distributions of dissolved organic carbon (doc) and chromophoric dissolved organic matter (cdom) in...

Estuarine, Coastal and Shelf Science 60 (2004) 133e149

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Distributions of dissolved organic carbon (DOC) andchromophoric dissolved organic matter (CDOM) in coastal

waters of the northern Tyrrhenian Sea (Italy)

S. Vignudellia, C. Santinellia, E. Murrub, L. Nannicinia, A. Serittia,)

aIstituto di Biofisica, Consiglio Nazionale delle Ricerche, Area della Ricerca di Pisa, Via Moruzzi 1, 56124 Pisa, ItalybInternational Marine Center, Localita Sa Mardini, 09072 Torregrande (Oristano), Italy

Received 23 June 2003; accepted 13 November 2003

Abstract

An investigation on the distribution of dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) incoastal waters of the northern Tyrrhenian Sea, affected by the Arno River discharge, is reported and discussed. Data refer toa survey carried out aboard of the R/V Urania in January 2000. The study period was characterized by a rather low river discharge

due to the particularly calm meteorological conditions. The river plume region extended to north of the Arno delta and it wasconfined to the inner shelf with depths less than 40 m and salinities !38.1. Surface DOC concentrations ranged in a narrow intervalof values (56e76 mM). Plume waters exhibited the highest levels of DOC (O70 mM). These values were slightly higher than those

found in the outer shelf (56e66 mM) which in turn, were generally comparable to those reported in the literature for open sea watersof different regions of the Mediterranean Sea. The distribution of CDOM was described by the two components of fluorescentmaterial, i.e., the ‘‘protein-like’’ (Fn(280)) and the ‘‘humic-like’’ (Fn(355)), respectively, characterized by different spectral regions for

both excitation and emission. The Fn(355) signature decreased towards the north and west generally mirroring the spatial pattern ofsalinity, temperature and DOC. On the contrary, the distribution of Fn(280) differed from that of Fn(355) without any apparentrelationship with the parameters above cited. This behavior supports the hypothesis that the ‘‘protein-like’’ fluorescent material

could be released to the coastal waters by the local activity of planktonic organisms. Conversely, DOC was conservatively exportedthrough the plume to the coastal waters and, similarly, Fn(355) appeared largely controlled by a conservative mixing. The correlationbetween DOC and Fn(355) (r

2 ¼ 0:78) represents a further confirmation of the terrestrial origin of DOC in the plume region. Verticalprofiles of DOC, Fn(280) and Fn(355) and those of the specific fluorescence, Fs(280), and Fs(355), in stations located off the shelf, showed

differences between northern and southern stations, suggesting a different composition of DOC in the two regions. Finally, althoughthese data refer to a single survey, they represent the first simultaneous DOC and CDOM view on the influence of the Arno River tothe carbon cycle in an estuarine Mediterranean region.

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Keywords: DOC; CDOM; estuarine and coastal waters; Arno River; Tyrrhenian Sea

1. Introduction

The coastal zone, especially in areas adjacent tomajor rivers, is a marine environment where keybiogeochemical processes take place. An ‘‘estuarineplume’’ is defined as the coastal region influenced byland-derived discharge from a river. An essentialcharacteristic of estuarine plumes is that they are highly

) Corresponding author.

E-mail address: [email protected] (A. Seritti).

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doi:10.1016/j.ecss.2003.11.023

dynamic regions, with significant salinity gradients(Morris et al., 1995). Moreover, mixing betweenterrestrial and coastal waters represents a key processfor biological productivity and carbon cycling, withstrong implications for the whole coastal systemfunctioning and ultimately on the fishery.

Dissolved organic carbon (DOC) represents themajor fraction (60%) of riverine organic matter (Spitzyand Ittekkot, 1991); in addition, in the region influencedby fresh waters, a local source of autochthonous DOCfrom phytoplankton and bacterial activity may be

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134 S. Vignudelli et al. / Estuarine, Coastal and Shelf Science 60 (2004) 133e149

observed (Sempere et al., 2000; Raymond and Bauer,2001). The high mixing rate, characterizing the estuariesand the plume zones, makes it difficult to discriminatebetween different DOC sources. Until now, the fate ofestuarine DOC has been unclear (Peterson et al., 1994;Cauwet, 2002). A possible hypothesis is that it behavesconservatively, so its decrease may be due only toa dilution of riverine waters, as suggested by the inverserelationship observed between DOC and salinity insome systems (Mantoura and Woodward, 1983; Moranet al., 1991; Cauwet, 2002). This implies an absence ofreactivity and a biological unavailability of DOC duringthe transit through the estuary. Another hypothesis,reported in recent years (Peterson et al., 1994; Raymondand Bauer, 2000, 2001), is that DOC in estuarine regionsis both removed and added. It may be removed byphysicochemical transformations such as flocculation,and precipitation of humic substances (Fox, 1993;Hedges and Keil, 1999). In addition, the photolysis ofcomplex organic compounds may produce labile DOCfrom biologically less reactive riverine DOC (Kieber andMopper, 1990; Amon and Benner, 1996). Furthermore,important DOC production processes may occur inestuaries. Both phytoplankton (Peterson et al., 1994;Raymond and Bauer, 2001) and benthic algae activities,as well as the plasmolysis of freshwater algae (Petersonet al., 1994), can surely cause DOC production. A largefraction of the DOC, locally produced or modified, maybe then rapidly assimilated by bacteria (Turner, 1978;Moran et al., 1999). The importance of these processes issupported by Peterson et al. (1994) who report that evenwhen DOC concentrations were conservatively distrib-uted in an estuary, the isotopic composition of DOCrevealed the existence of a dynamic cycle of DOC inputand removal. Raymond and Bauer (2001) report theoccurrence of non-conservative behavior of DOC in theYork River Estuary. Their model predicted that only20e38% of the DOC at the mouth was of estuary origin,while 38e56% was added internally.

