Nutrient, particulate matter and phytoplankton variability in the photic layer of the Otranto strait

Ž .Journal of Marine Systems 20 1999 381–398

Nutrient, particulate matter and phytoplankton variability in thephotic layer of the Otranto strait

Giorgio Socal a,), Alfredo Boldrin a, Franco Bianchi a, Giuseppe Civitarese b,Amelia De Lazzari a, Sandro Rabitti a, Cecilia Totti c, Margherita M. Turchetto a

a Istituto di Biologia del Mare C.N.R., Castello 1364ra, I-30122 Venice, Italyb Istituto Sperimentale Talassografico C.N.R., I-34100 Trieste, Italyc UniÕersity of Ancona, Faculty of Sciences, I-60131 Ancona, Italy

Received 10 September 1996; accepted 1 March 1997

Abstract

The distribution of nutrients, suspended matter and phytoplankton in the photic layer of the Otranto Strait, as observed inŽ .four seasonal situations February, May, August and November 1994 , is analysed in relation to hydrography. In winter, two

Ž . Ž .water masses were found: the Adriatic Surface Water ASW and the Ionian Surface Water ISW , located at the western andeastern side of the Strait, respectively. In the fresher and cooler ASW, nutrients, suspended matter and phytoplankton

Žcontents were higher than in the warmer and saltier ISW on average: N–NO s2.3 mM, POCs3.6 mM, chl.as0.4 mg3y3 y3 .dm in the ASW, against N–NO s1.3 mM, POCs2.5 mM, chl.as0.3 mg dm in the ISW . In the ASW, the mean3

Ž .N:P ratio 50 revealed an excess of nitrogen with respect to phosphorus; the nitrogen supply, as well as the significantpresence of diatoms, made us suppose that new production processes were occurring here. In summer, the main features

Ž . Ž . Ž .were: i high water column stability, ii small horizontal differences in hydrological and biological features, iii extremelyŽ . Ž .low concentration of nutrients at the surface and iv a deep chlorophyll maximum DCM at the nutricline level.

Phytoplankton summer communities, mainly consisting of nanoflagellates, coccolithophorids and small naked dinoflagel-Ž .lates, were present in low quantities. The low phytoplankton carbon:chlorophyll a ratio -20 , observed at DCM, indicated

a high chlorophyll content per single cell, probably as a physiological response to low irradiance. As to the phytoplanktonannual cycle, sediment traps and water column observations were highly correlated, evidencing two abundance peaks inspring and autumn, and a summer minimum. Hydro-chemical and biological data suggest that the winter spread of the ASWwas the main factor favouring the enhancement of phytoplankton growth and controlling the species composition in thestrait, while the DCM formation represents a more typical characteristic of ISW, with an oligotrophic feature similar to thatof other Eastern Mediterranean water masses. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Eastern Mediterranean; Otranto strait; nutrients; suspended matter; phytoplankton

) Corresponding author: Fax: q39-041-5204126; E-mail: [email protected]

0924-7963r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0924-7963 98 00075-X

( )G. Socal et al.rJournal of Marine Systems 20 1999 381–398382

1. Introduction

The Otranto Strait is the exchange area betweenthe Ionian and the Adriatic Seas. Here, the Adriatic

Ž .Surface Water ASW flowing southward along theItalian shelf encounters the Ionian Surface WaterŽ .ISW coming from the Ionian Sea along the Greek

Žand Albanian coasts Zore-Armanda, 1969; Ferenti-.nos and Kastanos, 1988 . The occasional presence of

Ž .the Modified Atlantic Water MAW , protruding intothe Adriatic from its meandering into the Ionian, has

Ž .also been described Robinson et al., 1992 .The Adriatic Surface Water and the Ionian Sur-

face Water are characterised by different basic fea-tures. The first, coming from a basin heavily sup-plied by rivers, features lower salinity and higherlevels of dissolved nutrients, as well as higher sus-

Žpended matter concentration Vilicic et al., 1989;Bregant et al., 1992; Franco and Michelato, 1992;

.Fonda Umani et al., 1992 , while the second exhibitshigher temperature and salinity and lower sestoncontent, these thermohaline features being similar to

Žthat of the Eastern Mediterranean basin Zore-Armanda, 1969; Michelato and Kovacevic, 1991;

.Rabitti et al., 1994 .In addition, as confirmed by satellite and direct

Žcurrent observations Michelato and Kovacevic,.1991; Artegiani et al., 1993; Poulain et al., 1996 ,

the spreading magnitude and spatial extension of thetwo water masses are subject to great seasonal vari-ability.

