Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology

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1 Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology Volume 42, Issue 2 ISSN X ( ) eissn DOI: /s x Original research paper Received: Accepted: July 13, 2012 November 22, 2012 Sedimentation of suspensions in the Vistula River mouth Ewa Szymczak *, Dorota Galińska Institute of Oceanography, University of Gdańsk Department of Marine Geology Al. Marszałka Piłsudskiego 46, Gdynia, Poland Key words: Suspended sediment concentration, salinity, Vistula River, Gulf of Gdańsk Abstract The aim of this study was to analyze the variability of the suspension concentration in the area where the Vistula River discharges into the Gulf of Gdańsk. While analyzing the relationship between suspension concentration and the distance from the river mouth and the spatial location in the water column, a number of other important factors were considered, i.e. salinity, temperature, the composition of suspension and in situ hydrodynamic conditions. The samples of surficial sediment were analyzed with regard to the content of organic matter and <0.063 mm size fraction. * Corresponding author: e.szymczak@ug.edu.pl INTRODUCTION River mouth areas are unique locations, characterized by the mixing of freshwater and seawater, where the interconnected and complex chemical, biological, hydrodynamic and sedimentation processes can be investigated. The Vistula water flow feeding into the Gulf of Gdańsk and carrying sedimentary material extends horizontally from 2 to 15 nautical miles from the river mouth, and has a vertical range of m (Cyberska, Krzymiński 1988, Cyberski, Krężel 1993, Grelowski, Wojewódzki 1996). The variability of the distance the riverine waters travel into the gulf mainly depends on the variability of hydrological and meteorological factors. A hydrological front, which has been defined as a transition layer between water masses characterized by different velocities and flow directions, forms in the mixing zone (Magnuszewski, Soczyńska 2001) and the changes in salinity and temperature values are visible. Under such variable conditions the boundary between freshwater and seawater becomes a trap that obstructs further movement of suspensions originating from riverine discharge (Leeder 1982). Clay minerals (<0.002 mm) introduced with the suspended material display specific properties in water that are characteristic of colloidal solutions. The most significant feature is the propensity to flocculate, which results from interactions between particles with certain electrical charges. Flocculation occurs due to ph change or the addition of a specific amount of salt (Stoch 1974). It is a process wherein particles clump together into large loose aggregates called flocs. The floc settling velocities are higher than those of single grains and particles (Kranck 1984). It is believed that flocculation is enhanced by the presence of strongly cohesive organic matter in suspension (Kranck 1979, Droppo et al. 1998). Besides the presence of clay

196 Ewa Szymczak, Dorota Galińska minerals, organic matter, and variable chemical conditions, the violent mixing of water masses, which increases the frequency of particle collisions, is also a

