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1.
The sensitivity of the Baltic Sea mean salinity to climatic changes of the freshwater supply is analyzed. The average salinity of the Baltic Sea is about 6‰. The low salinity is an effect of a large net freshwater supply and narrow and shallow connections with the North Sea. As a result of mixing in the entrance area, a large portion of the outflowing Baltic Sea water returns with the inflowing salty water and thus lowers the salinity of the Baltic Sea deep-water considerably. This recycling of the Baltic Sea water is a key process determining the salinity of today's Baltic Sea. The sensitivity of this recycling, and thus of the Baltic Sea salinity, to climatic changes in the freshwater supply is analyzed. A simple model is formulated for the variations of the Baltic Sea freshwater content. Historical data of the freshwater supply and the salinity in the Baltic Sea are used in the model to achieve an empirical expression relating variations of the recycling of Baltic Sea water to the variations of the freshwater supply. The recycling is found to be very sensitive to the freshwater supply. We find that an increase of freshwater supply of 30% is the level above which the Baltic Sea would turn into a lake. Recent climate modeling results suggest that river runoff to the Baltic Sea may increase dramatically in the future and thus possibly put the Baltic Sea into a new state.  相似文献   

2.
The dynamics controlling the response of the Baltic Sea to changed atmospheric and hydrologic forcing are reviewed and demonstrated using simple models. The response time for salt is 30 times longer than for heat in the Baltic Sea. In the course of a year, the Baltic Sea renews most of its heat but only about 3% of its salt. On the seasonal scale, surface temperature and ice-coverage are controlled by the atmospheric conditions over the Baltic Sea as demonstrated by e.g. the strong inter-annual variations in winter temperature and ice-coverage due to variations in dominating wind directions causing alternating mild and cold winters. The response of surface temperature and ice-coverage in the Baltic Sea to modest climate change may therefore be predicted using existing statistics. Due to the long response time in combination with complicated dynamics, the response of the salinity of the Baltic Sea cannot be predicted using existing statistics but has to be computed from mechanistic models. Salinity changes primarily through changes in the two major forcing factors: the supply of freshwater and the low-frequency sea level fluctuations in the Kattegat. The sensitivity of Baltic Sea salinity to changed freshwater supply is investigated using a simple mechanistic steady-state model that includes baroclinic geostrophic outflow from the Kattegat, the major dynamical factor controlling the freshwater content in the Kattegat and thereby the salinity of water flowing into the Baltic Sea. The computed sensitivity of Baltic Sea surface salinity to changes of freshwater supply is similar to earlier published estimates from time-dependent dynamical models with higher resolution. According to the model, the Baltic Sea would become fresh at a mean freshwater supply of about 60 000 m3 s−1, i.e. a 300% increase of the contemporary supply. If the freshwater supply in the different basins increased in proportion to the present-day supply, the Bothnian Bay would become fresh already at a freshwater supply of about 37 000 m3 s−1 and the Bothnian Sea at a supply of about 45 000 m3 s−1. The assumption of baroclinic geostrophic outflow from the Kattegat, crucial for the salinity response of the Baltic Sea to changed freshwater supply, is validated using daily salinity profiles for the period 1931–1977 from lightship Läsö Nord.  相似文献   

3.
Shelf seas such as the North Sea and the Baltic Sea are characterised by spatially and temporally varying stratification that is highly relevant for their physical dynamics and the evolution of their ecosystems. Stratification may vary from unstably stratified (e.g., due to convective surface cooling) to strongly stratified with density jumps of up to 10 kg/m3 per m (e.g., in overflows into the Baltic Sea). Stratification has a direct impact on vertical turbulent transports (e.g., of nutrients) and influences the entrainment rate of ambient water into dense bottom currents which in turn determine the stratification of and oxygen supply to, e.g., the central Baltic Sea. Moreover, the suppression of the vertical diffusivity at the summer thermocline is one of the limiting factors for the vertical exchange of nutrients in the North Sea. Due to limitations of computational resources and since the locations of such density jumps (either by salinity or temperature) are predicted by the model simulation itself, predefined vertical coordinates cannot always reliably resolve these features. Thus, all shelf sea models with a predefined vertical coordinate distribution are inherently subject to under-resolution of the density structure.To solve this problem, Burchard and Beckers (2004) and Hofmeister et al. (2010) developed the concept of vertically adaptive coordinates for ocean models, where zooming of vertical coordinates at locations of strong stratification (and shear) is imposed. This is achieved by solving a diffusion equation for the position of the coordinates (with the diffusivity being proportional to the stratification or shear frequencies). We will show for a coupled model system of the North Sea and the Baltic Sea (resolution ∼ 1.8 km) how numerical mixing is substantially reduced and model results become significantly more realistic when vertically adaptive coordinates are applied. We additionally demonstrate that vertically adaptive coordinates perform well in simulating the two dynamically different regions North Sea and Baltic Sea with a single parameter set.An analysis of the computational overhead of the adaptive coordinates indicates an increase of 5–8% in runtime. This is still less expensive than adding more sigma-layers to reduce spurious numerical mixing.  相似文献   