Another important approach proposed for the studyof estuarine processes is the assessment of the chromo-phoric dissolved organic matter (CDOM) (Blough et al.,1993; De Souza Sierra et al., 1997; Vodacek et al., 1997).CDOM is the component of dissolved organic matter(DOM) absorbing light; part of this light can be then

List of special characters

Greek ‘‘Mu’’m is used in mM which means 10�6 Molar

Greek ‘‘Lambda’’l is used as symbol for wavelength units

Greek ‘‘Theta’’is used as symbol for potential temperature

released as fluorescence. Two distinct groups of fluo-rophores, emitting in different spectral regions, havebeen recognized (Traganza, 1969; Laane, 1981; Mopperand Schultz, 1993; De Souza Sierra et al., 1997; Serittiet al., 1998; Baker et al., 2003). The first is excited ata wavelength of 280 nm (lex) and exhibits an emissionmaximum (lem (max)) at wavelengths ranging from 340 to360 nm; this group of fluorophores was associated withthe ‘‘protein-like’’ material, because its spectral charac-teristics are similar to those of the two fluorescingamino-acids: tryptophan and tyrosine (Mopper andSchultz, 1993; Mayer et al., 1999). The second group offluorophores, excited at a lex of 355 nm, exhibits alem (max) ranging from 440 to 470 nm; it was named‘‘humic-like’’ material, as its spectral characteristics aresimilar to those of humic material of different origins(Coble et al., 1990; Mopper and Schultz, 1993). On thisbasis, the fluorescence properties of CDOM have beenused as a tool to infer information on the processes affect-ing the fate of estuarine waters entering the sea (Bloughet al., 1993; Green and Blough, 1994; Blough and DelVecchio, 2002). Some authors (Laane and Koole, 1982;De Souza Sierra et al., 1994; Mayer et al., 1999) reportthat the CDOM fluorescence may be used as a tracer forinvestigating the mixing between riverine and sea waters.

Although the freshwater inputs significantly contrib-ute to the enhancement of the primary productivity incoastal waters, only few data are available on DOCconcentrations in estuarine environments of theMediter-ranean Sea (Cauwet et al., 1997; Seritti et al., 1998;Ferrari, 2000; Sempere et al., 2000; Pettine et al., 2001). Inparticular, only one paper (Seritti et al., 1998) refers to theTyrrhenian Sea. This basin represents not only a keypointfor the circulation of the northwesternMediterranean Sea(Astraldi andGasparini, 1992; Vignudelli et al., 2000) butit also receives the contribution of the Arno River in itsnorthern part. So it may be considered a region where thestudy of the origin and fate of DOC and CDOM ofterrestrial origin can substantially contribute to theknowledge of the role of estuaries in the biogeochemicalprocesses affecting the carbon cycle in coastal regions.

The region investigated (Fig. 1) is an almostrectangular-shaped area approximately 50 km long and25 km wide, located off the northern coast of Tuscany.The nearshore part over the shelf receives a significantterrestrial input through the discharge of the ArnoRiver, which forms a cuspate delta to the west of Pisa,with a reduced inner estuarine area and low salinities atthe mouth. The catchment covers a large drainage areaof about 8228 km2 (ARPAT, 1999), where anthropo-genic inputs are largely focused on the centers ofpopulation concentrated for most of the river length.The flow regime of the Arno River is quite irregular,varying from less than 4 m3 s�1 in summer (as occurredin 1931) to about 2300 m3 s�1 in the presence of a floodevent (as occurred on 4 November 1966).

135S. Vignudelli et al. / Estuarine, Coastal and Shelf Science 60 (2004) 133e149

Fig. 1. Study area and grid of sampling stations (filled circles). C, D, E (empty circles) refer to the sampling stations located inside the Arno River.

W (filled star) indicates the position of the wave buoy. Bathymetry contours (meters) are also shown.

Currents, waves, winds and volume of discharge arethe principal factors regulating the mixing processes inthe plume region. Over the shelf, currents essentiallyflow parallel to the shoreline from south to north, withaverage surface velocities of the order of 30 cm s�1

(Astraldi and Manzella, 1983). Tidal forces are weak(Tsimplis et al., 1995) and consequently the associatedcurrents exhibit minimal influence on the local physicalprocesses. Wave action is strong during autumn throughspring and it is related to occasional storms in summer.Most incident waves arrive from the southwest sector,around 240(, and exhibit wave heights lower than 3.5 m98% of the time (Inghilesi et al., 2000). The prevailingwinds are easterly from November through February,while in the remaining months, the dominant winddirection is from the western quadrant.

Based on monthly field campaigns carried out in 1989and 1990, Del Mancino et al. (1993) reported a firstspatial and temporal description of the hydrology of theArno estuarine area. In proximity of the mouth,a complete mixing of estuarine and ambient shelf watershas never been observed. The water column structureroughly reflects a stratified situation with a tendency tobecome partially mixed seaward. The Arno plume

spreads predominantly northward and it shows a narrowand quite long shape, as also confirmed by satelliteimagery (Maselli et al., 1992). Only during the warmseason does the plume move cross-shelf in proximity ofthe mouth or alternatively reverses southward, asreported by Gasparini et al. (1986). Seaward intrusionsof branches of low salinity waters were observed duringJanuary 1990 by Del Mancino et al. (1993).

