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Climate, hydrographic variations and marine benthic hypoxia in Koljö Fjord, Sweden
Journal of Sea Research 46 (2001) 187±200
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Climate, hydrographic variations and marine benthic hypoxia in KoljoÈ Fjord, Sweden Kjell Nordberg a,*, Helena L. Filipsson a, Mikael Gustafsson a, Rex Harland b, Per Roos c a
Department of Earth Sciences, Oceanography, GoÈteborg University, PO Box 460, SE-405 30 GoÈteborg, Sweden b Centre for Palynology, University of Shef®eld, Dainton Building, Brook Hill, Shef®eld S3 7HF, and DinoData Services, 50 Long Acre, Bingham, Nottingham NG13 8AH, UK c Department of Radiation Physics, University of Lund, SE-221 85 Lund, Sweden Received 27 July 2000; accepted 5 July 2001
Abstract Since the late 1970s, Scandinavian waters have been extensively investigated for human-induced marine pollution, especially marine eutrophication, oxygen de®ciency in bottom waters and subsequent benthic mortality. The most serious oxygen de®ciencies are noted in the sill fjords along the Swedish west coast, in southern Norway and in large areas of the southern Kattegat and Baltic Sea. One of these sill fjords, KoljoÈ Fjord, is located on the Swedish west coast. This fjord is characterised by frequently occurring episodes of hypoxia/anoxia which last for months or even years. Sediments are laminated and the fjord is generally regarded as seriously affected by human-induced eutrophication. We detail the environmental development of a welldocumented fjord by combining high resolution sediment records with long hydrographical and meteorological instrumental data, and we present ultra high-resolution sediment information together with long-term instrumental records of air-temperatures, NAO indices and hydrography from KoljoÈ Fjord. These data show, in contrast to the current opinion focusing on anthropogenic eutrophication, that natural causes are the most important factors controlling the marine environment in this sparsely populated area. Natural variables concerned are fjord physiography, weather and hydrography (including the macro-nutrients DIN and PO4-P), sediment laminations and organic carbon. Interactions between fjord physiography, weather and hydrography regulate the possibility for water exchange and deep-water renewals. The present study points to the importance of natural causes for the environmental status of sill basins and semi-enclosed areas along the west coast of Sweden. q 2001 Elsevier Science Ltd All rights reserved. Keywords: Sill fjord; Sweden; Skagerrak; Hypoxia; Laminated sediments; Hydrography; Climate; Eutrophication
1. Introduction Marine offshore eutrophication in Scandinavian waters has been the focus of research among a large number of marine scientists for about 20 years (Baden et al., 1990; Diaz and Rosenberg, 1995; Nixon, 1995 and references in these). The consensus is that there * Corresponding author. E-mail address: [email protected] (K. Nordberg).
has been an increase in human sewage and agricultural fertilisers in the North Sea, including the Skagerrak Kattegat and Baltic Sea since the 1960s and 1970s (Aure et al., 1996; Rosenberg et al., 1996). This has in some places, especially in areas subjected to local outlets, resulted in enhanced primary production, altered algal composition, decreased Secchi depth increased accumulation of oxygen-consuming organic material on the sea ¯oor, increased severity of oxygen de®ciency in bottom water and subsequent benthic
1385-1101/01/$ - see front matter q 2001 Elsevier Science Ltd All rights reserved. PII: S 1385-110 1(01)00084-3
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Fig. 1. Map of the investigation area with the core location (black dot), monitoring station (cross) and the sill areas (S1 S2 and S3) indicated. Core location: 588 13 0 62 N/118 34 0 25 E; Monitoring site: 588 13 0 83 N/118 34 0 80 E.
