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The hydrophobic fraction of organic matter in the Krka River Estuary
Marine Chemistry, 39 (1992) 251-267
251
Elsevier Science Publishers B.V., Amsterdam
The hydrophobic fraction of organic matter in the Krka River Estuary Vjero6ka Vojvodi6 and Bo~ena 12osovi6 Centre f o r Marine Research Zagreb, Ruder Bogkovi( Institute, Zagreb, Croatia
(Received 28 February 1991; revision accepted 28 April 1992)
ABSTRACT Vojvodi6, V. and 12osovi6, B., 1992. The hydrophobic fraction of organic matter in the Krka River Estuary. Mar. Chem., 39: 251-267. Surface active substances have been studied in the Krka River Estuary using an a.c. polarography method and various fractionation procedures. Total surface substances (SAT) were separated into (particulate) heterodispersed and dissolved fractions by filtration. The dissolved fraction was separated into hydrophobic basic and neutral components, hydrophobic acid and hydrophilic components by sorption on to XAD-8 resin. The content and the composition of SA x varied seasonally, depending on the biological activities taking place in the Krka River Estuary. The accumulation of SA T, with hydrophobic properties, occurred at the fresh/saline water interface along the vertical profile. Daily changes in the form of the surface active substances along the vertical profile were observed. The formation of the heterodispersed fraction, especially observable in the upper freshwater layer and at the halocline, resulted in the change of physico-chemical properties of surface active substances. Organic matter appeared mainly in a dissolved form in the deeper saline water layer, showing negligible oscillations. The electrochemical characterization of the fractionated SA T indicated that strong hydrophobicity of the surface active substances under natural pH conditions could be attributed to the significant contribution of the heterodispersed fraction of the organic material, which is the most reactive part of the surface active substances in the Krka River Estuary. The dissolved fraction of the surface active substances, prevailing in the deeper saline water layer, exhibited either hydrophilic or conditionally hydrophobic properties, depending on pH and ionic strength of the medium.
INTRODUCTION
The complex mixture of organic matter in the Krka River Estuary consists of organic solutes, colloids and particles coated with organic matter, as well
Correspondence to: V. Vojvodi6, Centre for Marine Research Zagreb, Ruder Bo~kovi6 Institute,
Zagreb, Croatia.
0304-4203/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
252
V. VOJVODIC AND B. COSOV1C
as various biological materials, such as phytoplankton, zooplankton, microorganisms and living and dead cells. The content, chemical composition and forms of organic matter in the Krka River Estuary depend on the river input, allochthonous organic material formed from excretion and decomposition of plankton and bacteria, as well as on dynamic transformations of various forms influenced by salinity, temperature and turbidity. Most biogeochemical processes in natural waters take place at natural phase boundaries (Parks, 1975; Hunter and Liss, 1981). Organic compounds with surface active properties are concentrated at water• atmosphere, water/sediment, and water]dispersed organic material interfaces (Liss, 1975; (2osovi6 et al., 1985; Marty et al., 1989). Surface active substances influence the structure and physico-chemical properties of natural interfaces and mediate the processes of mass and energy transfer between different phases. The fractionation of the organic substances in natural aquatic systems occurs owing to adsorption processes, resulting in elevated concentrations of strongly adsorbable substances in the interfacial region, the less adsorbable substances remaining in the bulk phase. Earlier relevant investigations carried out in other aquatic areas suggest that hydrophobic interactions play an important role in the adsorption of organic matter in natural waters (Tanford, 1973; Tschesche, 1983; Carlson et al., 1985). Bacterial adhesion on surfaces is influenced by both a hydrophobic attraction and an electrostatic repulsion (Loosdrecht et al., 1990). Regarding the origin of the organic matter, autochthonous organic material, formed by excretion and decomposition of plankton and bacteria, is presumably mainly hydrophobic, because of the lower content of dissociable groups than in the organic material derived by leaching of soil (Buffle, 1990). This work is based on a study of the seasonal and daily variations of the hydrophobic fraction of organic matter in a small stratified estuary with a low terrigeneous input. Organic substances were fractionated by sorption on a XAD-8 resin, while an electrochemical method was used for determination of the hydrophobic surfactant activity. Study area
The Krka River Estuary, located in the central part of the eastern Adriatic coast, belongs to the type of stratified estuaries characterized by a low terrigeneous and nutrient input (Skrivani6 and Gr2eti6, 1986; Gr2eti6, 1990). A sharp salinity gradient separates the upper freshwater layer from the lower saline water layer by a strong halocline ((2osovi6 et al., 1982; Zuti6 and Legovi6, 1988). Although the concentrations of dissolved organic carbon (DOC), particulate organic carbon (POC) and total suspended material (TSM) in the Krka River Estuary are considerably lower than the values reported for other
HYDROPHOBIC ORGANIC MATTER 1N THE KRKA ESTUARY
253
estuaries throughout the world, the maximum concentrations of these components are obtained at the fresh/saline water interface ((~osovi6 et al., 1982; Ketchum, 1983; Cauwet, 1991). The influence of the surface fresh water on the productivity of the deeper saline water has generally been detected, since the interfacial layer is biogeochemically a very active zone for phytogenic organic matter (Denant et al., 1991). A decrease in the chlorophyll/phaeophytin ratio was observed along the vertical profile, suggesting decomposition processes of the freshwater phytoplankton at the fresh/saline water interface. The halocline has been detected as a good barrier for the phytoplankton derived from the river water (Vili~i6 et al., 1989). The highest values for Chlorophyll a are obtained at the halocline, while a significant primary production characterizes a low-salinity water layer (Fuks et al., 1991). MATERIAL
AND
METHODS
Sampling sites and collection of samples Samples from various sites in the Krka River Estuary were collected by divers, along a longitudinal transect and in the vertical profile of the water column (Fig. 1) (Kniewald et al., 1987). A new technique of sample collection using a peristaltic pump at intervals of 2 cm in a short vertical scale around the halocline has also been applied (Cauwet, 1991). Station E-3, being under favourable hydrographic conditions and not directly subject to human activities, was chosen as a reference area.
Determination of surface active substances by a.c. polarography A phase-sensitive a.c. polarography method was used for determination of surfactant activity in the manner described elsewhere ((2osovi6 and Vojvodi6, 1982, 1987; (2osovi6 et al., 1985). High sensitivity has been achieved by enhancing the accumulation of the surfactants at the electrode surface by stirring the solution. The decrease of the capacity current (AI) for a particular accumulation period is a function of the surfactant concentration in the solution. The values of the surfactant activity are expressed in terms of surfactant equivalents of the non-ionic surface active substance Triton-X-100 (M = 600) (mg d m - 3). The capacity current measurement at the mercury electrode is a convenient method for the study of those organic substances which are adsorbed primarily owing to hydrophobic expulsion ((7osovi6, 1985). In comparison with other polarographic methods, for example the polarographic maximum method, which is better correlated with the DOC values (Hunter and Liss,
254
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1981), the phase-sensitive a.c. polarography method is more sensitive to the hydrophobic part of the organic matter (Cosovi6, 1985). The adsorbability at the mercury/water interface is found to be in proportion to the hydrophobicity of aliphatic fatty acids of different chain lengths, as determined by their octanol/water partition coefficients (Ulrich et al., 1988). Regarding the specific sensitivity of the a.c. polarography method to the hydrophobic surfactants, special attention has been paid to the hydrophobic part of the surface active substances in the Krka River Estuary.
