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Influence of density stratification on effluent plume dynamics

Lončar G., Beg Paklar G., Janeković I.

Oceanologia

2011

No. 53 (2)

565-585

In this study we modelled sea temperature (T), salinity (S) and density field dynamics using a 3D numerical model applied to Rijeka Bay (Croatia) in order to explore their effect on effluent plume dynamics in the vicinity of four submarine sewage outfalls when the bora wind (NE direction) is blowing. The vertical density stratification in the area studied is strongly related to the bora wind, which contributes significantly to the lowering of the pycnocline depth through enhanced mixing in the vertical, giving rise to changes in the neutral buoyancy level. The features of near-field plume dynamics were calculated with the use of a separate near-field numerical model, using information on the vertical density distribution previously calculated using a 3D numerical model. The results of the numerical simulations and statistical analysis of the wind data indicate a very low probability of complete water column homogenization and consequent effluent plume rise to the sea surface under the influence of the bora wind during the peak tourist season (May-September).

Diurnal variations in nitrogen, phosphorus and iron compounds in the southern Baltic Sea

Sikorowicz G., Falkowska L., Burska D., Dunajska D., Pryputniewicz D., Magulski R., Lewandowska A.

Oceanologia

2005

No. 47 (2)

243-263

In order to assess their short-term variability, nutrient concentrations were measured at standard depths at 2 h intervals in the deepest region of the Gdańsk Deep during the first ten days of June 2001. The mean concentrations of nutrients in the euphotic zone were: NN (NO_2-, NO_3-, NH_4+) - 1.93 žmol dm^-3, PO_4^3- - 0.12 žmol dm^-3 and Fetot - 0.11 žmol dm^-3. During daylight hours, when the rate of assimilation was fastest, concentrations of nitrogen, phosphorus and iron compounds were very low. The phosphate concentration fell to a minimum (0.03 žmol dm^-3) between 04:00 and 10:00 hrs, while total iron dropped to 0.01 žmol dm^-3 between 10:00 and 16:00 hrs. Both levels were below the limiting values for phosphorus and iron. At night, concentrations of NO_3- and PO_4^3- rose by 25%, those of NH_4+ and Fetot by 35%. The mean molar ratios of N_N:PO_4^3- and Fetot:PO_4^3- in the surface layer were subject to significant daily fluctuations. The molar N_N:PO_4^3- ratio was higher than the optimum value established for the Baltic Sea. Below the halocline, the concentrations of dissolved iron and phosphorus rose as a result of diffusion from sediments in response to changing redox conditions.