4.
One hundred years of hydrographic measurements in the Baltic Sea   总被引：1，自引：0，他引：1  
The first measurements of salinity of the deep water in the open Baltic Sea were made in the last decades of the 1800s. At a Scandinavian science meeting in Copenhagen in 1892, Professor Otto Pettersson from Sweden suggested that regular measurements of hydrographic parameters should be carried out at some important deep stations in the Baltic Sea. His suggestion was adopted and since that time we have rather complete hydrographical data from the Bornholm Deep, the Gotland Deep, and the Landsort Deep and from some stations in the Gulf of Bothnia. The measurements were interrupted in the Baltic Proper during the two World Wars. At the beginning only salinity, temperature and dissolved oxygen were measured and one or two expeditions were carried out annually, mostly in summer. In the 1920s also alkalinity and pH were occasionally measured and total carbonate was calculated. A few nutrient measurements were also carried out. After World War II we find results from four or more expeditions every year and intercalibration of methods was arranged. Results of temperature, salinity and dissolved oxygen measurements from the Bornholm Deep, the Gotland Deep, the Landsort Deep and salinity measurements from three stations in the Gulf of Bothnia, covering the whole 20th century are presented and discussed. The salinity distribution and the variations between oxygen and hydrogen sulphide periods in the deep water of the Gotland Deep and the Landsort Deep are demonstrated. Series of phosphate and nitrate distribution in the Gotland Deep are shown from the 1950s to the present and the effects of the stagnant conditions are briefly discussed. Two large inflows of highly saline water, the first during the First World War and the second in 1951, are demonstrated. The 20th century minimum salinity of the bottom water in the Baltic Proper in 1992 is discussed.  相似文献   

5.
Time series of freshwater runoff, seawater salinity, temperature and oxygen were used in transfer functions (TF) to model changes of mesozooplankton taxa in the Baltic Sea from the 1960’s to the 1990’s. The models were then compared with long term zooplankton monitoring data from the same period. The TF models for all taxa over the whole Baltic proper and at different depth layers showed statistically significant estimates in t-tests. TF models were further compared using parsimony as a criterion. We present models showing 1) r2 > 0.4, 2) the smallest residual standard error with the combination of exploratory variables, 3) the lowest number of parameters and 4) the highest proportional decrease in error term when the TF model residual standard error was compared with those of the univariate ARIMA model of the same response variable. Most often (7 taxa out of a total of 8), zooplankton taxa were dependent on freshwater runoff and/or seawater salinity. Cladocerans and estuarine copepods were more conveniently modelled through the inclusion of seawater temperature and oxygen data as independent variables. Our modelling, however, explains neither the overall increase in zooplankton abundance nor a simultaneous decrease found in the neritic copepod, Temora longicornis. Therefore, biotic controlling agents (e.g. nutrients, primary production and planktivore diets) are suggested as independent variables for further TF modelling. TF modelling enabled us to put the controlling factors in a time frame. It was then possible, despite the inherent multiple correlation among parameters studied to deduce a chain-of-events from the environmental controls and biotic feedback mechanisms to changes in zooplankton species. We suggest that the documented long-term changes in zooplankton could have been driven by climatic regulation only. The control by climate could be mediated to zooplankton through marine chemical and physical factors, as well as biotic factors if all of these were responding to the same external control, such as changes in the freshwater runoff. Increased runoff would explain both the increasing eutrophication, causing the overall increase of zooplankton, and the changes in selective predation, contributing to decline of Temora.  相似文献   