Fractionation of surface active substances The total surface active substances from the original sample were separated into particulate and dissolved fractions by filtration through a Whatman fibreglass filter (GF/F). Particulate organic matter generally represents particles which are larger than the pore sizes of the filter. This fraction also includes a large portion of the material dispersed in the sizes smaller than the filter pore
HYDROPHOBIC ORGANIC MATTER IN THE KRKA ESTUARY
255
dimension (Johnson and Wangersky, 1985). In this work, the fraction of the organic matter retained on the filter has been termed heterodispersed surface active substances (fraction H). The dissolved fraction (fraction D) also consists of most of the colloidal organic components (Thurman, 1985). After removing the heterodispersed fraction from the original sample, the remaining dissolved fraction represents a mixture of the hydrophobic basic and neutral components (fraction DI), hydrophobic acid (fraction D2) and hydrophilic components (fraction D3), which were then separated by sorption on to XAD-8 resin, according to the modified procedure described elsewhere (Leenheer and Huffman, 1979; Leenheer, 1981). The fractionation procedure of the dissolved fraction is illustrated in Fig. 2. Fraction D~ was separated from the mixture by sorption on to XAD-8 resin (lst column) at natural pH. The solution which remained after the sorption (fraction D 2 + D 3), was acidified to pH = 2 and applied to the 2nd column, on XAD-8 resin, in order to separate the hydrophobic acid components (fraction D2). The hydrophilic components (fraction D3), which were not sorbed on to a resin at both pH values, remained in the final effluent. The percentages of the fractions based on the surfactant activity measurements were calculated as follows: (1) Fractionation by filtration: SAT is total surfactant activity SAD is surfactant activity of dissolved fraction SAN = S A y -- SAD is surfactant activity of heterodispersed fraction (2) Fractionation of S A D by sorption on XAD-8 resin: pH = 7 SAD, = SAD - SA(D,- + D3) is surfactant activity of hydrophobic base and neutral fraction pH = 2 SAD, = SAID,- + D3) -- SAD3 is surfactant activity of hydrophobic acid fraction SAD3 is surfactant activity of hydrophilic fraction RESULTS
AND
DISCUSSION
Seasonal variation o f SAT values in the K r k a River Estuary
The values obtained for S A v in the samples collected at various estuarine stations in the Krka River Estuary during 1982-1988 (Fig. 3) are in the range between 0.04 and 0.16mgdm -3. These values are satisfactorily comparable with the values obtained in the coastal areas of the Adriatic Sea and in the North Adriatic Sea, with more intensive biological activities than in the offshore sea ((7osovi6 et al., 1985; Marty et al., 1989). The lowest SAT values were obtained in winter (0.02-0.04 mg dm-3). These values are in satisfactory agreement with the surfactant activity values
256
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Surfactant activity
Total surface active substances
SAT
I
filtration on filter ~
~
Fraction H:
Fraction D:
Heterodispersed organic matter
Dissolved organic matter
....
SAH, SAD
Fraction O1: < (adsorbed on resin) Hydrophobic base and neutral components
adsorption onto XAD-8 resin pH-7
....
SAD1
L
Fraction D2+ D3: (not adsorbed on resin)
_ _ _ SAD2+D 3
Hydrophobic acid and hydrophilic components
Fraction D2+D3: acidified to pH = 2
Fraction D2:
adsorbed onto XAD-8 resin :)i-1=2
(adsorbed on resin) Hydrophobic acid components
-I-
Fraclion D3: (not a d s o d ~ on resin)
....
....
SAD2
'~D3
Hydrophllic components
Fig. 2. Analytical scheme of fractionation of organic solutes by filtration and sorption on to XAD-8 resin.
obtained for offshore areas of the Mediterranean Sea ((~osovi6 et al., 1985; Marty et al., 1989). Such low values obtained in winter could be attributed to a small allochthonous input of organic matter and low biological activity ((~osovi6, 1985; Vili6i6 et al., 1989). The SA.r values increased during the period of intensive biological activity in summer (0.08-0.16 mg dm-3), while
HYDROPHOBIC ORGANIC MATTER IN THE KRKA ESTUARY
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SURFACTANTACTIVITY/mgdrr] 3 Frequency distribution of surfactant activity (SAT) values in the samples: (a) The Krka River Estuary in different seasons; (b) spring; (c) summer; (d) autumn; (e) winter during 1 9 8 2 - 1 9 8 8 . F i g . 3.
the SAT values obtained in an early spring were lower, usually not exceeding 0 . 0 8 m g d m -3. The largest range of the SAT values (0.04-0.32mgdm -3) occurred in autumn. The seasonal variations of the SAT values in the Krka River, in the samples collected from Station E-0 (during May 1986-December 1988) are illustrated in Fig. 4. The SAT values presented by curve 1 show an inverse relationship with the flow of the Krka River (curve 2). Similar variations of the SA T values were also obtained in the samples collected from the upper freshwater layer (Station E-3 - - located downstream of the estuary) (curve 3). It is evident that
258
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Fig. 4. Seasonal variations of surfactant activity ( S A T ) values in the samples. Krka River (curve I) at the depth of 0.5 m with corresponding values of the river flow (curve 2), and Station E-3 at the depth of 0.5 (curve 3).
the SAT values at both locations (Stations E-0 and E-3) do not exceed the value o f 0 . 2 4 m g d m 3 in equivalents of Triton-X-100.