This paper reports DOC and CDOM distributions inthe northern coastal region of the Tyrrhenian Sea both inthe plume region and off the slope, in order to investigatethe influence of the Arno River on the coastal system andto study the off slope behavior of these parameters.Correlations among DOC, CDOM and physical param-eters, temperature and salinity, were also examined anddiscussed in terms of possible processes.

2. Materials and methods

2.1. Metocean observations

Measurements of the Arno River discharge were pro-vided by the local office of the National Mareographic


and Hydrographic Service. Wave data were taken ata buoy located at station W (filled star in Fig. 1)operated by the National Mareographic and Hydro-graphic Service in the framework of the Italian WaveNetwork. The same institution provided sea level datafrom a gauge located in the Livorno harbor in theframework of the Italian Mareographic Network.Meteorological data (including wind speed and di-rection) were available from a coastal weather stationlocated in proximity of Livorno and managed byoperators of the chemical plant SOLVAY S.p.A.

2.2. Study area, sampling stations and fieldmeasurements

The field study was carried out on the R/V Uraniafrom 19 to 29 January 2000 in the framework of a moregeologically oriented mapping program. Study area andsampling stations are shown in Fig. 1. The samplingstrategy was generally restricted to the Arno delta regionand encompassed the northern outer shelf. The south-ernmost transect (B-5) was positioned in front of theArno River mouth. Water depths ranged from 20 m, inthe stations closest to the coast, to about 370 m, in thoselocated offshore. At each station continuous profiles ofsalinity and potential temperature were recorded bymeans of a ConductivityeTemperatureeDepth (CTD)SBE911 plus Sea-Bird probe, calibrated before and afterthe cruise. Water samples for DOC and CDOMdeterminations were collected with a General Oceanicrosette sampler mounted on the CTD system, at thesurface in all the stations, and at five depths at theoutermost stations (5e55).

Seawater samples were filtered onboard through0.2 mM membrane filters (Sartorius Minisart,SM16534K) and stored in amber glass bottles at 4 (Cin the dark until analysis. Additional surface sampleswere collected at the Arno mouth (station C) and justupstream (stations D, E) using a small rubber boat.Salinity and temperature inside the estuary weremeasured with a portable Hanna 9033 conductivityinstrument.

2.3. DOC measurements

DOC measurements were carried out by means ofa Shimadzu 5000 TOC analyzer according to Sharpet al. (1993). Fifty microliters of 50% H3PO4 was addedto 10 ml of the sample. It was then purged with anoxygen stream for 10 min to remove inorganic carbon,before high temperature catalytic oxidation. The mea-surements were performed in triplicate with a fixedinstrumental variance !2%. DOC concentrations werecalculated according to Thomas et al. (1995) usingMilli-Q water as blank and potassium hydrogenphthalate as standard for the calibration curve. The

accuracy of the measurements was daily verified witha DOC certified seawater sample (nominal value:44e45 mM), kindly provided by the Bermuda BiologicalStation (BBS, USA).

2.4. Fluorescence measurements

A Jasco Spectrofluorometer (model FP770) was usedfor fluorescence measurements; the slit-widths were setat 5 nm and 10 nm for excitation and emission wave-lengths, respectively. Two spectral regions were in-vestigated according to Traganza (1969) and Seritti et al.(1998). The first, defined as ‘‘protein-like’’ fluorescence(hereinafter Fn(280)), was excited at a wavelength of280 nm (lex) with emission recorded in the range of350G 5 nm (lem); the second, named ‘‘humic-like’’fluorescence (hereinafter Fn(355)), was excited atlex ¼ 355 nm reading the emission in the range of450G 5 nm (lem) (Vodacek et al., 1997). Milli-Q waterwas used as blank and its spectrum was subtractedthroughout. The fluorescence data were expressed asnormalized fluorescence units according to Hoge et al.(1993):

FnðlÞðN:Fl:U:Þ ¼ ðFðsÞ=RðsÞÞ=ðFðrÞ=RðrÞÞ;

where F and R are the fluorescence and the Raman bandof the sample (s) and the reference compound (r),respectively. A solution of 0.01 mg l�1 of 2-aminopyridine in 1 N H2SO4 was chosen as standard forFn(280) instead of tryptophan, which has highly temper-ature-dependent fluorescence. A solution of 0.01 mg l�1

of quinine sulfate in 1 N H2SO4 was used as standard forFn(355). Hence, the values of the fluorescence componentexcited at 355 nm were multiplied by a factor of 10 inorder to compare them with those of the literature,where a solution of 0.1 mg l�1 of quinine sulfate wasdefined as 100 normalized fluorescence units (Donardet al., 1989). Absorption measurements were also tenta-tively carried out by means of a Jasco UV/VIS spectro-photometer (mod. 7850) with a 10 cm quartz cell in therange 220e550 nm. However, as the signals were parti-cularly low and comparable to the instrumental detec-tion limit (0.0005 absorbance units), the results wereconsidered unreliable.

