Hydrobiologia 475/476: 151–159, 2002.E. Orive, M. Elliott & V.N. de Jonge (eds), Nutrients and Eutrophication in Estuaries and Coastal Waters.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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Nutrient (N, P, Si) fluxes between marine sediments and water column incoastal and open Adriatic

A. Baric, G. Kuspilic & S. MatijevicInstitute of Oceanography and Fisheries, 21000 Split, P.O. Box 500, Croatia

Key words: benthic flux, nutrient, marine sediment, Adriatic Sea

Abstract

Nutrient benthic fluxes, as well as sediment phosphorus concentration at the open sea and coastal water stations ofthe Central and South Adriatic were studied during 1997–98. The fluxes were in the ranges: 0.16–2.67 mmol m−2

d−1 (silicate); −0.031–0.164 mmol m−2 d−1 (phosphate); −0.51–2.03 mmol m−2 d−1 (ammonia); and −1.32–1.62 mmol m−2 d−1 (nitrate + nitrite). Silicate flux showed a gradient from the coastal area to the open sea.Ammonia was the main nitrogen species in the flux at the estuary and bay stations, while the sum of nitrate andnitrite was predominant at the open sea stations. Relationships between phosphate and ammonia fluxes (r = 0.699,p<0.01) as well as phosphate and silicate (r = 0.529, p<0.01) were established.

Introduction

As a consequence of organic matter oxidation in themarine sediment (Froelich et al., 1979) fluxes of inor-ganic nutrients between the sediment and the overly-ing seawater are established through the sediment-water boundary layer (Santschi et al., 1983; Jørgensen& Revsbech, 1985; Sundby et al., 1992). The benthicnutrient fluxes are, in general, diffusion controlled,but in shallow areas they can be enhanced throughinfaunal activity (Tahey et al., 1994) or hydrodynamicpressure (Vanderborght et al., 1977). The importanceof benthic fluxes as an important source of nutrientsfor the primary productivity was pointed out by nu-merous authors (e.g. Rowe et al., 1975; Hopkinson,1987). The fluxes can be also enhanced due to in-creased nutrient input to a particular area (Berelsonet al., 1998), but on a wider scale it was showed thatthrough hydrographical circumstances the continentalslopes (not the shelf areas) are the major depositioncentres of organic matter and areas of intensive re-mineralisation (Christensen, 1989; Lohse et al., 1995;Walsh, 1991).

Hammond et al. (1984) and Giordani & Ham-mond (1985) studied nutrient fluxes in the Northerneutrophic Adriatic. Their results showed that the ob-tained benthic flux was comparable to the input ofnutrient from the Po River, while further investiga-

tions (Giordani et al., 1992) showed that a macrofaunaplays important role in this area. In addition, Banzon& Hopkins (1995) investigated the estuary of the PoRiver, the entire Italian coast and the Jabuka Pit. Theirdata were comparable and lower than those reportedby Giordani et al. (1992). Established �Si:�P and�Si:�N ratios showed no consisted trend, suggest-ing that recycling of silicate is decoupled from theother nutrients. So far, there are no published data onnutrient fluxes for the east Adriatic.

Characteristics of the Adriatic Sea

The Adriatic Sea is the northernmost part of the Medi-terranean. Its length is 783 km and average widthis 243 km. According to the bathimetry (Fig. 1) theAdriatic may be divided into three parts: the shal-low Northern Adriatic with an average depth of 40m; the Central Adriatic with maximal depth of 266m; and the Southern Adriatic basin (max. depth 1233m). Due to the cyclonic circulation of water masses(Zore-Armanda et al., 1999), in-going streams of oli-gotrophic waters from the eastern Mediterranean isflowing along the east Adriatic coast and therefore,open waters of the Central and Southern Adriaticare very low in nutrient concentrations, particularlyorthophosphates (Zore-Armanda et al., 1991).

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Figure 1. Map of the Adriatic Sea showing the location of sampling stations.

Numerous islands are located parallel to the east-ern coast and form narrow channels which impair theexchange of water masses between the coastal areaand open sea. Despite the fact that river and urbanwastewater nutrient loads at the eastern Adriatic arelower compared to the load of the Po River and otherItalian rivers, effects of the eutrophication process arevisible in the coastal area. Since early seventies vari-ous phenomena, such as, the increase of planktonbiomass (Krstulovic et al., 1997), changes in the struc-ture of phytoplankton community (Nincevic, 2000),as well as ‘red-tide’ phenomena and bottom layer an-oxia (Marasovic et al., 1991) occurred. Areas withthe highest nutrient concentration and primary pro-ductivity are semi-enclosed bays with a high urbannitrogen and phosphorus loading (Baric et al., 1992),and salt-wedge estuaries, such as the Krka River estu-ary (Legovic et al., 1994), having nitrate and silicateconcentration for one to two orders of magnitude

higher than in open sea waters (Baric & Kuspilic,unpublished data).

This paper presents the study of benthic nutrientflux on 41 samples, collected during 1997–98 in dif-ferent areas of the Central and Southern Adriatic, suchas semi-enclosed bays, estuaries, channels and openwaters.

