| CHAPTER IV - State of the marine Environment of the Baltic Sea Regions |
Part 1 : Gulf of Bothnia (Bothnian Bay, Bothnian Sea, Åland Sea, Archipelago sea)
P. Sandén1, H. Pitkänen, K. Eloheimo
The Bothnian Bay has low phosphorus concentrations and comparatively high concentrations of nitrogen, while in the Bothnian Sea the phosphorus concentrations are considerably higher and the nitrogen concentrations, primarily inorganic species, are lower. This nutrient distribution favours a limitation of the lightsaturated planktonic production by the available phosphorus in the Bothnian Bay, while phosphorus and nitrogen might alternate as limiting substances in the Bothnian Sea.
Analyses of long-term changes in chemical variables during the period 1970-93 showed several significant trends. The concentrations of inorganic nitrogen have increased throughout the Gulf. Similar trends were also found for total nitrogen, which in part is due to the increase in inorganic nitrogen. Inorganic phosphorus concentrations have decreased in the Bothnian Bay, in both surface and bottom waters. Total phosphorus has increased in the surface waters of the Northern Quark, Bothnian Sea and åland Sea. Silicate concentrations have generally decreased in the Gulf. The long-term changes presented here agree with previous published studies (see REF : 589,592). Trend estimates on the riverine load to these basins show significant increases in total nitrogen and silicate loads, while the changes in phosphorus loads are not significant. These changes are also in accordance with previously published studies (see REF : 558,635). Changes in riverine loads can, therefore, to some extent explain the observed trends in the Gulf. The increasing loads were mainly due to higher run-off during the 1980s compared with the 1970s. The increased input of nitrogen from the Finnish drainage area was also due to an increased use of fertilisers in the 1970s and 1980s (see REF : 545).
Although there is only little measured evidence for increased primary production or phytoplankton biomass, several trends observed in hydrochemical variables during 1970-93 can be interpreted as an indication of increasing primary or bacterial production. The decrease of phosphate concentrations in the Bothnian Bay points in that direction, as does the increase in total phosphorus in large parts of the Gulf. Decreases in silicate and ammonium concentrations could also be caused by changes in the plankton production in the Gulf, although direct evidence is lacking. The decrease in concentrations is particularly hard to explain by changes in the load situation, because of the higher run-off in the later part of the period. However, reduction of coastal point sources during the 1970s and 1980s have contributed to the decrease of coastal phosphorus levels (see REF : 545).
The increase in nitrogen concentrations could be attributed to increased riverine load, as both concentration and flow were higher during the 1980s compared to the 1970s (see REF : 545,558,635). Increase in nitrogen concentrations in the Baltic Proper (see REF : 589), and increasing concentrations in the atmospheric deposition (see REF : 183) might also have contributed. Internal cycling of nutrient could be changing as well, leading to trends in the nutrient concentrations. The origin of the changes in observed concentrations can presently not be clearly determined. The complex interaction between different loads and processes within basins and lag effects occurring between the different factors contribute to the difficulty of defining single key factors.
The limited data restrain the analysis of the hydrochemistry in the open Gulf. A much denser sampling programme is needed for a proper spatial analysis. Analysis of seasonal variation and the ability to consider climatic variation from year to year is also hampered by the low sampling frequency at the open-sea stations. With a better harmonisation of the sampling programme, it should be possible to improve the data material for both types of analysis without incurring unreasonable. Within national programmes, several intensive stations are used in coastal waters, and this increases the ability to assess the state of these regions.
The major part of variations in the riverine load of nutrient inputs is due to variations in run-off. The average loads of nitrogen over the whole period 1970-93 were 44,000 t yr-1 to the Bothnian Bay and 48,000 t yr-1 to the Bothnian Sea. Corresponding data for total phosphorus are 2,800 and 2,300 t yr-1, respectively. Figure 4.1.6 presents yearly loads of total nitrogen and total phosphorus for the Bothnian Sea and Bothnian Bay (see REF : 634).
Trend tests on the riverine load data suggest a significant increase in nitrogen loads, while the increase in phosphorus is not significant. The tendency towards increasing loads of most chemical variables during the investigated period was primarily a result of the higher run-off during the 1980s compared with that in the 1970s. The use of fertilisers may have contributed to this trend on the Finnish side (see REF : 545). Regarding the Archipelago Sea, the input of nitrogen was also estimated to have increased, while the phosphorus load did not show a clear change.