2 196 Ewa Szymczak, Dorota Galińska minerals, organic matter, and variable chemical conditions, the violent mixing of water masses, which increases the frequency of particle collisions, is also a prerequisite for the occurrence of flocculation (Eisma 1991). Many important studies on the concentration and transport of suspensions in the Gulf of Gdańsk have been published (Jonasz 1993; Cyberski, Krężel 1993; Bradtke 1997; Bradtke et al. 1997; Burska, Graca 2011). MATERIALS AND METHODS Samples of seawater and surficial sediments were collected in the river mouth area of the Vistula Przekop during three scientific cruises, on March 23, June 23 and November 30, 2010, on board the cutter r/v Oceanograf II. The location of sampling stations varied from ca. 1 to 7 km away from the Vistula river mouth. The water depth at sampling sites ranged between 4 and 22 m (Fig. 1). Due to poor weather and technical limitations it was possible to collect samples only at sites deeper than 10 m. Water sampling stations were chosen based on the in situ hydrodynamic conditions (direction of surface currents) in this particular part of the Gulf of Gdańsk. At each sampling site, water samples were collected from the surface layer, a point half way between the surface and the bottom, and just above the bottom using a bathometer. The water temperature and conductivity were measured immediately after sampling by means of an Elmetron CC-411. Surface sediment was collected using the Van Veen grab. Laboratory analyses of seawater consisted of determinations of the amount of suspension, and the shares of mineral and organic components. The grain-size composition of bed sediments were determined with a sieve analysis which allowed the construction of a grain-size distribution curve, determination of sediment type, and the calculation of pertinent coefficients. The content of organic matter in suspension and sediment was determined by the loss-on-ignition (LOI) method. STUDY AREA The immediate study area was a stretch of the Vistula Przekop adjacent to the river mouth (Fig. 1) through which 90% of the river s water flows to the Gulf of Gdańsk. The Vistula Przekop is an artificial Fig. 1. Location of sampling sites and profiles in the outlet of Vistula Przekop. estuary, located near Świbno, that was constructed in the years by digging a canal between Przegalina and the Gulf of Gdańsk. The flow at the mouth of the Vistula Przekop ranges from 340 to 4100 m 3 s -1 with the mean value of 1054 m 3 s -1. Each year, the Vistula River discharges ca. 30 km³ of freshwater into the Gulf of Gdańsk, which constitutes 87% of the annual runoff estimated at 34.5 km³ by Majewski (1975). The Vistula River transports sedimentary material into the Gulf of Gdańsk. It has been estimated that the volume of the material carried by the Vistula River entering the estuary equals 5 t km -2 year -1 (Łajczak 1999). This gives the annual load ranging from ton (Łajczak 2005) to ton (Brański 1972), and amounts to 19.9% of all transported material discharged into the Baltic Sea. Grain-size analysis of the material transported along the lower stretch of the Vistula River (Brański 1975) and in the stretch between Tczew and Świbno (Cyberski 1982) shows that it consists of the size fraction between and 0.6 mm. The silt fraction constitutes the biggest share (over 55%), while medium and fine sands amount to 30%. Close to 15% of the material transported in suspension belongs to <0.002 mm size fraction. Material dragged by the river is composed of fine

Sedimentation of suspensions in the Vistula River mouth 197 and medium sands whose content is 35 and 45%, respectively. Coarse sands and gravel comprise the remaining 20%.

The load of material transported by the river varies throughout the year, and is related to the complex hydrological regime of the Vistula.

3 Sedimentation of suspensions in the Vistula River mouth 197 and medium sands whose content is 35 and 45%, respectively. Coarse sands and gravel comprise the remaining 20%. Along the stretch between Tczew and the Vistula river mouth, Cyberski (1982) observed a general tendency for decreasing grain size with decreasing distance from the estuary. The load of material transported by the river varies throughout the year, and is related to the complex hydrological regime of the Vistula. This in turn results from the specific geographical location of the catchment area; namely, in the temperate climatic zone characterized by transition between the oceanic and continental climate sub-types. When the Vistula River swells it discharges large amounts of transported material into the Baltic, at times exceeding the mean annual load. This phenomenon has been limited to a large extent by human activities, e.g. the Włocławek dam retains ca. 50% of the material transported by the river (Łajczak et. al 2008). The alluvial fan in the Vistula river mouth grows constantly because it receives a plentiful supply of solid material. A big share of the transported material remains there, which results in the formation of a new outer delta. Fig. 2. Suspension concentration (g m -3 ) in the water column along profile I. RESULTS AND DISCUSSION Suspension concentration in the water column in the frontal area of the Vistula outlet showed high variability during the study period. The values measured in specific sampling seasons in both surface water and in the near-bottom layer ranged widely (from 3.00 to 26.28, and from 2.32 to g m -3, respectively). In March, the content of suspension in surface waters varied from 8.3 to g m -3. The suspension concentration level at depths >6 m was similar, but with lower extreme values, i.e and 6.04 g m -3. In surface waters the isolines of suspension concentration were parallel (Fig. 2). A similar pattern was observed for isohalines although salinity values increased with increasing distance from the river mouth (Fig. 3). At depths >4 m suspension concentration increased with increasing distance from the river mouth, which suggests gradual settling of suspension from the surface waters. The highest values of ca. 5.5 g m -3 were observed 1.35 to 2.0 km away from the river mouth. The surface currents from the south and south eastern directions with velocities of ca. 5 cm s -1 enhanced the spreading of the riverine waters during the study period. Fig. 3. Salinity (PSU) distribution in the water column along profile I. The year 2010 was not typical for hydrological processes. In June, a high concentration of suspension was measured in connection with flood waters that had reached the Gulf of Gdańsk in May and June. Suspension concentration in this period varied widely from 1.08 to g m -3. Along the entire profile there was a visible decrease in suspension concentration with increasing distance

