High nutrient inputs from the Neva River in the eastern Gulf of Finland, Baltic Sea create steep ... more High nutrient inputs from the Neva River in the eastern Gulf of Finland, Baltic Sea create steep spatial gradients in water column concentrations of nutrients and of particulates. During July 1993 nutrient fluxes across the sediment-water interface were measured beginning in this eutrophication gradient and extending out into the open waters of the western Gulf. Dissolved oxygen (O 2 ) microgradients showed that O 2 concentrations were depleted within the first 2-4 mm of the sediment-water interface and calculated O 2 consumption ranged from 462 to 526 mol m 2 h 1 with no significant differences [analysis of variance (ANOVA); P >0·05] observed between stations. Spatial patterns, however, were evident with significant differences between stations observed in most nutrient fluxes (ANOVA; P<0·01) with the highest sediment-water fluxes observed at the most eutrophic station in the eastern Gulf of Finland. Sediment-water fluxes ranged between 9·37-34·7 mol m 2 h 1 for NH 4 + , 6·8-12·45 mol m 2 h 1 for NO 3 , 0·10-2·45 mol m 2 h 1 for NO 2 and 1·59-28·5 mol m 2 h 1 for dissolved inorganic phosphate. Dissolved silicate fluxes (71·8-83·5 mol m 2 h 1 ) were the only nutrient not to show significant differences (one-way ANOVA; P>0·05) between stations. Estimated denitrification rates from sediments ranged from 6·6-11·5 mol m 2 h 1 and are not particularly intense compared with other Baltic areas, however, given that these rates only include denitrification supported by water column NO 3 concentrations and not denitrification coupled to nitrification with the sediments, then they are not so far from the rates (17·8 mol m 2 h 1 ) estimated from mass balances in the Gulf of Finland. The sediment-water fluxes reported here include some of the highest rates reported for the Baltic Sea, but are typical of most eutrophic coastal marine environments. The high fluxes reported here contribute to the large reservoir of dissolved inorganic nutrients contained in the water column immediately below the mixed surface layer.
Biological, Physical and Geochemical Features of Enclosed and Semi-enclosed Marine Systems, 1999
The entire Baltic Sea, as well as many of its different sub-regions, are subject to eutrophicatio... more The entire Baltic Sea, as well as many of its different sub-regions, are subject to eutrophication due to high nutrient inputs. To plan expensive water management measures one needs a tool to quantify effects of different water management policy decisions. The tools implemented here are simulation models based on similar descriptions of biochemical interactions in the water and sediments but coupled to different hydrodynamical models. For the Baltic Proper a lD physical model with high vertical resolution but horizontally integrated was used. Simulations for 20 years made with 50% load reduction each 5 year show that for this domain and at these scales the recovery would take decades. The most effective is reduction of phosphorus, while reduction of only nitrogen leads to a dramatic increase in cyanobacteria blooms. For the Gulf of Finland a high-resolution 3D hydrodynamic model was coupled to a more crude 3D-box biogeochemical model describing concrete conditions during August and November 1991. In the Eastern Gulf of Finland the effects of a 50% load reduction from the St. Petersburg region are pronounced even after two weeks. Here, nitrogen reduction would be more beneficial than that of phosphorus, both locally and at a larger scale. The conclusion from these simulations is that the difference in effects of nitrogen versus phosphorus reduction is dependent on scales and locations of management.
Vatten, ISSN 0042-2886, vol. 50, nr. 3, 220-230 Based on the intensive hydrochemical measurements... more Vatten, ISSN 0042-2886, vol. 50, nr. 3, 220-230 Based on the intensive hydrochemical measurements during the Gulf of Bothnia Year 1991, we present descriptions of the regional variations of freshwater and nutrient inputs and hydrographic and nutrient conditions in the basins for this particular year. These data are then used to construct budgets of silicate, inorganic and organic nitrogen and phosphorus for the Bothnian Sea and Bay. The variations of internal sources and sinks and advective transports between the basins and with the Baltic proper show the relative importance and magnitude of biogeochemical and physical processes for the development of nutrient concentrations in these basins. Annual budget estimates show that the Bothnian Sea appears to function as a very efficient sink for nitrogen. There is a very small exchange of nitrogen with the Baltic proper, in contrast to the large amounts of phosphorus imported and silica exported. Overall N/P ratios show the high rates typ...