The present analysis is limited to the riverine load and does not include the atmospheric load, point sources at the coast and imports from adjacent basins. Available literature suggests an increase in atmospheric deposition of nitrogen (see REF : 183). The transport between basins might also increase the load on the Gulf as the concentration of both nitrogen and phosphorus increased in the Baltic Proper (see REF : 589).
Estimates on the origin of nutrients include uncertainties. It was suggested (see REF : 545) that roughly 50 % of the total phosphorus input and 40 % of the total nitrogen input entering the Gulf of Bothnia from Finland has an anthropogenic origin. Agriculture, identified as the largest single source, was considered responsible for 24 % of the total nitrogen and for 28 % of the total phosphorus input to the Gulf of Bothnia from the Finnish drainage area. The contribution from the agriculture is almost 50 % of the bioavailable nitrogen input (see REF : 545).
Spatial representations of the average surface (0-10 m) nutrient concentrations are given in contour plots (Figs. 4.1.7, 4.1.8). These graphs are based on data from stations with observations during 1989-93. The present analysis focuses on the second (April-June) and fourth quarters (October-December), defined as ‘spring’ and ‘autumn’, respectively. A division of seasons, based purely on climatic variation from year to year, cannot be justified because of problems with the scheduling of expeditions. The two quarters chosen represent one season with a high primary production and one season dominated by remineralization. However, four near-coast intensive stations provided some complementary information for a whole seasonal cycle (Figs. 4.1.9, 4.1.10).
Differences between basins - As previously pointed out (see REF : 729), Figures 4.1.7 and 4.1.8 show that the major differences in the Gulf are found between the Bothnian Sea and the Bothnian Bay. Variations within these sub-basins are usually smaller. The different nitrogen fractions, as well as silicate, are higher in the Bothnian Bay, possibly due to the higher loading of fresh water per volume. Only small spatial variations in the concentration of total nitrogen can be observed during the two seasons. The main part of this variation can be attributed to the inorganic fractions.
Compared to nitrogen, phosphate and total phosphorus show the opposite pattern during autumn, with higher concentrations in the Bothnian Sea. Very low phosphate concentrations are found throughout the Gulf during spring, without any notable spatial pattern. Taken together, the general pattern for nitrogen and phosphorus results in a rapid change in concentrations in the Northern Quark area, separating the two main sub-basins. The origin of the low phosphate is not yet clearly determined, but precipitation with iron has been suggested as one possible mechanism (see REF : 682).
The oxygen content of water is supersaturated during spring in most parts of the Gulf, but the highest values, around 115 % saturation, were found in the central part of the Bothnian Sea. Autumn-oxygen-saturation values, on the other hand, are <100 % (94-97 %) and exhibit only small variations throughout the Gulf.
Compared to the individual nutrients, the pattern for the inorganic nitrogen-to-phosphorus ratios (DIN:NIP) becomes more pronounced, while the silicate-to-inorganic-nitrogen ratio (DSi:DIN) shows a uniform pattern. According to basic ecological theories, the nutrient ratios have a potential to indicate the growth-limiting nutrient. In the Bothnian Sea, the DIN:DIP values are around the Redfield uptake ratio of 16:1 indicating balance between the two nutrients, while phosphorus is the limiting nutrient in the Bothnian Bay. Throughout the Gulf no instances of DSi:DIN ratios of ¨1:1, the uptake ratio for diatoms, can be observed. Silicate-limiting conditions are therefore unlikely.
Previous studies on the spatial distribution of nutrients within the entire Baltic Sea gave similar results (see REF : 591,729). Using data from 1980-89, statistical testing on differences between the different basins was made. The earlier findings correspond to the general pattern presented here (Figs. 4.1.7, 4.1.8). The differences between the sub-basins are most probably an effect of the higher loading of fresh water, rich in allochthonous organic carbon, to the Bothnian Bay. Here, the annual phytoplankton growth is pronouncedly hampered due to nutrient and/or light deficiency (see REF : 7,20,373), while the bacterial growth is sustained, compared to the Bothnian Sea, through the supply of allochthonous organic carbon (see REF : 373). The relative successful competition of bacterioplankton in this basin is due to the riverine loading of organic carbon. Therefore, the different, effective uptake kinetics shown by bacterioplankton may therefore contribute to the observed hydrochemical situation in the Bothnian Bay. The food web, dominated by small effectively suspended plankton organisms and sparsely occurring diatoms, also results in an extremely low sedimentation rate of carbon, nitrogen and phosphorus in the open Bothnian Bay (see REF : 705). The low vertical stability further promotes an efficient supply of oxygen also to the deep water. Combined with the low sedimentation rate, this fosters a well-oxygenated bottom environment.
