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.
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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
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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).
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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.
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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.
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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 calculated. Comparing these periods will show the development of the oxygen concentration in different water masses rather than specific places. For convenience the different water types will follow the same classification as the earlier study by Andersson 1996, these are given in Table 2.
A special analyze is also made with focus on the oxygen annual cycle and correlation with salinity and temperature.
Table 2: Water masses classifications in the Skagerrak and the Kattegat
Water type Salinity [psu]
Skagerrak coastal water 25 – 32 Skagerrak water 32 – 35 Atlantic Water 35 – 35.2
Kattegat surface water 15 – 30
Kattegat deep water 32 – 35
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Results
The Skagerrak
Mean annual cycles for oxygen concentration in the Skagerrak are given in Figure 2. A clear seasonal cycle is present in all water masses although the amplitude decreases with increasing salinity. Also note the differences when the minimum concentration occurs. In the coastal water the lowest concentration is found in August while in the Atlantic water the lowest value is found two months later. No clear trends between decades can be seen in the Skagerrak waters although a small decrease in minimum concentration can be seen in the coastal and Atlantic waters.
Skagerrak coastal water (25-32 psu) Skagerrak water (32-35 psu) 8.5 8.5 1971-1980 1971-1980 8 8 1981-1990 1981-1990 7.5 1991-2000 7.5 1991-2000 2001-2010 2001-2010 7 7
6.5 6.5
Oxygen [ml/l] Oxygen 6 [ml/l] Oxygen 6
5.5 5.5
5 5 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 Month Month
Atlantic water (35-35.2 psu) 8.5 1971-1980 8 1981-1990 7.5 1991-2000 2001-2010 7
6.5
Oxygen [ml/l] 6
5.5
5 1 2 3 4 5 6 7 8 9 10 11 12 Month
Figure 2: Oxygen concentration development in the Skagerrak waters. Mean annual cycle for four 10-year periods.
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At station Å17 the deep water at 300 meters shows a decrease of the oxygen concentration during the last 40 years (Figure 3). In average the decrease is about 6·10-3 ml/l yr-1 for both the total and for the most represented month (August), giving a total decrease of about 0.25 ml/l for the whole period. At 200 meters the trend is even clearer. For the later period, a clear positive trend can be seen at 300 meters with 8·10-3 ml/l yr-1 for all measurements and close to 20·10-3 ml/l yr-1 for the reference month. At 200 meters, as well as the rest of the water column, no similar positive trend can be seen.
Å17 (Skagerrak) Depth: 200 m (340) 7.5
7
6.5
6
Oxygen [ml/l] Oxygen 5.5
5
4.5 70 72 75 77 80 82 85 87 90 92 95 97 00 02 05 07 10 Year
Depth: 300 m (340) 7.5
7
6.5
6
Oxygen [ml/l] Oxygen 5.5
5
4.5 70 72 75 77 80 82 85 87 90 92 95 97 00 02 05 07 10 Year
Figure 3: Oxygen concentration at the two deepest standard depths at station Å17. Trend lines for all measurements (thick) and measurements made in August (thin). Broken lines shows trend since the year 2000.
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Station Å13 shows a significant decrease of the oxygen content at 75 meters with a mean of about 12·10-3 ml/l yr-1 for all data as well as for the reference month (Figure 4). After the year 2000 the concentration has stabilized and shows a smaller decrease of about 2·10-3 ml/l yr-1 for all measurements. Looking at the reference month during the last period a strong positive trend can be recognized with as much as 30·10-3 ml/l yr-1. The rest of the water column below the halocline has the same tendency except for the reference month during the last period where the trend is getting weaker with decreasing depth.
Å13 (Skagerrak) Depth: 50 m (85) 8
7.5
7
6.5
6
Oxygen [ml/l] 5.5
5
4.5 70 75 80 85 90 95 00 05 10 Year
Depth: 75 m (85) 8
7.5
7
6.5
6
Oxygen [ml/l] 5.5
5
4.5 70 75 80 85 90 95 00 05 10 Year
Figure 4: Oxygen concentration at the two deepest standard depths at station Å13. Trend lines for all measurements (thick) and measurements made in August (thin). Broken lines shows trend since the year 2000.
