Marchetti 1983)Generally, these blooms are linked to a highly strati-fied system that often reoccurs at the beginning andend of the summer period, but in almost every summersince 1975 they have given rise to red tide phenomena(Fonda Umani et al. 1990).

3. MATERIALS AND METHODS

During the PR94 cruise (8 to 21 November 1994), inorder to study the unforeseen flood event that tookplace at the Po River mouths between approximately 6and 24 November, temperature, dissolved oxygen andsalinity profiles of the water column were measuredusing a multiparametric probe (SBE 911 plus), andsediment box-corers were collected to sample bottomsediments. Stations were located in the main marineimpact zones of the Po River flood inputs, along tran-sects perpendicular to the coast line, with a frequencydistance between stations of each transect of about 5nautical miles near the Po River mouth and 12 nauticalmiles at the farthest extent (Fig. 1). Unfortunately, itwas not possible to measure turbidity and chlorophyll aprofiles owing to the lack of necessary equipment onboard. The multiparametric profiles were conductedwithin a few days in order to have a synoptic view ofthe physical-chemical properties of the water column,with the main aim of evaluating the persistence ofwater column stratification in a season that is usuallymarked by high hydrodynamics, with constant valuesof temperature and salinity throughout the wholewater column (Hendershott & Rizzoli 1976, Franco1983, Franco et al. 1983, Fonda Umani et al. 1990,Artegiani et al. 1997).

As far as bottom investigations are concerned, itshould be pointed out that we decided not to samplecoastal sands; our main purpose was to evaluate theextension mechanisms (in the direction of the opensea) of suspended material coming from the Po Riverduring the flood, and to assess evidence on the bottomof the autochthonous organic matter being deposited.

Sampling extended to zones further out to sea,where relict sands outcrop (Brambati et al. 1983), toevaluate the maximum extension of fine materialdeposition during exceptional floods. Each sedimentbox-corer was first described macroscopically to iden-tify sedimentary structures, grain-size, bioturbationand macrofauna presence. In particular, the thicknessof the recently deposited material, recognisable by thehigh degree of hydration and ochre colour, wererecorded. Next, sub-samples were collected at 2 levels(surficial and sub-surficial) in each box-corer. As far aspossible, the thickness of the surficial layer (0.5 to 1 cmthick) was chosen inside the thickness of the recentlydeposited material. The sub-surficial sample (about2 cm thick) was collected immediately below the floodlayer. This sampling strategy was based on the differ-ent information that surficial and sub-surficial samplescan give. In fact, while surficial sediments provideinformation on the last sedimentological event (on thecondition that sampling is carried out very soon afterthe event and that other processes, such as currents or

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wave motion, did not rework the sediment), sub-surfi-cial samples provide information on more sedimento-logical events, averaged over time, because of the dif-ferent reworking processes that act on sediments.Physical and chemical parameters (redox potential[Eh] and pH) were measured in each sub-sampleimmediately after the collection, and then each samplewas subdivided into aliquots for sedimentological,mineralogical and geochemical analyses.

Water content, grain-size and mineralogical compo-sition of sediment samples were determined on analiquot stored at 4°C. Organic C, total N, total, inor-ganic and organic P content analyses were performedon an aliquot stored at –20°C and then freeze dried.The water content was calculated as the differencebetween the weight of the wet and dried sedimentsamples; samples were dried at 70°C until they reacha constant weight (Gallignani & Magagnoli 1972).Grain-size analyses were conducted by wet sieving, inorder to separate the finest fractions (<63 µm) from thecoarser fractions (>63 µm), and then by X-ray sedigra-phy on the finer fractions. Mineralogical compositionwas determined through X-ray-diffrattometry, andpercentages of the most abundant minerals wereevaluated (Cook et al. 1975).

Total and organic C and total N (mainly organic N;Giordani & Angiolini 1983) were determined by meansof a CHNS elemental analyser. Organic C was deter-mined by decarbonisation with HCl 1 M and drying at60°C for 2 h (Froelich 1980, Hedges & Stern 1984).

Inorganic P was determined on the extract of sedi-ment samples that were stirred at room temperaturefor 16 h with HCl 1 M (Aspila et al. 1976). Total P wasdetermined on the extract after burning to ashes at550°C for 2 h and stirring for 16 h with HCl 1 M. Reac-tive phosphates in all extracts were determined usingcolorimetric technique (Strikland & Parson 1972,Griffiths 1973). The organic P content was calculatedas the difference between total and inorganic P.