Nutrient variability within the basins - The results of the present study agree with a previous detailed analysis of transects in the Bothnian Sea (see REF : 591). The concentrations of several elements are lower in the central basin as compared with those at stations closer to the coast (Figs. 4.1.7, 4.1.8). Furthermore, the concentration of inorganic phosphorus seems to be higher on the eastern side of the basin, whereas ammonium and silicate tend to be higher on the western side. During spring, total phosphorus deviates from this pattern, showing the highest concentrations in the central part of the Bothnian Sea.
Several processes contribute to produce these patterns. The general circulation in the Gulf is one important factor, producing a northward transport along the Finnish coast and a southward transport along the Swedish coast. This brings water with higher phosphorus concentrations from the Baltic Proper northwards, and waters with higher silicate and inorganic nitrogen concentrations move southwards. The higher concentrations closer to the coast can also be a result of the riverine input and direct emissions from communities and industries located along the coast. Nutrient-rich water from the deeper layers could also be an important source, as upward transport mechanisms are more efficient in shallower areas.
Nutrient variability in coastal waters - The Finnish coastal waters are exposed to pronounced riverine discharge. Therefore, they generally show 2-4 times higher concentrations of total nitrogen and phosphorus compared to waters from corresponding offshore areas (Figs. 4.1.9, 4.1.10; (see REF : 328,547)). The width of the zone, affected by the coastal effect, varies from several tens of kilometers in the Archipelago Sea, Northern Quark and northeastern Bothnian Bay to practically zero at open and less contaminated parts of the coast.
The area representing the most extensive coastal effect is the southernmost part of the Gulf, the Archipelago Sea, where during winter the average concentration of total nitrogen varies between 25 and 50 mmol m-3 and total phosphorus between 0.8 and 1.5 mmol m-3 ( Figure 4.1.9; (see REF : 301)). The inner archipelago is characterised by loading from intensive agriculture and municipalities. In the outer archipelago, load from fish farming has been assessed as substantial and the most prominent external source of nutrients in summer (see REF : 301).
The other Finnish coastal waters showing increased nutrient concentrations include areas such as those off Uusikaupunki, the Kokemäenjoki Estuary with adjacent waters, the Northern Quark Archipelago, the waters off Kokkola-Pietarsaari and the whole northeastern Bothnian Bay. Municipal and industrial waste waters, agriculture and intensive forestry are the most likely primary sources for the nutrient increase (see REF : 317,328,545), which has been reflected by an increase of the primary productivity and of chlorophyll a concentrations.
The description of the seasonal variation of nutrients at Finnish coastal stations is based on available literature and data from the Finnish intensive stations collected by the Regional Environment Centers and handled by the Finnish Environment Institute (Figs. 4.1.9, 4.1.10).
In the offshore regions of the Gulf of Bothnia, including most of the Finnish coastal zone, except the Archipelago Sea, the relatively low winter values for inorganic nitrogen and phosphorus suggest a low potential phytoplankton productivity (see REF : 7,317,394,728). In the open Finnish coastal waters of the southern Bothnian Sea, the winter values are somewhat higher (Figs. 4.1.9, 4.1.10). Only in the southernmost part of the Gulf, the Archipelago Sea, do higher winter values occur, which are similar to those in the eutrophic eastern Gulf of Finland, i.e., 20 mmol m-3 for nitrogen and 1.0 mmol m-3 for phosphorus (see REF : 544).
According to the observed concentration patterns, phosphorus limits the primary production in summer both in the open and in the coastal Bothnian Bay. In early summer, the coastal water phosphate concentrations are occasionally close to the detection limit, while DIN seldom sinks below 2.5 mmol m-3 in this basin ( Figure 4.1.10; (see REF : 7,317)). As the inorganic N/P ratios approach 100, this indicates that phosphorus predominates as the limiting nutrient. In the coastal waters of the Bothnian Sea, the lower summer concentrations of inorganic nitrogen, and consequently lower DIN:DIP ratios between 10 and 20, indicate a relatively balanced supply of nitrogen and phosphorus during the growing season (Figs. 4.1.9, 4.1.10). Caution should, however, be applied when interpreting hydrochemical data exclusively in terms of limiting nutrient, because in a regenerating summer food web the relative supply rate of the nutrients is the most important parameter.