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The Kattegat
Also in the Kattegat waters there is a clear annual cycle in the oxygen concentration with a range of about 2.5 ml/l between the maximum and minimum concentration (Figure 5). There is however a distinct difference of about 2 ml/l in the oxygen concentration between the two water masses clarifying the strong stratification in the Kattegat. Negative oxygen trends can be seen both in the coastal water and in the deep water. In the deep water the trend is clear throughout the whole year and particularly prominent in September when the oxygen concentrations are at the lowest level. The minimum peek below 2 ml/l represents a single day and is therefore not representative.
Kattegat coastal water (15-30 psu) Kattegat deep water (32-35 psu)
1971-1980 1971-1980 9 9 1981-1990 1981-1990 1991-2000 1991-2000 2001-2010 2001-2010 8 8
7 7
6 6
5 5 Oxygen [ml/l] Oxygen [ml/l] Oxygen
4 4
3 3
2 2
1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 Month Month
Figure 5: Oxygen concentration development in the Kattegat waters. Mean annual cycle for four 10-year periods.
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Moving into the Kattegat, station Fladen shows the same negative trend for the entire period as in the Skagerrak although with a smaller decrease of about 5·10-3 ml/l yr-1 in the deep water (Figure 6). No trend can be seen for measurements made in September. A bit further up in the water column, at a depth of 60 meters, the trend is stronger including all measurement with a decrease of 9·10-3 ml/l yr-1. Also here a significant negative trend can be seen in September. The trend in the later period shows a somewhat larger decrease for all measurements for both depths while the trend for September in this period shows a strong increase with 45·10-3 and 66·10-3 ml/l yr-1 at 70 and 60 meters respectively.
FLADEN (Kattegat) Depth: 60 m (75) 8
7
6
5
4 Oxygen [ml/l]
3
2 70 75 80 85 90 95 00 05 10 Year
Depth: 70 m (75) 8
7
6
5
4 Oxygen [ml/l]
3
2 70 75 80 85 90 95 00 05 10 Year
Figure 6: Oxygen concentration at the two deepest standard depths at station Fladen. Trend lines for all measurements (thick) and measurements made in September (thin). Broken lines shows trend since the year 2000.
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A small negative trend for the whole period can be seen at the deep water (50 meters) at station Anholt E (Figure 7).This trend is more significant at a depth of 40 meters showing a decrease of about 12·10-3 ml/l yr-1. The trend for the reference month September on the other hand shows a small positive trend at both 40 and 50 meters. For the later period, same but stronger trends are observed.
ANHOLT E (Kattegat) Depth: 40 m (55) 8
7
6
5
4
3
Oxygen [ml/l] Oxygen 2
1
0 84 86 88 90 92 94 96 98 00 02 04 06 08 10 Year
Depth: 50 m (55) 8
7
6
5
4
3
Oxygen [ml/l] Oxygen 2
1
0 84 86 88 90 92 94 96 98 00 02 04 06 08 10 Year
Figure 7: Oxygen concentration at the two deepest standard depths at station Anholt E. Trend lines for all measurements (thick) and measurements made in September (thin). Broken lines shows trend since the year 2000. Note that this station has shorter time interval.
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Öresund
The trend at station W Landskrona is negative for the whole period when including all measurements as well as for the reference month at both 30 and 40 meters depth, being as strong as 36·10-3 ml/l yr-1 for the reference month at 30 meters (Figure 8). The later period follows the same pattern as station Fladen and Anholt E with a significant negative trend including all measurements and a significant positive trend looking at the reference month. The indications here are stronger than the ones at Fladen and Anholt E.
W LANDSKRONA (Öresund) Depth: 30 m (50) 8
7
6
5
4
3
Oxygen [ml/l] 2
1
0 72 75 77 80 82 85 87 90 92 95 97 00 02 05 07 10 Year
Depth: 40 m (50) 8
7
6
5
4
3
Oxygen [ml/l] 2
1
0 72 75 77 80 82 85 87 90 92 95 97 00 02 05 07 10 Year
Figure 8: Oxygen concentration at the two deepest standard depths at station W Landskrona. Trend lines for all measurements (thick) and measurements made in September (thin). Broken lines shows trend since the year 2000.