We are using the coupled models of decision support system NEST, to evaluate the response of the ... more We are using the coupled models of decision support system NEST, to evaluate the response of the marine ecosystem to changes in external loads through various management options. The models address all the seven major marine basins and the entire drainage basin. A series of future scenarios have been developed, in close collaboration with HELCOM, to see the possible effects
... Variations of nitrogen, phosphorus and silicate in the Baltic 25 central station in the easte... more ... Variations of nitrogen, phosphorus and silicate in the Baltic 25 central station in the eastern Gothland Basin may well describe the average conditions. RESULTS In order to describe typicalseasonal and spatial distribution patterns, the interpolated monthly surface (0-5 m) mean ...
Bottom water and sediment characteristics and net sediment-water fluxes of oxygen, nitrogen, phos... more Bottom water and sediment characteristics and net sediment-water fluxes of oxygen, nitrogen, phosphorus and hydrogen sulphide were measured under oxic a n d experimentally induced anoxic conditions along a depth gradient (47 to 130 m) during summer in the Baltic proper. Temperature, salinity and dissolved nutrient concentrations (particularly phosphorus and nitrate) in bottom waters increased with depth while oxygen concentrations decreased sharply. Sediment organic content was much higher in sediments located beneath the permanent pycnocline (ca 65 m). Sediments at all stations were somewhat depleted in total N relative to total C (C.N -10). Sediments at the shallow station (47 m ) were highly enriched with total P relative to C or N (C:N P = 18:2:1) but were somewhat depleted in P at the deeper stations (C:N:P = 120:12:1). Under oxic conditions oxygen fluxes ranged from 214 to 777 pm01 0 m Q-' and decreased with depth. Phosphorus and nitrite fluxes were always very small and were directed either into or out of the sediments. Ammonium fluxes were small (1 to 30 pm01 N m-' h-') at all stations and did not exhibit a clear pattern with depth; 0 : N -N H , flux ratios were close to expected 'Redfield ratios' at the deep station (16:l) but were much higher (>55:1) at the shallower sites indicating that processes other than only an~monificatlon were taking place in sediments. Nitrate was always consumed by sediments (ca 1 to 16 ktn~ol N m-' h-') and fluxes were proportional to nitrate concentrations in the overlying water. Under anomc conditions there was a dramatic increase in P-PO4 fluxes (2 to 40 pm01 P m-' h-') and a smaller lncrease In N-NH, fluxes (14 to 35 pm01 N m-2 h ' ) . Large hydrogen sulphide fluxes (>40 pm01 S m-' h-') were observed in sediments from the deepest station only. Under oxic conditions sediment recycling of N and particularly of P were small compared to estimated rates of burial of P and burial plus denitrification of N However, under anoxic conditions, sediment recycling of both N and P were similar to or much greater than sedlment loss terms of burial and denitrification. Sediment regeneration under oxic conditions could supply 1 to 8 % and 0 to 2 % of estimated phytoplankton demand of N and P, respectively; under anoxic conditions 12 % of N and u p to 200 O/o of P demand could be met via sediment recycling.
Increasing nutrient loads have characterized the Baltic Sea during the last century. However, the... more Increasing nutrient loads have characterized the Baltic Sea during the last century. However, the detection of long-term trends in the water column has been difficult due to both paucity of data and high variability. Analysis of water quality data with robust non-parametric methods has shown statistically significant increases in total nitrogen, total phosphorus, nitrate (NO3), and dissolved inorganic phosphate, although with considerable spatial and temporal differences. Significant decreases in dissolved silicate (DSi) and ammonium (NH,) concentration have also been reported. We report here significant decreases in the DSi:DIN ratio (where DIN, dissolved inorganic nitrogen, is the sum of NO3, NO2, and NH4 concentrations) in the Baltic Sea from 1970 to 1990. The molar ratios prior to the formation of the spring bloom are now approaching un~ty, with further decreases expected with continued eutrophication of the Baltic Sea. This can be explained by an increased net sedimentation of biogenic silica due to increased primary production attributable to increased nutrient loading. While the Baltic proper is generally assumed to be N limited, declining DSi:DIN ratios indicate that spring diatom growth may become DSi limited in the near future, as the optimal DSi:DIN ratio for diatom growth is approximately 1:l. This decrease in the DSi:DIN ratio cannot be statistically detected in the river input to the Baltic proper. Only a few significant tests were found in the sea, with both upward and downward trends detected. Ecological implications of this observed reduction in the DS1:DIN ratio may include DSi-limited diatom growth and changes in species composition and, subsequently, food web dynamics.