The conditions in the Archipelago Sea clearly differ from those of the remaining part of the Gulf. Here both the general level and the annual variation of nitrogen and phosphorus concentrations, and the level of primary production, are much higher than in the open waters, i.e., up to 1.0 mmol m-3 P and 40 mmol m-3 N ( Figure 4.1.10; (see REF : 301)). This is due to the average winter inflow of nutrient-rich surface water from the Northern Baltic Proper and from the Gulf of Finland. It is also due to land-based nutrient inputs as well as to the restricted vertical mixing caused by the complex geomorphology of this area.
Rivers contribute nutrients to coastal waters especially in April/May during the spring flood. This increases the nutrient concentrations in estuaries and in the adjacent waters, especially in the northeastern Bothnian Bay and in the inner Archipelago Sea. High riverine loading appears to increase the inorganic N/P ratio in coastal waters, compared with open waters in the Bothnian Sea and Archipelago Sea, promoting phosphorus as a limiting nutrient off the river mouths and in the enclosed basins of the inner archipelagos (see REF : 545).
The largest seasonal variations in phosphate and inorganic nitrogen were found at a frequently sampled Swedish coastal station (NB1, 63°30’N, 19°48’E) ( Figure 4.1.11). Both variables approach zero during the summer. Other variables exhibited a less pronounced seasonal variation ( Figure 4.1.11).
The large variation in inorganic nitrogen and phosphorus is an obvious effect of the uptake by the osmotrophic phyto- and bacterioplankton. The increase in temperature and water column stability in spring leads to a pronounced increase in planktonic production. The production during summer is limited by the mineralisation rate of inorganic nutrients by micro- and macrozooplankton. The low seasonal variation in silicate indicates that this nutrient is available in amounts that clearly exceed the organisms’ requirements. Seasonal variations in total nitrogen and phosphorus are largely a result of the variation in the inorganic fraction. The observation that total phosphorus does not decrease as much as expected from the decrease in phosphate concentration might be a result of the increase in dissolved organic phosphorus during summer due to riverine inflow to the estuary ( Figure 4.1.11).
Inorganic nitrogen-to-phosphorus ratios showed a weak seasonal variation, indicating balance between these nutrients at NB1. A much more pronounced variation was found for the ratio between dissolved silica and inorganic nitrogen.
The data does not allow an evaluation of trends during 1989-93, since a period of at least ten years is needed to get reliable results from trend tests. This is due to the test statistics and the ability to separate short-term variations from changes in the system. The trend analysis was therefore made on two different periods, one covering the whole period of reliable data (1970-93) and the other covering the three HELCOM assessment periods from 1979 to 1993. Two different depths were considered, i.e., the trophogenic zone, here defined as the first 10 m, and the bottom sampling depth. Statistical results are presented in Table 4.1.1 and Table 4.1.2.. No differences in trends were discernable among seasons.
The development of total phosphorus, nitrate and silicate concentrations is shown for the surface water at station C1 in Figure 4.1.12. Aggregated with data from two other stations, the trend analysis for the Bothnian Sea resulted in significant trends for all three variables at the 1 % level. Figure 4.1.12 illustrates the high variability of the data used in the analysis.
Phosphorus - In general, trends of the order of 1-2 % yr-1 can be detected for phosphorus by applied statistics. Phosphate showed three significant trends, all of them with decreasing concentrations in the Bothnian Bay ( Table 4.4.1, Table 4.1.2). The results indicate that a change of >=2 % yr-1 is needed for a significant trend over the whole period covered by reliable data, or >=3 % yr-1 for the shorter period 1979-93. Significantly increasing concentrations of total phosphorus are seen during 1970-93 in the surface waters of all sub-basins but the Bothnian Bay.
The riverine load of nutrients to the Gulf has not changed systematically (phosphorus), or was nearly constant and even slightly increasing (nitrogen) ( Figure 4.1.6), and can therefore not explain the decrease of the concentrations in the Bothnian Bay. Both increasing phyto- or bacterioplankton production or increasing precipitation of metal (primarily iron) phosphates could also explain the observed decreasing trends. However, an increase of the potential primary production could not be found, and bacterioplankton and sedimentation series in offshore areas are too short to allow trend analyses. Since point sources at the coast contribute less than 10 % of the phosphorus load to this part of the Gulf (see REF : 225), this makes changes in this source an unlikely cause of the decline in nutrient levels. The increase of total phosphorus in the surface waters could be due to decreased vertical stability caused by a decreasing salinity in the deepwater. An increase of the riverine load in total phosphorus could also partly contribute to the observed changes.