Variations of SAT values in the vertical profile of the Krka River Estuary The variations of the SAT values in the samples collected at various depths along the vertical profile during 1982-1988, are illustrated in Fig. 5. The highest values obtained at the fresh/saline water interface along the vertical profile (0.08-0.32mgdm -3) are caused by the accumulation of SAv at the fresh/saline water interface. Significantly lower SAT values obtained in the deeper saline water did not exceed 0.08 mg dm -3. Total surfactant activities in the samples collected in the upper freshwater layer (0.5 m) and in the saline water layer (6m) are similar; the values are in the range between 0.04 and 0 . 1 6 m g d m -3. The same distributions of DOC, POC, phaeophytin and Chlorophyll a, as well as surfactant activity values determined by the polarographic maximum method, have already been obtained (Zuti6 and Legovi6, 1988; Cosovi6 and Vojvodi6, 1989; Vili6i6 et al., 1989). Since the stratified estuary is subject to dramatic changes in physico-chemical conditions on a short vertical scale at the halocline, the samples were collected along the salinity gradient at intervals of 2 cm. The samples were collected from Station E-2, on 16 May 1988. The results presented in Fig. 6 show that in the layer between 2.02 and 2.08 m, with a sharp salinity gradient (8-32%o), the SA T value increased from 0.27 to 0.38 m g d m -3. In comparison with the values observed in both upper freshwater layer and deeper saline layer (< 0 . 1 m g d m - 3 ) , the SAT values at the halocline are significantly higher. However, the values changed slightly on a short vertical scale at the halocline, in spite of the presence of a sharp salinity gradient. If one compares the SAT values with the DOC and POC measurements of the same samples (Cauwet, 1991) the following conclusion can be drawn. On
HYDROPHOBIC O R G A N I C M A T T E R IN THE KRKA ESTUARY
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Fig. 5. Frequency distribution of surfactant activity (SAT) values in the Krka River Estuary during 1982-1988 in the samples collected at different depths: (a) surface, 0.5 m; (b) fresh/saline interface; (c) saline water, 6.0m; and (d) more than 10.0m.
a short vertical scale at the halocline, the SAT values increased three to four times with respect to both the upper freshwater layer and deeper saline water layer. The DOC values increased less than two times, while the variations of the POC values were similar (but not identical) to those of the SAT values. The same effect can be observed comparing the SAT and SAD (filtered samples) values obtained along the vertical profile; theSAD values are generally lower than the SAT values, and the accumulation at the halocline is less visible. The results given in Table 1 show that more than 80% of the surfactant activity is in the dissolved fraction, in saline water layer (S = 35-37%o), i.e. the SAT values are negligibly higher than the SAD values. In the upper freshwater layer, with a large range of salinity values between 3.8 and 24%0, the SAT values are significantly higher than the SAD values; between 30 and 75% of the SA x originated from the fraction H. The organic material appearing in the heterodispersed form in the upper water layer accumulates at the halocline, where it can be removed by decomposition processes as well as by the river water flow. Since only a small part of the fraction H has been found in the saline water, especially in the deeper saline layer, it is evident that
260
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Fig. 6. Variation o f surfactant activity (SA T) values on a short scale o f the vertical profile at the halocline at Station E-2 on 16 May 1988, c o m p a r e d with the variations o f the D O C and P O C values observed by Cauwet (1991, Fig 7).
TABLE I Fractionation of surface active substances by filtration Date in 1988
12.05 13.05
15.05
16.05
Depth (m)
3.60 2.60 1.60 2.35 3.35 3.60 2.00 2.15 3.00 2.00 2.15
Salinity (%0)
37.0 10.5 3.8 10.0 36.0 37.0 5.0 24.0 35.0 5.0 24.0
Fractions (%) Heterodispersed
Dissolved
7. I 69.9 75.5 67.9 16.0 16.7 59.4 56.2 42.9 31.5
92.9 30.1 24.5 32.1 84.0 83.3 40.6 43.8 I00.0 57.1 68.5
H Y D R O P H O B I C O R G A N I C M A T T E R IN T H E K R K A E S T U A R Y
261
sinking of the aggregates through the fresh/saline water interface is below significance. It is important to note that the strong adsorbability and the hydrophobic character of the heterodispersed fraction in the Krka River Estuary are to a great extent responsible for the association and aggregation processes of the organic matter, and, probably, for the stability of the fresh/saline water interface.
Daily variations of S A t and SAo values in the Krka River Estuary Daily variations of the S A T and S A D values along the vertical profile were studied at Station E-3 for 24 h on 12/13 May 1988. The samples were collected every 6 h at the following depths: 0.3, 1.6, 2.5 m (fresh/saline water interface), 3.5, 5 and 20m. The salinity values of the samples collected at the depth of 3.5 m and deeper were higher than 35%0, while the values obtained for the samples collected at the depths of 0.3 and 1.6 m in the upper freshwater layer were between 2 and 4%o. The salinity values obtained at the fresh/saline water interface were between 10 and 12%o, with the exception of the samples collected at 18:00 and 00:00 h on 12 May when higher values were obtained, 20%0 and 28%o, respectively. The results of the SAy variations are illustrated in Fig. 7. In the saline water layer (depth of > 3.5 m) the SA-r values were constant during the period investigated, with a slight increase from the bottom at a depth of 20 m to the upper water layer at a depth of 3.5 m. A dramatic increase of the SAv values occurred between 0:00 and 06:00 h in the freshwater layer, at the depths of 0.3 and 1.6 m, especially at the fresh/saline water interface at a depth of 2.5 m. During the following 6 h, the SA T values decreased significantly, reaching the values previously obtained in the samples collected from the freshwater layer, but retaining a high value at the fresh/saline water interface. The oscillations of the SA T values at the fresh/saline water interface are in satisfactory agreement with the changes in the Chlorophyll a values obtained in the same samples (Fuks et al., 1991), indicating the importance of the biological activity in the SAT variation during the day. Daily variations of the SAT values along the vertical profile were compared with the DOC and POC values obtained by Cauwet (1991, Fig. 6). The variations of the POC values were similar to those of the SAT and Chlorophyll a values, while the variations of the DOC values obtained in the same, but filtered samples, differed from the SAT and Chlorophyll a values. The increase of the SAT and POC values in the upper freshwater layer and at the fresh/saline water interface during 24 h suggests the aggregation processes of the organic matter. The hydrophobic properties of this type of organic material (fraction H) are responsible for variations in the SAT values during the day, in spite of a low total organic matter content in the Krka River Estuary.
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Qualitative changes of the adsorbable organic substances during the day have also been observed. The capacity current-potential curves obtained in samples collected from a depth of 1.6m at 00:00, 06:00 and 12:00h are illustrated in Fig. 8. The shapes of polarographic curves of the samples collected at 00:00 and 12:00h (Figs. 8(a) and (c)) are similar; the same is evident for the SAT values given in Table 2. On the contrary, the capacity current-potential curves of the samples collected at 06:00 h (curves 1, 2 and 3 in Fig. 8(b)) changed visibly, indicating different physico-chemical properties of the surface active material. In these samples the SAT value increased about four times in comparison with the samples collected during other sampling periods. The SA D values showed no variations during the day. Fractionation of surface active substances
Total surface active substances in the samples collected along the vertical profile from Station E-3 on 18 May 1988, were separated into H, D, D~, D2 and D 3 fractions, according to the procedure illustrated in Fig. 2. The results
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TABLE 2 Variations of S A 1988 Time (h)
00:00 06:00 12:00
T
and
S A D values
in the samples collected at station E-3 from a depth of 1.6 m on 13 May
Surfactant activity in equivalents of T-X-100 (mgdm 3) SA T
SA D
O. 11
O. 11
0.44 0.13
O. 11 0.12
V. VOJVODI(~ AND a. ~'OSOVIC
264 TABLE 3
Fractionation of organic solutes by sorption on XAD-8 resin in the samples collected at station E-3 on 15 May 1988 Depth (m)
Salinity (%o)
Surfactant activity in equivalents of T-X-100 ( m g d m ~) SA v
2.0
5.0
0.114
Fractions (%) H
DI
D2
D3
43.0
19.7
23.6
13.7
SAn ~
0.065
(o. 128) 2.15 fresh/saline interface 3.00
24.0
0.270
O. 185 (0.250)
31.5
16.6
19.1
32.8
31.0
0.065
0.085 (0.160)
0
17.5
64.0
18.5
~'Values given in parentheses are obtained by a.c. polarography at pH = 2 .
