Bornholm Basin

Direct estimates of diapycnal exchange coefficients have been made by Kullenberg (1977) from dispersion measurements of injected dye tracer in the thermoline and halocline of the Arkona Basin and the Bornholm Basin in the Baltic Sea. 

The forerunners of the later standardized seasonal cniises started in May 1955 with synchronous measurements by the r/v Joh. L. Kruger and the r/v Magnetologe between the Fehmarnbelt and the Bornholm Basin. Finally, the programme of seasonal cruises had started in August 1957. It included about 50 fixed stations and comprised the combination of longitudinal and transversal sections between the Fehmarnbelt and the Bornholm Basin. More diurnal anchor stations each in the Fehmarnbelt and in the southern entrance of the Sound were included for investigations on temporal variabilities. The parameters measured were temperature, salinity, and oxygen at fixed depths. At some stations, current measurements were carried out from the anchored ship (cf. Section 3.2.1). 

Due to the great success of the IBY, the Baltic oceanographers recommended at the conference in Helsinki in 1970 the voluntary continuation of the IBY programme for the subsequent years. The GDR followed this recommendation. In combination with the national monitoring programme covering the Baltic Sea between Fehmarnbelt and Bornholm Basin, the IfM Warnemunde expanded its seasonal cruises to the Gotland Deep... 

The intensity of MBls can be estimated during the inflow events by the salinity conditions at the Baltic entrance sills (cf. Section 10.4). The effects of MBls in the central Baltic deep water depends on the density of the inflowing water, the mixing with the ambient water on its way through the Baltic Sea (cf. Fig. 10.1), and the density of the deep water in the various basins. The pathways of the inflowing water propagating from the sills through the Arkona Basin into the Bornholm Basin have been studied in detail by Lass et al. (2005). 

Attention must also be paid to the baroclinic summer inflows (cf. Section 10.5). These inflows below the MBI category are caused by long-lasting calm weather conditions over central Europe, which most likely occur in late summer and early autumn. Strong summer inflows can transport substantial amounts of exceptionally warm and saline water across the Darss Sill into the intermediate layers of the central Baltic Sea (Feistel et al., 2003c, 2004a). A typical example for weak baroclinic inflows and their effects between the Darss Sill and the Bornholm Basin is illustrated in Fig. 10.2. 

The first deep basin downstream of the entrance sills is the Bornholm Basin (Fig. 10.1). This basin has a maximum depth of more than 90 m and is separated from the next downstream basin by the Slupsk Sill (sill depth 60 m). The buffering properties of the Bornholm Basin play an essential part for the effectiveness of MBls in other central basins. The thermohaline conditions in the Bornholm Basin are also considerably important for the evolution of stagnation in the central Baltic deepwater. In general, there is a frequent inflow of lower amounts of highly saline water that penetrates across the sills into the ArkonaBasin during each baroclinic or weak barotropic inflow event. This water is trapped into the... 

Bornholm Basin renewing the ambient deepwater to a certain extent and causing seasonal variations (Matthaus, 1977, 1978 Franck, 1985). 

The salinity of the Bornholm Basin below the permanent halocline is a measure for the estimation of the impact of weak inflows on the central Baltic deep water. During periods of low inflow activity, salinity and thus density decreases in the deep water of the Bornholm Basin. Depending on the volume of saline water and its density, inflows below the MBI magnitude — but sometimes even MBIs — fill up only that basin, and the saline water does not pass to a greater extent the Slupsk Sill downstream through the Slupsk Channel into the Gotland Basin (cf. Fig. 10.2). 

When the buffering capacity of the Bornholm Basin is exhausted, weak inflows of saline and oxygen-rich water can pass that basin in depths of 50-60 m. This water frequently interleaves just below the halocline at the level of neutral buoyancy and propagates through... 

Fonselius (1962) started the detailed description of the effects of MBIs in the central Baltic deep water. He identified MBIs studying long-term variations, mainly of salinity in the Bornholm Basin. Fonselius described the dynamic processes between the Baltic deep basins after MBIs, the importance of the Bornholm Basin as buffer basin, and the overflow of the Eastern Gotland Basin in intermediate depths downstream into the Landsort Deep. 

The January 2003 event was a very specific one, never observed before in the Baltic Sea. Similar to January 1993, this MBI was followed by weak inflows in March and May 2003, not reaching the magnitude of a major event. Both postinflows enhanced the MBI because the Bornholm Basin was already filled with saline water favoring the rapid eastward propagation into the central Baltic. While the March inflow had almost the same temperature and oxygen content as the MBI, the inflow in May was significantly warmer (5-10°C). 

In the Bornholm Basin, which was filled with dense, cold water from the January MBI, the warm jet appeared on top of it. A very detailed study, by fortunate coincidence, of its passage over the Slupsk Sill was canied out by Paka et al. (2006), revealing even features of the... 

The Bornholm Basin deepwater, which is frequently influenced by each baroclinic or weak barotropic inflow event (cf. Section 10.2), is renewed annually to a certain extent. Consequently, anoxic conditions cannot be observed as often and persistent as in the subsequent basins. The phosphate and nitrate concentrations mirror these frequent changes between oxic and anoxic conditions. The relatively low phosphate values in the 1960s suggest that oxic conditions prevailed during this period. 

Not all inflows are able to propagate into the central Baltic basins because the Bornholm Basin acts as a buffer (cf. Section 10.2). Moreover, the effects of MBls are reduced, and they proceed more slowly in the subsequent basins due to the increasing volume of deepwater. Therefore, reactions of the nutrient regime to MBls can be best studied in these basins. The changes in the nutrient distribution are thus described in detail more for the near-bottom water layer of the Eastern Gotland Basin (Fig. 10.18). 

Mohrholz, V., Dutz, J., Kraus, G., 2006. The impact of exceptionally warm summer inflow events on the environmental conditions in the Bornholm Basin. Journal of Marine Systems, 60, 285-301. 

As the second instmctive example, the salinity at three depth levels in the Bornholm Basin (15°E, 55°N) between 1950 and 2005 is displayed in Fig. 11.5. 

FIGURE 11.5 Monthly time series of salinity at three depth levels 0, 50, and 80 m in the BALTIC cell at 15°E, 55°N, Bornholm Basin. 

FIGURE 12.3 Long-term variations of nitrate layer (0-10 m) for four stations of the Bornholm Basin (c,d). 

FIGURE 12.5 Distribution of hydrographic and chemical variables during a deepwater renewal in the Bornholm Basin (February 28, and March 1, 1969). 

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