Silicate - Silicate concentrations in the Bothnian Sea and åland Sea were observed to decrease by about 3 % yr-1 in surface waters. At the åland Sea station, decreasing concentrations were also found in the deep waters. The Bothnian Bay exhibited increasing concentrations during 1979-93, but there was no significant trend for 1970-93. Trends of the order of 1-2 % yr-1 can be detected. The riverine load of silica increased during the period, and the decreasing concentrations in the Gulf must therefore be related to other factors. The most probable cause is an increase in primary production. Changes in the internal silica cycling and in the exchange of silicate with the Baltic Proper can, however, not be excluded.
Nitrogen - Total nitrogen concentrations increased throughout the Gulf, both at the surface and close to the bottom. This was evident for both tested periods, with only single non-significant results for the shorter period in the Bothnian Bay and in the Northern Quark. The order of the change was about 1 % yr-1 for the whole Gulf. A significant part of the increase was due to the increase in nitrate, especially in the deep water ( Table 4.1.1, Table 4.1.2).
Oxidised forms of nitrogen (nitrate and nitrite) increased throughout the Gulf of Bothnia both in the surface layer and close to the bottom. At the surface, the trend slope was steepest for the northern areas of the Gulf. The Bothnian Bay showed a slope >2 % yr-1 for both periods. In the southern areas, the slope value was considerably lower, i.e., only about 0.3 % yr-1. Trends as low as 0.3 % yr-1 can be detected given, the characteristics of the present data set. For all sub-basins, the analysis in the surface waters indicated steeper slopes during 1979-93 compared with 1970-93. Bottom waters showed a reverse picture, with a steeper slope during the longer period and in the southern parts of the Gulf.
Trend analyses of the reduced form of nitrogen (ammonium) revealed three significant trends. For the 1970-93 period in the southern parts of the Gulf, a decrease could be seen in surface waters. A significant increase of ammonium could be shown for the bottom waters during the shorter period in the Bothnian Sea. Quite steep slopes, of the order of 5 % yr-1, are needed to get significant trends in ammonium concentrations.
Most of the changes in nitrogen concentration can be explained by increasing loads from the surrounding rivers and from the atmosphere ( Figure 4.1.6; (see REF : 545,635)). Increasing trends for nitrogen in the Baltic Proper and changes in the vertical stability of the water column in the Gulf of Bothnia may, however, also have contributed.
Oxygen saturation - Only limited changes in the oxygen saturation could be found in the time series. Increases in the saturation in the bottom waters of the Bothnian Bay and the Northern Quark were the only significant changes. These trends were <0.5 % yr-1 and were probably connected to the lower vertical stability with decreasing salinities.
Nutrient ratios - Analyses of trends in the inorganic nitrogen-to-phosphorus ratio revealed only a few significant trends in the surface water, whereas all but one test showed significant increases in the bottom waters. The latter is primarily due to the increase in nitrogen. For surface water, the observed trends were increases during both periods in the Bothnian Bay, and during the shorter period in the åland Sea. The silicate-to-inorganic nitrogen ratio, on the other hand, decreased throughout the Gulf. Only two non-significant cases were observed. The change in the Si:N ratios was due to both the increase in nitrogen and, in most sub-areas, also due to the decrease of silica, both in the deep and in the surface water.
Differences between the assessment periods - The data set is too limited to make a thorough statistical evaluation on differences between the different assessment periods, 1979-83, 1984-88 and 1989-93. Given the present sampling frequency, sample variability and statistical methods, one cannot expect to find changes in a system by using trend analysis on such short data series. Apart from the long-term trends presented above, no major changes in the hydrochemistry could be seen between the periods.
By the application of linear regression analysis, a significant increase in nitrogen and phosphorus for most parts of the Archipelago Sea was found for the period 1970-93 (see REF : 301). The increasing trends were explained by increased loading from agriculture, fish-farming and via the atmosphere during the 1970s and 1980s. In contrast, phosphorus decreased significantly in the innermost archipelago, possibly due to intensified purification of municipal waste waters in Turku and neighbouring towns.
In the open area between the Archipelago Sea and the åland Islands, the increase of nutrients was mainly due to a similar increase in the Baltic Proper (see REF : 589). In the Finnish coastal waters affected by the pulp and paper industry, e.g., off Oulu in the southeastern Bothnian Bay, the loading of phosphorus had decreased by 100 t yr-1 at the beginning of the 1990s (see REF : 328). This caused lower phosphorus concentrations at some locations in these areas. Time series of nutrients at the Swedish coast are too few and too short to allow a trend analysis.