3. Results and discussion

3.1. Environmental conditions

The environmental conditions, before and during thefield study (19e29 January 2000), were substantiallycalm and stable. Sea level data confirmed the limitedtidal range and the absence of abnormal surge events.The study area (Fig. 1) was characterized by a weaksurface wave height regime (0.2e1.9 m). Meteorological


data showed that close to 60% of the wind originatedfrom the northern sector (315e345(), whereas, south-westerly to westerly winds were observed only onJanuary 21 and 29. Moreover, 57% of the wind speedwas substantially below 3 m s�1, 39% ranged between 2and 6 m s�1 and 4% was higher. Therefore, a relativelycalm regime characterized the wind conditions duringthe survey.

The daily mean discharge of the river during thesurvey was quite irregular with a dominance of flowrates in the range 44e100 m3 s�1, with an averagearound 63 m3 s�1. For this period of the year, thesevalues were only about half of the long-term average(Fig. 2). A weak-flow regime characterized the wholeyear 2000. In such low discharge conditions and underunfavorable wind directions, the plume was expected tohave little seaward extension.

3.2. Satellite observations

Satellite imagery (Fig. 3) yielded a synoptic coverageof the area during the period of the field study andprovided valuable information on the distribution oftemperature and color of surface waters occurring onthe continental shelf. Fig. 3 reports a Sea SurfaceTemperature map (Fig. 3a) (spatial resolution of 1.1 km,from the AVHRR instrument, aboard the NOAA seriesof polar-orbiting environmental satellites) and an oceancolor map (Fig. 3b) (spatial resolution of 1.1 km, fromthe SeaWiFS mission). The images used here originatefrom the MCHSST AVHRR archive at German remotesensing center (Dech, 1995) and from the SeaWiFSarchive at NASA’s Goddard (Fu et al., 1998). The bestcoverage, in terms of clear sky, occurred on January 19.Both sensors provided a clear visualization of a cold andsediment-laden tongue, which extends northward fromthe Arno mouth, and it was restricted near shore

exhibiting an elongated shape. The most distinct featurewas the presence of a sharp boundary separating coastalwaters from those offshore. The same stable pattern, inthe form of a coastal attached envelope, was alsodetected in the other available cloud-free imagescollected during the observational period. Despite thelow resolution, it was reasonable to consider them asrepresentative of the fluvial discharge influence, en-hanced by the very shallow coastal region (less than10 m depth). The direction of the plume waters wasconsistent with the long-shore current regime encoun-tered during the winter season (Astraldi and Manzella,1983). Because of low winds, the plume was spatiallyquite stable.

3.3. General hydrological features

As shown by satellite imagery, the grid of stationswas not optimal for a complete monitoring of the plumeregion. However, the draft of the vessel, compared tothe low depth of the stations closer to the coast,hindered a more accurate choice of the stations.Nevertheless, this investigation represented an opportu-nity to get the first DOC data on the coastal waters ofthe northern Tyrrhenian Sea affected by the Arno River.

Fig. 4a reports salinity distribution and Fig. 4breports potential temperature distribution in the surfacelayer. A frontal boundary clearly separated the waters ofestuarine origin from those of the shelf ambient. Salinityshowed values lower than 37.0 extended north of theArno delta along the coast, while values higher than 38.0were present at offshore stations, where the influence ofthe riverine system decreased. The salinity gradientswere relatively weaker than those of other river plumeregions in the Mediterranean Sea (Broche et al., 1998).Moreover, the low-flow river regime was unfavorable toincrease these gradients. The surface water temperature

Fig. 2. Mean daily discharge of the Arno River for the year 2000 (black line) compared with the average calculated for a long-term period (grey line).

The black box indicates the period of the year when the survey was carried out.


Fig. 3. Snapshots of the river-influenced region of the northern Tyrrhenian Sea on 19 January 2000: NOAA AVHRR image of sea surface

temperature (a) and SeaWiFS color image (b). The white box indicates the study area.

(Fig. 4b) was around 13.7 (C in the open sea waters andgradually decreased to reach values of about 9.8 (C inthe plume region and about 6 (C in the river mouthwaters. The lowest values of salinities (S! 37:0) andtemperatures (w! 10:4 (C) were not found immediately

at the Arno River mouth, but in a narrow regionbetween the coast and 10 km offshore. In addition, twooffshore intrusions of cold and low salinity waters,centered around stations 9e12 and 29, were detected(Fig. 4). This behavior on the inner shelf may be related


Fig. 4. Surface distribution of salinity (a) and potential temperature (b) in the study area. Black squares indicate the locations where CTD data were

collected. The value at the river mouth refers to the station E in Fig. 1.


to the fluctuating regime of the Arno River outflow. Thealongshore displacement of both isotherms and isoha-lines suggested a well organized coastal current, flowingparallel to the coast, in the northwest direction.

3.4. DOC surface distribution

Fig. 5 reports DOC distribution in the surface waters.The pattern closely resembles those found for salinityand temperature. In fact, the highest DOC values(O70 mM) were detected in correspondence to thewaters of estuarine origin, as evidenced by the lowvalues of salinity and temperature reported above. Incontrast to the other parameters, the southwesterndistribution of DOC showed a more pronouncedintrusion towards the open sea. DOC concentrationsincreased toward the shore below 43.90(N. At higherlatitudes, this influence became negligible and the DOCsignature gradually decreased to reach values of about64 mM at a distance of about 15 km from the coast. Atthe offshore stations, DOC exhibited a rather homoge-neous structure, with values ranging from 56 to 66 mM.In the plume region, DOC ranged from 70 to 76 mMwhich are values slightly higher than those found on theouter shelf, but quite lower than those reported in theliterature for other Mediterranean estuarine and coastalwaters. Cauwet et al. (1997) found DOC concentrations