4. RESULTS

The November 1994 Po River flood was caused byintense rainfall in a zone to the southwest of the PoRiver’s drainage basin, in Piedmont (northwest Italy).In this region, precipitation reached values of between50 and 175 mm in 24 h (Autorità di Bacino del Fiume Po1994) between 5 and 6 November 1994. Such intenserainfall caused a huge flood in the upper reaches of thePo River that was slightly reduced when it arrived atthe sea mouths for normal discharges of tributaries intothe lower part of the Po River. The flood reached thePo mouths 1 d after the rainfall began. The runoff intothe sea began around 6 November (2290 m3 s–1) and

finished about 24 November (2270 m3 s–1) (Fig. 2); themaximum flood discharges, reached on 10 November,was 8630 m3 s–1; and average flood flow was 5479 m3

s–1, which was considerably above the average of thePo River’s discharge between 1917 and 1994 (1500 m3

s–1, Ufficio Idrologico e Mareografico pers. comm.) andthat of the average discharge for 1994 (1870 m3 s–1).The flood brought into the sea 5.1 × 106 t of suspendedsolids (equal to 48% of the annual amount), 3297 t oftotal P (26%) and 27600 t of N (25%) (Marchetti etal. 1996).

In the survey period (8 to 21 November 1994), whichcoincided with the arrival at the sea of the flood waters,the weather-sea conditions of the northern AdriaticSea were characterised by generally low hydrody-namic with regard to both geostrophic current inten-sity and wave motion, as observed by other field inves-tigations in the same period in the northwesternAdriatic Sea (ELNA Project cruise; Hopkins et al. 1999,Hopkins 2001).

The salinity, temperature and oxygen profiles mea-sured in the water column allowed us to reconstructthe state of the basin at the moment of the flood dis-charge. Salinity, temperature and oxygen data wereelaborated to obtain longitudinal transects located per-pendicularly to the coastline, immediately to the southof the Po River delta (Transect A, Fig. 3), in front of thedelta (Transect B, Fig. 4) and to the north (Transect C,Fig. 5) and also surface and bottom distributionsof these parameters in front of the Po River delta(Figs. 6 & 7).

The salinity and temperature transect just south ofthe Po River delta (Transect A, Figs. 1 & 3a,b) highlightsa strong stratification of the water column, with a sur-face layer (thickness ~10 m) of colder and less salinewater, relative to underlying water. The temperatureand salinity values inside the upper layer increasegradually both from the surface down towards thelower layer and in a land-to-sea direction (Figs. 3a,b &6a,b). This layer extends for over 40 km offshore in thezone to the southeast of the Po River delta (Fig. 6a,b)

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Frascari et al.: Po River anomalous flood event

while it is confined to the coast, witha less accentuated stratification inzones in front of and immediatelynorth of the delta (Figs. 4a,b, 5a,b &6a,b). This situation is similar to thatshown by Artegiani et al. (1997) forthe autumn season, and by Franco(1983) with regard to the buoyancy ofriver waters over basin water, butwith a major offshore extension of theless salty water front owing to the ex-ceptional character of the flood rela-tive to the averaged data of Artegianiet al. (1997). Moreover, the tem-perature transects show that in thedeeper and eastern zones, an addi-tional layer near the bottom ischaracterised by lower temperatures(Figs. 3a, 4a & 5a). The deep colderwater layer is also visible in thecentral part of the basin in the bot-tom temperature areal distribution(Fig. 7a). Over the whole area, thesalinity between the intermediateand deeper waters increases with amore gradual and regular trendthan does variation in temperature(Figs. 3a, 4a, 5a & 7a), reaching amaximum in the eastern part of thestudied area and forming a deeplayer with both high salinity and lowtemperature. The dissolved oxygendistributions along the transects(Figs. 3c, 4c & 5c) allow us to note anoxygen-rich surface layer that ex-tends as far as the external limits ofthe flood plumes (Fig. 6c) and a bot-tom hypoxic layer that correspondsto a north-south elongated areaextending to the external limits ofthe flood plume (Fig. 7c). Both thedeep hypoxic layer and the well-oxygenated surface layer tend to be-come thicker and more evident in thefrontal zones where the Po Riverfresh waters mix with the saltierwaters of the basin (Figs. 3c, 4c & 5c).