6.
A. A. Maximov 《Oceanology》2006,46(2):185-191
The data on the bottom concentrations of dissolved oxygen in the eastern part of the Gulf of Finland obtained in 1923–1939 and in 1962–1989 were analyzed. No statistically reliable differences were found between the two periods studied. It was found that, during the 20th century, the fluctuations in the oxygen concentration were caused by the interannual variability of the winter severity and water salinity. A strong oxygen deficit in the summer was recorded after cold winters, when early freezing suppressed the vertical mixing, and/or in the years of a significant inflow of salt waters from the Baltic Sea with low oxygen contents. It is likely that the long-term dynamics of the oxygen concentration near the bottom in the open parts of the bay is determined by the large-scale variability of the hydrometeorological parameters.  相似文献   

7.
Recent decadal salinity changes in the Greenland-Scotland overflow-derived deep waters are quantified using CTD data from repeated hydrographic sections in the Irminger Sea. The Denmark Strait Overflow Water salinity record shows the absence of any net change over the 1980s–2000s; changes in the Iceland–Scotland Overflow Water (ISOW) and in the deep water column (σ0 > 27.82), enclosing both overflows, show a distinct freshening reversal in the early 2000s. The observed freshening reversal is a lagged consequence of the persistent ISOW salinification that occurred upstream, in the Iceland Basin, after 1996 in response to salinification of the northeast Atlantic waters entrained into the overflow. The entrainment salinity increase is explained by the earlier documented North Atlantic Oscillation (NAO)-induced contraction of the subpolar gyre and corresponding northwestward advance of subtropical waters that followed the NAO decline in the mid-1990s and continued through the mid-2000s. Remarkably, the ISOW freshening reversal is not associated with changes in the overflow water salinity. This suggests that changes in the NAO-dependent relative contributions of subpolar and subtropical waters to the entrainment south of the Iceland–Scotland Ridge may dominate over changes in the Nordic Seas freshwater balance with respect to their effect on the ISOW salinity.  相似文献   

8.
Based on measurements of the 18O isotope composition of 247 samples collected over a 3-year period we have assessed the oxygen isotope composition of water masses in the North Sea. This is the first δ18O data set that covers the entire North Sea basin. The waters lie on a mixing line: δ18O (‰VSMOW) = −9.300 + 0.274(S) with North Atlantic sub-polar mode water (SPMW) and surface waters, and Baltic Sea water representing the saline and freshwater end members respectively. Patterns exhibited in surface and bottom water δ18O distributions are representative of the general circulation of the North Sea. Oxygen-18 enriched waters from the North Atlantic enter the North Sea between Scotland and Norway and to a lesser extent through the English Channel. In contrast, oxygen-18 depleted waters mainly inflow from the Baltic Sea, the rivers Rhine and Elbe, and to a lesser degree, the Norwegian Fjords and other river sources. Locally the δ18O–salinity relationship will be controlled by the isotopic composition of the freshwater inputs. However, the range of local freshwater compositions around the North Sea basin is too narrow to characterise the relative contributions of individual sources to the overall seawater composition. This dataset provides important information for a number of related disciplines including biogeochemical research and oceanographic studies.  相似文献   

9.
The impact of the Major Baltic Inflows (MBIs) of 1976, 1993, and 2003 on cod reproduction in the Gotland Basin of the Baltic Sea is considered based on calculations of the eastern Baltic cod egg abundance and offspring survival indices. The reproductive success (1976–1977) was defined by the elevation of the 11 psu isohaline and deepening of the 2 mL/L isooxygene surface, which provided the formation of a water layer suitable for cod spawning at a depth of 90–130 m. Prerequisites of the appearance of the reproductive layer in 1976–1977 were a previously high salinity level and penetrating two successive MBIs in one year. The cause of the weak MBI impact in 1993 was the previous salinity minimum; in 2003 it was rapid oxygen depletion due to an increase in temperature. As a result of their weaker influence, small and fragmented cod spawning biotopes appeared in the Gotland Basin.  相似文献   