ABSTRACT A diagnostic advection-diffusion equation system in a ‘natural coordinate’ frame of refe... more ABSTRACT A diagnostic advection-diffusion equation system in a ‘natural coordinate’ frame of reference is investigated in order to improved its robustness. The dependence of the leading-order solutions on higher-order features of the hydrographic data has been decreased and a differential equation with and analytical solution is obtained. The properties of the lowest-order equation are compared with previous studies where the undegenerated equations were used to estimate the distribution of both the diapycnal salt flux and the oxygen consumption within the Baltic proper. The agreement found is within the acceptable bounds. Finally a pilot study of a silicate budget of the Baltic proper has been performed.
The brackish Baltic Sea has been seen as particularly suitable for studies of food webs. Compared... more The brackish Baltic Sea has been seen as particularly suitable for studies of food webs. Compared to fully marine ecosystems, it has low species diversity, which means fewer trophic linkages to analyse. The Baltic Sea is also one of the best-studied areas of the world, suggesting that most data requirements for food web models should be fulfilled. Nevertheless, the influence of physical and biological factors on trophic interactions and biogeochemical patterns varies spatially in the Baltic Sea, adding considerable complexity to food web studies. Food web structure and processes can be described and compared quantitatively between areas by estimating the flow of matter or energy through the organisms. Most such models have been based on carbon, though studies of complementary flows of other elements limiting production, such as nitrogen and phosphorus would be desirable. However, since ratios between carbon and other elements are used in calculating these flows, it is crucial, as a first step, to quantify the flows of carbon as accurately as possible.
Nitrogen as the limiting nutrient for primary production in the eutrophic Baltic proper has been ... more Nitrogen as the limiting nutrient for primary production in the eutrophic Baltic proper has been under debate. Based on only a limited number of actual measurements, nitrogen fixation has been assumed to be the only significant internal nitrogen source. It is then assumed that about one fifth of the net nitrogen load to the Baltic proper comes from nitrogen Ž . fixation by cyanobacteria. An alternative or additional source is utilisation of dissolved organic nitrogen DON .
External nutrient inputs to the major basins of the Baltic Sea a century ago were reconstructed f... more External nutrient inputs to the major basins of the Baltic Sea a century ago were reconstructed from various literature and data sources. The reconstructed input of total nitrogen was less than a half and that of total phosphorus was about a third of their contemporary values. The simulated "pre-industrial" conditions are validated by comparison to actual historical data on the water transparency, oxygen concentration, primary production, and net sediment accumulation. The "pre-industrial" trophic state could have been more phosphorus limited than today because simulated basin-wide annual averages of dissolved inorganic phosphorus concentrations of 0.06-0.3 µM P are about 40-80% of their present day values, while dissolved inorganic nitrogen concentrations of 2-4 µM N are almost the same as today or even slightly higher.
The environmental conditions of the Baltic Sea have been of concern for approximately the last th... more The environmental conditions of the Baltic Sea have been of concern for approximately the last three decades. In spite of this concern, the Sea is far from being in good ecological condition. One important reason may be the lack of an overview of costs and benefits associated with the pollution reductions needed for the restoration of the Sea. The purpose
Management of eutrophication in marine ecosystems requires a good understanding of nutrient cycle... more Management of eutrophication in marine ecosystems requires a good understanding of nutrient cycles at the appropriate spatial and temporal scales. Here, it is shown that the biogeochemical processes controlling large-scale eutrophication of the Baltic Sea can be described with a fairly aggregated model: simple as necessary Baltic long-term large scale (SANBALTS). This model simulates the dynamics of nitrogen, phosphorus, and silica driven by the external inputs, the major physical transports, and the internal biogeochemical fluxes within the seven major sub-basins. In a long-term hindcast , the model outputs reasonably matched observed concentrations and fluxes. The model is also tested in a scenario where nutrient inputs are reduced to levels that existed over 100 years ago. The simulated response of the Baltic Sea trophic state to this very large reduction is verified by a similar simulation made with a much more complex processoriented model. Both models indicate that after initial, rather rapid changes the system goes into much slower evolution, and nutrient cycles would not become balanced even after 130 years.