The distribution of water content insurficial sediments (Fig. 8a) shows 2areas characterised by higher values.The 2 areas correspond to the 2mouths of Po di Goro and Po dellaPila. The first of these areas extendssouth-southeast, whilst the second—after initially moving towards the

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Frascari et al.: Po River anomalous flood event

open sea—heads southeast. They mark the main dis-persion path into the sea of the Po River fine sedimentinputs distributed from these 2 mouths; in fact, highlyhydrated sediments indicate fine particles that hadsettled shortly before and were thus not greatly com-pacted. The thickness of this layer ranged from about20 cm in the area near the Po River mouths to 1 or 2 cmnear the farther border of the study area. In the sub-surficial sediments (Fig. 8b), the water content waslower because of their older deposition and hencehigher compactness.

The grain-size fine-fraction distribution in the surfi-cial sediments (Fig. 9) shows a prevalence of clay frac-tions (<8 φ in the same areas in which highly hydratedsediments deriving from the Po River were deposited.

The mineralogical analyses performed on surficialand sub-surficial sediment samples confirm the pres-ence of (1) a silico-clastic facies (Dinelli et al. 1997)deriving from the Po River, in front of and south of thePo River delta, and (2) a carbonate facies, derivingfrom rivers further north. Serpentine and dolomite areamong the minerals that can be used as tracers of these

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2 facies. The former prevails in the fine Po River inputsand the latter in the particulate of rivers north of thePo River (Nelson 1971, Dinelli et al. 1997, Tomadin &Varani 1998). The serpentine distribution, in both sur-ficial (Fig. 10a) and sub-surficial (Fig. 10b) sediments,has higher concentrations in an area in front of andsouth of the Po delta and in another area further off-shore, to the southeast; in a northeasterly direction,concentrations lower than those in the first 2 areas arefound in an intermediate north-south belt. In this beltthe dolomite concentration increases, marking thesouthward transition zone of northern river sediments.In the sub-surficial sediments, the serpentine distribu-tion pattern shows that the belt with greater concentra-tions becomes more limited towards the coast, in frontof the delta mouths, whilst to the southeast it is almostabsent (Fig. 10b).

The distribution of organic C (which includes bothterrigenous and autochthonous organic matter) in sur-ficial sediments (Fig. 11a) shows a high concentrationzone in the area in front of the delta, which extendsboth northwards and southwards in the central areas

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Frascari et al.: Po River anomalous flood event

of the northern Adriatic basin. The highest concentra-tions are found in the most easterly and southerly zoneof the study area. In sub-surficial sediments, organic Chas the same distribution pattern but with less pro-nounced southward and northward lobes (Fig. 11b).

Furthermore, to distinguish the nature and origin ofthe organic matter, the organic C:total N ratio wasconsidered: it indicates, for high values (>9 to 10), acontinental origin or an advanced degradation oforganic matter, and, for low values (<7 to 8), anautochthonous marine organic matter with limiteddegradation (Müller 1977, Müller & Mathesius 1999).The C:N ratio presents a zone in surficial sediments(Fig. 12a) with high values—representative of preva-lently terrigenous organic matter—that is located infront of the river mouths and is extended furtheroffshore than in sub-surficial sediments (Fig. 12b).Low surficial sediment C:N ratio values are present inareas far from the Po River mouths.

Organic P shows a different distribution betweensurficial and sub-surficial sediment. In surficial sedi-ment (Fig. 13a), the organic P presents 2 enrichmentareas: one in front of the Po River delta with a south-east protuberance, and another offshore, characterisedby a south-southwest extension of the limits of themuddy deposits distributed from the Po River. The sub-surficial sediments have their maxima in the areas to

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Clim Res 31: 151–165, 2006160

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12°00’E 13°00’E12°30’E 12°00’E 13°00’E12°30’E

Fig. 11. Organic C content distribution in (a) surficial and (b) sub-surficial sediments around the Po River delta

Longitude Longitude

Latit

ude

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Frascari et al.: Po River anomalous flood event 161

ISMAR ñ MARINE GEOLOGY ISMAR ñ MARINE GEOLOGY

0 10 20 30 40 km 0 10 20 30 40 km

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Fig. 12. C:N molar ratio distribution in (a) surficial and (b) sub-surficial sediments around the Po River delta