Other water masses present in this area are theŽ .Levantine Intermediate Water LIW , defined as wa-

ter more saline than 38.75 PSU, extending in theintermediate layer with the core from 200 to 400 m

Ž .in depth, and the Adriatic Deep Water ADW ,Žoccupying the bottom layer Buljan and Zore Ar-

.manda, 1976; Orlic et al., 1992 . The intermediateŽwater, which is relatively rich in nutrients Civitarese

.et al., 1994 , may assume great relevance for pri-mary productivity when vertical mixing occurs.

Autotrophic communities and their seasonal cycleŽhave been scarcely investigated in this area Bianchi

.et al., 1992; Vilicic et al., 1995 , since biologicalŽsamplings have not been performed regularly Vil-

.icic et al., 1989 . Current information about spatialand temporal dynamics of phytoplankton can beimproved with the employment of sediment traps

Ž .Wassmann and Floderus, 1994 ; a link betweensuspended particulate matter and particulate mattercollected in sediment traps has been observed by

Žseveral authors Wassmann et al., 1990; Riebesell et.al., 1995; Andreassen et al., 1996 .

The MAST-MTP ‘Hydrodynamics and Geochem-ical fluxes in the Strait of Otranto’ project providedthe opportunity to gain a better knowledge abouthydrodynamics, nutrients, particle fluxes and biolog-ical characteristics across the strait of Otranto. Themain objectives of the present paper are:

-to describe the vertical and horizontal pattern ofnutrient, suspended matter and phytoplankton inrelation to the hydrographic properties of the wa-ter column;-to characterise the main environmental factorswhich control the distribution of phytoplanktonbiomass and species composition;-to merge trap and water column information, inorder to evaluate the phytoplankton cycle through-out the year.

2. Materials and methods

Four seasonal surveys relevant to this paper wereŽcarried out in the Otranto Strait on February OTR1

. Ž . Ž .cruise , May OTR2 , August OTR3 and Novem-Ž .ber 1994 OTR4 . The cruises were performed on

board the Italian RrV ‘Urania’ and the Greek RrV‘Aegaio’. The locations of the stations is illustratedin Fig. 1.

Continuous vertical profiles of temperature, salin-ity, light transmittance and in-situ fluorescence wereperformed with a SeaBird SBE 16 probe, coupledwith a Sea-Tech transmissometer and fluorometer.

Discrete samples down to 200 m depth werecollected by means of a rosette sampler with Niskinbottles, at standard or selected depths, to measure

Ždissolved oxygen, nutrients N–NO , N–NO , P–2 3. Ž .PO , Si–SiO , total suspended matter TSM , parti-4 4

cle concentration and size spectra, particulate organicŽ . Ž .carbon POC , total particulate nitrogen PN ,

Ž .chlorophyll a and b chl.a, chl.b , phytoplanktonabundance and species composition. For POCand PN, the contribution of different size fractionsŽ .-2 mm, 2–10 mm,)10 mm were determined inselected samples.

( )G. Socal et al.rJournal of Marine Systems 20 1999 381–398 383

Fig. 1. Sampling stations along five sections in the Otranto Strait; squares evidence sediment trap locations.

Ž .Fig. 2. Horizontal distribution of surface temperature in winter and summer 1994 from Gacic et al., 1996 .


Dissolved oxygen was determined in accordancewith the Winkler method; dissolved nutrients wereanalysed on board by means of a Chemlab Analyser,

Ž .following Grasshoff et al. 1983 appropriatelyadapted for automatic analyses.

TSM, POC, PN and chlorophyll samples werefiltered through Whatman GFrF glass fibre filters

and immediately stored at y208C until the analyseswere to be carried out. For POC and PN size frac-tions, samples were pre-filtered with 2 and 10 mmpolycarbonate Nuclepore filters and analysed in thesame way as the total. All filters were previously

Ž .pre-washed and pre-combusted 4808C . Dry weightdeterminations were performed with the gravimetric

Ž .Fig. 3. Vertical distribution of temperature, phosphate, nitrate and chlorophyll a as in-situ fluorescence at section 300 in February 1994.

https://www.researchgate.net/publication/40930912_Methods_of_Sea-Water_Analysis?el=1_x_8&enrichId=rgreq-26a2b995-511a-4a47-8759-f570f02ede6a&enrichSource=Y292ZXJQYWdlOzIyMzY4MTI5MDtBUzo5NzI1NTY5NjYzMzg2OEAxNDAwMTk4OTczNjU2


Ž .method Strickland and Parsons, 1972 . POC and PNwere analysed with a Perkin Elmer 2400 CHN ele-mental analyser; inorganic carbon was removed fromthe filters after exposure to HCl vapours for 24 h at

Ž .room temperature Hedges and Stern, 1984 . Particlenumber and size spectra were determined with a

Ž .Coulter Counter TA II Sheldon et al., 1972 imme-diately after sampling, with an orifice tube of 100mm; the examined size range was 2–40 mm.