of the surfactant activity values and percentages of all the fractions determined by an a.c. polarography method are given in Table 3. Fraction H was higher in the upper freshwater layer than at the halocline, while fraction D prevailed in the deeper saline water, i.e. the heterodispersed fraction has not been detected. Higher surfactant activity values of fraction D were mainly observed at pH = 2, which is in satisfactory agreement with the behaviour of naturally occurring polyelectrolytes, such as humic and fulvic organic material (Cleven, 1984; Cosovi~ and Vojvodi~, 1989). The percentages of fraction D, adsorbable on a resin at natural pH, were similar in all three samples, not exceeding the value of 20%. The highest value of the fraction D 2, adsorbable at pH = 2, was obtained in the saline water layer. The fraction D 3 value was not higher than 20% in the freshwater and saline water layers, whereas it was around 30% at the fresh/saline interface. The data obtained show that total surface active substances in the Krka River Estuary are mainly hydrophobic under natural conditions owing to the significant contribution of fraction H. On the contrary, fraction D, consisting mainly of the fractions D2 and D3, shows either hydrophilic or conditionally hydrophobic properties depending on pH and on ionic strength of the medium. In spite of low concentrations of organic matter in the Krka River Estuary, very high oscillations of the SAT values observed especially at the halocline, are influenced by the hydrophobic character of fraction H. The vertical variations of fraction D, which contains only 20% of the hydrophobic components, are much lower and comparable with the variations of the DOC
HYDROPHOBIC ORGANIC MATTER IN THE KRKA ESTUARY
265
values. Hydrophobicity of the fraction D probably originates from the colloidally dispersed organic matter, which is the subject of further studies. CONCLUSIONS
The concentrations of the total surface active substances in the Krka River Estuary are generally low, but show significant oscillations. The seasonal variations of the SAT values strongly depend on the biological activity; the same applies for the karstic Krka River, where the SAT values also vary with the flow of the river. Surface active substances are spontaneously fractionated in the vertical profile by accumulating at the fresh/saline water interface of the Krka River Estuary. The daily changes of the form of the surface active substances along the vertical profile were observed. The process of the formation of the heterodispersed fraction, especially observable in the upper freshwater layer and at the halocline, results in a change of the physico-chemical properties of the surface active substances. Organic matter prevails in a dissolved form in the deeper saline water layer, showing negligible oscillations. Electrochemical characterization of the fractionated SA T in the Krka River Estuary shows that the surface active substances in the upper freshwater layer and at the halocline are mostly hydrophobic under natural conditions owing to the significant contribution of the heterodispersed fraction of the organic matter. Fraction H is mostly responsible for the variations in total surfactant activity of the samples and, thus, represents the most reactive part of the organic matter in the Krka River Estuary. The dissolved fraction of the surface active substances, which prevails in the deeper saline water layer, exhibits either hydrophilic or conditionally hydrophobic properties depending on pH and an ionic strength of the medium. ACKNOWLEDGEMENTS
We wish to thank Zeljko Kwokal, Slobodan Macura and Donat Petricioli for underwater sampling and the crew of the R/V "Vila Velebita" for collection of samples in May 1988. The financial support of the Authority for Scientific Research of the Republic of Croatia, U N E P / F A O (Mediterranean Action Plan) and the Commission of European Communities Directorate General for Science Research and Development (Project Cll-0333-YU) is gratefully acknowledged. REFERENCES Buffle, J., 1990. The ecological role of aquatic organic and inorganic components. Deduced from
266
v. VOJVODIC AND B. COSOV1C
their nature, circulation and interactions. In: J.A.-C. Broekaert, S. Gucer and F. Adams (Editors), Metal Speciation in the Environment. NATO ASI Series, Vol. G23. Springer, Berlin, pp. 469-501. Carlson, D.J., Mayer, L.M., Brann, M.L. and Mague, T.M., 1985. Binding of monomeric organic compounds to macromolecular dissolved organic matter in seawater. Mar. Chem., 16:141-153. Cauwet, G., 1991. Carbon inputs and associated biogeochemical processes in a stratified estuary: Krka river (Yugoslavia). Mar. Chem., 32: 269-283. Cleven, R., 1984. Heavy metal/polyacid interaction. Ph.D. Thesis, Agricultural University of Wageningen. (~osovi~, B., 1985. Aqueous surface chemistry. Adsorption characteristic of organic solutes. Electrochemical evaluation. In: W. Stumm (Editor), Chemical Processes in Lakes. Wiley, New York, pp. 55-80. (~osovib, B. and Vojvodi6, V., 1982. The application of a.c. polarography to the determination of surface-active substances in seawater. Limnol. Oceanogr., 27: 361-369. (?osovi6, B. and Vojvodi/:, V., 1987. Direct determination of surface active substances in natural waters. Mar. Chem., 22: 363-373. t~osovi6, B. and Vojvodi6, V., 1989. Adsorption behaviour of the hydrophobic fraction of organic matter in natural waters. Mar. Chem., 28: 183-198. t~osovi6, B., Zuti6, V. and Cauwet, G., 1982. Determination of organic matter in the Krka estuary. In: 28th Meeting CIESM, Cannes. t~osovi~, B., Zuti6, V., Vojvodi~, V. and Plebe, T., 1985. Determination of surfactant activity and anionic detergents in seawater and sea surface microlayer in the Mediterranean. Mar. Chem., 17: 127-139. Denant, V., Saliot, A. and Mantoura, R.F.C., 1991. Distribution of algal chlorophyll and carotenoid pigments in a stratified estuary: The Krka River, Adriatic Sea. Mar. Chem., 32: 285-297. Fuks, D., Devescovi, M., Precali, R., Krstulovi6, N. and ~oli6, M., 1991. Bacterial abundance and activity in the highly stratified estuary of the Krka River. Mar. Chem., 32: 333-346. Gr~eti6, Z., 1990. Basic hydrological and chemical properties of the Krka Estuary. Ph.D. Thesis, University of Zagreb. Hunter, K.A. and Liss, P.S., 1981. Polarographic measurement of surface active material in natural waters. Water Res., 15: 203-215. Johnson, B.D. and Wangersky, P.J., 1985. Seawater filtration: particle flow and impaction considerations. Limnol. Oceanogr., 30: 966-971. Ketchum, B.H., 1983. Estuaries and Enclosed Seas, Ecosystems of the World, Vol. 23. Elsevier, Amsterdam. Kniewald, G., Kwokal, Z. and Branica, M., 1987. Marine sampling by scuba diving. 3. Sampling procedures for measurement of mercury concentrations in estuarine waters and seawater. Mar. Chem., 22: 343-353. Leenheer, J.A., 1981. Comprehensive approach to preparation, isolation and fractionation of dissolved organic carbon from natural waters. Environ. Sci. Technol., 15: 578-587. Leenheer, J.A. and Huffman, Jr., E.W.D., 1979. Analytical method for dissolved-organic carbon fractionation. U.S. Geol. Surv., Water-Resour. Invest. 79: 1-16. Liss, P.S., 1975. Chemistry of the sea surface microlayer. In: J.P. Riley and Skirrow (Editors), Chemical Oceanography, Vol. 2. Academic, New York, pp. 245-299. Loosdrecht, M.C.M., Norde, W., Lyklema, J. and Zehnder, A.J.B., 1990. Hydrophobic and electrostatic parameters in bacterial adhesion. Aquat. Sci. 52:103-114. Marty, J.C., Zuti6, V., Precali, R., Saliot, A., t~osovi6, B., Smodlaka, N. and Cauwet, G., 1989. Organic matter characterization in the Northern Adriatic Sea with special reference to the sea surface microlayer. Mar. Chem., 26: 313-330. Parks, G.A., 1975. Adsorption in the environment. In: J.P. Riley and Skirrow (Editors), Chemical Oceanography, Vol. 1. Academic, London, pp. 241-308.