Overall budgets for nutrient and humus are described for the Baltic Sea as well as for the subsys... more Overall budgets for nutrient and humus are described for the Baltic Sea as well as for the subsystems, i.e., the Baltic proper, the Bothnian Bay and the Bothnian Sea. The residence times for total phosphorus, total nitrogen, silicate and humus are 13.3, 5.5, 11.2 and 9.6 years respectively, compared to 21.8 years for a conservative substance (salt). About 90% of the nutrient losses are due to biogeochemical sinks within the Baltic Sea. Thus only about 10% is exported to external areas (the Kattegat/Belt Sea). For humus the corresponding figures are about 75 and 25%, respectively. This means that the Baltic Sea to a large extent can be regarded as a closed system and perturbations in the water exchange with the North Sea should have little effect on the nutrient budgets. The sinks are parameterized by an expression borrowed from limnology where the net nutrient loss is a function of the winter surface concentration. A budget model is run in a prognostic, hindcast mode with the assumed timedependent phosphorus and nitrogen loading of the Baltic proper. The computed development of the winter surface concentrations of total P and total N for the period 1950-1988 appears quite realistic. The possibility of having variable sinks which are functions of the surface winter concentrations of nitrogen and phosphorus is described using calculations based on data from the different Baltic subareas. Such sinks should significantly decrease the winter N:P ratio in the surface water when the nutrient loading increases with time. With better descriptions of in particular the pools of nutrients in the sediment, it would be possible to model future changes of nutrient concentrations in the water column in relation to loading.
High nutrient inputs from the Neva River in the eastern Gulf of Finland, Baltic Sea create steep ... more High nutrient inputs from the Neva River in the eastern Gulf of Finland, Baltic Sea create steep spatial gradients in water column concentrations of nutrients and of particulates. During July 1993 nutrient fluxes across the sediment-water interface were measured beginning in this eutrophication gradient and extending out into the open waters of the western Gulf. Dissolved oxygen (O 2 ) microgradients showed that O 2 concentrations were depleted within the first 2-4 mm of the sediment-water interface and calculated O 2 consumption ranged from 462 to 526 mol m 2 h 1 with no significant differences [analysis of variance (ANOVA); P >0·05] observed between stations. Spatial patterns, however, were evident with significant differences between stations observed in most nutrient fluxes (ANOVA; P<0·01) with the highest sediment-water fluxes observed at the most eutrophic station in the eastern Gulf of Finland. Sediment-water fluxes ranged between 9·37-34·7 mol m 2 h 1 for NH 4 + , 6·8-12·45 mol m 2 h 1 for NO 3 , 0·10-2·45 mol m 2 h 1 for NO 2 and 1·59-28·5 mol m 2 h 1 for dissolved inorganic phosphate. Dissolved silicate fluxes (71·8-83·5 mol m 2 h 1 ) were the only nutrient not to show significant differences (one-way ANOVA; P>0·05) between stations. Estimated denitrification rates from sediments ranged from 6·6-11·5 mol m 2 h 1 and are not particularly intense compared with other Baltic areas, however, given that these rates only include denitrification supported by water column NO 3 concentrations and not denitrification coupled to nitrification with the sediments, then they are not so far from the rates (17·8 mol m 2 h 1 ) estimated from mass balances in the Gulf of Finland. The sediment-water fluxes reported here include some of the highest rates reported for the Baltic Sea, but are typical of most eutrophic coastal marine environments. The high fluxes reported here contribute to the large reservoir of dissolved inorganic nutrients contained in the water column immediately below the mixed surface layer.
The recently reported tendencies toward decreasing total amounts of silicate in the Baltic Sea ar... more The recently reported tendencies toward decreasing total amounts of silicate in the Baltic Sea are investigated by use of non-parametric trend analysis. The period 1968-1986 showed significant falling trends in surface waters from the whole system. The deepest parts of the Baltic proper did in contrast reveal strong increasing trends. These trends are more pronounced during the latter part of the analysis period which is characterized by stagnant conditions in the Baltic proper. These conditions have been prevailing since the major inflow in 1976. The causes of the observed changes are unclear. The increased load of nutrients and accompanying increase in primary production is, however, one factor. Another is coupled to the stagnation conditions of the Baltic Proper.