Materials and methods

Sediment sampling was done between June 1997 andSeptember 1998 at stations showed on Figure 1. Aplastic gravity corer of internal diameter of 6.5 cm andlength of 100 cm was used for the sampling of threeparallel sediment cores for the flux study, and onemore for the organic matter content and granulometry.At the same time, 10 l of seawater was sampled fromthe bottom layer by a plastic Nansen bottle. The sea-

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water samples were used for the exchange of seawaterabove the sediment samples, as well as for the determi-nation of dissolved oxygen in the bottom layer. CDTprobe was used for the determination of temperatureand salinity in the bottom layer.

Immediately after the collection, sediment sampleswere checked visually regarding compactness and reg-ular stratification. Samples collected by the verticalpenetration of the corer were used for the further ana-lysis. The plastic tubes containing 40–60 cm longsediment samples and overlying seawater were closedwith plastic caps and connected with Tygon tubes toa peristaltic pump and than placed in a dark thermo-static chamber with a constant temperature (bottomlayer temperature). Seawater in the tubes above thesediment was first reduced to the volume of 200 ml andthan slowly replaced by the flow of sampled bottomseawater at the constant rate of 100 ml min−1. Thereplacement lasted about 30 min. Finally, the volumeabove the sediment samples, was adjusted to approx-imately 500 ml. This volume as the maximal for theaccurate analytical determination of nutrient concen-tration changes, especially phosphate, was chosen on aseries of previous experiments. The air above the sea-water was removed by a continuous argon flow. Thefirst 50 ml sample of seawater for the determinationof nutrient concentration was taken 60 min after thestart of incubation, consecutive ones in the intervalsbetween 90 and 120 min, during the incubation, whichlasted between 9 and 12 h. The seawater samples wereanalyzed on board, immediately after sampling.

Nutrient concentration was analyzed by anAutoAnalyzer II, using standard spectrophotometricmethods (Strickland & Parsons, 1972; Grasshoff,1976; Oudot & Montel, 1988) modified for this study.Wako Pure Chemical Industries (Japan) ‘CSK Stand-ard Solution’ were used for the instrument calibration.

The quantity of nutrients (n) released, or uptake bysediments as a function of time (t) was calculated bythe following equations (1–4):

n0 = C0 × V0, (1)

n1 = n0 + (C1 − C0) × V0, (2)

n2 = n1 + (C2 − C1) × V1, (3)

n3 = n2 + (C3 − C4) × V2, (4)

etc. where n0 is initial quantity; n1 quantity at t1, etc.C0 is initial concentration; C1 concentration at t1, etc.,and V0 is initial chamber volume; V1 chamber volumeafter first subsampling, etc.

Benthic nutrient flux was calculated using thegraphical method of linear regression by plotting thequantities (n; µmol) as the function of time (t; min).From the linear fitting slopes (k), the sediment sur-face area (A=33.166 cm2), and a conversion factor(F=1.44∗104; µmol cm−2 min−1 to mmol m−2 d−1)replicate fluxes (J ) were established by Equation (5):

J = k∗A−1∗F. (5)

For the further calculations, an average value of thethree replicate fluxes was used.

Granulometric composition of sediment sampleswas done according to Shepard (1954). Organic mat-ter content (in 5 cm surface sediment samples) wasdetermined gravimetrically after oxidation with 30%hydrogen peroxide solution and ignition at 450 ◦C for6 h (Vdovic et al., 1991).

During the same sampling period, sedimentsamples for the determination of inorganic and totalphosphorus concentration (IP, TP) were collected. Thesediment samples (n = 38) were collected by grav-ity corer (i.d. = 3.2 cm) and the top (10 cm) of thesediment cores were divided into subsamples each1 cm thick. The phosphorus concentrations (mmol Pkg −1 dry sediment) were determined by the methodproposed by Aspila et al. (1976).

Results

For the presentation and interpretation of obtained res-ults, the sampling stations were grouped into threeareas: estuary and bay stations (EBS), channel waterstations (CWS), and open sea stations (OSS). Stationdepths, sediment type, organic matter content, IP andTP sediment concentrations as well as temperature, sa-linity and oxygen saturation in the bottom water layerare presented in Tables 1 and 2, respectively.

The open sea stations are characterised by fine-sized grain sediment, except OSS 2 where sand pre-dominates. An increase of sand and gravel content wasfound at the channel water stations. At the stations inKrka estuary (EBS 1) and Neretva estuary (EBS 3),a portion of coarser sediment particles is higher thanat Kastela Bay station (EBS 2). In the organic mat-ter content, there were no distinct differences betweenspecific areas, although high values were noticed inthe each group of stations as the result of specificecological conditions. Accordingly, EBS 1 is locatedin the eutrophic Krka estuary, CWS 3 in the coastalarea under strong terrigenous inputs and the deepest

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Table 1. Depths of stations, sediment type according to Shepard (1954), mean organic matter content ± STD, inorganic and totalphosphorus concentrations ± STD through the 1997/98

Station Depth Sediment Organic IP TP

(m) type matter (mmol P kg−1 (mmol P kg 1

(%) d. w.) d. w.)