198 Ewa Szymczak, Dorota Galińska from the river mouth and increasing depth. The drop was very pronounced for depths >4 m at a distance of ca. 200 m from the river mouth (Fig. 4).

5). In June, currents from N and NNE directions with velocities between 5 and 15 cm s -1 were dominant; as a result, the mixing of freshwater and seawater was more intense.

Because of westerly surface currents a drop in suspension concentration from 8.6 to 3.2 g m -3 was observed in the easterly direction from the outlet (Fig. 6).

4 198 Ewa Szymczak, Dorota Galińska from the river mouth and increasing depth. The drop was very pronounced for depths >4 m at a distance of ca. 200 m from the river mouth (Fig. 4). A large inflow of freshwater in connection with flooding events resulted in decreased salinity (<2 PSU) of surface waters. The bottom waters were characterized by higher salinity, i.e. >8 PSU (Fig. 5). In June, currents from N and NNE directions with velocities between 5 and 15 cm s -1 were dominant; as a result, the mixing of freshwater and seawater was more intense. The measurements obtained in November also indicated a gradual dissipitation of the transported material with increasing distance from the river mouth. Because of westerly surface currents a drop in suspension concentration from 8.6 to 3.2 g m -3 was observed in the easterly direction from the outlet (Fig. 6). As in other cases, surface waters remained under the influence of the Vistula River freshwater. Therefore salinity in the surface layer was decreased (Fig. 7). Fig. 6. Suspension concentration (g m -3 ) in the water column along profile III. Fig. 4. Suspension concentration (g m -3 ) in the water column along profile II. Fig. 7. Salinity (PSU) distribution in the water column along profile III. Fig. 5. Salinity (PSU) distribution in the water column along profile II. Regardless of the season, the highest suspension concentrations were measured in surface waters. When analyzing the graphs, it should be noted that a drop in suspension concentration in the water column with higher salinity is sharper than in surface waters. Based on the analysis of changes in suspension concentration and salinity with depth at specific sampling sites (Fig. 8), it was noticed that at a depth of 2-6 m (max. 8 m) the increase in salinity was accompanied by decreased suspension concentration, while at 6-12 m depth suspension concentration increased gradually. The increased chloride concentration in this zone influenced stronger interactions between particles which had formed

$The effects of flocculation of mineral particles were less pronounced because benthic sediments in the vicinity of the Vistula river mouth contain very little clay and silt fraction.$

