132
W. Schramm
5.2 Physical and Chemical Characterization
The Baltic Sea including the transition zones to the North Sea (Belt Sea,
Sound) comprises an area of 3.95 x 10 5 km 2 and has a volume of
2.12 x 10 4 km 3 (Fig. 5.1). The average depth is only 54 m, with a maximum depth of about 400 m north of the Island of Gothland. The entire
Baltic can be described as a series of shallow basins, separated from each
other by sills.
A salinity gradient can be observed, with decreasing salinities from
polyhaline conditions in the Belt Sea (15-18 PSU) to oligohaline conditions in the northernmost Bothnian Bay with a mere 2-3 PSU (Fig. 5.2).
The salinity gradient is determined by the irregular inflow of salt-rich
North Sea Water (32 PSU) and the land run-off of freshwater from the
drainage basins around the Baltic. Usually, the freshwater discharge
exceeds the seawater inflow, resulting in a continuous outflow of lowsalinity surface water from the Baltic into the Kattegat.
As far north as the Aland Sea, the Baltic is stratified, with a permanent
primary halo cline between 50 -70 m depth. During summer, a rather
stable thermocline occurs at 10-15 m depth, below which a layer of cold
"winter water" extends down to the permanent halocline.
The relative stability of these discontinuity layers determines the typical seasonal variation of the nutrient situation in the open Baltic. In
spring, with increasing light intensities, plankton blooms develop, causing in the upper layer down to the thermocline a rapid decrease of
nutrient concentrations close to detection limits in early summer.
Usually, the breakdown of the spring bloom is followed by nutrient
regeneration processes in the water column, often causing a second
phytoplankton bloom in late summer. During autumn and winter,
storms and thermal convection cause mixing of the nutrient-rich deep
water from below the discontinuity layers with the surface layer, replenishing nutrients in the water column (von Bodungen 1986).
A different nutrient pattern has been observed in macrophytic communities, probably as a result of intensive nutrient regeneration through
benthic heterotrophic activity with the possibility of rapid recycling
within the phytobenthic communities. Compared to the surface water in
offshore stations, in red algae or Fucus communities from Kiel Bight, for
example, nutrient levels were significantly higher, and the spring-early
summer minimum occurred several weeks later compared to the surface
water in offshore stations (Fig. 5.3; Schramm et al. 1988). Kautski and
Wallentinus (1980) showed that in red algal communities in the archipelago along the Swedish east coast (Asko area), nutrient regeneration
W. Schramm
5.2 Physical and Chemical Characterization
The Baltic Sea including the transition zones to the North Sea (Belt Sea,
Sound) comprises an area of 3.95 x 10 5 km 2 and has a volume of
2.12 x 10 4 km 3 (Fig. 5.1). The average depth is only 54 m, with a maximum depth of about 400 m north of the Island of Gothland. The entire
Baltic can be described as a series of shallow basins, separated from each
other by sills.
A salinity gradient can be observed, with decreasing salinities from
polyhaline conditions in the Belt Sea (15-18 PSU) to oligohaline conditions in the northernmost Bothnian Bay with a mere 2-3 PSU (Fig. 5.2).
The salinity gradient is determined by the irregular inflow of salt-rich
North Sea Water (32 PSU) and the land run-off of freshwater from the
drainage basins around the Baltic. Usually, the freshwater discharge
exceeds the seawater inflow, resulting in a continuous outflow of lowsalinity surface water from the Baltic into the Kattegat.
As far north as the Aland Sea, the Baltic is stratified, with a permanent
primary halo cline between 50 -70 m depth. During summer, a rather
stable thermocline occurs at 10-15 m depth, below which a layer of cold
"winter water" extends down to the permanent halocline.
The relative stability of these discontinuity layers determines the typical seasonal variation of the nutrient situation in the open Baltic. In
spring, with increasing light intensities, plankton blooms develop, causing in the upper layer down to the thermocline a rapid decrease of
nutrient concentrations close to detection limits in early summer.
Usually, the breakdown of the spring bloom is followed by nutrient
regeneration processes in the water column, often causing a second
phytoplankton bloom in late summer. During autumn and winter,
storms and thermal convection cause mixing of the nutrient-rich deep
water from below the discontinuity layers with the surface layer, replenishing nutrients in the water column (von Bodungen 1986).
A different nutrient pattern has been observed in macrophytic communities, probably as a result of intensive nutrient regeneration through
benthic heterotrophic activity with the possibility of rapid recycling
within the phytobenthic communities. Compared to the surface water in
offshore stations, in red algae or Fucus communities from Kiel Bight, for
example, nutrient levels were significantly higher, and the spring-early
summer minimum occurred several weeks later compared to the surface
water in offshore stations (Fig. 5.3; Schramm et al. 1988). Kautski and
Wallentinus (1980) showed that in red algal communities in the archipelago along the Swedish east coast (Asko area), nutrient regeneration