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

Bottom-mounted acoustic Doppler current profiler measurements indicate that the net transport of water (844 m(3) 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.