10.
Correlations have been revealed between the distribution of the different ichthyoplankton species and the fluctuations of the environmental factors along transects in the Baltic Sea during the autumn of 2005 and spring and autumn of 2006. The early ontogenetic stages of the cod and other bottom fish species were confined to the Bornholm Basin located in the southwestern part of the sea with near-bottom salinity of more than 16?? and the absence of an oxygen deficiency in the isohaline zone of 11?C14??. Eggs and larvae of the Baltic sprat were distributed in June throughout all the deep-sea regions with near-bottom water salinity of no less than 8%., up to the western Gulf of Finland. In the spring of 2006, the abundance of sprat eggs was found to drop significantly compared with the average long-term data for the recent period; this was due to the cold winter of 2006.  相似文献   

11.
A 3D,time-dependent,baroclinic,hydrodynamic and salinity model was implemented and applied to the Oujiang River estuarine system in the East China Sea.The model was driven by the forcing of tidal elevations along the open boundaries and freshwater inflows from the Oujiang River.The bottom friction coefficient and vertical eddy viscosity were adjusted to complete model calibration and verification in simulations.It is demonstrated that the model is capable of reproducing observed temporal variability in the water surface elevation and longitudinal velocity,presenting skill coefficient higher than 0.82.This model was then used to investigate the influence of freshwater discharge on residual current and salinity intrusion under different freshwater inflow conditions in the Oujiang River estuary.The model results reveal that the river channel presents a two-layer structure with flood currents near the bottom and ebb currents at the top layer in the region of seawater influenced on north shore under high river flow condition.The river discharge is a major factor affecting the salinity stratification in the estuarine system.The water exchange is mainly driven by the tidal forcing at the estuary mouth,except under high river flow conditions when the freshwater extends its influence from the river’s head to its mouth.  相似文献   

12.
Hydrographic changes in the Labrador Sea, 1960–2005   总被引：1，自引：0，他引：1  
The Labrador Sea has exhibited significant temperature and salinity variations over the past five decades. The whole basin was extremely warm and salty between the mid-1960s and early 1970s, and fresh and cold between the late 1980s and mid-1990s. The full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s. The freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the Labrador Sea sustained throughout the subsequent years. It took only a decade for the whole water column to lose most of its excessive freshwater, reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s. If the recent tendencies persist, the basin’s storages of salt and heat will fairly soon, likely by 2008, exceed their historic highs.The main process responsible for the net cooling and freshening of the Labrador Sea between 1987 and 1994 was deep winter convection, which during this period progressively developed to its record depths. It was caused by the recurrence of severe winters during these years and in its turn produced the deepest, densest and most voluminous Labrador Sea Water (LSW1987–1994) ever observed. The estimated annual production of this water during the period of 1987–1994 is equivalent to the average volume flux of about 4.5 Sv with some individual annual rates exceeding 7.0 Sv. Once winter convection had lost its strength in the winter of 1994–1995, the deep LSW1987–1994 layer lost “communication” with the mixed layer above, consequently losing its volume, while gaining heat and salt from the intermediate waters outside the Labrador Sea.While the 1000–2000 m layer was steadily becoming warmer and saltier between 1994 and 2005, the upper 1000 m layer experienced another episode of cooling caused by an abrupt increase in the air-sea heat fluxes in the winter of 1999–2000. This change in the atmospheric forcing resulted in fairly intense convective mixing sufficient to produce a new prominent LSW class (LSW2000) penetrating deeper than 1300 m. This layer was steadily sinking or deepening over the years following its production and is presently overlain by even warmer and apparently less dense water mass, implying that LSW2000 is likely to follow the fate of its deeper precursor, LSW1987–1994. The increasing stratification of the intermediate layer implies intensification in the baroclinic component of the boundary currents around the mid-depth perimeter of the Labrador Sea.The near-bottom waters, originating from the Denmark Strait overflow, exhibit strong interannual variability featuring distinct short-term basin-scale events or pulses of anomalously cold and fresh water, separated by warm and salty overflow modifications. Regardless of their sign these anomalies pass through the abyss of the Labrador Sea, first appearing at the Greenland side and then, about a year later, at the Labrador side and in the central Labrador Basin.The Northeast Atlantic Deep Water (2500–3200 m), originating from the Iceland–Scotland Overflow Water, reached its historically freshest state in the 2000–2001 period and has been steadily becoming saltier since then. It is argued that LSW1987–1994 significantly contributed to the freshening, density decrease and volume loss experienced by this water mass between the late 1960s and the mid 1990s via the increased entrainment of freshening LSW, the hydrostatic adjustment to expanding LSW, or both.  相似文献   