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Correlation and annual cycle
In order to determine if the changes in oxygen concentration are related to movements of water masses the oxygen concentration is correlated with salinity and temperature over the later period. Figure 9 shows plots and trends for oxygen concentration and salinity for the four stations closest to the Swedish coast. Calculations shows no significant correlation between the two for this period except for station W Landskrona which has a small negative correlation, see Table 3. However, at several times a distinct correlation can be seen for shorter time scales. Drops in salinity are prominent in 2005, 2007 and 2008 at both station Anholt E and W Landskrona and correlating with increasing oxygen levels. No significant oxygen trend can be seen at stations Å13 and Fladen while Anholt E and W Landskrona shows a significant decrease. At the same time the salinity shows an increasing trend at all stations apart from Station Å13 where no trend can be seen.
Å13 (Skagerrak) 75 m (85) FLADEN (Kattegat) 70 m (75) 7.5 36 7 35.5
6 6.5 35 34.5
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5.5 34 33.5 Oxygen [ml/l] Salinity [psu] Oxygen [ml/l] 4 Salinity [psu]
4.5 33 3 32.5 01 02 03 04 05 06 07 08 09 10 11 01 02 03 04 05 06 07 08 09 10 11 Year Year
ANHOLT E (Kattegat) 50 m (55) W LANDSKRONA (Öresund) 40 m (50) 7 35 8 35
6 33 34 6 5 31
4 33 4 29
3 27 Oxygen [ml/l] 32 Salinity [psu] Oxygen [ml/l] 2 Salinity [psu] 2 25
1 31 0 23 01 02 03 04 05 06 07 08 09 10 11 01 02 03 04 05 06 07 08 09 10 11 Year Year
Figure 9: Oxygen concentration (full line) and salinity (dotted line) with trend lines for the later period. Depths are deepest standards depth at each station.
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Table 3: Correlation coefficients
Station Correlation: oxygen/salt Correlation: oxygen/temperature
Å13 -0.2335 -0.7031 Fladen 0.0635 -0.7696 Anholt E -0.0087 -0.4539 W Landskrona -0.3994 -0.7279
Oxygen concentration correlates well with temperature with the highest concentration when the temperature is low, see Table 3. All stations also show an increasing trend in temperature (Figure 10).
Å13 (Skagerrak) 75 m (85) FLADEN (Kattegat) 70 m (75) 7.5 15 7 15
13 13 6 6.5 11 11
9 5 9
5.5 7 7 Oxygen [ml/l] Oxygen [ml/l] 4 Tempearture (C) Tempearture (C) Tempearture 5 5
4.5 3 3 3 01 02 03 04 05 06 07 08 09 10 11 01 02 03 04 05 06 07 08 09 10 11 Year Year
ANHOLT E (Kattegat) 50 m (55) W LANDSKRONA (Öresund) 40 m (50) 7 15 8 15
6 13 12 6 5 11
4 9 4 9
3 7 Oxygen [ml/l] Oxygen [ml/l]
6 Tempearture (C) 2 Tempearture (C) 2 5
1 3 0 3 01 02 03 04 05 06 07 08 09 10 11 01 02 03 04 05 06 07 08 09 10 11 Year Year
Figure 10: Oxygen concentration (full line) and temperature (dotted line) with trend lines for the later period. Depths are deepest standards depth at each station.
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Plotting all stations together (Figure 11) clearly shows the common annual cycle but also interesting differences between the stations. Since the oxygen concentration at saturation is larger for cold water it is relevant to plot the oxygen saturation in order to separate the effect of biological oxygen consumption from the temperature effect. This reveals that the oxygen is at saturation or very close to at the Skagerrak station in the end of winter and spring. This shows that this water has recently been in contact with the atmosphere probably originating in the outer Skagerrak and the North Sea. The Kattegat deep water, originating in the Skagerrak, does not reach the same degree of saturation in the spring. The reason for this is probably that some of the oxygen has been consumed along the way. Biological decomposition mainly takes place in the summer and this is clearly seen in the plots. Oxygen depletion starts in the end of spring and reaches minimum values in the beginning of fall. The extent of oxygen consumption clearly increases towards the south and in the southern Kattegat and Öresund oxygen levels drops below 2 ml/l at several times in the period indicating hypoxia conditions in the area.
