HELSINKI COMMISSION HELCOM MONAS … COMMISSION HELCOM MONAS 4/2002 Monitoring and Assessment Group...

HELSINKI COMMISSION HELCOM MONAS 4/2002Monitoring and Assessment GroupFourth MeetingWarnemünde, Germany, 21-25 October 2002

Note by Secretariat: FOR REASONS OF ECONOMY, THE DELEGATES ARE KINDLY REQUESTED TO BRING THEIR OWNCOPIES OF THE DOCUMENTS TO THE MEETING

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Agenda Item X

Document code: To be filled in by the Secretariat

Date: To be filled in by the Secretariat

Submitted by:

HELCOM MONAS Indicator Report.

1. Title of Indicator: 1. Eutrophication (Hydrography and Oxygen in the deep basins)Responsible Institute: SMHI & FIMRContact Person: Bertil Håkansson (SMHI)

HELCOM MONAS 4/2002, Document 2/1

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2. Graphs

Figure 1.

Figure 2.


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Figure 3.

Figure 4.


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Figure 5.

3. Title(s) of graphs

Figure 1 The Baltic Proper, divided into basins, with principle hydrographic sampling stationsmarkedFigure 2 Time series of buoyancy frequency with depth, West Gotland Basin. Regions of strongstratification show in red.Figure 3 Time series of the maximum winter-spring buoyancy frequency (upper panel), and meandepth of the maximum (lower panel).Figure 4 Histogram of oxygen levels in the basins of the Baltic Proper (‘Starred’ values for theNorthern Baltic Proper, 2000 and 2001, were calculated using data from November – December, as noAugust – October data were available).Figure 5 Oxygen concentrations (2 mg/l and 0 mg/l shaded) in bottom water, 1990-2001, based onICES and SMHI data (1990-1999) and FIMR and SMHI data (2000 & 2001).


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4. Key MessageAll basins show a freshening of the surface (0 – 10 metres depth) waters since 1990, at an average rateof 0.04 psu per year. Deep (below 80 metres depth) water salinities have increased in all basins exceptfor the Southern Baltic Proper (see Table 1).

Oxygen levels declined from a peak during 1992-1994. Conditions are worst in the East Gotland Basinand Northern Baltic Proper, where almost 30% of the basin volume is affected. In autumn, hydrogensulphide is present in all basins of the Baltic Proper, though not in the Gulf of Finland.

Figure 1 NoneFigure 2 Buoyancy frequency shows effect of summer stratification (in upper 40 metres) and widthof permanent halocline (60 – 90 metres)Figure 3 Upper: Stable values of the winter-spring (chosen to remove the effects of summerstratification) buoyancy frequency particularly in the Northern Baltic Proper and West Gotland Basin.Higher values, and also greater variability, in the Southern Baltic Proper.

Lower: Deepening of the permanent halocline in the Southern Baltic Proper, from 45 toclose to 60 metres. Shallowing of permanent halocline in Northern Baltic Proper from 80 to 60 metres.In East and West Gotland Basins, halocline depth appears stable.Figure 4 Effect of 1993 inflow to the Baltic on oxygen levels in the deep basins is clear. Since 1993-94, basins have been affected by steadily worsening oxygen levels.Figure 5 Extent of depleted oxygen levels (below 2 mg/l) in bottom water. The effect of the 1993inflow can be seen, reducing the extent of low- and anoxic waters. The extent of depleted oxygenlevels in bottom water has since increased.

5. Results and Assessment5.1 Relevance of the indicator for describing developments in the environment

Baltic surface waters are strongly influenced by land run-off. Changes in run-off alter the surface salinity whileinflows through The Sound (Öresund) and Belt Sea control the salinity of the deeper waters. Stratificationbetween the upper and lower layers inhibits turbulent eddies from mixing surface and deep waters together,preventing the oxygenated surface water being mixed below the halocline, making the ventilation of deep waterdependent on fresh inflows from the Kattegat. The stratification also hinders the transfer of phosphorous (whichis abundant in the deep water) to the surface waters. This in turn limits the algal productivity.

Buoyancy frequency is a measure of the strength of water column stratification. This is the frequency at which a’parcel’ of water would oscillate around its original position if displaced slightly. A low frequency suggests thatthe parcel oscillates slowly, because the density difference between it and the surrounding water is small. This inturn implies that the water around our ’parcel’ is well (and easily) mixed. If the frequency is high, the water isstrongly stratified. In well mixed water, turbulent eddies can grow, and mix the water column further, while inregions of strong stratification, the growth of eddies is inhibited, reducing mixing. Additionally, a region ofstrong stratification inhibits mixing between water above and below.

Changes in buoyancy frequency with time show the effect of seasonal stratification (Figure 2). In summer, thestrongest stratification is found in shallow water, due to the effect of the (warm) fresh water run-off from theland, and from solar heating. As the year progresses , the depth of the stratification maximum increases, due toincreasing volumes of fresh water, further solar heating, and in the autumn, by mixing by waves, and surfacecooling leading to downward convection. In winter and spring, the surface water becomes mixed down to thelevel of the permanent halocline (at about 80 metres depth).

The mean winter - spring value and depth of the buoyancy frequency maximum indicates the strength of thelonger-term stratification in the basin, and the depth of the surface layer – without the ’noise’ introduced by theintra-annual variability. Figure 3 shows the changes in each of the basins between 1990 and 2002. Changes in


the depth, and magnitude of the buoyancy frequency could be expected due to changes in run-off from land, inthe salinity of the deep water, temperature changes and also changes in storminess.