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~krivani6, A. and Gr~eti6, Z., 1986. Basic hydrographic parameters and nutrients. In: Long-Term Programme for the Pollution Monitoring and Research in the River Krka Estuary and Kornati Archipelago (Adriatic Sea). MED POL Phase II, Annual Reports for 1985, Centre for Marine Research, Rudjer Bo~kovi~ Institute, Zagreb. Tanford, C., 1973. In: The Hydrophobic Effect: Formation of Micelles and Biological Membranes. Wiley-Interscience, New York, Chapter 6. Thurman, E.M., 1985. In: M. Nijhoff and W. Junk (Editors), Organic Chemistry of Natural Waters. Kluwer, Lancaster. Tschesche, H., 1983. The chemical structure of biologically important macromolecules. In: W. Hope et al. (Editors), Biophysics. Springer, Berlin, pp. 20-36. Ulrich, H.J., Stumm, W. and Cosovi~, B., 1988. Adsorption of aliphatic fatty acids on aquatic interfaces. Comparison between two model surfaces: The mercury electrode and p-AI203colloids. Environ. Sci. Technol., 22: 37-41. Vili6i6, D., Legovi6, T. and Zuti6, V., 1989. Vertical distribution of phytoplankton in a stratified estuary. Aquat. Sci., 51: 31-46. 7.uti6, V. and Legovi6, T., 1988. A film of organic matter at the fresh/sea-water interface of an estuary. Nature, 328: 612-614.
251
Elsevier Science Publishers B.V., Amsterdam
The hydrophobic fraction of organic matter in the Krka River Estuary Vjero6ka Vojvodi6 and Bo~ena 12osovi6 Centre f o r Marine Research Zagreb, Ruder Bogkovi( Institute, Zagreb, Croatia
(Received 28 February 1991; revision accepted 28 April 1992)
ABSTRACT Vojvodi6, V. and 12osovi6, B., 1992. The hydrophobic fraction of organic matter in the Krka River Estuary. Mar. Chem., 39: 251-267. Surface active substances have been studied in the Krka River Estuary using an a.c. polarography method and various fractionation procedures. Total surface substances (SAT) were separated into (particulate) heterodispersed and dissolved fractions by filtration. The dissolved fraction was separated into hydrophobic basic and neutral components, hydrophobic acid and hydrophilic components by sorption on to XAD-8 resin. The content and the composition of SA x varied seasonally, depending on the biological activities taking place in the Krka River Estuary. The accumulation of SA T, with hydrophobic properties, occurred at the fresh/saline water interface along the vertical profile. Daily changes in the form of the surface active substances along the vertical profile were observed. The formation of the heterodispersed fraction, especially observable in the upper freshwater layer and at the halocline, resulted in the change of physico-chemical properties of surface active substances. Organic matter appeared mainly in a dissolved form in the deeper saline water layer, showing negligible oscillations. The electrochemical characterization of the fractionated SA T indicated that strong hydrophobicity of the surface active substances under natural pH conditions could be attributed to the significant contribution of the heterodispersed fraction of the organic material, which is the most reactive part of the surface active substances in the Krka River Estuary. The dissolved fraction of the surface active substances, prevailing in the deeper saline water layer, exhibited either hydrophilic or conditionally hydrophobic properties, depending on pH and ionic strength of the medium.
INTRODUCTION
The complex mixture of organic matter in the Krka River Estuary consists of organic solutes, colloids and particles coated with organic matter, as well
Correspondence to: V. Vojvodi6, Centre for Marine Research Zagreb, Ruder Bo~kovi6 Institute,
Zagreb, Croatia.