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7
6
5
4
3 Oxygen [ml/l]
2
1
0 01 02 03 04 05 06 07 08 09 10 11 Year
110
100
90
80
70
60
50
Saturation (%) 40
30
20
10 01 02 03 04 05 06 07 08 09 10 11 Year
Figure 11: Oxygen concentration [ml/l] and saturation [%] at Å13 (-), Fladen (--), Anholt E (-.) and W Landskrona (..). Data from each stations deepest standard depth.
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Discussion and conclusions
With respect to the whole period a negative oxygen trend is clear at most stations. The lack of continuous data in the earlier stages will affect the linear regression depending on when in the year measurements are made. However, using the reference month gives similar trends making the result still reliable. Although there might be indications that the oxygen levels have stabilized or even increased in the last decade the linear regression is very delicate to changes. With the later period being only eleven years adding or removing one year will most likely give a different result. Also if another period of this length was chosen the result could be very different. With this in mind and the fact that there is less data in the early years, additional trends where calculated for the period 1982- 2011 (the period for station Anholt E). The results was mainly compared with the trends for the whole period and gave about the same or slightly less negative trend for most stations when including all measurements. In addition the reference month at stations Å13 and Fladen turn to show small positive trends for this period. This shows that one has to be careful with the choice of time period before making any conclusions in oxygen trends.
Although the trend is negative it is not nearly as strong as the ones found in the studies by Andersson & Rydberg 1988 and Andersson 1996. What is clear is that the decrease of about 50·10-3 ml/l yr-1 stated in these reports has not continued after 1990. It is possible that the oxygen conditions have improved the last 20 years although the main reason most likely is the time over which the trend has been calculated over. The fluctuations in the oxygen concentrations from one year to another can be very strong in the area and short time trends are therefore very delicate to changes.
The residence time in the deep waters is not very long being about three months in the Skagerrak and varying between one and four months in the Kattegat (Andersson, 1996). The steady annual cycle in Figure 11 confirms this not indicating any long stagnant periods. The area is constantly ventilated with new water and the high oxygen content and saturation levels, especially in the winter, shows that this water has recently been equilibrated with the atmosphere. The annual cycles are reinforced to different extent depending on the degree of organic decomposition. Good oxygen conditions in the inflowing water from the Atlantic gives the Skagerrak deep water a high oxygen level and a relatively high saturation level throughout the year. Water that is entering the Kattegat deep water from the Skagerrak is subject to a more extensive decomposition of organic matter along its way. This is why the Kattegat deep water is showing a lower degree of saturation and lower oxygen levels. The Öresund deep water is ventilated with water from the Kattegat that has already been depleted from some of its oxygen. This together with a higher degree of decomposition and/or less ventilation is resulting in lower oxygen levels and at several times leads to hypoxic conditions.
Despite an often sharp halocline inhabiting mixing in the Kattegat and Öresund oxygen is supplied to the deep water regularly. This indicates consistent inflows of oxygen rich water to the areas. However, stations W Landskrona and Anholt E, at several times, show heavy outflows of low saline water from the Baltic Sea correlate well with increasing oxygen levels indicating strong mixing of the water column (Figure 9). These outflows have to be extensive since the effect of the mixing is reaching down as deep as 50 meters.
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References Andersson, L. (1996). Trends in nutrient and oxygen concentrations in the Skagerrag‐Kattegat. Journal of Sea Research , 35 (1‐3), 63‐71.
Andersson, L., & Rydberg, L. (1988). Trends in nutrient and oxygen conditions within the Kattegat: effects on local nutrient supply. Estuarine, Coastal and Shelf Science , 26, 559‐579.
Gustavsson, B. (1997). Interaction between Baltic Sea and North Sea. German Journal of Hydrography , Number 49, 165‐182.
Gustavsson, B., & Stigebrandt, A. (1996). Dynamics of the freshwater‐influenced surface layers in the Skagerrak. Journal of Sea Research , 35 (1‐3), 39‐53.
Havsmiljöinstitutet. (2011). Havet 2011 Om miljötillståndet i svenska havsområden. Havsmiljöinstitutet.
Mattsson, J. (1996). Some comments on the barotropic flow through the Danish Straits and the division of the flow between the Belt Sea and the Öresund. Tellus A , vol 48 (no 3), 456‐464.
Rodhe, J. (2012). Nationalencyklopedin. Retrieved februari 23, 2012, from ne.se: http://www.ne.se/kattegatt
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