Chlorophyll filters were homogenised, extractedŽin a 90% acetone solution Strickland and Parsons,

. Ž1972 , then injected into a HPLC system Beckman.System Gold , equipped with a C18 Ultrasphere

ODS reverse-phase column. The analytical condi-Ž .tions provided a methanol–water 80–20% gradient;

only chlorophyll a and b concentrations were calcu-lated, after standardisation against pure standardsŽ .Sigma .

Phytoplankton samples, fixed in hexamethylene–tetramine buffered formalin, were counted using aZeiss Axiovert 35 inverted microscope, after settling

Ž .in 100 ml chambers for 70 h Utermohl, 1958 .¨About 200 cells were counted for each sample. Cell

Ž .size and volume were determined Edler, 1979 andŽ .phytoplankton biomass phytoplankton carbon, PPC

was obtained by multiplying cell or plasma volumeby factors of 0.11 for diatoms, coccolithophorids andflagellates and by 0.13 for thecate dinoflagellatesŽ .Smetacek, 1975 . To evidence a phytoplankton sea-

Ž .sonal cycle, a Multi-Dimensional Scaling MDSordination method was used on the species-samplematrix, after a double-square transformation of abun-

Ž .dance data Clarke and Warwick, 1994; Carr, 1996 .Undetermined nanoflagellates were excluded, owingto the fact that they were ubiquitous in our study.

Sediment traps coupled with current meters wereŽ .moored on the Italian shelf station 301 and slope

Ž .station 304 . An automated time-series TechnicapPPS 4r3 trap with a collecting area of 0.05 m2 wasmoored at station 301 at 118 m depth from February1994 to May 1995. The initial sampling period wasset to 7 days, then to 20 days long starting fromAugust 1994. At station 304, a Technicap PPS 5r2

Ž 2 .trap collecting areas1 m was moored at 603 mdepth from May 1994 to May 1995, with a samplingperiod of 7 days. The sampling bottles were filledwith a preservative solution consisting of a high

Ž y3density mixture 50 g NaCl dm in pre-filtered.seawater with the addition of buffered formalin.

After recovery, all samples were stored at q48C

Table 1Ž . Ž .Simple statistics for each parameter in the sampled stations during winter February 1994 , divided into Adriatic Surface Water ASW and

Ž .Ionian Surface Water ISW

Parameters Units ASW ISW

n min max avg std n min max avg std

Temperature 8C 102 11.2 13.0 12.4 0.4 273 12.5 15.2 14.0 0.4Salinity PSU 102 36.9 38.4 37.9 0.3 273 38.0 38.8 38.3 0.2

y3Density kg m 98 28.2 29.0 28.8 0.2 253 28.5 29.1 28.8 0.2Dissolved Oxygen mM 102 208.0 283.4 258.5 10.3 270 196.2 267.9 244.5 19.9Relative Oxygen % 98 80 105 98 3 246 77 106 96 8Nitrite mM 101 0.02 0.59 0.23 0.12 268 0.01 0.24 0.08 0.06Nitrate mM 101 1.2 4.4 2.3 0.7 269 0.1 5.2 1.3 1.5Orthophosphate mM 101 0.02 0.15 0.05 0.02 272 0.01 0.20 0.05 0.05Orthosilicate mM 101 1.4 5.1 2.4 0.5 272 1.1 6.2 2.4 1.2

y3Total particles n cm 34 4668 21 742 15 587 4585 85 1146 12 555 5022 2614y3TSM mg dm 16 0.19 0.84 0.56 0.22 23 0.10 0.53 0.12 0.13

Organic Matter % 16 9 78 33 23 14 18 96 58 23POC mM 19 1.4 9.2 3.6 2.1 52 0.5 8.6 2.5 1.5PN mM 19 0.2 1.4 0.5 0.3 52 0.1 1.0 0.3 0.2CrN mol 19 5.1 9.5 6.8 1.1 52 5.0 13.5 7.6 1.7

y3Chlorophyll a mg dm 14 0.1 0.9 0.4 0.3 39 0.1 0.4 0.3 0.1

nsnumber of observations, minsminimum, maxsmaximum, avgsaverage, stdsstandard deviation.


until laboratory analyses were performed. OpticalŽ .Utermohl, 1958 and SEM observations were car-¨ried out on subsamples of the settled material. Arough estimate of the cell flux was performed fordiatoms, dinoflagellates, coccolithophorids and sili-coflagellates.

3. Results

3.1. Hydrochemistry and suspended matter

Ž .In February 1994 OTR1 cruise , the hydrologicalsituation was characterised by the presence of two

Fig. 4. Mean vertical profiles of temperature, salinity, dissolved oxygen, nitrite, nitrate, phosphate, silicate, particle number, chlorophyll a,Ž . Ž .particulate organic carbon POC , particulate nitrogen PN and C:N in February, in ASW and ISW.


