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Long Term Oxygen Trends in the Skagerrak and Kattegat Deep Waters

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Long Term Oxygen Trends in the Skagerrak and Kattegat Deep Waters UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre Long term oxygen trends in the Skagerrak and Kattegat deep waters Magnus Wenzer ISSN 1400-3821 B685 Bachelor of Science thesis Göteborg 2012 Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Long term oxygen trends in the Skagerrak and Kattegat deep waters Magnus Wenzer Abstract Oxygen data from the Skagerrak and the Kattegat for the period 1970-2011 has been investigated with focus on long term trends in the deep water. All stations show negative trends looking at the entire period. However, when looking at data based on a single month most stations have clear positive trends in the last decade. The choice of time period is of big importance for the trend analysis hence including one extra year will give a different result. When comparing monthly average for different 10-year periods no trends can be seen except for the Kattegat deep water where the latest decade has the lowest oxygen levels of the entire investigated period. The annual cycle in both oxygen concentration and saturation are clear at all studied stations indicating that the deep water in the area is relatively well-ventilated throughout the year. Biological decomposition increases towards the south and in the Öresund the oxygen concentration has dropped below 2 ml/l at several times the last decade indicating hypoxia in the area. 1 Table of Contents Introduction ............................................................................................................................................. 3 Data used in the study ......................................................................................................................... 5 Data analysis ....................................................................................................................................... 6 Results ..................................................................................................................................................... 7 The Skagerrak...................................................................................................................................... 7 The Kattegat ...................................................................................................................................... 10 Öresund ............................................................................................................................................. 13 Correlation and annual cycle ............................................................................................................. 14 Discussion and conclusions ................................................................................................................... 17 References ............................................................................................................................................. 18 2 Introduction In the publication “Havet 2011, Om miljötillståndet i svenska havsområden”, which is published by Havsmiljöinstitutet, it is stated that the oxygen levels in the deep water in the Skagerrak and the Kattegat have decreased during the de last forty years. Earlier investigations have been made looking at the development of oxygen and nutrient concentrations for different water masses in the area. A study by Andersson & Rydberg 1988, showed a decrease in the oxygen concentration in the Kattegat deep water between 1971 and 1982 with about 50·10-3 ml/l yr-1. That study was followed by Andersson 1996, who found similar results looking at the periode 1971-1990. He also found the same negative trends for autum concentrations in all studied water masses in the Skagerrak. The aim with this report is to make a detailed investigation of the development of oxygen concentration at some specific locations. Focus is in the deep water with some fixed stations representing the Skagerrak, the Kattegat and Öresund. As a complement the oxygen concentration related to different water masses in the area is also presented. The Skagerrak, the Kattegat and the Danish Strait together form the area that connects the North Sea with the Baltic Sea (Figure 1). Kattegat has the deepest connection to the Baltic Sea through the Danish Straits with a sill depth of about 18 meters. The connection through Öresund is shallower with a sill depth of 8 meters. Moving into the Kattegat the depth average is 23 meters with the deepest areas located in a trench along the Swedish coast (Gustavsson, 1997). The maximum depth of the Kattegat is about 130 meters (Rodhe, 2012). The Skagerrak is in general much deeper with some 100 meters on the border to the Kattegat and with a mean depth of about 210 meters. The maximum depth, 710 meters can be found in the Norwegian trench along the Norwegian south coast (Gustavsson, 1997). 3 Å13 Å17 Skagerrak Å15 100 m 50 m 20 m Fladen N14 Falkenberg Kattegat Anholt E W Landskrona Figure 1: Map of the studied area. Main stations marked with stars. Dots show extra stations. 4 Two main sills, the Drogden sill in Öresund and the Darss sill in the Belt Sea, prevent major inflow of denser water from the Kattegat to enter the Baltic. This Kattegat surface water together with net outflow of fresh water causes the Belt Sea and the Öresund to be strongly stratified creating a sharp halocline with a 5-10 psu difference between surface and bottom water (Gustavsson, 1997). As the low saline water from the Baltic makes its way through the Belt Sea and Öresund the water becomes saltier due to mixing in the narrow and shallow straits and when the water reaches the Kattegat the salinity is in the range of 10-14 psu, with the saltier coming from the Great Belt due to heavier mixing. In the Kattegat, strong vertical entrainment of bottom water into the surface layer causes an import of dense Skagerrak water (32-35 psu) to the deep basin simultaneously raises the salinity of the Kattegat surface water as it moves north (Gustavsson, 1997). Despite the heavy mixing a distinct front can be found between the Kattegat water, now reaching a salinity of 15-25 psu and the much saltier Skagerrak water. Although this front is usually very sharp it moves a lot in short time periods mainly due to the extent of barotropic flow through the Danish straits and the degree of wind mixing. The front is usually seen from Cape Skagen stretching north-east (Gustavsson & Stigebrandt, 1996). Even in the Skagerrak the surface water originating from the Baltic Sea can be recognized propagating along the Swedish and Norwegian coasts. The bottom water of the Skagerrak consists of Atlantic water coming from the North Sea while the rest of the water masses are a mixture of different water types. Typically there is a strong horizontal gradient in the surface water stretching from 15 psu in the south to 30 psu in the north (Andersson, 1996) Data used in the study All data used in this paper is downloaded from the SMHI data base SHARK (Svenskt HavsARKiv). Data is gathered by Swedish environmental monitoring coordinated by Naturvårdsverket. Out of seven used stations five where chosen as main stations for extensive analysis (Figure 1). Basic data for the main stations are given in Table 1. In general the data shows less frequent measurement in the earlier period. Typically measurements are more frequent after 1990 although at some stations there is a total lack of data during some years in the 1990s. The distribution of measurements over the year also varies. In the Skagerrak measurements are made more frequently in May and August compared to other months. In the northern Kattegat more measurements are made in September while in the southern Kattegat the measurements are more evenly distributed over the year although there are somewhat more observations in August and September. In Öresund the frequency is more uneven with more measurements in January and August and less in July and October. Some special depths in the profiles represent standard depths in the monitoring program since they include considerably more data than other depths in between. About one percent of the data has obvious errors and has therefore been excludes from the analysis. Measurements of oxygen concentration along with salinity are well represented in most stations. 5 Table 1: Main stations for analysis Station: Latitude (N): Longitude (E): Ca depth at station: Å 17 58°16'5.00" 10°30'8.00" 340m Å 13 58°20'2.00" 11° 1'60.00 85m Fladen 57°11'5.00" 11°40'0.00" 75m Anholt E 56°40'0.00" 12° 7'0.00" 55m W Landskrona 55°51'60.00" 12°45'0.00" 50m Data analysis Data is used in two different ways in this report. First, oxygen tendencies over time are calculated in the deep water at specific location. Here only the standard depths are used in order to include the maximum amount of data at one particular depth. This means that although the aim is to analyze water close to the sea floor, the deepest available measurements are not used since they are observed less frequent. Simple linear regression is used to detect trends for the whole period as well as for the period from the year 2000 and forward; this period will here on be referred to as the later period. A special regression is also made for the month that includes most data in each time series. Secondly, data is categorized into different water types according to their salinity. Monthly mean for four 10-year periods are then
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