High nutrient inputs from the Neva River in the eastern Gulf of Finland, Baltic Sea create steep ... more High nutrient inputs from the Neva River in the eastern Gulf of Finland, Baltic Sea create steep spatial gradients in water column concentrations of nutrients and of particulates. During July 1993 nutrient fluxes across the sediment-water interface were measured beginning in this eutrophication gradient and extending out into the open waters of the western Gulf. Dissolved oxygen (O 2 ) microgradients showed that O 2 concentrations were depleted within the first 2-4 mm of the sediment-water interface and calculated O 2 consumption ranged from 462 to 526 mol m 2 h 1 with no significant differences [analysis of variance (ANOVA); P >0·05] observed between stations. Spatial patterns, however, were evident with significant differences between stations observed in most nutrient fluxes (ANOVA; P<0·01) with the highest sediment-water fluxes observed at the most eutrophic station in the eastern Gulf of Finland. Sediment-water fluxes ranged between 9·37-34·7 mol m 2 h 1 for NH 4 + , 6·8-12·45 mol m 2 h 1 for NO 3 , 0·10-2·45 mol m 2 h 1 for NO 2 and 1·59-28·5 mol m 2 h 1 for dissolved inorganic phosphate. Dissolved silicate fluxes (71·8-83·5 mol m 2 h 1 ) were the only nutrient not to show significant differences (one-way ANOVA; P>0·05) between stations. Estimated denitrification rates from sediments ranged from 6·6-11·5 mol m 2 h 1 and are not particularly intense compared with other Baltic areas, however, given that these rates only include denitrification supported by water column NO 3 concentrations and not denitrification coupled to nitrification with the sediments, then they are not so far from the rates (17·8 mol m 2 h 1 ) estimated from mass balances in the Gulf of Finland. The sediment-water fluxes reported here include some of the highest rates reported for the Baltic Sea, but are typical of most eutrophic coastal marine environments. The high fluxes reported here contribute to the large reservoir of dissolved inorganic nutrients contained in the water column immediately below the mixed surface layer.
Biological, Physical and Geochemical Features of Enclosed and Semi-enclosed Marine Systems, 1999
The entire Baltic Sea, as well as many of its different sub-regions, are subject to eutrophicatio... more The entire Baltic Sea, as well as many of its different sub-regions, are subject to eutrophication due to high nutrient inputs. To plan expensive water management measures one needs a tool to quantify effects of different water management policy decisions. The tools implemented here are simulation models based on similar descriptions of biochemical interactions in the water and sediments but coupled to different hydrodynamical models. For the Baltic Proper a lD physical model with high vertical resolution but horizontally integrated was used. Simulations for 20 years made with 50% load reduction each 5 year show that for this domain and at these scales the recovery would take decades. The most effective is reduction of phosphorus, while reduction of only nitrogen leads to a dramatic increase in cyanobacteria blooms. For the Gulf of Finland a high-resolution 3D hydrodynamic model was coupled to a more crude 3D-box biogeochemical model describing concrete conditions during August and November 1991. In the Eastern Gulf of Finland the effects of a 50% load reduction from the St. Petersburg region are pronounced even after two weeks. Here, nitrogen reduction would be more beneficial than that of phosphorus, both locally and at a larger scale. The conclusion from these simulations is that the difference in effects of nitrogen versus phosphorus reduction is dependent on scales and locations of management.
Vatten, ISSN 0042-2886, vol. 50, nr. 3, 220-230 Based on the intensive hydrochemical measurements... more Vatten, ISSN 0042-2886, vol. 50, nr. 3, 220-230 Based on the intensive hydrochemical measurements during the Gulf of Bothnia Year 1991, we present descriptions of the regional variations of freshwater and nutrient inputs and hydrographic and nutrient conditions in the basins for this particular year. These data are then used to construct budgets of silicate, inorganic and organic nitrogen and phosphorus for the Bothnian Sea and Bay. The variations of internal sources and sinks and advective transports between the basins and with the Baltic proper show the relative importance and magnitude of biogeochemical and physical processes for the development of nutrient concentrations in these basins. Annual budget estimates show that the Bothnian Sea appears to function as a very efficient sink for nitrogen. There is a very small exchange of nitrogen with the Baltic proper, in contrast to the large amounts of phosphorus imported and silica exported. Overall N/P ratios show the high rates typ...