Oxygen depletion is widely used as an indicator for the indirect effects of nutrient enrichment. Oxygen levelsabove 6 mg/l are considered to cause no problems. Levels below this cause increasing stress to organisms.Lowest oxygen levels are experienced at the end of summer, between August and October, when detritus frombiological activity in the surface waters sinks, and is broken down by bacteria, which consume the availableoxygen. When oxygen concentrations fall below about 1.5 mg/l, bacteria start to use anaerobic processes, whichproduce hydrogen sulphide as a by-product. This hydrogen sulphide is toxic, and its concentration is described interms of negative oxygen – the concentration of oxygen that is required to oxidise it.

5.2 Policy relevance and policy references

5.3 Assessment

Figure 3 shows the changes in buoyancy frequency in each basin between 1990 and 2002. The buoyancyfrequency is highest in the Southern Baltic Proper, due to the large density difference between the (relatively)saline bottom water, and the overlying, brackish water flowing out from the remainder of the Baltic. The depthof the maximum is also reduced compared to the other basins. Higher values of buoyancy frequency areexpected if there is increased fresh water outflow, strong saline inflows, and/or higher surface temperatures.There is no clear trend in the strength of stratification. It is possible that there is an slight increase in the valuesfrom the West Gotland Basin and Northern Baltic Proper – as could be expected from changes in salinity – butthese are very small. The depth of the maximum, shown in the lower plot, appears unchanged in the WestGotland Basin, but shallows in the Northern Baltic Proper. In the southern Baltic, the depth of the maximumappears to increase from around 40 metres in 1994, to about 60 in 2002. This could be attributed to a reduction inthe inflow of salt water from theKattegat, and a steady dilution of thebasin by the fresher surface waters. Timeseries of surface salinity between 1990and 2002 show a steady freshening ofthe surface 10 metres of the BalticProper, at an average rate of 0.04practical salinity units (psu) per year.Deep-water salinity between 1990 and2002 shows the opposite trend. Below80 metres, salinity increased by 0.09 psuper year, with the highest rates ofincrease in the Northern Baltic Properand the West Gotland Basin.

The surface salinity changes arereasonable, given the increasing freshwater input – particularly since 2000. Thedeep water changes suggest the movementof saline water in the deep basins of thenorth and west, which is not replaced with’new’ salt water in the Southern BalticProper.

For each of the basins, autumn (August,September and October) oxygen profileswere examined using all data from SMHIand ICES (between 1990-1999) and SMHIand FIMR (2000-2001). For 2000 – 2001,the period was extended to December inthe Northern Baltic Proper, due to a paucity

Changes in salinity (psu/year)Surface

(top 10 metres)Deep

(bottom 80 metres)Southern Baltic Proper -0.03 (-0.11) -0.05 (-0.06)East Gotland Basin -0.05 (-0.02) +0.05 (-0.05)West Gotland Basin -0.05 (-0.18) +0.13 (+0.04)Northern Baltic Proper -0.02 (-0.01) +0.12 (-0.13)Gulf of Finland 0.00 (-0.25) No data

Table 1 Rates of change of surface and deepwater salinity in theBaltic Proper, 1990 – 2002. Bracketed values show data from 2000-2001

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of autumn data. For each profile, the depth at which the oxygen

Region Standarddeviation

Northern Baltic Proper 3.1%East Gotland Basin 10.8%West Gotland Basin 2.6%Southern Baltic Proper 6.4%Gulf of Finland 3.9%

Table 2 Standard deviation, as a percentage of basin volume,in the estimates of the mean basin volume affected by reducedoxygen levels in the Baltic Proper, 1990 - 2001


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concentration fell below certain levels (0, 2,4,5 and 6 mg/l) were calculated, and these depths were used toestimate the volume of water in each basin affected by reduced oxygen levels. These were then averaged, to givean annual basin-mean volume at each oxygen level, and are presented in Figure 4 as a percentage of the totalvolume of each basin. The standard deviation from these averages gives an idea of the variation within eachbasin. Table 2 shows the standard deviation within each basin.All basins show minimum oxygen depletion between 1992 and 1994. This was due to the inflow of oxygenatedsaline water from the Kattegat, spreading out first through the Southern Baltic Proper, and then into the deepregions of the other basins. Since this last, large inflow, oxygen concentrations have decreased in all basins. TheEast Gotland Basin and Northern Baltic Proper are worst affected, with between 20 and 40% of the total basinvolume experiencing reduced oxygen levels, and almost 30% of the Northern Baltic Proper suffering acutetoxicity in 2001. Hydrogen sulphide is present in 10% of the water by 2001, and area affected is increasing. Theproportions of affected water in the Gulf of Finland, Southern Baltic Proper and West Gotland Basin are lower.These basins have a higher proportion of shallow areas compared to the East Gotland Basin and Northern BalticProper, while their surface waters (which make up a larger proportion of their volume) are regularly replaced (byfresh water flowing out).

6. References

7. DataHydrographic data was collected by SMHI from the research vessel Argos, between 1990 and 2002. Oxygendata came from the Swedish Meteorological and Hydrological Institute data archive (SHARK), from the ICESdatabank, and from the Finnish Institute for Marine Research.

8. Meta data

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