All the measurement data from 1993 to 2007, for pH from Swedish sea areas held in SMHI’s SHARK database, have been analysed for trends. These analyses show that the pH has decreased in most sea areas.
Analyses were carried out for the Skagerrak, the Kattegat, the southern Baltic Proper, the northern and central Baltic Proper, the Bothnian Sea and the Bay of Bothnia.
In addition a division has been made between surface water and deep water. The surface waters follow a more or less annual cycle which is connected to algal growth, while the situation in the deep water is more stable.
In the deep water of the Baltic Proper where there is sometimes a lack of oxygen. Here the pH varies due to the quantities of hydrogen sulphide in the water. In these regions the water has been divided into three layers – surface water, intermediate water and deep water.
The results show that the biggest changes have occurred in the surface and deep waters of the Bothnian Sea, in the surface water of the Bothnian Bay and in the intermediate and bottom waters of the southern Baltic Proper.
- In the surface water of the Kattegat the pH has dropped by 0.06 units during the period, and the deep water by 0.11 units.
- The trends in the intermediate and deep waters of the southern Baltic Proper are of a similar size: 0.20 units.
- In the Baltic Proper, the change in the intermediate layer was 0.14 units and 0.09 units in the deep water.
- In the Central and Northern parts of the Baltic Proper, the intermediate level has changed by 0.14 units and the deep water by 0.09 units.
- The surface water of the Bothnian Sea shows the greatest change of 0.44 units while the deep waters there have dropped by 0.27 units.
- In the Bay of Bothnia the surface water pH has dropped by 0.20 units and the deep water by 0.18 units.
Changes in the other water areas can not be statistically verified.
The results are mainly based on monthly measurements from a number of stations in each sea area. A monthly average was calculated based on all measurements in the different sea areas and in the various depth layers.
An annual mean was also calculated, although not for the Bothnian Sea or Bay since the underlying data was not as homogenous as in other areas. In the Skagerrak no measurements were taken during the period 2001-2007, so there is a big gap in the data for this area.
The trends and significant levels were calculated with a simple linear regression. Results were only shown if they were significant to 95% (p<0.05). All changes concern the period between 1993 and 2007.