0304-4203/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
252
V. VOJVODIC AND B. COSOV1C
as various biological materials, such as phytoplankton, zooplankton, microorganisms and living and dead cells. The content, chemical composition and forms of organic matter in the Krka River Estuary depend on the river input, allochthonous organic material formed from excretion and decomposition of plankton and bacteria, as well as on dynamic transformations of various forms influenced by salinity, temperature and turbidity. Most biogeochemical processes in natural waters take place at natural phase boundaries (Parks, 1975; Hunter and Liss, 1981). Organic compounds with surface active properties are concentrated at water• atmosphere, water/sediment, and water]dispersed organic material interfaces (Liss, 1975; (2osovi6 et al., 1985; Marty et al., 1989). Surface active substances influence the structure and physico-chemical properties of natural interfaces and mediate the processes of mass and energy transfer between different phases. The fractionation of the organic substances in natural aquatic systems occurs owing to adsorption processes, resulting in elevated concentrations of strongly adsorbable substances in the interfacial region, the less adsorbable substances remaining in the bulk phase. Earlier relevant investigations carried out in other aquatic areas suggest that hydrophobic interactions play an important role in the adsorption of organic matter in natural waters (Tanford, 1973; Tschesche, 1983; Carlson et al., 1985). Bacterial adhesion on surfaces is influenced by both a hydrophobic attraction and an electrostatic repulsion (Loosdrecht et al., 1990). Regarding the origin of the organic matter, autochthonous organic material, formed by excretion and decomposition of plankton and bacteria, is presumably mainly hydrophobic, because of the lower content of dissociable groups than in the organic material derived by leaching of soil (Buffle, 1990). This work is based on a study of the seasonal and daily variations of the hydrophobic fraction of organic matter in a small stratified estuary with a low terrigeneous input. Organic substances were fractionated by sorption on a XAD-8 resin, while an electrochemical method was used for determination of the hydrophobic surfactant activity. Study area
The Krka River Estuary, located in the central part of the eastern Adriatic coast, belongs to the type of stratified estuaries characterized by a low terrigeneous and nutrient input (Skrivani6 and Gr2eti6, 1986; Gr2eti6, 1990). A sharp salinity gradient separates the upper freshwater layer from the lower saline water layer by a strong halocline ((2osovi6 et al., 1982; Zuti6 and Legovi6, 1988). Although the concentrations of dissolved organic carbon (DOC), particulate organic carbon (POC) and total suspended material (TSM) in the Krka River Estuary are considerably lower than the values reported for other
HYDROPHOBIC ORGANIC MATTER 1N THE KRKA ESTUARY
253
estuaries throughout the world, the maximum concentrations of these components are obtained at the fresh/saline water interface ((~osovi6 et al., 1982; Ketchum, 1983; Cauwet, 1991). The influence of the surface fresh water on the productivity of the deeper saline water has generally been detected, since the interfacial layer is biogeochemically a very active zone for phytogenic organic matter (Denant et al., 1991). A decrease in the chlorophyll/phaeophytin ratio was observed along the vertical profile, suggesting decomposition processes of the freshwater phytoplankton at the fresh/saline water interface. The halocline has been detected as a good barrier for the phytoplankton derived from the river water (Vili~i6 et al., 1989). The highest values for Chlorophyll a are obtained at the halocline, while a significant primary production characterizes a low-salinity water layer (Fuks et al., 1991). MATERIAL
AND
METHODS
Sampling sites and collection of samples Samples from various sites in the Krka River Estuary were collected by divers, along a longitudinal transect and in the vertical profile of the water column (Fig. 1) (Kniewald et al., 1987). A new technique of sample collection using a peristaltic pump at intervals of 2 cm in a short vertical scale around the halocline has also been applied (Cauwet, 1991). Station E-3, being under favourable hydrographic conditions and not directly subject to human activities, was chosen as a reference area.
Determination of surface active substances by a.c. polarography A phase-sensitive a.c. polarography method was used for determination of surfactant activity in the manner described elsewhere ((2osovi6 and Vojvodi6, 1982, 1987; (2osovi6 et al., 1985). High sensitivity has been achieved by enhancing the accumulation of the surfactants at the electrode surface by stirring the solution. The decrease of the capacity current (AI) for a particular accumulation period is a function of the surfactant concentration in the solution. The values of the surfactant activity are expressed in terms of surfactant equivalents of the non-ionic surface active substance Triton-X-100 (M = 600) (mg d m - 3). The capacity current measurement at the mercury electrode is a convenient method for the study of those organic substances which are adsorbed primarily owing to hydrophobic expulsion ((7osovi6, 1985). In comparison with other polarographic methods, for example the polarographic maximum method, which is better correlated with the DOC values (Hunter and Liss,
254
v. VOJVODI(7AND a. COSOVIC
30 (~
25 i Cz
2O 15 10 DISTANCE / krn Es E~. STAT ION
E3 E9
5
0 i E2
i E~
Fig. 1. The Krka River Estuary and the position of the sampling stations.
1981), the phase-sensitive a.c. polarography method is more sensitive to the hydrophobic part of the organic matter (Cosovi6, 1985). The adsorbability at the mercury/water interface is found to be in proportion to the hydrophobicity of aliphatic fatty acids of different chain lengths, as determined by their octanol/water partition coefficients (Ulrich et al., 1988). Regarding the specific sensitivity of the a.c. polarography method to the hydrophobic surfactants, special attention has been paid to the hydrophobic part of the surface active substances in the Krka River Estuary.
Fractionation of surface active substances The total surface active substances from the original sample were separated into particulate and dissolved fractions by filtration through a Whatman fibreglass filter (GF/F). Particulate organic matter generally represents particles which are larger than the pore sizes of the filter. This fraction also includes a large portion of the material dispersed in the sizes smaller than the filter pore
HYDROPHOBIC ORGANIC MATTER IN THE KRKA ESTUARY
255
dimension (Johnson and Wangersky, 1985). In this work, the fraction of the organic matter retained on the filter has been termed heterodispersed surface active substances (fraction H). The dissolved fraction (fraction D) also consists of most of the colloidal organic components (Thurman, 1985). After removing the heterodispersed fraction from the original sample, the remaining dissolved fraction represents a mixture of the hydrophobic basic and neutral components (fraction DI), hydrophobic acid (fraction D2) and hydrophilic components (fraction D3), which were then separated by sorption on to XAD-8 resin, according to the modified procedure described elsewhere (Leenheer and Huffman, 1979; Leenheer, 1981). The fractionation procedure of the dissolved fraction is illustrated in Fig. 2. Fraction D~ was separated from the mixture by sorption on to XAD-8 resin (lst column) at natural pH. The solution which remained after the sorption (fraction D 2 + D 3), was acidified to pH = 2 and applied to the 2nd column, on XAD-8 resin, in order to separate the hydrophobic acid components (fraction D2). The hydrophilic components (fraction D3), which were not sorbed on to a resin at both pH values, remained in the final effluent. The percentages of the fractions based on the surfactant activity measurements were calculated as follows: (1) Fractionation by filtration: SAT is total surfactant activity SAD is surfactant activity of dissolved fraction SAN = S A y -- SAD is surfactant activity of heterodispersed fraction (2) Fractionation of S A D by sorption on XAD-8 resin: pH = 7 SAD, = SAD - SA(D,- + D3) is surfactant activity of hydrophobic base and neutral fraction pH = 2 SAD, = SAID,- + D3) -- SAD3 is surfactant activity of hydrophobic acid fraction SAD3 is surfactant activity of hydrophilic fraction RESULTS
AND
DISCUSSION
Seasonal variation o f SAT values in the K r k a River Estuary
The values obtained for S A v in the samples collected at various estuarine stations in the Krka River Estuary during 1982-1988 (Fig. 3) are in the range between 0.04 and 0.16mgdm -3. These values are satisfactorily comparable with the values obtained in the coastal areas of the Adriatic Sea and in the North Adriatic Sea, with more intensive biological activities than in the offshore sea ((7osovi6 et al., 1985; Marty et al., 1989). The lowest SAT values were obtained in winter (0.02-0.04 mg dm-3). These values are in satisfactory agreement with the surfactant activity values
256
V. VOJVODI(~AND B. (?OSOVIC Original sample:
Surfactant activity
Total surface active substances
SAT
I
filtration on filter ~
~
Fraction H:
Fraction D:
Heterodispersed organic matter
Dissolved organic matter
....
SAH, SAD
Fraction O1: < (adsorbed on resin) Hydrophobic base and neutral components
adsorption onto XAD-8 resin pH-7
....