We are using the coupled models of decision support system NEST, to evaluate the response of the ... more We are using the coupled models of decision support system NEST, to evaluate the response of the marine ecosystem to changes in external loads through various management options. The models address all the seven major marine basins and the entire drainage basin. A series of future scenarios have been developed, in close collaboration with HELCOM, to see the possible effects
... Variations of nitrogen, phosphorus and silicate in the Baltic 25 central station in the easte... more ... Variations of nitrogen, phosphorus and silicate in the Baltic 25 central station in the eastern Gothland Basin may well describe the average conditions. RESULTS In order to describe typicalseasonal and spatial distribution patterns, the interpolated monthly surface (0-5 m) mean ...
Bottom water and sediment characteristics and net sediment-water fluxes of oxygen, nitrogen, phos... more Bottom water and sediment characteristics and net sediment-water fluxes of oxygen, nitrogen, phosphorus and hydrogen sulphide were measured under oxic a n d experimentally induced anoxic conditions along a depth gradient (47 to 130 m) during summer in the Baltic proper. Temperature, salinity and dissolved nutrient concentrations (particularly phosphorus and nitrate) in bottom waters increased with depth while oxygen concentrations decreased sharply. Sediment organic content was much higher in sediments located beneath the permanent pycnocline (ca 65 m). Sediments at all stations were somewhat depleted in total N relative to total C (C.N -10). Sediments at the shallow station (47 m ) were highly enriched with total P relative to C or N (C:N P = 18:2:1) but were somewhat depleted in P at the deeper stations (C:N:P = 120:12:1). Under oxic conditions oxygen fluxes ranged from 214 to 777 pm01 0 m Q-' and decreased with depth. Phosphorus and nitrite fluxes were always very small and were directed either into or out of the sediments. Ammonium fluxes were small (1 to 30 pm01 N m-' h-') at all stations and did not exhibit a clear pattern with depth; 0 : N -N H , flux ratios were close to expected 'Redfield ratios' at the deep station (16:l) but were much higher (>55:1) at the shallower sites indicating that processes other than only an~monificatlon were taking place in sediments. Nitrate was always consumed by sediments (ca 1 to 16 ktn~ol N m-' h-') and fluxes were proportional to nitrate concentrations in the overlying water. Under anomc conditions there was a dramatic increase in P-PO4 fluxes (2 to 40 pm01 P m-' h-') and a smaller lncrease In N-NH, fluxes (14 to 35 pm01 N m-2 h ' ) . Large hydrogen sulphide fluxes (>40 pm01 S m-' h-') were observed in sediments from the deepest station only. Under oxic conditions sediment recycling of N and particularly of P were small compared to estimated rates of burial of P and burial plus denitrification of N However, under anoxic conditions, sediment recycling of both N and P were similar to or much greater than sedlment loss terms of burial and denitrification. Sediment regeneration under oxic conditions could supply 1 to 8 % and 0 to 2 % of estimated phytoplankton demand of N and P, respectively; under anoxic conditions 12 % of N and u p to 200 O/o of P demand could be met via sediment recycling.
Increasing nutrient loads have characterized the Baltic Sea during the last century. However, the... more Increasing nutrient loads have characterized the Baltic Sea during the last century. However, the detection of long-term trends in the water column has been difficult due to both paucity of data and high variability. Analysis of water quality data with robust non-parametric methods has shown statistically significant increases in total nitrogen, total phosphorus, nitrate (NO3), and dissolved inorganic phosphate, although with considerable spatial and temporal differences. Significant decreases in dissolved silicate (DSi) and ammonium (NH,) concentration have also been reported. We report here significant decreases in the DSi:DIN ratio (where DIN, dissolved inorganic nitrogen, is the sum of NO3, NO2, and NH4 concentrations) in the Baltic Sea from 1970 to 1990. The molar ratios prior to the formation of the spring bloom are now approaching un~ty, with further decreases expected with continued eutrophication of the Baltic Sea. This can be explained by an increased net sedimentation of biogenic silica due to increased primary production attributable to increased nutrient loading. While the Baltic proper is generally assumed to be N limited, declining DSi:DIN ratios indicate that spring diatom growth may become DSi limited in the near future, as the optimal DSi:DIN ratio for diatom growth is approximately 1:l. This decrease in the DSi:DIN ratio cannot be statistically detected in the river input to the Baltic proper. Only a few significant tests were found in the sea, with both upward and downward trends detected. Ecological implications of this observed reduction in the DS1:DIN ratio may include DSi-limited diatom growth and changes in species composition and, subsequently, food web dynamics.