Ž .different water masses Gacic et al., 1996 . Tempera-ture and salinity values showed a west–east differen-tiation due to the water masses involved in the upper

Ž .circulation in the area Fig. 2 . The western end ofŽthe strait featured the cooler and fresher ASW Ts

.12.48C, Ss37.9 PSU , while in the eastern end theŽ .warmer and saltier ISW Ts14.08C, Ss38.3 PSU

occurred. From 150 to 200 m in depth, the thermoha-line characteristics were homogeneous in the whole

Ž .area Ts148C, Ss38.7 PSU; Fig. 3 .The ASW extension decreased further southward,

but it was still present in the central part of theŽ X .southernmost section at 39830 N, Fig. 2 . An oblique

Ž .gradient of temperature thermal front was observedŽ .Fig. 3 .

Hydrochemical and suspended matter statistics ofthe two water masses are reported in Table 1, whileFig. 4 shows the mean vertical profiles.

Concerning the winter hydrochemistry, the sur-face layer was characterised by oxygen oversatura-tion, while nitrite and nitrate distribution revealed astrong dissymmetry between the western and the

Ž .eastern ends of the section Fig. 3 , the former beingŽricher up to 0.4 mM for nitrite and 3.0 mM for. Žnitrate than the latter 0.15 mM and less than 1.0

.mM, respectively . Phosphate and silicate distribu-tion did not show a similar behaviour. Phosphatevalues varied from 0.05 mM at surface to 0.20 mMat 200 m in depth, whereas silicate values ranged

Ž .from 2 to 6 mM Fig. 4 .

Ž . Ž . Ž .Fig. 5. Particulate organic carbon POC percent fractions above and particle size distribution below at stations along the Adriatic seaŽ . Ž .water ASW, Station 201 and Ionian sea water ISW, Station 211 in February 1994.


On average, ASW exhibited higher TSM valuesŽ y3 y3.than ISW 0.6 mg dm against 0.1 mg dm , with

respectively 33% and 58% of the total belonging toŽ .the organic fraction Table 1 . For the former water

mass, mean particle concentration was about 16 000cmy3, with a mode of approximately 3–4 mm, while

Žin the ISW concentration values were lower 5000y3 .particles cm , with a prevailing fraction of around

Ž .14 mm Fig. 5 . The ASW was also richer in POCand PN than ISW. Carbon content in the various size

Ž .fractions Fig. 5 revealed that the smaller fractionŽ .-2 mm contained more or less the same amountof carbon, while the contribution to the POC wasdifferent in the other fractions: 39% was supportedby the intermediate range in ASW, while 41% con-

Ž .sisted of larger fraction )10 mm in ISW.Secchi disk readings were higher in the eastern

half of the strait, reaching a maximum value of 35 min the ISW and 10 m in the ASW.

Chlorophyll a and in-situ fluorescence measure-Ž .ments Fig. 3 also showed a significant differentia-

tion: ASW was characterised by higher chlorophyllŽ y3 y3content than ISW 0.9 mg dm and 0.3 mg dm ,

.respectively . In addition, the relative chlorophyllmaximum shifted gradually from the surface in the

Ž .ASW to 50 m depth in the ISW Fig. 4 .A cyclonic circulation was observed in the north-

Ž .ern–central part of the channel Gacic et al., 1996 ;this produced an upwelling, well evidenced by the

Ž .nitrate content 2 mM , and an increase in biologicalactivity at the surface, revealed by the fluorescenceprofile as well as by discrete chlorophyll samplingsŽ y3 .0.3 mg dm ; Fig. 6 .

Ž .In August 1994 OTR3 cruise , only minor hori-zontal differences in hydrological features were foundŽ .Fig. 2 ; surface heating was the dominant factor inthe whole basin, as this produced a seasonal thermo-cline, with temperatures )208C from surface to 30

Ž .m and decreasing down to 13.88C at 75 m Fig. 7 .Data principal statistics and mean vertical profiles

Ž .from August 1994 OTR-3 cruise are reported inTable 2 and Fig. 8.

Ž .Fig. 6. Vertical distribution of nitrate and chlorophyll a as in-situ fluorescence at section 100 in February 1994.


Ž .Fig. 7. Vertical distribution of temperature, phosphate, nitrate and chlorophyll a as in-situ fluorescence at section 300 in August 1994.

The oxygen pattern showed a subsurface maxi-Žmum at about 40 m in depth 258 mM, saturations

. Ž121% . The surface layer was nutrient depleted Fig..7 , with concentration values of total inorganic nitro-

gen -1 mM, phosphate -0.03 mM and silicate ofabout 1.3 mM. The nutricline started at 50–75 m,then concentrations increased up to 5, 0.23, and 6.6mM, respectively for nitrate, phosphate and silicate

at 200 m depth. Nitrite showed a maximum value atŽ .75 m 0.07 mM , then decreasing again to lowerŽ .levels Fig. 8 .