EBS 1 38 clayey silt 7.91±0.96 27.34±5.83 43.16±8.58

EBS 2 38 silty clay 5.88±0.43 13.11±3.67 28.05±5.12

EBS 3 20 clayey silt 6.25±0.38 12.99±3.35 23.33±3.90

CWS 1 38 clayey silt 5.13±0.99 5.89±1.57 13.01±3.70

CWS 2 52 clayey silt 4.91±1.34 7.75±1.59 14.17±2.32

CWS 3 66 clayey silt 8.23±1.17 n.a.a n.a.a

CWS 4 88 clayey silt 5.41±0.93 n.a.a n.a.a

OSS 1 206 silt 5.90±1.01 11.04±1.27 22.03±3.36

OSS 2 103 sand 2.91±0.48 13.64±4.10 19.75±2.68

OSS 3 178 clayey silt 5.63±0.39 10.51±1.97 22.51±5.67

OSS 4 100 silty clay 4.72±0.42 15.97±6.24 31.47±2.01

OSS 5 1010 silty clay 9.58±1.22 19.76±4.18 30.14±4.30

aNot analysed.

OSS 5 in the Southern Adriatic Pit. The only one sta-tion with the sandy sediment type is OSS 2 where thelowest organic matter content was determined. Theobtained results showed very important differencesin phosphorus concentrations in sediment of differentstations. The lowest phosphorus sediment concentra-tions (both TP and IP) were found on CWS, while thehighest value was at EBS 1. This extremely high TPconcentration is the result of high IP content probablyas the consequence of impacts from a nearby harbourterminal for phosphate ores and fertilizers.

The estimated benthic fluxes (J) at the investigatedareas are shown in Table 3 for EBS, Table 4 for CWS,and Table 5 for OSS. Area flux means for the coldperiod of year (CP), from November to April, andthe warm period (WP), from May to October, are alsogiven.

The silicate flux was in the range from 0.16 to 2.67mmol m−2 d−1. The highest silicate fluxes were es-tablished at EBS 1 (river Krka estuary) which is undersignificant freshwater impact and known as the areawith intensive diatom blooms (Legovic et al., 1996)and at EBS 2 located in eutrophic Kastela Bay (Mara-sovic et al., 1991; Baric et al., 1992). An increase ofsilicate fluxes in WP (in all investigated areas), as wellas a decrease of fluxes from EBS to OSS area in thesame period can be observed from the area flux means.The obtained values of phosphate fluxes were in therange between −0.031 and 0.164 mmol m−2 d−1. Thehighest positive fluxes were established in EBS, partic-

Table 2. Temperature (T), salinity (S) and oxygen saturation (O2)in bottom water layer at investigated areas in cold and warm period(1997/98)

Parameter Cold period Warm period

T (◦C)EBS 12.4–15.8 14.2–17.9

CWS 12.0–15.3 14.1–17.6

OSS 12.7–15.1 12.0–15.1

S (‰)EBS 37.98–38.40 38.21–38.27

CWS 38.01–38.45 38.01–38.39

OSS 38.49–38.65 38.48–38.82

O2 (%)EBS 91.9–104.0 85.3–96.9

CWS 93.7–97.5 76.1–102.3

OSS 92.0–96.0 77.0–87.5

ularly in WP. Negative phosphate fluxes were obtainedonly in CP, except once at CWS 1 (June 1997). Theammonia fluxes were between −0.51 and 2.03 mmolm−2 d−1. Area means were mainly positive, althoughthe negative fluxes were also obtained. The nitrate +nitrite fluxes (range: −1.32 to 1.62 mmol m−2 d−1)showed differences between particular areas. Based onthe area means, the marine sediment in OSS area is thesource of ammonia and sink of nitrate+nitrite through

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Figure 2. Ammonia:total nitrogen fluxes ratio at EBS, CWS andOSS stations in warm and cold period of the year.

the year. In CWS area, the marine sediment in CP isthe source of ammonia and sink of nitrate + nitrite,while in WP the situation is opposite. In EBS area,sediment is source of ammonia while nitrate + nitritefluxes means are zero (in CP) or positive (in WP).

Contribution of ammonia in the total benthic ni-trogen exchange at the investigated areas is shown onFigure 2. In CP, the ammonia percentage in the totalnitrogen exchange for all areas is 50% (remaining isnitrate + nitrite). In WP the ammonia contribution in-creases up to 80% in EBS and decreases to 20% inOSS.

Correlation analyses between positive nutrientfluxes (Fig. 3a, b) were statistically significant forJPO4 and JNH4(r = 0.699, p<0.01, n =16), as wellas for JPO4 and JSi (r = 0.529, p<0.01, n = 24).

Discussion

The obtained nutrient fluxes in the Central and South-ern Adriatic are comparable with and lower thanworldwide published data (Hopkinson, 1987) as wellas for the Northern Adriatic (Giordani et al., 1992).Low nutrient fluxes at the open sea area correspondto the low sedimentation rates (Kuptsov et al., 1981;Giordani et al., 1992; Faganeli et al., 1994).