13.
Labrador Sea convection was most intense and reached the greatest depths in the early 1990s, followed by weaker, shallower, and more variable convection after 1995. The Simple Ocean Data Assimilation (SODA) version 2.0.2/2.0.4 assimilation model is used to explore convective activity in the North Atlantic Ocean for the period from 1992 to 2007. Hydrographic conditions, which are relatively well observed during this period, are used to compare modeled and observed winter mixed-layer depths and water mass anomalies in relation to Deep Western Boundary Current transports and meridional overturning circulation (MOC) changes at the exit of the subpolar basin. The assimilation differs markedly from local observations in the March mixed-layer depth, which represents deep convection and water mass transformation. However, mean MOC rates at the exit of the subpolar gyre, forced by stratification in the mid-latitudes, are similar to estimates based on observations and show no significant decrease during the 1992–2007 period. SODA reproduces the deep Labrador Sea Water formation in the western North Atlantic without any clear indication of significant formation in the Irminger Sea while the lighter upper Labrador Sea Water density range is reached in the Irminger Sea in the 1990s, in agreement with existing assumptions of deep convection in the Irminger Sea and also supported by computed lag correlations with the Labrador Sea. Deep Water transformation mainly takes place in the eastern North Atlantic. The introduction of CFC-11 into the SODA model as a tracer reproduces the mean and multiyear variations of observed distributions.  相似文献   

14.
Atlantic Water flow through the Barents and Kara Seas   总被引：2，自引：0，他引：2  
The pathway and transformation of water from the Norwegian Sea across the Barents Sea and through the St. Anna Trough are documented from hydrographic and current measurements of the 1990s. The transport through an array of moorings in the north-eastern Barents Sea was between 0.6 Sv in summer and 2.6 Sv in winter towards the Kara Sea and between zero and 0.3 Sv towards the Barents Sea with a record mean net flow of 1.5 Sv. The westward flow originates in the Fram Strait branch of Atlantic Water at the Eurasian continental slope, while the eastward flow constitutes the Barents Sea branch, continuing from the western Barents Sea opening.About 75% of the eastward flow was colder than 0°C. The flow was strongly sheared, with the highest velocities close to the bottom. A deep layer with almost constant temperature of about −0.5°C throughout the year formed about 50% of the flow to the Kara Sea. This water was a mixture between warm saline Atlantic Water and cold, brine-enriched water generated through freezing and convection in polynyas west of Novaya Zemlya, and possibly also at the Central Bank. Its salinity is lower than that of the Atlantic Water at its entrance to the Barents Sea, because the ice formation occurs in a low salinity surface layer. The released brine increases the salinity and density of the surface layer sufficiently for it to convect, but not necessarily above the salinity of the Atlantic Water. The freshwater west of Novaya Zemlya primarily stems from continental runoff and at the Central Bank probably from ice melt. The amount of fresh water compares to about 22% of the terrestrial freshwater supply to the western Barents Sea. The deep layer continues to the Kara Sea without further change and enters the Nansen Basin at or below the core depth of the warm, saline Fram Strait branch. Because it is colder than 0°C it will not be addressed as Atlantic Water in the Arctic Ocean.In earlier decades, the Atlantic Water advected from Fram Strait was colder by almost 2 K as compared to the 1990s, while the dense Barents Sea water was colder by up to 1 K only in a thin layer at the bottom and the salinity varied significantly. However, also with the resulting higher densities, deep Eurasian Basin water properties were met only in the 1970s. The very low salinities of the Great Salinity Anomaly in 1980 were not discovered in the outflow data. We conclude that the thermal variability of inflowing Atlantic water is damped in the Barents Sea, while the salinity variation is strongly modified through the freshwater conditions and ice growth in the convective area off Novaya Zemlya.  相似文献   