Mean particle concentration values revealed aŽmarked decrease when compared to winter always

y3 . Ž-5000 particles cm . POC and PN mean range:.0.9–4.8 and 0.1–0.6 mM, respectively showed a

maximum at the surface and a minimum at 200 m


Table 2Ž .Simple statistics for each parameter in the sampled stations during summer August 1994; see Table 1

Parameters Units n min max avg std

Temperature 8C 428 13.2 28.5 18.8 5.7Salinity PSU 433 37.0 38.8 38.2 0.4

y3Density kg m 397 24.4 29.1 27.7 1.6Dissolved Oxygen mM 370 182.4 295.4 225.3 27.9Relative Oxygen % 424 75 121 97 13Nitrite mM 415 0.01 0.20 0.03 0.03Nitrate mM 417 0.1 5.7 1.7 2.2Orthophosphate mM 423 0.01 0.27 0.08 0.07Orthosilicate mM 423 0.5 8.8 2.9 2.0

y3Total particles n cm 164 1229 24 823 4193 2388y3TSM mg dm 24 0.03 1.01 0.17 0.19

POC mM 86 0.5 9.0 3.5 1.6PN mM 86 0.1 1.2 0.5 0.2CrN mol 86 3.8 15.1 7.4 1.7

y3Chlorophyll a mg dm 66 0.0 1.0 0.3 0.2

Ž .Fig. 8 , the fraction -2 mm representing aboutŽ .50% of POC Fig. 9 . C:N molar ratio presented a

minimum of 6.6 at 75 m depth.ŽHigh Secchi disk values about 30 m, with a max.

.of 46 m were found. A deep chlorophyll maximumŽ .DCM was found everywhere, with chlorophyll aconcentrations ranging from 0.1 mg dmy3 at thesurface to 0.5 mg dmy3 at 75 m depth, in fairly good

Žagreement with the in-situ fluorescence profiles Fig..7 .

Appreciable chlorophyll b concentrations wereŽdetected, especially at 50–75 m depth 0.2–0.3 mg

y3 .dm .

3.2. Phytoplankton biomass and species composition

In February 1994, phytoplankton data evidenced aŽ .surface maximum in the ASW Fig. 10 , with abun-

dance )2=105 cells dmy3 and biomass )8 mg Cdmy3 ; under the surface, abundance decreased to3=104 cells dmy3 at 100 m depth, correspondingto a biomass of 1.4 mg C dmy3. In the ISW, surfacevalues were lower, with a minimum of about 2=104

cells dmy3 and 1.3 mg C dmy3. PhytoplanktonŽ 5increased at 20 m depth abundances1.3=10 cells

y3 y3.dm , biomasss2.4 mg C dm .In May, the highest values of abundance and

Ž 5biomass were found at station 306 3.3=10 cellsy3 y3 .dm , 11 mg C dm ; Fig. 10 . The vertical distri-

bution presented decreasing values downward

Ž 4 y3 y3around 5=10 cells dm and 1.9 mg C dm at.100 m depth .

The lowest values of the year were observed inŽ 4 y3August mean abundances7=10 cells dm ,

y3 .biomasss3 mg C dm . The relative maximumŽ 5 y3 y3about 1.4=10 cell dm , 5.3 mg C dm , Fig..10 was found at 50–75 m depth in the whole

section, according to the fluorescence and chloro-phyll a profiles.

In autumn, a peak was observed in the surfaceŽ .layer 0–20 m , with a maximum in the western

Ž 5 y3coastal station 2.5–2.6=10 cell dm , 8–11 mg Cy3 .dm and decreasing values up to the bottom of the

Ž 4 y3 y3photic layer 1–2=10 cells dm , 1 mg C dm ;.Fig. 10 .

Linear correlation coefficients between phyto-Ž .plankton biomass PPC and other parameters were

Ž .computed for the whole data set Table 3 . Theinverse correlation between PPC and nutrients pro-vided evidence of the nutrient depletion due to thephytoplankton uptake, while nitrite was directly cor-related to PPC. High phytoplankton biomass wasstrictly related to ASW, as demonstrated by theinverse PPCrsalinity correlation. PPC vrs PN andPPC vrs POC gave the best correlation coefficients.The POCrPPC linear regression showed that theintercept at 0 PPC was around 18 mg POC dmy3

Ž .Fig. 11 .PPC represented 12% of POC, whereas PPC:chl.a

ratio was variable, with a mean value of 23, similar


Fig. 8. Mean vertical profiles of temperature, salinity, dissolved oxygen, nitrite, nitrate, phosphate, silicate, particles, chlorophyll a,Ž . Ž .particulate organic carbon POC , particulate nitrogen PN and C:N in August 1994.