in the range 70e100 mM in a section going from themouth of the Rhone River to 250 km offshore. HigherDOC values (92e180 mM) were also reported by Pettineet al. (2001) for a study on the seasonal influence of PoRiver to the coastal waters of the Adriatic Sea.However, it must be pointed out that both riversexhibited annual average discharges more than oneorder of magnitude higher (1374e1965 m3 s�1) than thatof the Arno River (w75 m3 s�1). This particular lowvalue of the discharge of Arno River can also explain thesmall differences observed between the DOC concen-trations in the plume region and those of the offshorestations which in turn are very similar to those detectedin open sea waters of both eastern (Sempere et al., 2002;Seritti et al., 2003) and western (Doval et al., 1999;Dafner et al., 2001; Santinelli et al., 2002) Mediterra-nean Sea. Moreover, it must be taken into account thatthe biological activity during winter is generally at thelowest annual levels and consequently also the localproduction of DOC is quite low.

3.5. CDOM surface distribution

The surface distribution of the ‘‘humic-like’’ fractionof CDOM, as determined by fluorescence excited at355 nm, is given in Fig. 6a. Its signature decreased grad-ually towards the north and west generally mirroring

Fig. 5. Surface distribution of DOC in the study area. The values measured at each station are also reported above the symbols of the stations. The

value at the river mouth refers to the station E in Fig. 1.


Fig. 6. Surface distribution of CDOM expressed as fluorescence excited at 355 nm (Fn(355)) (a) and fluorescence excited at 280 nm (Fn(280)) (b). Data

are expressed in normalized fluorescence units (N.Fl.U.). The value at the river mouth refers to the station E in Fig. 1.


the spatial pattern of salinity and temperature. Thehighest values of Fn(355) were found in the plume region,in correspondence with the maximum of DOC. Thisindicates a clear terrestrial origin of this type offluorescent material which can be attributed to theriverine load of organic compounds such as lignine andlong chain lipids, which are derived from vascular planttissues (Meyers-Schulte and Hedges, 1986; Opshal andBenner, 1997). The Fn(355) data reported here arecomparable to those of Seritti et al. (1998) for coastalstations located on the south of the Arno River mouthwhere average values of 0.77 N.Fl.U. were found. Incontrast, higher values (mean value 24.98 N.Fl.U.) thanthose of this study were found inside the Arno River inthe survey carried out in the summer 1997 (Seritti et al.,1998). This must be attributed to the particularly lowdischarge of the Arno River in that period as can also beinferred by the plot of long-term average river dischargereported in Fig. 2. A comparison between the levels ofthe ‘‘humic-like’’ fluorescence found in this study andthose of the literature, referred to other world areas,showed similarities and differences. Vodacek et al.(1997), in an investigation on the seasonal variation ofDOC and CDOM in the Middle Atlantic Bight, reportFn(355) values ranging from 0.85 to 1.02 N.Fl.U., andDOC from 70 to 140 mM, in surface waters of stationslocated on the shelf and slope along a section extendingfrom the mouth of the Delaware Bay to the SargassoSea; on the basis of these results they conclude thatphoto-oxidation processes play a relevant role, especial-ly in the summer period, on the conservative or non-conservative behavior of the system. Del Castillo et al.(2000) in a study, carried out by using the three-dimensional excitationeemission matrix technique(EEM), on CDOM distribution in different stationslocated in the West Florida Shelf, report DOC valuesranging from 90 to 305 mM and emission maximumclose to 0.9 ppb of quinine sulfate for the fluorescencepeak at Ex/Emmax of 305/394 nm, which are thecharacteristic wavelengths of humic material in marinesurface waters (Coble, 1996). Rochelle-Newall andFisher (2002) report Fn(355) and DOC average valuesof 4.2 N.Fl.U. and 138 mM for coastal waters ofChesapeake Bay, monitored in a three year period.Except the last reference, where both values are higherthan those presented here, the apparent disagreementbetween the similarity of fluorescence values and thesubstantially higher DOC concentrations, found in thestudies above reported, suggests the possible occurrenceof two different mechanisms: a noticeable incidence ofa photo-bleaching of the ‘‘humic-like’’ fluorescent bandand the presence of a considerable extent of non-fluorescent organic material, which instead substantiallycontributes to DOC; both processes could take placeat the same time especially in the summer months.In Section 3.8 the evidence of a certain extent of

non-fluorescent organic material was confirmed on thebasis of the correlation between DOC and Fn(355).

The spatial surface pattern of the ‘‘protein-like’’fluorescence is reported in Fig. 6b. In contrast with theFn(355) trend, Fn(280) data differed from those of salinity,temperature and DOC. In particular, it exhibiteda maximum in the region centered around the station40, which was positioned northward with respect to thearea where the other parameters peaked. This behaviorsuggested that the ‘‘protein-like’’ fluorescence could berelated to a marine source, indirectly linked to the ArnoRiver discharge, such as a nutrient input to the coastalwaters, transferred northward by currents; nutrientscould induce a production of local planktonic organismswhich represent a source of organic material of recentorigin. No Fn(280) data of estuarine and coastal waters ofdifferent world regions were found in the literatureexcept those of Seritti et al. (1998) referring to a tractof the Tyrrhenian Sea coast, on the south of theArno River mouth, where higher average values(0.07 N.Fl.U.) were found. According to the previoushypothesis that mostly a marine source could beresponsible for this type of fluorescence, the higherFn(280) values observed in the summer 1997 than those ofthe winter 2000 can be connected to the higherbiological activity which was surely higher in Septemberthan in January.