Among all the nutrient fluxes, only silicate fluxshowed a regular increase from OSS to EBS area,which reflects the existing plankton density gradi-ent in the Central Adriatic (Krstulovic et al., 1997)with a decreasing diatom portion in the phytoplank-

Figure 3. The relationship between phosphate and ammonia flux (a)and phosphate and silica flux (b).

ton community (68% EBS and 35% OSS; Nincevic,2000). The silicate flux increase in WP also coincidewith plankton maximas in this period (Baranovic etal., 1992; Krstulovic, 1992; Nincevic, 2000), but inshallow areas the contribution of opal solubility in-crease, due to higher seawater bottom temperature(Hurd, 1973) in this period of year, is also possible(Table 2). The contribution of radiolarian skeleton dis-solution to the silicate fluxes in the Adriatic may beconsidered as negligible in areas shallower than 100m, because these species are present in deeper areas(Krsinic, 1998). The only one unexpected high fluxvalue (JSi=1.97 mmol m−2 d−1) was recorded on OSS5 (March 1998). This flux increase with respect toSeptember 1997 flux value was very probably, induced

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Table 3. Benthic fluxes ± STD at EBS area

Date Station JSi JPO4 JNO3+NO2 JNH4

(mmol m−2 d−1) (mmol m−2 d−1) (mmol m−2 d−1) (mmol m−2 d−1)

17 Jun 1997 EBS 2 2.33 ± 0.14 n.a.b 0.28 ± 0.07 2.03 ± 1.50

20 Jun 1997 EBS 3 0.85 ± 0.22 0.013 ± 0.003 0.04 ± 0.01 −0.39 ± 0.12

27 Jun 1997 EBS 1 1.94 ± 0.25 0.013 ± 0.002 0.17 ± 0.04 −0.43 ± 0.15

16 Sep 1997 EBS 2 1.30 ± 0.11 n.a.b 0.18 ± 0.05 0.41 ± 0.09

25 Sep 1997 EBS 3 0.52 ± 0.32 n.a.b 0.09 ± 0.01 −0.12 ± 0.07

15 Dec 1997a EBS 1 1.67 ± 0.71 0.008 ± 0.001 0.18 ± 0.01 0.10 ± 0.02

30 Jan 1998a EBS 2 0.62 ± 0.32 −0.031 ± 0.004 −0.38 ± 0.04 0.00 ± 0.02

01 Mar 1998a EBS 2 0.66 ± 0.29 0.002 ± 0.001 −0.11 ± 0.03 0.12 ± 0.09

31 Mar 1998a EBS 3 0.73 ± 0.11 0.011 ± 0.011 0.14 ± 0.0 −0.20 ± 0.02

06 Apr 1998a EBS 2 0.89 ± 0.38 −0.011 ± 0.007 0.05 ± 0.06 0.35 ± 0.20

09 Apr 1998a EBS 1 1.14 ± 0.90 0.089 ± 0.055 0.35 ± 0.43 0.25 ± 0.24

27 Apr 1998a EBS 2 1.71 ± 0.20 0.076 ± 0.043 −0.08 ± 0.05 0.32 ± 0.19

28 Jun 1998a EBS 1 2.67 ± 0.24 0.164 ± 0.051 −0.16 ± 0.07 −0.36 ± 0.13

Area mean CP 0.96 ± 0.41 0.022 ± 0.049 0 ± 0.25 0.14 ± 0.21

± STD WP 1.60 ± 0.65 0.064 ± 0.087 0.10 ± 0.15 0.19 ± 0.95

aSimultaneously determined sediment phosphorus concentrations and phosphate fluxes.bNot analysed.

Table 4. Benthic fluxes ± STD at CWS area

Date Station JSi JPO4 JNO3+NO2 JNH4

(mmol m−2 d−1) (mmol m−2 d−1) (mmol m−2 d−1) (mmol m−2 d−1)

