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1

Transport and hydraulically-induced recycling of phosphorus in the North Sea-Baltic Sea transition zone

Kepp R.R., Struve A., Christiansen C., Lund-Hansen L.C., Nielsen M.H., Vang T.

Oceanologia

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2006

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No. 48 (2)

175-191

EN

Bottom-mounted acoustic Doppler current profiler measurements indicate that the net transport of water (844 m3 s-1) in the Little Belt makes up only 6% of the total transport between the Baltic Sea and the North Sea. This is a smaller percentage than the 9% commonly found in the literature. Owing to barotropic and tidal currents the gross transport is 5 times larger. The net transport is directed towards the North Sea mainly in the top 32 m of the water column but towards the Baltic Sea it occurs in the lower 5 m of the water column. The resulting transport of phosphorus is strongly affected by vertical mixing in an area of hydraulic control in the narrow part of the Little Belt. Comparisons of phosphorus profiles in stratified waters and in the mixing area indicate a yearly entrainment of 15 tonnes P from the bottom water to the surface layer. This vertical transport of P forms part of an internal loop in the general transport between the Baltic Sea and the North Sea. Compared to the transport observed 15-16 years ago, the present net phosphorus transport of 163 tonnes yr-1 from the Baltic Sea through the Little Belt is substantially lower.

2

Development and dynamics of a coastal sub-surface phytoplankton bloom in the southwest Kattegat, Baltic Sea

Lund-Hansen L.C.

Oceanologia

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2006

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No. 48 (3)

433-446

EN

The study was based on CTD-casts performed on 5 consecutive days at 7 positions along a shallow- to deep-water transect during the spring bloom in the southwest Kattegat. The development, dynamics and fate of the sub-surface chlorophyll maximum (SCM) are described. The SCM developed at a constant rate of 1.4 mg chl a m-3 d-1, which is equal to a carbon fixation rate of 49.0 mg C m-3 d-1. The SCM developed at 6 m depth at low irradiance levels (10-43.0 žE m-2 s-1). Chl a concentrations reached up to 23 mg chl a m-3 in the centre of the SCM. Growth rates in the SCM were estimated at 0.82 d-1. The water column was only weakly stratified, with nutrients in excess in the whole of the water column. The SCM was relocated vertically to depths below the photic zone by wind-induced advection between the bay and the outer Kattegat. There were indications of a horizontal relocation of the SCM by an internal standing wave. Application of high spatial and temporal resolution made it possible to estimate key SCM parameters, such as growth rates [mg chl a m-3 d-1] and carbon fixation rate [mg C m-3 d-1], on the basis of chl a variations in time and space.

3

Deposition of organic matter and particulate nitrogen and phosphorus at the North Sea-Baltic Sea transition - a GIS study

Jansen D. L., Lundqvist D. P., Christiansen C., Lund-Hansen L.C., Balstrom T., Leipe T.

Oceanologia

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2003

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No. 45 (2)

283-303

EN

A GIS (Geographical Information System) based study on deposition in the North Sea-Baltic Sea transition area has been carried out. The study is based on (i) a digital bathymetry model, (ii) 93 available 210Pb / 137Cs sedimentation rate estimations, (iii) grain-size distributions, organic matter, C, N and P content of 64 top 1 cm sediment samples from the study area, and (iv) GIS-based modelling of resuspension potentials based on wind statistics. With the use of regression statistics on depth, resuspension potential and sediment characteristics, results are extrapolated area-wide from the 64 sampling positions. The area is divided into sediment types and classified as accumulation or erosion/transport bottoms. Model results show good agreement with existing maps of sediment distributions, indicating that the sediment distribution is governed to a large extent by wind-induced waves. Correlations of sediment types, their deposition rates and their N and P contents were used to estimate spatial deposition rates. In all, the yearly deposition in the study area amounts to 2.8 million tons of organic matter, 0.14 million tons of total nitrogen, and 0.035 million tons of total phosphorus. Correlations of sediment types and dry bulk densities were used to infer spatial inventories of organic matter and total nitrogen and phosphorus in the top 1 cm of the sediments. A total of 100 million tons of organic matter, 4 million tons of total nitrogen, and 0.019 million tons of total phosphorus are contained in the top 1 cm of the sediments in the study area. In general, the deep parts of the study area with low resuspension potentials act as sinks for the fine-grained sediments and their associated particulate nutrients.

4

Near-bottom fluxes and composition of suspended matter in the Pomeranian Bay

Laima M. J., Lund-Hansen L.C., Pazdro K., Christiansen C., Emeis K. C.

Oceanologia

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1999

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No. 41 (3)

335-353

EN

The quality and composition of suspended organic matter in near-bottom fluxes was determined at a mooring station (Odas Tonne) 20 km north-north-west of the Odra river mouth from June to December 1997. Salinity data and high concentrations of suspended matter near the bottom showed that the material entering the Pomeranian Bay from the Odra flood was recognisable for about three weeks. Vertical sediment fluxes, however, were low ~ 40 g m-2 d-1 compared to those measured later in the year ~ 60 g m-2 d-1. On the other hand, average molar CNP ratios in sediment trap material decreased from June to December 1997. These results may have been a combined effect of dilution and material transport in a layer close to the sediment surface. Fluff layers sampled at Odas Tonne in August 1997 contained a very high proportion of branched fatty acids of bacterial origin, indicating high rates of bacterial degradation. Long-chain fatty acids indicated an origin from higher terrestrial plants. The saturated fatty acid content was high in the surface sediment and the traps, increasing towards the top trap. The percentage composition of fatty acids indicated that the lowest trap was fed mainly by material from the underlying sediment. Low salinities, variability in molar ratios for major elements, higher than usual bacterial activities and detection of fatty acids characteristic of land plants during the June-August deployment show a relationship with the Odra flood of summer 1997.