3.6. Influence of the Arno River

Surface data related to the stations located in thelower part of the Arno River (close to the mouth)showed very different characteristics with respect tothose of the plume region. The relatively low dischargeconditions allowed the landward intrusion of salinewaters. The salinity dropped from 37.3 at station B to15.6 at station C, just inside the river, and it decreased to10.5 and 8.9 in stations D and E, respectively. A similartrend was observed for temperature, which was 11.7 (Cat station B with a decrease to values of about 6.0 (C atstations C, D and E. This behavior could be related toa quite high extent of stratification in the water column,with the thickness of the saline interface decreasingupstream along the river.

Concerning DOC, the Arno River representeda carbon source for the contiguous coastal area withconcentrations nearly 290 mM at station C and close to320 mM in stations D and E. These concentrations wereslightly lower than those (280e402 mM) reported bySeritti et al. (1998) for stations located in similarpositions in the Arno River mouth. These relativelylow differences can surely be attributed to the differentperiod of surveys (September instead of January), withconsequent different river discharges (15 instead of63 m3 s�1), as well as different salinity gradients in thelast part of the river mouth. A comparison with other


rivers entering the Mediterranean Sea is difficult to dobecause no DOC data were found inside the rivers.Nevertheless, the DOC concentrations, detected in thecoastal waters affected by the plumes of Po (Pettineet al., 2001) and Rhone (Cauwet et al., 1997) rivers, werein the range 92e180 mM and 70e250 mM, respectively.In the survey presented here, DOC dropped from about300 mM to 65 mM, just a few kilometers outside theArno River mouth (station B), in correspondence toa strong increase of salinity. This DOC value is com-parable to those of the stations not subjected to theplume influence.

The Arno mouth waters also showed very high valuesof CDOM (Fig. 6). Fn(355) exhibited values of 1.4 N.Fl.U.at station B and an increase of 9.8e9.9 N.Fl.U. atstations C, D and E. However, this component ofCDOM showed very similar values (9.8e9.9 N.Fl.U.) atall the river mouth stations (C, D and E), while, in thecoastal station B facing the Arno mouth and in the plumeregion, values were about 1.2e1.4 N.Fl.U. A similarbehavior was also observed for Fn(280) which exhibitedan increase from 0.015 N.Fl.U., measured at station Bjust outside the Arno mouth, to 0.67 N.Fl.U. observedat station C. An additional increase, to values of 0.77 and0.93 N.Fl.U., was detected going into the river at stationsD and E, respectively. These results demonstrate thatthe Arno River surely represented a source of fluores-cent material, both ‘‘humic-like’’ and ‘‘protein-like’’,but while the former, going towards the sea, showeda trend similar to that of salinity, the latter decreasedmuch more rapidly suggesting that the ‘‘protein-like’’material of terrestrial origin was rapidly degraded in theestuary area.

3.7. Vertical distributions of salinity,DOC and CDOM

As reported above (Figs. 4e6), the influence of ArnoRiver on physical and chemical parameters of surfacewaters, was largely constrained to the stations located atlongitude higher than 10.0(E and latitude lower than43.90(N. In order to investigate whether and how theriver waters affected intermediate and deep waters, thevertical distribution of salinity was studied along allthe eastewest transects.

Fig. 7 depicts three longitudinal sections of salinityfor the transects: B-5 located in front of the Arno mouth(Fig. 7a), 20-16 (Fig. 7b) in the middle part of the studyarea and 40-36 (Fig. 7c) located in the northern part ofthe station grid reported in Fig. 1. All the transects fromB-5 (Fig. 7a) to 40-36 (Fig. 7c) were characterized by thepresence of low salinity waters (S! 38:0) in a narrowregion on the coastal side (left part of the plots). Thelayer of fresher waters, due to the influence of the river,decreased with distance from the coast going from south(Fig. 7a) to north (Fig. 7c). In particular, in the transect

B-5 (Fig. 7a), it was restricted within 12 km of theshoreline; in the transect 20-16 (Fig. 7b) it decreased to5 km, while in the transect 40-36 (Fig. 7c) it was limitedwithin 2 km. The average thickness of the layer wasrestricted to the upper 40 m in all the transects wherethis low salinity layer was present (Fig. 7aec). Incontrast, no low salinity cores were observed in the morenorthern transects (not reported). This behavior repre-sents a confirmation that a large part of the estuarinewaters were mostly deflected to the north rather than tothe west and they were confined near shore.

The salinity distribution in the surface layer(0e100 m) of the outer stations (55-5; Fig. 7d) exhibiteda behavior representative of a typical winter condition,with a relatively large degree of horizontal and verticalhomogeneity. By contrast, a progressive increase ofsalinity with depth, to reach values O38.50 below300 m, was observed in the southern stations (5e25;Fig. 7d), while values close to 38.3 were detected in thedeep waters of the northern stations (26e46; Fig. 7d).