ABSTRACT A diagnostic advection-diffusion equation system in a ‘natural coordinate’ frame of refe... more ABSTRACT A diagnostic advection-diffusion equation system in a ‘natural coordinate’ frame of reference is investigated in order to improved its robustness. The dependence of the leading-order solutions on higher-order features of the hydrographic data has been decreased and a differential equation with and analytical solution is obtained. The properties of the lowest-order equation are compared with previous studies where the undegenerated equations were used to estimate the distribution of both the diapycnal salt flux and the oxygen consumption within the Baltic proper. The agreement found is within the acceptable bounds. Finally a pilot study of a silicate budget of the Baltic proper has been performed.
The brackish Baltic Sea has been seen as particularly suitable for studies of food webs. Compared... more The brackish Baltic Sea has been seen as particularly suitable for studies of food webs. Compared to fully marine ecosystems, it has low species diversity, which means fewer trophic linkages to analyse. The Baltic Sea is also one of the best-studied areas of the world, suggesting that most data requirements for food web models should be fulfilled. Nevertheless, the influence of physical and biological factors on trophic interactions and biogeochemical patterns varies spatially in the Baltic Sea, adding considerable complexity to food web studies. Food web structure and processes can be described and compared quantitatively between areas by estimating the flow of matter or energy through the organisms. Most such models have been based on carbon, though studies of complementary flows of other elements limiting production, such as nitrogen and phosphorus would be desirable. However, since ratios between carbon and other elements are used in calculating these flows, it is crucial, as a first step, to quantify the flows of carbon as accurately as possible.
Nitrogen as the limiting nutrient for primary production in the eutrophic Baltic proper has been ... more Nitrogen as the limiting nutrient for primary production in the eutrophic Baltic proper has been under debate. Based on only a limited number of actual measurements, nitrogen fixation has been assumed to be the only significant internal nitrogen source. It is then assumed that about one fifth of the net nitrogen load to the Baltic proper comes from nitrogen Ž . fixation by cyanobacteria. An alternative or additional source is utilisation of dissolved organic nitrogen DON .
External nutrient inputs to the major basins of the Baltic Sea a century ago were reconstructed f... more External nutrient inputs to the major basins of the Baltic Sea a century ago were reconstructed from various literature and data sources. The reconstructed input of total nitrogen was less than a half and that of total phosphorus was about a third of their contemporary values. The simulated "pre-industrial" conditions are validated by comparison to actual historical data on the water transparency, oxygen concentration, primary production, and net sediment accumulation. The "pre-industrial" trophic state could have been more phosphorus limited than today because simulated basin-wide annual averages of dissolved inorganic phosphorus concentrations of 0.06-0.3 µM P are about 40-80% of their present day values, while dissolved inorganic nitrogen concentrations of 2-4 µM N are almost the same as today or even slightly higher.
The environmental conditions of the Baltic Sea have been of concern for approximately the last th... more The environmental conditions of the Baltic Sea have been of concern for approximately the last three decades. In spite of this concern, the Sea is far from being in good ecological condition. One important reason may be the lack of an overview of costs and benefits associated with the pollution reductions needed for the restoration of the Sea. The purpose
Management of eutrophication in marine ecosystems requires a good understanding of nutrient cycle... more Management of eutrophication in marine ecosystems requires a good understanding of nutrient cycles at the appropriate spatial and temporal scales. Here, it is shown that the biogeochemical processes controlling large-scale eutrophication of the Baltic Sea can be described with a fairly aggregated model: simple as necessary Baltic long-term large scale (SANBALTS). This model simulates the dynamics of nitrogen, phosphorus, and silica driven by the external inputs, the major physical transports, and the internal biogeochemical fluxes within the seven major sub-basins. In a long-term hindcast , the model outputs reasonably matched observed concentrations and fluxes. The model is also tested in a scenario where nutrient inputs are reduced to levels that existed over 100 years ago. The simulated response of the Baltic Sea trophic state to this very large reduction is verified by a similar simulation made with a much more complex processoriented model. Both models indicate that after initial, rather rapid changes the system goes into much slower evolution, and nutrient cycles would not become balanced even after 130 years.