21 Jun 1997 CWS 4 1.70 ± 0.06 0.025 ± 0.001 1.62 ± 0.18 −0.51 ± 0.01

26 Jun 1997 CWS 1 0.81 ± 0.25 −0.019 ± 0.009 0.03 ± 0.01 0.62 ± 0.16

24 Sep 1997 CWS 2 2.49 ± 0.41 n.a.b 0.09 ± 0.01 −0.09 ± 0.01

26 Sep 1997 CWS 4 1.08 ± 0.23 n.a.b −0.25 ± 0.08 −0.07 ± 0.05

28 Sep 1997 CWS 3 1.37 ± 0.75 n.a.b 0.04 ± 0.03 −0.03 ± 0.01

14 Dec 1997a CWS 1 0.46 ± 0.10 0.006 ± 0.003 0.16 ± 0.01 0.12 ± 0.02

16 Dec 1997 CWS 3 0.59 ± 0.33 −0.020 ± 0.008 −0.18 ± 0.12 0.56 ± 0.25

21 Dec 1997 CWS 2 0.70 ± 0.29 0.019 ± 0.007 0.26 ± 0.02 0.12 ± 0.03

24 Jan 1998 CWS 4 0.58 ± 0.06 −0.016 ± 0.002 0.07 ± 0.14 0.19 ± 0.13

24 Jan 1998 CWS 3 0.78 ± 0.37 0.042 ± 0.005 0.29 ± 0.05 0.25 ± 0.02

25 Feb 1998 CWS 3 0.79 ± 0.07 0.014 ± 0.002 0.20 ± 0.06 0.07 ± 0.02

26 Feb 1998 CWS 4 1.07 ± 0.52 0.014 ± 0.002 −0.15 ± 0.07 0.03 ± 0.01

28 Feb 1998 CWS 2 0.97 ± 0.03 0.015 ± 0.011 −0.28 ± 0.17 0.07 ± 0.06

30 Mar 1998 CWS 4 0.32 ± 0.04 0.066 ± 0.011 0.16 ± 0.04 0.45 ± 0.11

02 Apr 1998a CWS 2 0.16 ± 0.12 0.045 ± 0.013 −0.19 ± 0.15 0.22 ± 0.09

23 Apr 1998 CWS 2 0.66 ± 0.09 0.057 ± 0.008 −0.07 ± 0.03 0.43 ± 0.26

Area mean CP 0.66 ± 0.36 0.035 ± 0.072 −0.05 ± 0.30 0.21 ± 0.21

± STD WP 1.49 ± 0.65 0.005 ± 0.038 0.31 ± 0.75 −0.02 ± 0.41

aSimultaneously determined sediment phosphorus concentrations and phosphate fluxes.bNot analysed.

by the settling of particulate organic matter from thetrapped cold Northern Adriatic water (Zore-Armanda,1969) originating from one of the most productive

areas in the Mediterranean (Sournia, 1973). This em-phasises the higher organic matter content, averageTP concentrations (Table 1) and phosphate flux (0.164

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Table 5. Benthic fluxes ± STD at OSS area

Date Station JSi JPO4 JNO3+NO2 JNH4

n (mmol m−2 d−1) (mmol m−2 d−1) (mmol m−2 d−1) (mmol m−2 d−1)

22 Jun 1997 OSS 4 1.11 ± 0.24 0.008 ± 0.006 −0.09 ± 0.03 0.04 ± 0.06

17 Sep 1997 OSS 3 0.81 ± 0.16 n.a.b 0.28 ± 0.17 0.06 ± 0.03

27 Sep 1997 OSS 4 0.84 ± 0.37 n.a.b −0.04 ± 0.07 −0.02 ± 0.01

27 Sep 1997 OSS 5 0.53 ± 0.40 n.a.b −1.32 ± 0.74 −0.05 ± 0.02

25 Jan 1998 OSS 4 0.53 ± 0.17 0.022 ± 0.009 −0.09 ± 0.04 −0.13 ± 0.04

28 Jan 1998 OSS 3 0.73 ± 0.22 0.014 ± 0.007 0.16 ± 0.05 −0.19 ± 0.21

26 Feb 1998 OSS 4 0.52 ± 0.20 −0.012 ± 0.008 0.15 ± 0.04 −0.04 ± 0.04

28 Mar 1998 OSS 4 0.81 ± 0.04 n.a.b 0.24 ± 0.10 0.21 ± 0.12

28 Mar 1998a OSS 5 1.97 ± 0.02 0.164 ± 0.051 −0.39 ± 0.08 0.34 ± 0.11

31 Mar 1998a OSS 3 0.39 ± 0.08 −0.003 ± 0.001 −0.48 ± 0.08 0.20 ± 0.12

25 Jun 1998a OSS 3 1.14 ± 0.19 0.029 ± 0.036 −0.34 ± 0.15 −0.09 ± 0.01

28 Jun 1998a OSS 1 1.51 ± 0.48 0.017 ± 0.001 −0.80 ± 0.14 0.23 ± 0.06

Area mean CP 0.83 ± 0.58 0.037 ± 0.072 −0.06 ± 0.30 0.07 ± 0.21

± STD WP 0.99 ± 0.33 0.018 ± 0.010 −0.39 ± 0.58 0.01 ± 0.12

aSimultaneously determined sediment phosphorus concentrations and phosphate fluxes.bNot analysed.