15.
A three-dimensional, eddy-permitting ocean circulation model with implemented bottom boundary layer model and flux-corrected transport scheme is used to calculate the pathways and ages of various water masses in the Baltic Sea. The agreement between simulated and observed temperature and salinity profiles of the period 1980–2004 is satisfactory. Especially the renewal of the deep water in the Baltic proper by gravity-driven dense bottom flows is better simulated than in previous versions of the model. Based upon these model results details of the mean circulation are analyzed. For instance, it is found that after the major Baltic inflow in January 2003 saline water passing the Słupsk Furrow flows directly towards northeast along the eastern slope of the Hoburg Channel. However, after the baroclinic summer inflow in August/September 2002 the deep water flow spreads along the southwestern slope of the Gdansk Basin. Further, the model results show that the patterns of mean vertical advective fluxes across the halocline that close the large-scale vertical circulation are rather patchy. Mainly within distinct areas are particles of the saline inflow water advected vertically from the deep water into the surface layer. To analyze the time scales of the circulation mean ages of various water masses are calculated. It is found that at the sea surface of the Bornholm Basin, Gotland Basin, Bothnian Sea, and Bothnian Bay the mean ages associated to inflowing water from Kattegat amount to 26–30, 28–34, 34–38, and 38–42 years, respectively. Largest mean sea surface ages of more than 30 years associated to the freshwater of the rivers are found in the central Gotland Basin and Belt Sea. At the bottom the mean ages are largest in the western Gotland Basin and amount to more than 36 years. In the Baltic proper vertical gradients of ages associated to the freshwater inflow are smaller than in the case of inflowing saltwater from Kattegat indicating an efficient recirculation of freshwater in the Baltic Sea.  相似文献   

16.
Several time series in the Norwegian Sea indicate an upper layer decrease in temperature and salinity since the 1960s. Time series from Weather Station “M”, from Russian surveys in the Norwegian Sea, from Icelandic standard sections, and from Scottish and Faroese observations in the Faroe–Shetland area have similar trends and show that most of the Norwegian Sea is affected. The reason is mainly increased freshwater supply from the East Icelandic Current. As a result, temperature and salinity in some of the time series were lower in 1996 than during the Great Salinity Anomaly in the 1970s. There is evidence of strong wind forcing, as the NAO winter index is highly correlated with the lateral extent of the Norwegian Atlantic Current. Circulation of Atlantic water into the western Norwegian and Greenland basins seems to be reduced while circulation of upper layer Arctic and Polar water into the Norwegian Sea has increased. The water-mass structure is further affected in a much wider sense by reduced deep-water formation and enhanced formation of Arctic intermediate waters. A temperature rise in the narrowing Norwegian Atlantic Current is strongest in the north.  相似文献   

17.
The dynamics in the transition zone between the North Sea and Baltic Sea are analyzed here using data from a 22-year-long climatic simulation with a focus on the periods 1992–1994 and 2001–2003 when two recent major inflow events occurred. Observations from gauges and in situ measurements are used to validate the model. Parameters, which cannot be easily measured, such as water and salt transports through straits, have been compared against similar previous estimates. The good performance of simulations is attributed to the finer resolution of the model compared to earlier set ups. The outflow in the Kattegat, which is an analogue of the tidal outflows, tends to propagate to the North over the shallows without showing a substantial deflection to the right due to the Earth's rotation. The inflow follows the topography. The different inflow and outflow pathways are explained as a consequence of the specific combination of bathymetry, axial and lateral processes. The circulation in Kattegat is persistently clockwise with an eastern intensification during inflow and a western one during outflow regimes. The tidal wave there propagates as Kelvin wave, keeping the coast on its right. The flows in the two main straits reveal very different responses to tides, which are also highly asymmetric during inflow and outflow conditions. The circulation has a typical two-layer structure, the correlation between salinity and velocity tends to increase the salt transport in the salinity conveyor belt. The transversal circulation in the entrance of the Sound enhances the vertical mixing of the saltier North Sea water. The long-term averaged ratio of the water transports through the Great Belt and the Sound is ∼2.6-2.7 but this number changes reaching lower values during the major inflow in 1993. The transports in the straits are asymmetric. During inflow events the repartition of water penetrating the Baltic Sea is strongly in favor of the pathway through the Sound, which provides a shorter connection between the Kattegat and Baltic proper. The wider Great Belt has a relatively larger role in exporting water from the Baltic into the North Sea. A demonstration is given that the ventilation of the Baltic Sea deep water is not only governed by the dynamics in the straits and the strong westerly winds enhancing the eastward propagation of North Sea water (a case in 1993), but also by the clockwise circulation in the Kattegat acting as a preconditioning factor for the flow-partitioning.  相似文献   