495

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0 10 20 30 40 km 0 10 20 30 40 km

a bISMAR ñ MARINE GEOLOGYISMAR ñ MARINE GEOLOGY

45°30’N

44°30’

45°00’

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Fig. 13. Organic P content distribution in (a) surficial and (b) sub-surficial sediments around the Po River delta

Longitude Longitude

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Clim Res 31: 151–165, 2006

the south and offshore of the Po River delta (Fig. 13b).These areas are located in the same areas whereorganic C is also high. The maximum values of organicP in surficial and sub-surficial sediment are mainly dis-tributed along the edges of the various plumes visiblefrom the areal distribution of surface salinity and tem-perature (Fig. 6a,b).

Table 1 shows mean values and ranges, in thesurficial and sub-surficial sediments, of parametersdiscussed in the text.

5. DISCUSSION

The surface water salinity is minimal near the PoRiver mouths and becomes gradually higher offshorefor the continuous mixing of river and basin waters(Fig. 6a).

The temperature transects (Figs. 3b, 4b & 5b) andbottom temperature areal distribution (Fig. 7b) high-light a vertical stratification in the water column. Thestratification shows a division into 3 layers that arevery clear in Transect A (Fig. 3b): (1) the surface layerpresents cold waters coming from the Po River; (2) theintermediate layer is characterized by the warmestwaters, which are still affected by the marine summerwarming; and (3) the deepest and eastern layer againrepresents cold waters, corresponding to the densewater of the central northern Adriatic Sea charac-terised by low temperature and high salinity (Franco etal. 1982, Malanotte Rizzoli & Bergamasco 1983, Orel etal. 1993), which had remained confined to the bottomfrom the previous winter. The salinity and temperaturesurface stratifications are consequences of the freshwater from the Po River that floats offshore andfans out over the basin waters, mainly eastwards andsouthwards (Figs. 3, 4 & 6). The strong floating and thespreading eastward and southward of the fresher andcolder river waters are consequences of the high speedand quantity of the Po River runoff in a basin charac-terised by low hydrodynamics, with no geostrophic

currents and wave motion. In fact, the latter cause adisruption of the water column stratification when theybecome established in fall (Hendershott & Rizzoli 1976,Franco 1983, Franco et al. 1983, Fonda Umani et al.1990, Artegiani et al. 1997).

The dissolved oxygen concentration pattern could beused as an indicator of depositional processes ofdecomposing organic particulate, mainly from primaryproduction. In fact, the low oxygen concentrations inthe bottom waters are ascribed to oxygen consumptionby the microbic degradation of reactive organic matter.These hypoxic conditions are more intense and extendupwards in the zones where the Po River and basinwaters mix together (Figs. 3c & 4c). This is an indica-tion that oxygen consumption is due mainly to thedegradation of newly formed organic material settlingon the bottom, the latter being generated by the sur-face algal blooms in the mixing zones of the Po Rivernutrient-rich waters with the warmer basin waters.This increase in surface primary production in thefrontal zones of the river plume is confirmed by anincrease in dissolved oxygen in the surface layer nearto the front zones (Figs. 3c & 4c).

The parameters measured in surficial sedimentsallow us to obtain direct information on depositionalprocesses, as well as indirect observation and confir-mation of the processes that took place in the watercolumn when the survey cruise was carried out and theflood was underway. In contrast, the same parametersmeasured in the sub-surficial sediments allow us todeduce the processes that occurred in the study area,averaged over time, before the flood occurred.

The muddy material of early deposition from the riverplume is very rich in water, because it is the result offlocculation that occurs when fresh water mixes withsalt water (Olsen 1982). Generally, the flocculated ma-terial is deposited in the proximal part of the prodeltaarea (Barbanti 1989). In our case, owing to the fact thatfresh river water was floating on salty basin water, thefine and flocculated material contained in the floatingwater was spread out to sea in directions of river waterdistribution currents, and deposited all over the areacovered by the flood plume as more hydrated material.Indeed, the water content distribution map of the surfi-cial sediments (Fig. 8a) can be seen as traces of theflood plumes of the Po della Pila and Po di Goro mouths,which develop southward and south-southeastward.The origin of these zones with high water content isconfirmed by the sub-surficial sediments, which areless hydrated over the whole area, having not been in-fluenced by recent floods. Furthermore, the elevatedthickness of the flood layer (between 20 cm near the PoRiver mouths and 1 to 2 cm near the farther borders ofthe study area) indicates a very high particulate loadcoming out from the Po River during the flood event.