Žto values found for other oligotrophic waters Rabitti.et al., 1994 .

The phytoplankton communities consisted mainlyŽof small flagellates 2–10 mm, mainly undeter-

. Žmined , coccolithophorids mainly Emiliania hux-.leyi and small naked dinoflagellates.

Diatoms represented an important fraction of phy-Ž .toplankton only in winter 14%, Fig. 12 and in the


Ž . Ž .Fig. 9. Particulate organic carbon POC percent fractions aboveŽ .and particle size distribution below at the surface in August

1994.

western waters, where Skeletonema costatum wasthe dominant species; in summer, diatoms were par-

Ž 3 y3.ticularly scarce around 2–3=10 cells dm .Diatom assemblage appeared similar to that de-

Žscribed for the southern Adriatic Vilicic et al., 1989;.Vilicic et al., 1995 . Dinoflagellates showed an op-

Table 3Linear correlation coefficients between phytoplankton biomassŽ .PPC and the indicate parameters, after logarithmic transforma-tion of biological data

Parameters r p df

Temperature 0.13 n.s. 69Salinity y0.64 -0.01 69Density y0.25 -0.05 69Dissolved oxygen 0.42 -0.01 69Nitrite 0.27 -0.05 68Nitrate y0.36 -0.01 68Orthophosphate y0.47 -0.01 68Orthosilicate y0.45 -0.01 68Fluorescence 0.25 n.s. 38POC 0.68 -0.01 61PN 0.69 -0.01 59CrN 0.14 n.s. 59Chlorophyll a 0.54 -0.01 55Chlorophyll b 0.36 -0.01 52

r scorrelation coefficient, ps level of significance, df sdegreesof freedom.

posite trend, representing a relevant fraction of totalŽ .abundance mainly in August 18%, Fig. 12 .

Prasinophyceans presented an increase in abundanceat 50–75 m in summer, corresponding to a chloro-phyll b peak: the correlation between chlorophyll b

Žand prasinophyceans biomass was significant rs.0.816, dfs14 . Conversely, cryptophyceans were

abundant in the ASW mainly in winter.At DCM small nanoflagellates, coccolithophorids

and small naked dinoflagellates prevailed, as ob-Ž .served for the Levantine basin by Kimor 1990 .

The analysis of MDS plot, obtained from aBray–Curtis similarity matrix, showed a marked di-versification between winter and summer samples,

Ž .Fig. 10. Mean vertical profiles of phytoplankton biomass PPC during the four cruises in 1994; in February the biomass profiles areseparated for ASW and ISW.


Fig. 11. Linear regression between particulate organic carbonŽ . Ž .POC and phytoplankton biomass PPC in the Otranto strait in1994.

Fig. 12. Relative abundance of main phytoplankton groups duringŽ .the four cruises 2–200 mm cell size range .

Ž .while middle seasons partly overlapped Fig. 13 . Awinter community, composed of coccolithophorids,diatoms, cryptophyceans, and a summer one, withdinoflagellates, prasinophyceans and coccolitho-

Ž .phorids, can be outlined Table 4 .As regards the annual phytoplankton cycle, phyto-

plankton abundance in the water column and settledŽcells in sediment traps showed a strong link Fig.

.14 .During the winter, sediment traps showed that the

main portion of the total mass flux consisted ofinorganic material, whereas the organic fraction in-creased in spring–summer, concurrently with the

Ždevelopment of biological processes De Lazzari et. Ž .al., this issue . At the shelf station st. 301 , the

winter peak appeared more correlated with dynamicprocesses at the bottom, like resuspension, than tophytoplankton growth.

The phytoplankton cells settled in the sedimenttrap presented two abundance peaks in spring and

Ž .autumn with a summer minimum Fig. 14 .ŽDiatoms were the dominant group about 75% of

.the total in station 301 and 82% in station 304 .Coccolithophorids, as intact cells, were found mainly

Fig. 13. MDS of 71 phytoplankton samples collected during theŽfour cruises in 1994 FsFebruary; MsMay; A sAugust; Ns

.November .


Table 4Ž y3 .Dominant species during the four cruises: relative occurrence, average and maximum abundance cells dm . Bold values show the highest

abundances in the indicated cruises with respect to the others

Fig. 14. Vertical flux of phytoplankton cells measured by sedi-Ž . Žment traps at station 301 shelf area and at station 304 slope

. Žarea compared with phytoplankton abundance excluding.nanoflagellates found at the same stations during the seasonal

cruises.

in February and November in station 301. CoccolithsŽ .mostly of E. huxleyi were frequently found inSEM observations, also associated to faecal material.