Overall budgets for nutrient and humus are described for the Baltic Sea as well as for the subsys... more Overall budgets for nutrient and humus are described for the Baltic Sea as well as for the subsystems, i.e., the Baltic proper, the Bothnian Bay and the Bothnian Sea. The residence times for total phosphorus, total nitrogen, silicate and humus are 13.3, 5.5, 11.2 and 9.6 years respectively, compared to 21.8 years for a conservative substance (salt). About 90% of the nutrient losses are due to biogeochemical sinks within the Baltic Sea. Thus only about 10% is exported to external areas (the Kattegat/Belt Sea). For humus the corresponding figures are about 75 and 25%, respectively. This means that the Baltic Sea to a large extent can be regarded as a closed system and perturbations in the water exchange with the North Sea should have little effect on the nutrient budgets. The sinks are parameterized by an expression borrowed from limnology where the net nutrient loss is a function of the winter surface concentration. A budget model is run in a prognostic, hindcast mode with the assumed timedependent phosphorus and nitrogen loading of the Baltic proper. The computed development of the winter surface concentrations of total P and total N for the period 1950-1988 appears quite realistic. The possibility of having variable sinks which are functions of the surface winter concentrations of nitrogen and phosphorus is described using calculations based on data from the different Baltic subareas. Such sinks should significantly decrease the winter N:P ratio in the surface water when the nutrient loading increases with time. With better descriptions of in particular the pools of nutrients in the sediment, it would be possible to model future changes of nutrient concentrations in the water column in relation to loading.
High nutrient inputs from the Neva River in the eastern Gulf of Finland, Baltic Sea create steep ... more High nutrient inputs from the Neva River in the eastern Gulf of Finland, Baltic Sea create steep spatial gradients in water column concentrations of nutrients and of particulates. During July 1993 nutrient fluxes across the sediment-water interface were measured beginning in this eutrophication gradient and extending out into the open waters of the western Gulf. Dissolved oxygen (O 2 ) microgradients showed that O 2 concentrations were depleted within the first 2-4 mm of the sediment-water interface and calculated O 2 consumption ranged from 462 to 526 mol m 2 h 1 with no significant differences [analysis of variance (ANOVA); P >0·05] observed between stations. Spatial patterns, however, were evident with significant differences between stations observed in most nutrient fluxes (ANOVA; P<0·01) with the highest sediment-water fluxes observed at the most eutrophic station in the eastern Gulf of Finland. Sediment-water fluxes ranged between 9·37-34·7 mol m 2 h 1 for NH 4 + , 6·8-12·45 mol m 2 h 1 for NO 3 , 0·10-2·45 mol m 2 h 1 for NO 2 and 1·59-28·5 mol m 2 h 1 for dissolved inorganic phosphate. Dissolved silicate fluxes (71·8-83·5 mol m 2 h 1 ) were the only nutrient not to show significant differences (one-way ANOVA; P>0·05) between stations. Estimated denitrification rates from sediments ranged from 6·6-11·5 mol m 2 h 1 and are not particularly intense compared with other Baltic areas, however, given that these rates only include denitrification supported by water column NO 3 concentrations and not denitrification coupled to nitrification with the sediments, then they are not so far from the rates (17·8 mol m 2 h 1 ) estimated from mass balances in the Gulf of Finland. The sediment-water fluxes reported here include some of the highest rates reported for the Baltic Sea, but are typical of most eutrophic coastal marine environments. The high fluxes reported here contribute to the large reservoir of dissolved inorganic nutrients contained in the water column immediately below the mixed surface layer.
The recently reported tendencies toward decreasing total amounts of silicate in the Baltic Sea ar... more The recently reported tendencies toward decreasing total amounts of silicate in the Baltic Sea are investigated by use of non-parametric trend analysis. The period 1968-1986 showed significant falling trends in surface waters from the whole system. The deepest parts of the Baltic proper did in contrast reveal strong increasing trends. These trends are more pronounced during the latter part of the analysis period which is characterized by stagnant conditions in the Baltic proper. These conditions have been prevailing since the major inflow in 1976. The causes of the observed changes are unclear. The increased load of nutrients and accompanying increase in primary production is, however, one factor. Another is coupled to the stagnation conditions of the Baltic Proper.
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Papers by Fredrik Wulff