162

Table 1. Mean values (range) of parameters analysed in surfi-cial and sub-surficial sediments collected during the PR94

cruise

Parameter Surficial Sub-surficialsediments sediments

H2O (%) 47.2 (23.3–64.7) 42.8 (25.9–60.3)Silt (%) 36.5 (5.9–87.1) 37.6 (4.6–80.9)0Clay (%) 32.4 (0–71.5) 0 32.3 (0–69)0000Serpentine (%) 1.02 (0–4)0000 1.07 (0–3)00000Organic C (%) 0.85 (0.2–1.5)0 0.86 (0.2–1.7)00C:N 8.02 (4.5–12.2) 8.34 (5.1–12.9)0Organic P (µg g–1) 245.3 (67.5–757.0) 239.2 (83.3–666.4)

Frascari et al.: Po River anomalous flood event

These observations are also confirmed by the clayand serpentine surficial distributions (Figs. 9 & 10a),which show—while deposition is underway—a higheroffshore extension, which is unlike that under normalconditions when higher geostrophic currents occurand the fine Po River load is deposited near the coast.The spatial irregularities of the serpentine surficial dis-tribution and, in particular, the lobed northward exten-sion, in coincidence with the northward protuberanceof clay and organic C (as traces of plume extension),indicate the low strength of general circulation cur-rents during the flood that allowed the formation of ananti-cyclonic secondary gyre.

In contrast, the organic C distribution in the surficialsediments (Fig. 11a) and its relationship with surficialN concentrations (Fig. 12a) highlights algal bloom pro-cess formation that is not related to the flood plumedistribution pattern. The maximum organic C values,concomitant with low C:N ratios encountered in thezones most easterly and southerly of the Po Riverprodelta, may be explained by the deposition oforganic matter of primary production. The productionof organic matter is fostered by the mixing of nutrient-rich river waters with the warmer basin waters alongthe plume front, as can be seen by the high oxygenvalue distribution pattern; this mixing also favours thedepositional processes in the front zones due to the dis-appearance of the pycnocline, which maintains theparticulate in suspension (Franco et al. 1989). Deposi-tional processes are also favoured in the frontal areaowing to the retarded intruding river currents, whichreach minimum velocities in this region.

The distribution of organic P in surficial sediments(Fig. 13a) confirms the presence of newly formedorganic matter at the plume front. Furthermore, thehigh organic P concentrations, coinciding with highorganic C contents and concomitant with low C:Nratios in surficial sediments, are found in areas of lowbottom oxygen concentrations—namely where newlyproduced degrading organic matter is settling. OrganicP and organic C are thus excellent tracers, not only foridentifying zones affected by the flood plume but alsofor defining the areal limits of the plume itself and ofthe main processes of primary production in fall. Theseprocesses produce, in the presence of a stratified watercolumn, a general hypoxia or anoxia in the densebottom waters.

The distribution of the C:N ratio in surficial sedi-ments (Fig. 12a) shows high values in a zone near theriver mouths, which extend southwards; these highvalues indicate the zones subject to the deposition oforganic matter of continental origin, as a result of theearly deposition of coarser material.

In light of our findings deduced from the surfacesediment parameter distributions and the temperature

and salinity distributions in the water column, it is pos-sible to reconstruct the depositional processes of the PoRiver suspended particulate during the 1994 flood. Thedifference in salinity between the surface layer and thedeeper basin waters, the high velocity of the riverwater inputs (rich in suspended solids) and the lowhydrodynamics of the basin have favoured the buoy-ancy of the fresh river waters over the salty basinwaters that are spread out towards the central basinareas. This has increased the surface transport of par-ticulate coming from the river, which has depositedalong the plume dispersion axes and particularly at thefronts where fresh water and salt water mix and wherethe reducing current speed, the disappearance of thepycnocline and the higher weight of the flocculatedmaterial favour the settling of the particulate. In thismanner, only a small portion of the finer suspendedparticulate of river origin has undergone a process ofearly deposition that usually occurs just outside thePo River mouths as a result of flocculation processesinitiated by the mixing of fresh and salt water.