Dinoflagellates were observed in spring and sum-mer. Silicoflagellates were present in both stations,but were relatively more frequent at station 304.

4. Discussion

Nutrient availability, suspended matter and phyto-plankton standing stocks in the Otranto area pre-sented in February a strong heterogeneity betweenthe western and eastern flanks of the strait. The deepwinter mixing occurred only at the eastern half of thestrait, the remaining part being involved in ASWoutflow, showing an increased vertical gradient oftemperature associated to a marked pycnocline.

One of the most striking features is that theAdriatic side appeared selectively enriched in nitritesand nitrates. The other nutrients, namely phosphateand silicate, showed a different behaviour. In particu-lar, phosphate was completely depleted, its concen-tration being close to the detection limit. The result-


Fig. 15. Vertical distribution of N:P molar ratio at section 300 in February 1994.

y 3y Ž . Ž .ing NO :PO ratio N:P distribution Fig. 153 4

showed a strong signal in correspondence with theŽ .Adriatic waters N:P up to more than 60 and in a

Ž .small lens in the eastern half of the strait N:P)30due to the mesoscale cyclonic recirculation of ASW,while in ISW the N:P ratio was 22, a value consid-

Žered typical for the Mediterranean Coste et al.,.1988; Bethoux et al., 1992 . This nitrate excess is

probably an intrinsic characteristic of the AdriaticŽ .waters i.e., advected within the Adriatic waters ,

imprinted by the freshwater inputs from its northernbasin and the active role of the Adriatic shelf in theefficient removal of phosphorus from the water col-

Žumn via burial in the bottom sediments Degobbisand Gilmartin, 1990; Franco and Michelato, 1992;

.Giordani et al., 1992 . The same behaviour may beŽ .found in the Adriatic Deep Water ADW , in which

the nitrate excess with respect to phosphate producedan increased N:P ratio, the strength of which ap-peared to be associated with the magnitude of the

Ž .outflow through the strait Civitarese et al., 1998 .Considering that the N:P ratio in the water columnhas been used in previous studies to establish which

Želement is limiting Codispoti, 1989; Krom et al.,.1991 , the high values of N:P found in our samples

confirmed the view of a strongly P-limited AdriaticSea, further suggesting its possible role in supportingthe high N:P ratios reported in the Eastern Mediter-

Ž .ranean Berland et al., 1980; Civitarese et al., 1998 .As regards the hydrochemical and biological data,

the emerging winter picture appears to resemble atypical transition of phytoplankton community fromhigher to lower biomass proceeding from the westtowards the eastern end of the strait, in which the

ASW represented the phase of maximum increase ofbiomass, while the ISW resembled an oligotrophicstatus. Nutrient availability in the ASW would sug-gest that new production processes are occurringhere.

The presence of transient cyclonic circulation inŽ .the southern Adriatic Pit Barale et al., 1984 , associ-

ated with upwelling phenomena, produced seasonalŽ .phytoplankton blooms Vilicic et al., 1989 . This

dynamic structure triggered the biological activityand, consequently, the energy transfer along the

Ž .trophic chain Vilicic et al., 1994 . We observed aŽ .similar pattern in winter 1994 Gacic et al., 1996 ,

which led to phytoplankton growth in the northern–central area of the strait.

In summer, an anticyclonic structure, evidencedin the northern Ionian Sea, limited the water ex-change through the strait in the upper thermoclineand determined small scale structures in the current

Ž .field Gacic et al., 1996 . Both the effects of thehydrological feature and the biological activity deter-mined the very low dissolved inorganic nutrient con-

Ž .centration in the upper layer 50 m , the organicphase representing the only source of nutrients in

Žthis layer Dissolved Organic Phosphorus and Dis-.solved Organic Nitrogen; Civitarese et al., 1998 .

Water column stability induced by surface heat-ing, as well as the lack of nutrients in the upperlayer, led to the formation of a deep chlorophyll

Ž .maximum DCM at about 50–75 m depth, in corre-spondence with the nutricline. The DCM formationcan be considered as a summer sinking of epipelagicspecies, composed by small cells and many dinoflag-ellates down to deeper levels, when the surface layer


Ž .was warming up Kimor, 1990 . In our data, DCMappeared deeper than the maximum in abundance, as

Žalso observed in the Levantine basin Yacobi et al.,.1995 . The reason for this discrepancy was due to

the high cellular content of chlorophyll, as a physio-logical response to low irradiance and to great nutri-ent availability, confirmed by the phytoplanktonbiomass:chl.a ratios -20 at DCM, as found by