18.
The food choice of juvenile flounder (Platichthys flesus) and turbot (Scophthalmus maximus) was studied in the northern Baltic Sea during the years 1988, 1989, 1994 and 1995. The diet included organisms from 30 species/taxa in flounder (n = 306) and 10 species/taxa in turbot (n = 41). Flounder 45 mm mainly consumed meiofauna (dominating taxon: Harpacticoida, Copepoda) and larger fish (46–101 mm) consumed macrofauna (dominating taxa: Oligochaeta, Amphipoda and Chironomidae). In terms of biomass, macrofauna dominated for all sizes of flounders, and meiofauna was important only for the smallest fish. A strong seasonal variation could be detected in the diet. In spring, macrofauna dominated for all size classes of fish (only fish > 30 mm were caught in spring), while in summer and autumn meiofauna dominated the diets for fish 45 mm in size. Juvenile turbot (22–88 mm) consumed macrofauna and small fish. Turbot 30 mm consumed mainly amphipods, while > 30 mm turbot consumed mysid shrimps, amphipods and fish.The ontogenetic shift from meio- to macrofauna-sized prey in flounders occurs at a larger fish size in the northern Baltic Sea than reported in other areas, possibly depending on the increased relative importance of meiofauna in the northern Baltic. The seasonal variation in the diet could be due to seasonally changing abundances in the zoobenthos, or for the small fish (1-group, spring), to switching from meio- to macrofauna in order to optimize their energy gain. The 0-group flounders consumed meiofauna for a long period, possibly due to a learning-process or simply due to easy availability of meiofauna. Turbot has a much larger mouth gap than flounders, thus allowing them to consume macrofauna from the beginning of their benthic life.  相似文献   

19.
The Baltic Sea is a semi-enclosed sea with a steady salinity gradient (3‰–30‰). Organisms have adapted to such low salinities, but are suspected to be more susceptible to stress. Within the frame of the integrated environmental monitoring BONUS + project “BEAST” the applicability of immune responses of the blue mussel was investigated in Danish coastal waters. The sampling sites were characterised by a salinity range (11–19‰) and different mixtures of contaminants (metals, PAHs and POPs), according to chemical analysis of mussel tissues. Variation partitioning (redundancy analysis) was applied to decompose salinity and contamination effects. The results indicated that cellular immune responses (total and differential haemocyte count, phagocytic activity and apoptosis) were mainly influenced by contaminants, whereas humoral factors (haemolytic activity) were mainly impacted by salinity. Hence, cellular immune functions may be suitable as biomarkers in monitoring programmes for the Baltic Sea and other geographic regions with salinity variances of the studied range.  相似文献   

20.
Within the framework of the integral model in isopycnic coordinates including the upper mixed layer, we study the rearrangement of temperature and salinity fields in the Dead Sea. It is forced by the variability of atmospheric factors and mass fluxes through the lateral boundaries of the basin, which are connected with the freshwater run-off to the sea and the outflow from the sea to the shallow south bay, where evaporator installations for salt extraction are located. Such factors as solar radiation, wind velocity, air temperature, and relative humidity are prescribed. Evaporation is calculated with the use of a bulk formula, whereas precipitation intensity and discharge through the lateral boundaries are determined as functions of the sea level obtained from observations. This model reflects the specific character of the Dead Sea and takes into account the effect of water salinity on its evaporation, salt concretion, and mechanical evaporation. Comparison with the data of observations shows that the proposed model fairly precisely describes the observed specific features of the thermohaline water structure in the meromictic (1992–1996) and holomictic (1996–2000) periods.  相似文献   

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