6. CONCLUSION

It can be seen from the PR94 cruise data and alsofrom the literature (Hopkins et al. 1999, Hopkins 2001)that the geostrophic north-south current, generallypresent in this area, was very slow during the 1994Po River flood and, therefore, that the typical cyclonicautumn-winter Adriatic circulation had not yet set in.This is deduced in particular from the strong tem-perature and salinity stratification shown in the land-offshore transects—in fact, the north Adriatic Seawater column stratification is disrupted when the N-Adcurrent is present (Hendershott & Rizzoli 1976, Franco1983, Franco et al. 1983, Fonda Umani et al. 1990,Artegiani et al. 1997). The fresh and cold waters com-ing from the Po River spread out offshore, floatingabove the basin waters and undergoing subjacent andfrontal mixing processes at the pycnocline limit. Themain fresh water branch, coming out from the Po dellaPila (as deduced from sediment and water columndata), tends first to be distributed directly offshore, asfar as ~40 km from the coast, and then to split into 2branches: the main branch proceeds southwards whilethe less pronounced branch heads northwards. Waterscoming from mouths south of the Po della Pila are dis-tributed southeastwards and south-southeastwards.The low hydrodynamic of the basin together with thehigh speed and high amount of Po River runoff favourthe large extension of the Po River plume into thebasin,

The Po River water distribution pattern in the basinis confirmed by a comparison of the parameters mea-

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Clim Res 31: 151–165, 2006

sured in the surficial and sub-surficial sediments. Thedistribution of high values of water content, serpentine,organic C and organic P in the surficial sediments(compared to sub-surficial sediments) in areas extendedsoutheastwards and eastwards mark the fresh deposi-tion of terrigenous particulate. In particular, the areaoccupied by the main plumes and the frontal masses offlood waters can be superimposed on the fine highlyhydrated surficial sediments.

The elevated thickness and extension of the freshlydeposited hydrated sediment shows that water comingfrom the Po River during the flood must have been par-ticularly rich in suspended particulate. Furthermore,because of the considerable concentration of organicmatter (high organic C) with a low C:N ratio and highorganic P content, whose maxima were found in thebottom sediments in areas at the frontal zones, it ishypothesised that water coming from the Po Riverflood was rich in nutrients. In these areas, the fresh andcold nutrient-rich river waters mix on the surface withthe saltier and warmer basin waters, and this favoursthe growth of algal blooms.

The formation of algal blooms in the surface watersalong the plume fronts, and their subsequent reminer-alisation within the water column through the pycno-cline down to the bottom waters and the bottom sedi-ments, is confirmed by the (1) gradually decreasingdissolved oxygen concentration from bottom waterto the surface plume fronts, and (2) accumulation oforganic matter with low C:N ratio and high organic Pcontents in the surface sediment of the area located atthe plume fronts.

The reduced oxygenation of the confined bottomwaters is increased by the fresh organic mater mineral-ization processes. The persistence of the stratificationof the water column after this event (Regione EmiliaRomagna 1994) has led to a generalised anoxia of thebottom waters of the central Adriatic basin. The anoxiaof the bottom water was resolved only with the earlyfall storms and with the onset of the general winterAdriatic Sea circulation, which disrupted the watercolumn stratification.

In contrast, the lack of zones containing high concen-trations of organic C, a low C:N ratio and high concentra-tions of organic P in the sub-surficial sediments indicatethat, in the medium term, the particulate that accumu-lates in front of the Po River delta is reworked and redis-tributed by the currents and wave motion in more exten-sive areas, especially to the south. In this way, the PoRiver prodelta bottom sediments act as temporary reser-voirs of autochthonous organic matter and inorganicterrigenous particulate. These solid materials are thenredistributed to other parts of the basin located furthersouth, after degradation processes of the more reactiveorganic matter had already partially taken place.

Acknowledgements. This work was conducted under thePRISMA I Project program. We thank Dr. Cristina Bergaminiand Dr. Barbara Garrone for their field and laboratory sup-port and for advice on interpretation, Vladimiro Landuzzi,Gabriella Rovatti, Angelo Magagnoli and Maurizio Mengoli,for their analyses, and Dr. Fabio Zaffagnini for help inconstructing figures. This paper is Contribution No. 1380 ofISMAR, Sezione di Geologia Marina, CNR, Bologna, Italy.

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Submitted: November 21, 2004; Accepted: March 20, 2006 Proofs received from author(s): July 11, 2006