mmol m−2 d−1). Since the phosphate and nitrogen(ammonia, nitrate + nitrite) flux means have not al-ways followed plankton distribution it seems that Nand P regeneration is more influenced by specific pro-cesses taking place in the surface sediment layer thanby the particulate organic matter sedimentation. Prob-ably the most interesting result of established phos-phate fluxes in this part of the Adriatic is the groupingof negative fluxes in the CP. This is in the accordanceto Fisher et al. (1982) who detected phosphate sorptionas well as negative phosphate fluxes at lower watertemperatures. We believe that the most important rolein phosphate flux regulation in the Adriatic have Feoxide-hydroxides in the oxic surface layer, represent-ing active adsorption sites and sequestering phosphateas it was shown by Sundby et al. (1992) and Jensenet al. (1995). Since the changes in the vertical ironspeciation in Adriatic sediment through the year areunknown, we assume, on the basis of oxygen bottomlayer saturation ranges (Table 2), that in the CP thesediment surface layer is more oxidised than in WP.Therefore, due to a higher number of sorption sitesand reduced settling of particulate organic matter onthe sea floor the pore-water phosphate concentrationdecrease and subsequent negative P fluxes occurred atOSS 4, CWS 4 and EBS 2. The established relation-ships between JPO4 and JNH4 as well as between JPO4and JSi, in the cases of positive fluxes (Fig. 3a, b),indicate that phosphate regeneration and recovery to

the water column is (after fulfilment of the sedimentadsorption capacity) linear correlated to ammonia andsilicate regeneration and fluxes. Similar correlationfor the phosphate and ammonia flux was also foundby Fisher et al. (1982) and Nixon (1981), while thecorrelation of silicate and phosphate was stated byRedfield, et al. (1963). The intercept of the straightlines at y axes (Fig. 3a, b) are at relative equal value(−0.0046 mmol m−2 d−1: PO4–NH4 and −0.0035mmol m−2 d−1: PO4-Si) and represents an averageadsorption term in Adriatic sediment, while the slopesthemselves could be an indicator of relative mobilit-ies of phosphate, ammonia and silicate in sedimentnutrient fluxes (Fisher et al., 1982).

The ammonia flux area means show uniform lowfluxes in OSS area (maximal flux at OSS 5/ March1998) and somewhat higher in EBS area. The rela-tionship between JPO4 and JNH4 (Fig. 3a) with an�JPO4:�JSi ratio of 1:4.4 which is significant lowerto the Redfield ratio (Redfield et al., 1963) points tosignificant removal of ammonia from the pore-waterby adsorption (Rosenfeld, 1979) or nitrification. Themain characteristic of ammonia fluxes in the Adriaticis occurrence of negative fluxes. The occurrence ofnegative ammonia fluxes has been reported by otherauthors (Simon, 1988; Rowe & Phoel, 1992; Lohseet al., 1995). However, an explanation for this phe-nomenon was only provided by Simon (1988). He re-lated negative fluxes to the sediment resuspension. An

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occasional ammonia sediment uptake was proposed byLaima (1992) studying seasonal changes of ammoniapore water profiles and sediment adsorption capacityfor ammonia in the coastal area of Denmark. Hedetermined a high sediment adsorption capacity (ad-sorption K-value up to 108), but the relation betweensediment ammonia uptake and sediment characterist-ics was not found. Our obtained negative ammoniafluxes are probably the result of different factors spe-cific for the each studied area. In OSS area, with lowprimary production rate (∼280 mg C m−2 d−1) (Mis-eta, 1999) ammonia is probably completely oxidisedinto nitrite and nitrate at the oxic surface sedimentlayer. In the shallow channel area, the negative ammo-nia flux is related to the higher silicate flux (Table 3).If we use the increased silicate flux as the indicator ofincreased organic matter settlement, than the ammoniaadsorption onto detritic organic matter would be themain controlling process in the direction of ammoniafluxes either into or out of the sediment, as proposedby Rosenfeld (1979). In addition, the ammonia up-take is probably stimulated by the relative high bottomcurrent (indicated by prevailing coarsesized sedimentparticles) causing the sediment resuspension, as pro-posed by Simon (1988). In the estuary and bay areas(Table 4) the negative fluxes were obtained at stationsEBS 1 and EBS 3 but not at station EBS 2. The differ-ence may be related to the rivers impact. Krka River(EBS 1) and Neretva River (EBS 3) are much largerrivers than Jadro River (EBS 2) and granulometric sed-iment analysis at EBS 1 and EBS 3 showed higherportion of coarsesized particles (Table 1) which indic-ates the existence of significant counter-current in thebottom layer and consequent sediment resuspension.

Similar to the ammonia, nitrate + nitrite showedalso negative fluxes, especially in the OSS area in WP(area with lowest oxygen saturation ranges; Table 2).Such negative fluxes are the indicators of intensive de-nitrification, which occur after the reduction of oxygenand Mn (IV) (Froelich et al., 1979), but as shown byBrandes & Devol (1995) denitrification can take placeeven in the presence of oxygen.

Acknowledgements

This study forms part of the research project “Nutri-ent Cycle in the Water Columns and Exchanges at theWater-Sediment Interface” supported by the Ministryof Science and Technology, Republic of Croatia. Weare grateful to Mr Sc. Danijela Bogner for her invalu-

able assistance in the sediment characterisation, andthe crew of R.V “Bios” for the assistance in sedimentsampling.

References

Aspila K. I., H. Agemian & A. S. Y. Chau, 1976. A semi-automatedmethod for the determination of inorganic, organic and totalphosphate in sediments. Analyst 101: 187–197.

Banzon, V. & T. S. Hopkins, 1995. An estimate of pelagic andbenthic oxygen consumption and nutrient regeneration ratesfrom Elna data in the Adriatic Sea. Rapp. Comm. int. Mer Medit.34: 54.

