HELSINKI COMMISSION HELCOM MONAS 4/2002 Monitoring and Assessment Group Fourth Meeting Warnemünde, Germany, 21-25 October 2002 Agenda Item X Document code: Date: Submitted by: To be filled in by the Secretariat To be filled in by the Secretariat HELCOM MONAS Indicator Report. 1. Title of Indicator: 1. Eutrophication (Hydrography and Oxygen in the deep basins) Responsible Institute: SMHI & FIMR Contact Person: Bertil Håkansson (SMHI) Note by Secretariat: FOR REASONS OF ECONOMY, THE DELEGATES ARE KINDLY REQUESTED TO BRING THEIR OWN COPIES OF THE DOCUMENTS TO THE MEETING Page 1 of 7
2. Graphs Figure 1. Figure 2. Page 2 of 7
Figure 3. Figure 4. Page 3 of 7
Figure 5. 3. Title(s) of graphs Figure 1 The Baltic Proper, divided into basins, with principle hydrographic sampling stations marked Figure 2 Time series of buoyancy frequency with depth, West Gotland Basin. Regions of strong stratification show in red. Figure 3 Time series of the maximum winter-spring buoyancy frequency (upper panel), and mean depth of the maximum (lower panel). Figure 4 Histogram of oxygen levels in the basins of the Baltic Proper ( Starred values for the Northern Baltic Proper, 2000 and 2001, were calculated using data from November December, as no August October data were available). Figure 5 Oxygen concentrations (2 mg/l and 0 mg/l shaded) in bottom water, 1990-2001, based on ICES and SMHI data (1990-1999) and FIMR and SMHI data (2000 & 2001). Page 4 of 7
4. Key Message All basins show a freshening of the surface (0 10 metres depth) waters since 1990, at an average rate of 0.04 psu per year. Deep (below 80 metres depth) water salinities have increased in all basins except for the Southern Baltic Proper (see Table 1). Oxygen levels declined from a peak during 1992-1994. Conditions are worst in the East Gotland Basin and Northern Baltic Proper, where almost 30% of the basin volume is affected. In autumn, hydrogen sulphide is present in all basins of the Baltic Proper, though not in the Gulf of Finland. Figure 1 None Figure 2 Buoyancy frequency shows effect of summer stratification (in upper 40 metres) and width of permanent halocline (60 90 metres) Figure 3 Upper: Stable values of the winter-spring (chosen to remove the effects of summer stratification) 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 to close 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 1993 inflow can be seen, reducing the extent of low- and anoxic waters. The extent of depleted oxygen levels in bottom water has since increased. 5. Results and Assessment 5.1 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 while inflows through The Sound (Öresund) and Belt Sea control the salinity of the deeper waters. Stratification between 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 water dependent on fresh inflows from the Kattegat. The stratification also hinders the transfer of phosphorous (which is 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 that the parcel oscillates slowly, because the density difference between it and the surrounding water is small. This in turn implies that the water around our parcel is well (and easily) mixed. If the frequency is high, the water is strongly stratified. In well mixed water, turbulent eddies can grow, and mix the water column further, while in regions of strong stratification, the growth of eddies is inhibited, reducing mixing. Additionally, a region of strong stratification inhibits mixing between water above and below. Changes in buoyancy frequency with time show the effect of seasonal stratification (Figure 2). In summer, the strongest stratification is found in shallow water, due to the effect of the (warm) fresh water run-off from the land, and from solar heating. As the year progresses, the depth of the stratification maximum increases, due to increasing volumes of fresh water, further solar heating, and in the autumn, by mixing by waves, and surface cooling leading to downward convection. In winter and spring, the surface water becomes mixed down to the level 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 the longer-term stratification in the basin, and the depth of the surface layer without the noise introduced by the intra-annual variability. Figure 3 shows the changes in each of the basins between 1990 and 2002. Changes in Page 5 of 7
the depth, and magnitude of the buoyancy frequency could be expected due to changes in run-off from land, in the 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 levels above 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 from biological activity in the surface waters sinks, and is broken down by bacteria, which consume the available oxygen. When oxygen concentrations fall below about 1.5 mg/l, bacteria start to use anaerobic processes, which produce hydrogen sulphide as a by-product. This hydrogen sulphide is toxic, and its concentration is described in terms 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 buoyancy frequency 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 depth of the maximum is also reduced compared to the other basins. Higher values of buoyancy frequency are expected 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 values from the West Gotland Basin and Northern Baltic Proper as could be expected from changes in salinity but these are very small. The depth of the maximum, shown in the lower plot, appears unchanged in the West Gotland Basin, but shallows in the Northern Baltic Proper. In the southern Baltic, the depth of the maximum appears to increase from around 40 metres in 1994, to about 60 in 2002. This could be attributed to a reduction in the inflow of salt water from the Kattegat, and a steady dilution of the basin by the fresher surface waters. Time series of surface salinity between 1990 and 2002 show a steady freshening of the surface 10 metres of the Baltic Proper, at an average rate of 0.04 practical salinity units (psu) per year. Deep-water salinity between 1990 and 2002 shows the opposite trend. Below 80 metres, salinity increased by 0.09 psu per year, with the highest rates of increase in the Northern Baltic Proper and the West Gotland Basin. The surface salinity changes are reasonable, given the increasing fresh water input particularly since 2000. The deep water changes suggest the movement of saline water in the deep basins of the north and west, which is not replaced with new salt water in the Southern Baltic Proper. 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 the Baltic Proper, 1990 2002. Bracketed values show data from 2000-2001 Region Standard deviation 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% For each of the basins, autumn (August, Table 2 Standard deviation, as a percentage of basin volume, September and October) oxygen profiles in the estimates of the mean basin volume affected by reduced were examined using all data from SMHI oxygen levels in the Baltic Proper, 1990-2001 and ICES (between 1990-1999) and SMHI and FIMR (2000-2001). For 2000 2001, the period was extended to December in the Northern Baltic Proper, due to a paucity of autumn data. For each profile, the depth at which the oxygen Page 6 of 7
concentration fell below certain levels (0, 2,4,5 and 6 mg/l) were calculated, and these depths were used to estimate the volume of water in each basin affected by reduced oxygen levels. These were then averaged, to give an annual basin-mean volume at each oxygen level, and are presented in Figure 4 as a percentage of the total volume of each basin. The standard deviation from these averages gives an idea of the variation within each basin. 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 oxygenated saline water from the Kattegat, spreading out first through the Southern Baltic Proper, and then into the deep regions of the other basins. Since this last, large inflow, oxygen concentrations have decreased in all basins. The East Gotland Basin and Northern Baltic Proper are worst affected, with between 20 and 40% of the total basin volume experiencing reduced oxygen levels, and almost 30% of the Northern Baltic Proper suffering acute toxicity in 2001. Hydrogen sulphide is present in 10% of the water by 2001, and area affected is increasing. The proportions 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 Baltic Proper, while their surface waters (which make up a larger proportion of their volume) are regularly replaced (by fresh water flowing out). 6. References 7. Data Hydrographic data was collected by SMHI from the research vessel Argos, between 1990 and 2002. Oxygen data came from the Swedish Meteorological and Hydrological Institute data archive (SHARK), from the ICES databank, and from the Finnish Institute for Marine Research. 8. Meta data Page 7 of 7