Ž .Hobson and Lorenzen 1972 in the Atlantic Ocean.This summer picture in the Otranto strait repre-

sented a typical feature of an oligotrophic system,Ž .where DCM ubiquitous for large part of the year is

characterised by low chlorophyll, as well as primaryŽ .production values Estrada et al., 1993 . This occurs

for example in the major gyres of the Atlantic andŽPacific Oceans Gieskes et al., 1978; Eppley et al.,

.1988; Strass and Woods, 1991 and in the westernŽMediterranean Sea Estrada, 1985; Raimbault et al.,

.1993 , being pronounced in the extremely olig-Žotrophic waters of the Eastern Mediterranean Ber-

.man et al., 1984; Yacobi et al., 1995 . A similarvertical pattern of nitrite and chlorophyll a, whosepeaks were found at 50–75 m depth, was also re-

Žported in the western Mediterranean Estrada et al.,.1993 , evidencing excretion phenomena by bacteria

Ž .and phytoplankton Kiefer et al., 1976; Dortch, 1987 .The occurrence of chlorophyll b at DCM was corre-lated to the presence of prasinophyceans, confirming

Ž .the observations made by Jeffrey 1976 in the Pa-cific Ocean. A possible contribution to the increaseof chlorophyll b could be also attributed to the

Ž .presence of prochlorophyceans Yacobi et al., 1995 ,here not determined.

As regards the carbon content, POC and chloro-phyll vertical profiles did not match very well andthe POC fraction, represented by PPC, was lowŽ .about 12% . These considerations would suggestthat bacteria, protozooplankton and organic detrituscontribution to POC may be significant, as observed

Žin the Adriatic and Ionian Seas Faganeli et al.,.1989; Rabitti et al., 1994 . At DCM, the phytoplank-

ton contribution to POC increased, as shown by theŽminimum POC:chl.a and C:N ratios POC:chl.a-50

.and C:Ns6.6, at 75 m depth , the latter close to theŽtheoretical value reported for phytoplankton Re-

.dfield et al., 1963 .Picoplankton represented a significant fraction of

primary producers in the Otranto Channel as re-

vealed by the maximum carbon content in the -2mm size fraction, also reported by many authors in

Žthe Mediterranean Sea Berman et al., 1984; Dowidar,.1984; Azov, 1986 ; low PPC:chl.a ratios seem to

confirm this hypothesis. In the Levantine basin, Ya-Ž .cobi et al. 1995 estimated that 60% of chlorophyll

belongs to particles -2 mm, giving emphasis to thecontribution of picoplankton, mainly cyanobacteriaand prochlorophyceans, to primary production. Thelack of nutrients could favour a trophic web consist-ing of small cells as producers, as pico- andnanoplankton are more efficient in the utilisation of

Žthe resources in oligotrophic systems Thingstad and.Sackshaug, 1990 . In these conditions, regenerative

Ž .processes appear to be prevalent Malone, 1980 andŽ .dissolved organic phosphorus DOP and nitrogen

Ž .DON may represent an important source of nutri-ents. In fact, the presence of DOP at the surface hasbeen documented in this area typically in the spring

Ž .period Civitarese et al., 1998 .In conclusion, it can be assumed that the phyto-

plankton dynamics in the Otranto area is mainlyŽ .driven by: i the presence in winter of ASW, whose

western spread represents a way to enhance thegrowth of phytoplankton in the strait; the nutrientcontent in these waters favours larger phytoplanktonlike diatoms, which are able to exert ‘new produc-tion’ processes in the upper layer. When the ASWplume extends towards the east, following a transientcyclonic gyre, some pulses of biological activity also

Ž .occur on the eastern side of the strait; ii the verticalstability in summer, with inorganic nutrient depletionin the upper warmer layer which leads to low phyto-plankton biomass. The presence of DON and DOP inthe upper layer suggests that only regenerative nutri-ents can be utilised here by phytoplankton for a

Ž .weak primary production; iii the summer DCMŽ .formation at the nutricline 75 m depth , typical of

an oligotrophic system, with ‘deep communities’consisting of small cells, with a relatively high cellu-lar chlorophyll content, as a physiological responseto low irradiance levels.

Acknowledgements

This study was supported by the EU MAST-MTPproject entitled ‘Hydrodynamics and Geochemicalfluxes in the Strait of Otranto’, Contract No. MAS2-


CT93-0068. We would like to thank the OsservatorioŽ .Geofisico Sperimentale OGS -Trieste for CTD data,

and M. Marin, G. Penzo, G. Stocco and S. Tortatofor their assistance during the samplings and for theirwork in the laboratory. We are indebted to L.Craboledda and B. Cavalloni for their chlorophyllanalysis work, to A. Luchetta for nutrients analysisand finally to the anonymous referees for their help-ful suggestions.

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Nutrient, particulate matter and phytoplankton variability in the photic layer of the Otranto strait