5 Sedimentation of suspensions in the Vistula River mouth 199 (Fig. 9). Organic matter content in benthic sediment ranged from 0.6 to 5.5%. The top values of organic matter concentration can be observed in north western and eastern directions from the river mouth. The effects of flocculation of mineral particles were less pronounced because benthic sediments in the vicinity of the Vistula river mouth contain very little clay and silt fraction. The mean share of this size fraction reached ca. 2.2%. A comparison of the content of <0.063 mm fraction and organic matter content showed that the share of clay-silt fraction increases with increasing organic matter content in sediment (Fig. 9. and Fig. 10). This relationship is particularly strong northeast and east northeast from the river mouth, i.e. along the two dominant directions in which the Vistula River waters fan out from the outlet. Fig. 8. Changes of suspension concentration and salinity with the depth. loose aggregates. Aggregate settling velocities are higher than those of single grains and particles (Kranck 1984). A slight increase in suspension concentration was observed in the bottom layer. Suspensions were also analyzed in relation to the percentage content of mineral and organic components. The data showed that mineral suspension dominated over organic; its concentration ranged from 55 to 97%. During its descent to the bottom the strongly cohesive organic suspension participated in flocculation processes, and therefore it significantly affected the amount and quality of total suspension in the water column. The percentage content of organic suspension in total suspension in the water column was the highest close to the water surface and in the near-bottom layer. The increased level of suspension concentration above the bottom could result from the resuspension of a mobile layer which is located on the surface of the benthic sediment. The layer is deposited under calm weather conditions. It consists of loose aggregates characterized by high porosity and low density which, in consequence, allows for easy resuspension by currents with low velocities (Emeis et al. 2002). Resuspension of the sediments occurred due to both currents and wave action in shallow areas of this part Gulf of Gdańsk. In the Vistula river mouth organic particles probably undergo flocculation, resulting in the increased organic matter content in benthic sediment Fig. 9. Share (%) of silt and clay fraction in the surface layer of bottom sediments. Fig. 10. Share (%) of organic matter in the surface layer of bottom sediments. Also the CM Diagram (Passega, Byramjee 1969) gave indications of the mechanism of sediment deposition in the Vistula river mouth. Most of the sediment samples, taken in March and June, fall in section I (Fig. 11), which indicates rolling transport with some suspension. Fields IV and V are samples taken in June and November. These fields suggest transportation under a graded suspension regime. The mechanism of sediment deposition is related to the higher discharge and larger mass of sediment

6 200 Ewa Szymczak, Dorota Galińska transported as bed load in spring. In autumn, due to a lower water discharge, sediment is transported mainly in suspension. The probable location of the hydrological front (Fig. 12) was plotted based on the analysis of suspension concentration, salinity, organic matter content in sediment and in suspension, and the share of <0.063 mm size fraction in sediment. Our research suggests that the front positioned 2 to 4 km away from the Vistula river mouth. This location coincides with the hydrological front defined by Nowacki and Matciak (1996). Those authors positioned the front close to PSU isohalines using salinity measurements. CONCLUSIONS Fig. 11. CM diagram of Vistula river mouth deposits. Suspension concentration in the Gulf of Gdańsk waters within the Vistula river mouth area displays seasonal variability which results from the annual cycle of suspended load transport. Suspension concentration level decreases with increasing distance from the river mouth, and it is depth-dependent. The observed distribution of salinity and suspension concentration indicates that freshwater is spreading into a fan shape in the surface layer. The dissipation of suspension occurs along the dominant directions of the river water movement, i.e. northwest and east northeast. Under this scenario suspended fractions (<0.5 mm) carried by water flow are transported away from the river outlet while undergoing gradual sedimentation. The changes in suspension concentration and the removal of suspended particles (<0.063mm) from the water column are caused by flocculation in the zone of a density jump. Strongly cohesive organic matter tends to exist as flocs when entrained into suspension. Below the aforementioned zone, suspension concentration in the water column decreases. It increases again in the near-bottom layer which may result from sediment resuspension. Bottom sediments located below the zone of salinity, temperature, and suspension concentration jumps become enriched in clay and silt fractions. REFERENCES Fig. 12. Defined hydrological front in the Vistula River mouth area. Bradtke K. (1997). Simulation of suspended particulate matter transport in the Gulf of Gdańsk during Oceanological Studies XXVI (4), Bradtke K., Latała A. & Czabański P. (1997). Temporal and spatial variations In particle concentrations and size distributions in the Gulf of Gdańsk. Oceanological Studies XXVI (2-3), Brański J. (1972). The suspended sediment budget along the Vistula River. Prace PIHM 3. (in Polish) Brański J. (1975). Evaluation of the Vistula river basin denudation based on the results of measurements of suspended sediment. Prace PIHM 6, (in Polish) Burska D. Graca B. (2011). Carbon and biogenic substances in suspended matter. In Sz. Uścinowicz (Eds.), Geochemistry of