Fig. 8 reports the vertical distribution of DOC(Fig. 8a), Fn(280) (Fig. 8b) and Fn(355) (Fig. 8c) in theoutermost stations. Different symbols were used todistinguish southern (empty triangles), northern (filledsquares) stations, as well as station 55 (empty squares). Insurface waters, DOC exhibited concentrations rangingfrom 56 to 66 mM at all the stations. These values were inagreement with those reported in the literature for watersof other areas of the Mediterranean Sea, where DOCsurface concentrations commonly ranged from 56 to70 mM (Copin-Montegut and Avril, 1993; Doval et al.,1999; Santinelli et al., 2002). Below the surface layer, atdepths between 200 and 300 m, DOC decreased to valuesof 44e48 mM in the southern stations 5e25 and 55, whileit showed concentrations of 60e65 mM in the northernstations 26e46. The relatively high DOC values(O65 mM) found at a depth of 200e300 m in thenorthern stations are difficult to explain, however theycannot be attributed to the occurrence of a different watermass because of the low depths of these stations(!350 m). Below 300 m to the bottom, DOC exhibitedvalues of about 45e48 mM in the stations 5e25 and 55and of about 52e62 mM in the northern ones (26e46). Itmust be pointed out that below 150 m at each depth, thesouthern stations showed systematically lower valuesthan the northern ones, with the exception of the station55 which exhibited DOC concentrations much moresimilar to the southern than the northern stations(Fig. 8a). Salinity data (Fig. 7d) showed a higher extentof stratification, coupled to higher values of S (O38.5) inthe bottom waters of the southern stations 5e25 and 55,compared to those of the northern part (26e46), whichexhibited lower bottom values of S (w38.3) and weakerstratification.

Fig. 8b reports the vertical profiles of the ‘‘protein-like’’ fluorescence (Fn(280)) for the same stations. The


Fig. 7. Vertical distribution of salinity in three eastewest transects: B-5 (a), 20-16 (b), 40-36 (c) and in the outer northesouth transect 55-5 (d) (see

inset map). The heavy contour lines refer to the values of 38.0 (salinity limit of the river influence) and 38.5 (maximum of salinity).

values ranged from 0.010 to 0.017 N.Fl.U. in mostsurface samples. In the stations 5e25 and 55 a maxi-mum of about 0.020 N.Fl.U. was found at a depthof 150e200 m, then a decrease to reach values close to0.002 N.Fl.U. near the bottom, was detected. In thenorthern stations (26e46), below 100 m, the valuesranged from 0.010 to 0.015 N.Fl.U. However, fromFig. 8b, it is possible to observe that Fn(280) showedmuch less marked differences between the two groups ofstations than DOC.

The vertical distribution of the ‘‘humic-like’’ fluores-cence (Fn(355)) is reported in Fig. 8c. Most of surfacesamples showed values ranging from 0.45 to0.55 N.Fl.U., then, Fn(355) increased at about 150 m tovalues close to 0.6 and 0.7 N.Fl.U. in the northern andsouthern stations, respectively. Below 150 m, the north-ern stations Fn(355) showed more constant values(ranging from 0.55 to 0.62 N.Fl.U.) than the otherstations (5e25) in which a decrease until a minimum of

about 0.50 N.Fl.U. was observed at 250 m, followed byan increase to reach values of 0.65e0.70 N.Fl.U., at thebottom.

In order to investigate the differences observedbetween northern and southern stations, the specificfluorescence, defined, similarly to the specific absorption(Blough et al., 1993), as:

FsðlÞ ¼ FnðlÞ=DOC

was calculated for both groups of fluorescent material.Fig. 9 reports the vertical profiles of Fs(280) (Fig. 9a) andFs(355) (Fig. 9b), related to the southern (emptytriangles), northern (filled squares) and 55 (emptysquares) stations. A different behavior was observed inboth cases. In particular, the northern stations showeda quite constant vertical profile for both Fs(280) andFs(355). In contrast, the southern stations showed a widelyscattered vertical distribution of both parameters. Now,assuming a different contribution of various fluorescent


Fig. 8. Vertical profiles of DOC (a), Fn(280) (b) and Fn(355) (c) in the outermost stations. Errors in DOC are !2%; estimated errors for Fn(355) and

Fn(280) are !4% and 5%, respectively.

material to DOC, the trends reported in Figs. 8 and 9suggested that, in the northern stations, DOC couldprobably contain a higher proportion of non-fluorescentor weakly fluorescent (low Fs(l)) material, than in thesouthern stations. These observations suggested theoccurrence of DOC with different chemical character-istics, and different biological reactivity, in the tworegions. In particular, in the northern stations, DOCmight be characterized by a more complex composition,in which different molecular branches of non-fluorescentmaterial, previously bonded to aromatic rings, might beremoved from the original fluorescent compound to givealiphatic structures, such as polysaccharides and/orlipids, which probably represent the semi-labile fractionof DOC (Carlson, 2002). This could explain the higherDOC values, together with the quite constant values ofFs(l), found in the northern than in the southernstations. The scatter of the Fs(280) and Fs(355) values,along the water column of the southern stations, as wellas the differences of DOC profiles between the twogroups of stations, seem to confirm the moleculardifferences above hypothesized. The atypical behaviorof station 55, positioned in the most northern part ofsection 5e55, which exhibits DOC and fluorescence datamore similar to the southern than northern stations,

could be connected to the salinity profile of station 55,which is more similar to that of southern stations(bottom waters with salinity O38.5) than that of thenorthern ones (Fig. 7d).

On the basis of all these statements, a direct influenceof the Arno River on the depth profiles of DOC andfluorescence in the outer stations, should be excluded.

3.8. Relationships in the estuarine coastal region

One of the most debated questions in the inves-tigations of estuarine environments was their classifica-tion in terms of conservative or non-conservativebehavior, i.e., the assessment of the fate of thebiogeochemical parameters during their transfer fromthe riverine system to the coastal zone (Peterson et al.,1994).