Baranovic, A., T. Vucetic & T. Pucher-Petkovic, 1992. Long-termfluctuations of zooplankton in the middle Adriatic Sea. (1960–1982) Acta Adriat. 33: 85–120.

Baric, A., I. Marasovic & M. Gacic, 1992. Eutrophication phe-nomenon with special reference to the Kastela Bay. Chem. Ecol.6: 51–68.

Berelson, W. M., D. Heggie, A. Longmore, T. Kilgore, G. Nicholson& G. Skyring, 1998. Benthic nutrient recycling in Port PhillipBay, Australia. Estuar. coast. shelf Sci. 46: 917–934.

Brandes, J. A. & A. H. Devol, 1995. Simultaneous nitrate andoxygen respiration in coastal sediments: Evidence for discretediagenesis. J. mar. Res. 33: 771–797.

Christensen, J. P., 1989. Sulphate reduction and carbon oxidationrates in continental shelf sediments, an examination of offshelfcarbon transport. Cont. Shelf Res. 9: 223–246.

Faganeli J., J. Pezdic, B. Ogorelec, M. Misic & M. Najdek, 1994.The origin of sedimentary organic matter in the Adriatic, Cont.Shelf Res. 14: 365–384.

Fisher. T. R., P. R. Carlson & R. T. Barber, 1982. Sediment nutrientregeneration in three Northern Carolina estuaries. Estuar. coast.shelf Sci. 14: 101–116.

Froelich, P. N., G. Klinkhammer, M. L. Bender, N. A. Luedtki, G.R. Heath, D. Cullen, P. Dauphin, D. Hammond & B. Hartman,1979. Early oxidation of organic matter in pelagic sedimentsof the eastern equatorial Atlantic: suboxic diagenesis. Geochim.Cosmochim. Acta 43: 1075–1095.

Giordani, P. & D. E. Hammond, 1985. Techniques for measur-ing benthic fluxes of 222Rn and nutrients in coastal waters.CNR/IGM (Bologna, Italy) Tech. Rep. No. 20: 33 pp.

Giordani P., D. E. Hammond, W. M. Berelson, G. Montanari, R.Poletti, A. Milandri, M. Frignani, L. Langone, M. Ravaioli, G.Rovatti & E. Rabbi, 1992. Benthic fluxes and nutrient budgetsfor sediments in the Northern Adriatic Sea: burial and recyc-ling efficiencies. In Hamilton, E. I. & J. O. Nriagu (eds), Sci.Tot. Environ. Supplement 1992. Elsevier Science Publishers,Amsterdam: 251–275.

Grasshoff, K., 1976. Methods of Seawater Analysis. Verlag Chemie,Weinheim: 307 pp.

Hammond, D. E., P. Giordani, G. Montanari, A. Rinaldi, R. Poletti,G. Rovatti, M. Astorri & M. Ravaioli, 1984. Benthic flux meas-urements in NW Adriatic coastal waters. Memoirs of the Societyfor Geology Italy 27: 461–467.

Hopkinson, C. S. Jr., 1987. Nutrient regeneration in shallow-watersediments of the estuarine plume region of the nearshore GeorgiaBight, U.S.A. Mar. Biol. 94: 127–142.

Hurd, D. C., 1973. Interactions of biogenic opal, sediment and sea-water in the Central Equatorial Pacific. Geochim. Cosmochim.Acta 37: 2257–2282.

159

Jensen, H. S., P. B. Mortensen, F. Ø. Andersen, E. Rasmussen & A.Jensen, 1995. Phosphorus cycling in a coastal marine sediment,Aarhus Bay, Denmark. Limnol. Oceanogr. 40: 908–917.

Jørgensen, B. B. & N. P. Revsbech, 1985. Diffusive boundary lay-ers and the oxygen uptake of sediments and detritus. Limnol.Oceanogr. 30: 111–122.

Krsinic, F., 1998. Vertical distribution of protozoan and microcope-pod communities in the Southern Adriatic Pit. J. Plankton Res.20: 1033–1060.

Krstulovic, N., 1992. Bacterial biomass and production rates in thecentral Adriatic. Acta Adriat. 33: 49–65.

Krstulovic, N., M. Solic & I. Marasovic, 1997. Relationshipbetween bacteria, phytoplankton and heterotrophic nanoflagel-lates along the trophic gradient. Helgoländerwiss Meeresunters.51: 433–443.

Kuptsov V. M., E. M. Emelyvanov, K. M. Shinkus, I. V. Gzakova &S. W. Cehbashvili, 1981. New radiocarbon datings of sedimentsin the Mediterranean Sea and sedimentation rates. Oceanology21: 507–515.

Laima, M. J. C., 1992. Extraction and seasonal variation of NH4pools in different types of coastal marine sediments. Mar. Ecol.Prog. Ser. 82: 75–84.

Legovic, T., V. Zutic, Z. Grzetic, R. Precali & D. Vilicic, 1994.Eutrophication in the Krka estuary. Mar. Chem. 46: 203–215.