7 Sedimentation of suspensions in the Vistula River mouth 201 Baltic Sea surface sediments (pp ), Warsaw: Polish Geological Institute. Cyberska B. & Krzymiński W. (1988). Extension of the Vistula River water in the Gulf of Gdańsk. 16 th Conference of Baltic Oceanographers, September 1988 (pp ). Kiel, Institute of Marine Research, Kiel University. Cyberski J. (1982). Hydrological characteristics. In B. Augustowski (Eds.), The lower Vistula River valley (pp ). Gdańsk GTN: Ossolineum. (in Polish) Cyberski J., Krężel A. (1993). Influence of the Vistula river on suspended matter content in the Gulf of Gdańsk waters. Studia i Materiały Oceanologiczne 64, Marine Pollution 3, Droppo I. G., Jeffries D., Jaskot C. & Backus S. (1998). The prevalence of freshwater flocculation in cold regions: a study from Mackenzie River Delta, Northwest Territories, Canada, Arctic 51 (2), Eisma D. (1991). Particle size of suspended matter in estuaries. Geo-Marine Letters 11, Emeis K., Christiansen C., Edelvang K., Jähmilch S., Kozuch J., Laima M., Leipe T., Löffler A., Lund-Hansen L. C., Miltner A., Pazdro K., Pempkowiak J., Pollehne F., Shimmield T., Voss M. &Witt G. (2002). Material transport from the near shore to the basinal environment in the southern Baltic Sea. II: Synthesis of the data on origin and properties of material. Journal of Marine System 35, Grelowski A. & Wojewódzki T. (1996). The impact of the Vistula River on the hydrological conditions In the Gulf of Gdańsk in Bulletin of the Sea Fisheries Institute 137, Jonasz M. (1983). Particle size distribution in the Baltic. Tellus 35B, 5, Kranck K. (1979). Dynamics and distribution of suspended particulate matter in the St. Lawrence Estuary. Naturaliste Canada 106, Kranck K. (1984). The role of flocculation in the filtering of particulate matter in estuaries. In V. S. Kennedy (Eds.), The estuary as a filter,(pp ). Orlando, Academic Press. Leeder M. R. (1982). Sedimentology process and products. London, George Allen & Unwin. Łajczak A. (1999). Recent transport and sedimentation of suspended material in the Vistula River and major tributaries. Monografia Komitetu Gospodarki Wodnej PAN 15, Warszawa, Warszawska Drukarnia Naukowa Polskiej Akademii Nauk. (in Polish) Łajczak A. (2005). The role of the Vistula and Odra River in the delivery of suspended sediment to the Baltic Sea. In J.P. Girjatowicz & Cz. Koźmiński (Eds.), Hydrographic and meteorological aspects of research on the Baltic coast and in selected areas of Poland (pp ), Szczecin, Uniwersytet Szczeciński, Instytut Nauk o Morzu, Polskie Towarzystwo Geograficzne Oddział Szczeciński. (in Polish) Magnuszewski A. & Soczyńska U. (2001). International dictionary of hydrology. Warsaw, Wyd. Nauk. PWN. (in Polish) Majewski A. (1975). Hydrological characteristics of Polish estuary waters. Prace PIHM 105, (in Polish) Nowacki J. & Matciak M. (1996). Hydrological conditions in the area of the Vistula River water front. Przegląd Geofizyczny 41(4), (in Polish) Passega R. (1964). Grain-size representation by CM patterns as a geological tool. Journal of Sedimentary Petrology 34, Passega R. (1977). Significance of CM diagrams of sediments deposited by suspension. Sedimentology 24, Passega R., Byramjee R. (1969). Grain size image of clastic deposits. Sedimentology 13, Stoch L. (1974). Clay minerals. Warsaw, Wydawnictwo Geologiczne. (in Polish)

Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology

Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology Volume 42, Issue 2 ISSN 1730-413X (216 224) eissn 1897-3191 2013 DOI: 10.2478/s13545-013-0074-y Short communication