The relationships between DOC and salinity(Fig. 10a), Fn(355) and salinity (Fig. 10b), Fn(280) andsalinity (Fig. 10c) and DOC and Fn(355) (Fig. 10d) wereinvestigated only for the stations exhibiting a riverineinfluence (S! 38:1). Almost stable conditions were as-sumed over the observational period.

DOC and salinity (Fig. 10a) exhibited a significantinverse correlation (r2 ¼ 0:88, N ¼ 20, p! 0:00001).


Fig. 9. Vertical profiles of specific fluorescence (Fs(280) (a) and Fs(355) (b)) in the outermost stations.

This finding suggested a conservative behavior of DOCin the estuarine Arno environment as a consequence ofthe mixing of the plume with the surrounding waters.Generally, DOC in the estuaries behaves conservatively(Mantoura and Woodward, 1983; Moran et al., 1991)although recent studies report a completely differentfunctioning in the outer regions of Danube (Cauwetet al., 2002) and Columbia rivers (Hill and Wheeler,2002). A trend similar to that of DOC was observed forthe ‘‘humic-like’’ fluorescence Fn(355) (Fig. 10b). Itsintensity decreases in a rather conservative mode(r2 ¼ 0:79, N ¼ 20, p! 0:00001) as a result of thesimple dilution.

In contrast to the behaviors of DOC and Fn(355), the‘‘protein-like’’ fluorescence, Fn(280) (Fig. 10c), showed norelationship to salinity. As previously hypothesized, theabsence of conservative behavior of Fn(280) in the riverplume could be attributed to a specific marine source ofdissolved proteins which might be only indirectly linkedto the Arno River discharge.

DOC also directly correlated with Fn(355) (r2 ¼ 0:78,

N ¼ 20, p! 0:00001) (Fig. 10d). Because DOC repre-sents the sum of fluorescent DOC plus non-fluorescent

DOC and the latter corresponds to the x-intercept of theline of best fit, it resulted in a non-fluorescent DOCconcentration of 47 mM. This value agreed with thatreported by Ferrari (2000) for a sea area close to theRhone Estuary. Now, if this value is subtracted fromthat of total DOC, a relationship between fluorescentDOC and Fn(355) which passes through the origin, witha slope close to 0.07, was obtained. This suggested thatthe specific ‘‘humic-like’’ fluorescence, Fs(355), in theplume region was constant and derived from terrestrialinputs.

4. Concluding remarks

The results of this field study showed that the Arnoriverine system, in January 2000, represented a source ofDOC in a restricted area of the coastal zone of thenorthern Tyrrhenian Sea. The limited distribution of theplume was probably connected to the survey periodwhich was characterized by relatively low river dischargeand calm meteorological conditions. Once the riverwaters reached the coast, they undergo rapid dilution


Fn(355) vs. Salinity

y = -0.5313x + 20.849 r 2= 0.7937p < 0.00001

0.0

0.4

0.8

1.2

1.6

36.5 37.0 37.5 38.0 38.5Salinity

F n(3

55) (

N.F

l.U.)

Fn(280) vs. Salinity

0.00

0.02

0.04

0.06

0.08

36.5 37.0 37.5 38.0 38.5

Salinity

F n(2

80) (

N.F

l.U.)

DOC vs. Salinity

y = -7.3645x + 343.38r 2 = 0.8842p < 0.00001

60

64

68

72

76

80

36.5 37.0 37.5 38.0 38.5Salinity

DO

C (µ

M)

DOC vs. Fn(355)

y = 0.0674x - 3.603r 2 = 0.7827p < 0.00001

0.0

0.4

0.8

1.2

1.6

60 65 70 75 80DOC (µM)

F n(3

55)

(N.F

l.U.)

(a) (b)

(c) (d)

Fig. 10. Relationships between DOC and salinity (a), Fn(355) and salinity (b), Fn(280) and salinity (c) and DOC and Fn(355) (d). Only samples with

S! 38:1 were used for the plots (N ¼ 20).

with seawater and dispersed to the north. Satelliteimagery confirmed the location of the estuarine region,which extended north of the Arno confluence and it wasconfined near shore.

Meanders of low salinity and temperature werepresent well out of the Arno River delta together withrelatively high values of DOC. The increase of ‘‘humic-like’’ fluorescence in the plume region supported theterrestrial source of DOC in the estuarine area. TheArno estuarine system seemed to behave conservativelyfor DOC and ‘‘humic-like’’ fluorescence. In contrast, the‘‘protein-like’’ fluorescence seemed to be connected toa marine source, indirectly linked to a freshwater nu-trient input.

DOC and CDOM vertical profiles, in the stationslocated on the outer slope, showed different character-istics when compared with those recorded in the plumeregion.

Finally, although further efforts are essential to definethe influence of Arno input on the coastal biogeochem-ical processes, the data set obtained from this surveyrepresented the first study on the role of the Arno Riveron DOC and CDOM distributions in the coastal watersof the northern Tyrrhenian Sea.

Acknowledgements

The Captain and the Crew of the R/V Urania aregratefully acknowledged. The authors wish to thank

Ing. Bernardo Mazzanti (National Mareographic andHydrographic Service) for supplying the Arno Riverflow data and Luciano Mattera (SOLVAY S.p.A.) forproviding wind data. The authors are indebted to Prof.Sarah Green (University of Michigan) and Dr. GianPietro Gasparini (CNR-ISMAR) for their commentsand suggestions which greatly improved the manuscript.

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