Legovic T., V. Zutic, D. Vilicic & Z. Grzetic, 1996. Transport ofsilica in a stratified estuary. Mar. Chem. 53: 69–80.

Lohse L., J. F. P. Malschaert, C. P. Slomp, W. Helder & W. VanRaaphorst, 1995. Sediment-water fluxes of inorganic nitrogencompounds along the transport route of organic matter in theNorthern Sea. Ophelia 41: 173–197.

Miseta, M. A., 1999. Phytoplankton primary production and bio-mass on the Middle Adriatic coastal and open sea waters. MasterThesis, Faculty of Science, University of Zagreb, Zagreb: 128pp.

Marasovic, I. M. Gacic, V. Kovacevic, N. Krstulovic, G. Kuspilic,T. Pucher-Petkovic, N. Odzak & M. Solic, 1991. Developmentof the red tide in the Kastela Bay (Adriatic Sea). Mar. Chem. 32:375–385.

Nincevic, Z., 2000. Size-fractionated phytoplankton by biomassin the Middle Adriatic. Ph. D. Thessis, Faculty of Science,University of Zagreb: Zagreb: 128 pp.

Nixon, S. W., 1981. Remineralization and nutrient cycling in coastalmarine systems. In Neilson, B. J. & L. E. Cronin (eds), Estuariesand Nutrients. Clifton, New Jersey: Humana Press: 111–138.

Oudot, C. & Y. Montel, 1988. A high sensitivity method for thedetermination of nanomolar concentrations of nitrate and nitritein seawater with a Technicon Autoanalyzer II. Mar. Chem. 24:239–252.

Redfield, A. C., B. Ketchum & F. Richard, 1963. The influence oforganisms on the composition of seawater. In Hill, M. (ed.), TheSea. Vol. 2. Interscience, New York: 26–77.

Rosenfeld, J. K., 1979. Ammonia adsorption in nearshore anoxicsediments. Limnol. Oceanogr. 24: 356–364.

Rowe, G. T., C. H. Clifford & K. L. Smith Jr, 1975. Benthic nu-terient regeneration and its coupling to primary productivity incoastal waters. Nature 255: 215–217.

Rowe, G. T. & W. C. Phoel, 1992. Nutrient regeneration and oxygendemand in Bering Sea continental shelf sediments. Cont. ShelfRes. 12: 439–449.

Santschi, P. H., P. Bower, U. P. Nyffeler, A. Azevedo & W. S.Broecker, 1983. Estimates of the resistance to chemical transportposed by the deep-sea boundary layer. Limnol. Oceanogr. 28:899–912.

Shepard F. P., 1954. Nomenclature based on sand-silt-clay relations.J. Sediment. Petro. 24: 151–158.

Simon N. S., 1988. Nitrogen cycling between sediment and the shal-low water column in the transition zone of the Potomac Riverand estuary. I. Nitrate and ammonia fluxes. Estuar. coast. shelfSci. 26: 483–497.

Sournia, A., 1973. The primary productivity in the MediterraneanSea. Essai de mise a jour. Bull. Etud. Commun. Medit. Monaco5: 1–128.

Strickland, J. D. & T. R. Parsons, 1972. A practical handbook ofseawater analysis. Bull. Fish. Res. Bd Can. 167: 310 pp.

Sundby, B. C. Gobeil, N. Silverberg & A. Mucci, 1992. The phos-phorus cycle in coastal marine sediments. Limnol. Oceanogr. 37:1129–1145.

Tahey, T. M., G. C. A. Duineveld, E. M. Berghuis & W. Helder,1994. Relation between sediment–water fluxes of oxygen andsilicate and faunal abundance at continental shelf, slope anddeep-water stations in the Northwest Mediterranean. Mar. Ecol.Prog. Ser. 104: 119–130.

Vanderborght, J-P., R. Wollast & G. Billen, 1977. Kinetic mod-els of diagenesis in disturbed sediments. Part 1. Mass transferproperties and silica diagenesis. Limnol. Oceanogr. 22: 787–793.

Vdovic N., J. Biscan & M. Juracic, 1991. Relationship betweenspecific surface area and some chemical and physical propertiesof particulates: study in the Northern Adriatic. Mar. Chem. 36:317–328.

Walsh, J. J., 1991 Importance of continental margins in the marinebiogeochemical cycling of organic carbon and nitrogen. Nature350: 53–55.

Zore-Armanda, M., 1969. Water exchange between the Adriatic andthe Eastern Mediterranean. Deep-Sea Res. 16: 171–178.

Zore-Armanda, M., M. Bone, V. Dadic, M. Morovic, D. Ratkovic,L. Stojanoski & I. Vukadin, 1991. Hydrographic properties of theAdriatic Sea in the period from 1971 through 1983. Acta Adriat.32: 6–544.

Zore-Armanda, M., B. Grbec & M. Morovic, 1999. Oceanographicproperties of the Adriatic Sea – A point of view. Acta Adriat. 40(Suppl.): 39–54.