SAD1
L
Fraction D2+ D3: (not adsorbed on resin)
_ _ _ SAD2+D 3
Hydrophobic acid and hydrophilic components
Fraction D2+D3: acidified to pH = 2
Fraction D2:
adsorbed onto XAD-8 resin :)i-1=2
(adsorbed on resin) Hydrophobic acid components
-I-
Fraclion D3: (not a d s o d ~ on resin)
....
....
SAD2
'~D3
Hydrophllic components
Fig. 2. Analytical scheme of fractionation of organic solutes by filtration and sorption on to XAD-8 resin.
obtained for offshore areas of the Mediterranean Sea ((~osovi6 et al., 1985; Marty et al., 1989). Such low values obtained in winter could be attributed to a small allochthonous input of organic matter and low biological activity ((~osovi6, 1985; Vili6i6 et al., 1989). The SA.r values increased during the period of intensive biological activity in summer (0.08-0.16 mg dm-3), while
HYDROPHOBIC ORGANIC MATTER IN THE KRKA ESTUARY
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SURFACTANTACTIVITY/mgdrr] 3 Frequency distribution of surfactant activity (SAT) values in the samples: (a) The Krka River Estuary in different seasons; (b) spring; (c) summer; (d) autumn; (e) winter during 1 9 8 2 - 1 9 8 8 . F i g . 3.
the SAT values obtained in an early spring were lower, usually not exceeding 0 . 0 8 m g d m -3. The largest range of the SAT values (0.04-0.32mgdm -3) occurred in autumn. The seasonal variations of the SAT values in the Krka River, in the samples collected from Station E-0 (during May 1986-December 1988) are illustrated in Fig. 4. The SAT values presented by curve 1 show an inverse relationship with the flow of the Krka River (curve 2). Similar variations of the SA T values were also obtained in the samples collected from the upper freshwater layer (Station E-3 - - located downstream of the estuary) (curve 3). It is evident that
258
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1986
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Fig. 4. Seasonal variations of surfactant activity ( S A T ) values in the samples. Krka River (curve I) at the depth of 0.5 m with corresponding values of the river flow (curve 2), and Station E-3 at the depth of 0.5 (curve 3).
the SAT values at both locations (Stations E-0 and E-3) do not exceed the value o f 0 . 2 4 m g d m 3 in equivalents of Triton-X-100.
Variations of SAT values in the vertical profile of the Krka River Estuary The variations of the SAT values in the samples collected at various depths along the vertical profile during 1982-1988, are illustrated in Fig. 5. The highest values obtained at the fresh/saline water interface along the vertical profile (0.08-0.32mgdm -3) are caused by the accumulation of SAv at the fresh/saline water interface. Significantly lower SAT values obtained in the deeper saline water did not exceed 0.08 mg dm -3. Total surfactant activities in the samples collected in the upper freshwater layer (0.5 m) and in the saline water layer (6m) are similar; the values are in the range between 0.04 and 0 . 1 6 m g d m -3. The same distributions of DOC, POC, phaeophytin and Chlorophyll a, as well as surfactant activity values determined by the polarographic maximum method, have already been obtained (Zuti6 and Legovi6, 1988; Cosovi6 and Vojvodi6, 1989; Vili6i6 et al., 1989). Since the stratified estuary is subject to dramatic changes in physico-chemical conditions on a short vertical scale at the halocline, the samples were collected along the salinity gradient at intervals of 2 cm. The samples were collected from Station E-2, on 16 May 1988. The results presented in Fig. 6 show that in the layer between 2.02 and 2.08 m, with a sharp salinity gradient (8-32%o), the SA T value increased from 0.27 to 0.38 m g d m -3. In comparison with the values observed in both upper freshwater layer and deeper saline layer (< 0 . 1 m g d m - 3 ) , the SAT values at the halocline are significantly higher. However, the values changed slightly on a short vertical scale at the halocline, in spite of the presence of a sharp salinity gradient. If one compares the SAT values with the DOC and POC measurements of the same samples (Cauwet, 1991) the following conclusion can be drawn. On
HYDROPHOBIC O R G A N I C M A T T E R IN THE KRKA ESTUARY
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259
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Fig. 5. Frequency distribution of surfactant activity (SAT) values in the Krka River Estuary during 1982-1988 in the samples collected at different depths: (a) surface, 0.5 m; (b) fresh/saline interface; (c) saline water, 6.0m; and (d) more than 10.0m.
a short vertical scale at the halocline, the SAT values increased three to four times with respect to both the upper freshwater layer and deeper saline water layer. The DOC values increased less than two times, while the variations of the POC values were similar (but not identical) to those of the SAT values. The same effect can be observed comparing the SAT and SAD (filtered samples) values obtained along the vertical profile; theSAD values are generally lower than the SAT values, and the accumulation at the halocline is less visible. The results given in Table 1 show that more than 80% of the surfactant activity is in the dissolved fraction, in saline water layer (S = 35-37%o), i.e. the SAT values are negligibly higher than the SAD values. In the upper freshwater layer, with a large range of salinity values between 3.8 and 24%0, the SAT values are significantly higher than the SAD values; between 30 and 75% of the SA x originated from the fraction H. The organic material appearing in the heterodispersed form in the upper water layer accumulates at the halocline, where it can be removed by decomposition processes as well as by the river water flow. Since only a small part of the fraction H has been found in the saline water, especially in the deeper saline layer, it is evident that
260
V. VOJVODIC AND B. COSOVIC 10
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Fig. 6. Variation o f surfactant activity (SA T) values on a short scale o f the vertical profile at the halocline at Station E-2 on 16 May 1988, c o m p a r e d with the variations o f the D O C and P O C values observed by Cauwet (1991, Fig 7).
TABLE I Fractionation of surface active substances by filtration Date in 1988
12.05 13.05
15.05
16.05
Depth (m)
3.60 2.60 1.60 2.35 3.35 3.60 2.00 2.15 3.00 2.00 2.15
Salinity (%0)
37.0 10.5 3.8 10.0 36.0 37.0 5.0 24.0 35.0 5.0 24.0
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Dissolved
7. I 69.9 75.5 67.9 16.0 16.7 59.4 56.2 42.9 31.5
92.9 30.1 24.5 32.1 84.0 83.3 40.6 43.8 I00.0 57.1 68.5
H Y D R O P H O B I C O R G A N I C M A T T E R IN T H E K R K A E S T U A R Y
261
sinking of the aggregates through the fresh/saline water interface is below significance. It is important to note that the strong adsorbability and the hydrophobic character of the heterodispersed fraction in the Krka River Estuary are to a great extent responsible for the association and aggregation processes of the organic matter, and, probably, for the stability of the fresh/saline water interface.
Daily variations of S A t and SAo values in the Krka River Estuary Daily variations of the S A T and S A D values along the vertical profile were studied at Station E-3 for 24 h on 12/13 May 1988. The samples were collected every 6 h at the following depths: 0.3, 1.6, 2.5 m (fresh/saline water interface), 3.5, 5 and 20m. The salinity values of the samples collected at the depth of 3.5 m and deeper were higher than 35%0, while the values obtained for the samples collected at the depths of 0.3 and 1.6 m in the upper freshwater layer were between 2 and 4%o. The salinity values obtained at the fresh/saline water interface were between 10 and 12%o, with the exception of the samples collected at 18:00 and 00:00 h on 12 May when higher values were obtained, 20%0 and 28%o, respectively. The results of the SAy variations are illustrated in Fig. 7. In the saline water layer (depth of > 3.5 m) the SA-r values were constant during the period investigated, with a slight increase from the bottom at a depth of 20 m to the upper water layer at a depth of 3.5 m. A dramatic increase of the SAv values occurred between 0:00 and 06:00 h in the freshwater layer, at the depths of 0.3 and 1.6 m, especially at the fresh/saline water interface at a depth of 2.5 m. During the following 6 h, the SA T values decreased significantly, reaching the values previously obtained in the samples collected from the freshwater layer, but retaining a high value at the fresh/saline water interface. The oscillations of the SA T values at the fresh/saline water interface are in satisfactory agreement with the changes in the Chlorophyll a values obtained in the same samples (Fuks et al., 1991), indicating the importance of the biological activity in the SAT variation during the day. Daily variations of the SAT values along the vertical profile were compared with the DOC and POC values obtained by Cauwet (1991, Fig. 6). The variations of the POC values were similar to those of the SAT and Chlorophyll a values, while the variations of the DOC values obtained in the same, but filtered samples, differed from the SAT and Chlorophyll a values. The increase of the SAT and POC values in the upper freshwater layer and at the fresh/saline water interface during 24 h suggests the aggregation processes of the organic matter. The hydrophobic properties of this type of organic material (fraction H) are responsible for variations in the SAT values during the day, in spite of a low total organic matter content in the Krka River Estuary.
262
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Qualitative changes of the adsorbable organic substances during the day have also been observed. The capacity current-potential curves obtained in samples collected from a depth of 1.6m at 00:00, 06:00 and 12:00h are illustrated in Fig. 8. The shapes of polarographic curves of the samples collected at 00:00 and 12:00h (Figs. 8(a) and (c)) are similar; the same is evident for the SAT values given in Table 2. On the contrary, the capacity current-potential curves of the samples collected at 06:00 h (curves 1, 2 and 3 in Fig. 8(b)) changed visibly, indicating different physico-chemical properties of the surface active material. In these samples the SAT value increased about four times in comparison with the samples collected during other sampling periods. The SA D values showed no variations during the day. Fractionation of surface active substances
Total surface active substances in the samples collected along the vertical profile from Station E-3 on 18 May 1988, were separated into H, D, D~, D2 and D 3 fractions, according to the procedure illustrated in Fig. 2. The results
HYDROPHOBIC ORGANIC MATTER 1N THE KRKA ESTUARY
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TABLE 2 Variations of S A 1988 Time (h)
00:00 06:00 12:00
T
and
S A D values
in the samples collected at station E-3 from a depth of 1.6 m on 13 May
Surfactant activity in equivalents of T-X-100 (mgdm 3) SA T
SA D
O. 11
O. 11
0.44 0.13
O. 11 0.12
V. VOJVODI(~ AND a. ~'OSOVIC
264 TABLE 3
Fractionation of organic solutes by sorption on XAD-8 resin in the samples collected at station E-3 on 15 May 1988 Depth (m)
Salinity (%o)
Surfactant activity in equivalents of T-X-100 ( m g d m ~) SA v
2.0
5.0
0.114
Fractions (%) H
DI
D2
D3
43.0
19.7
23.6
13.7
SAn ~
0.065
(o. 128) 2.15 fresh/saline interface 3.00
24.0
0.270
O. 185 (0.250)
31.5
16.6
19.1
32.8
31.0
0.065
0.085 (0.160)
0
17.5
64.0
18.5
~'Values given in parentheses are obtained by a.c. polarography at pH = 2 .
of the surfactant activity values and percentages of all the fractions determined by an a.c. polarography method are given in Table 3. Fraction H was higher in the upper freshwater layer than at the halocline, while fraction D prevailed in the deeper saline water, i.e. the heterodispersed fraction has not been detected. Higher surfactant activity values of fraction D were mainly observed at pH = 2, which is in satisfactory agreement with the behaviour of naturally occurring polyelectrolytes, such as humic and fulvic organic material (Cleven, 1984; Cosovi~ and Vojvodi~, 1989). The percentages of fraction D, adsorbable on a resin at natural pH, were similar in all three samples, not exceeding the value of 20%. The highest value of the fraction D 2, adsorbable at pH = 2, was obtained in the saline water layer. The fraction D 3 value was not higher than 20% in the freshwater and saline water layers, whereas it was around 30% at the fresh/saline interface. The data obtained show that total surface active substances in the Krka River Estuary are mainly hydrophobic under natural conditions owing to the significant contribution of fraction H. On the contrary, fraction D, consisting mainly of the fractions D2 and D3, shows either hydrophilic or conditionally hydrophobic properties depending on pH and on ionic strength of the medium. In spite of low concentrations of organic matter in the Krka River Estuary, very high oscillations of the SAT values observed especially at the halocline, are influenced by the hydrophobic character of fraction H. The vertical variations of fraction D, which contains only 20% of the hydrophobic components, are much lower and comparable with the variations of the DOC
HYDROPHOBIC ORGANIC MATTER IN THE KRKA ESTUARY
265
values. Hydrophobicity of the fraction D probably originates from the colloidally dispersed organic matter, which is the subject of further studies. CONCLUSIONS
The concentrations of the total surface active substances in the Krka River Estuary are generally low, but show significant oscillations. The seasonal variations of the SAT values strongly depend on the biological activity; the same applies for the karstic Krka River, where the SAT values also vary with the flow of the river. Surface active substances are spontaneously fractionated in the vertical profile by accumulating at the fresh/saline water interface of the Krka River Estuary. The daily changes of the form of the surface active substances along the vertical profile were observed. The process of the formation of the heterodispersed fraction, especially observable in the upper freshwater layer and at the halocline, results in a change of the physico-chemical properties of the surface active substances. Organic matter prevails in a dissolved form in the deeper saline water layer, showing negligible oscillations. Electrochemical characterization of the fractionated SA T in the Krka River Estuary shows that the surface active substances in the upper freshwater layer and at the halocline are mostly hydrophobic under natural conditions owing to the significant contribution of the heterodispersed fraction of the organic matter. Fraction H is mostly responsible for the variations in total surfactant activity of the samples and, thus, represents the most reactive part of the organic matter in the Krka River Estuary. The dissolved fraction of the surface active substances, which prevails in the deeper saline water layer, exhibits either hydrophilic or conditionally hydrophobic properties depending on pH and an ionic strength of the medium. ACKNOWLEDGEMENTS
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