mortality. In spite of ambitious sewage treatment, no clear improvements have been noted in the fjords or along the Swedish west coast. Despite a lack of signi®cant local sources of pollutants in the fjords, the human-induced eutrophication is considered to be ongoing, and even worsening the so-called largescale eutrophication (Rosenberg, 1990; Lindahl et al., 1998). Twenty years of studies focusing on human-induced marine eutrophication have partly resulted in a reduced awareness of the importance of the physiography of the fjord basins, the sill depth and variations in weather conditions in¯uencing exchange of basin water. The aim of this study was to detail the environmental development of a well-documented fjord by combining ultra-high resolution sediment records with instrumental data on hydrography and meteorology. The periodic hypoxia/anoxia and the
laminated sediments in the fjord have created a natural high-resolution environmental archive in the sediments. Scandinavian sill fjords with a limited oxygen supply represent a unique environmental archive in which the continuous accumulation of ®ne sediments and the minimal tidal activity and bioturbation create a natural laboratory for high spatial and temporal resolution studies. KoljoÈ Fjord on the Swedish west coast (Fig. 1) is one of several fjords that are considered to be heavily affected by marine eutrophication (Rosenberg, 1985, 1990; Josefson and Rosenberg, 1988; Haamer, 1995; Edebo et al., 2000). The region is characterised by crystalline bedrock, clay-®lled valleys and mixed forests. The KoljoÈ Fjord area has few local sewage outlets, since, like so many other Scandinavian fjords, it is located in a sparsely populated area. Most settlements in the KoljoÈ Fjord area
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and in the nearby Havstens fjord area are summerhouses with no signi®cant in¯uence on the marine environment. In addition, the hinterland is little farmed with only scattered small holdings. KoljoÈ Fjord is part of an open-ended fjord system (Fig. 1) and has a maximum depth of 56 m. To the west the fjord is connected with the Skagerrak by a sill of 8 m depth (S1) while to the east, between the KoljoÈ Fjord and Havstens Fjord, the sill depth is 12 m (S2). The Havstens Fjord, in turn, is separated from the sea to the south by a sill at 20 m depth (S3). KoljoÈ Fjord is characterised by brackish conditions (salinities: 20±30), a strati®ed water column, temporal oxygen de®ciency in the deep-water, laminated sediments, Beggiatoa bacterial mats covering the sea-¯oor and a lack of a benthic macrofauna (Gustafsson and Nordberg, 1999). A strong pycnocline at between 15 and 25 m separates the deep-water from the surface water. Tidal activity is very low (0.15±0.2 m) and of minor importance for the exchange of deep-water in the fjord (BjoÈrk et al., 2000). Deep-water renewal usually takes place during the winter and early spring at intervals of one to several years (Gustafsson and Nordberg, 1999). This renewal occurs when the thermocline is weak and episodes of upwelling along the Skagerrak coast lead to more saline water spilling over the fjord sills and ®lling the deeper parts of the fjords with oxygenated water. Prevailing winter winds on the Swedish west coast are westerly but deep-water exchange only takes place after periods of strong offshore winds between north and east, when the surface water is forced from the coast allowing upwelling from depth. Following a renewal event, the oxygen content decreases and when it is below 2 cm 3 dm 23 the benthic macrofauna starts to show signs of hypoxia (Rosenberg and Loo, 1988); below 0.5 cm 3 dm 23 most of the benthic macrofauna dies (Rosenberg et al., 1991). The lack of benthic fauna and consequent bioturbation is a prerequisite for the formation and preservation of laminated sediments. An unusually long record of hydrographic measurements is available for KoljoÈ Fjord, dating back to the mid-1930s with a downtime for World War II. These data are used here for the ®rst time to compare directly our proxy information with direct measurements.
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2. Material and methods Twelve short high-quality sediment cores were collected in September 1998 at a single site in 43 m of water from KoljoÈ Fjord (Fig. 1) at lat. 588 13 0 62 N long. 118 34 0 25 E. In a previous investigation (1993± 1994) on living benthic foraminifera, the same core location had been sampled (Gustafsson and Nordberg, 1999). We used a Multiple Corer Mark III-400 (Barnett et al., 1984) which gives a virtually undisturbed sediment surface. After collection the sediment cores were X-rayed on board ship using an Andrex BV (155 140 kV/10 mA) portable X-ray machine. A core collected in September 1995, at the same core location, is presented here in comparison with the cores collected in September 1998. 2.1. Sediment analyses After X-raying, one of the 1998 cores was sliced at 1-cm intervals and analysed for organic carbon (Corg) using a Carlo Erba NA 1500 analyser and dated by the 210 Pb method. At the same site we had previously dated two cores and investigated one core for its organic carbon content. The samples analysed for organic carbon were pre-treated with hydrochloric acid to remove any calcium carbonate. 2.2.
210
Pb dating
Sediment samples of 0.5±1 g were completely dissolved using hot HF/HNO3/HCl in the presence of 209Po acting as a yield determinant for the 210Pb daughter isotope 210Po. The polonium isotopes were plated on polished nickel discs from dilute hydrochloric acid in the presence of ascorbic acid to complex the iron. The discs were analysed for 210Po and 209Po using alpha spectrometry counting over at least 2 days. The mass of each sample was corrected for its salt content by using known porosity and assuming a salinity of 28. Radium-226 was measured by gamma spectrometry in order to determine the supported 210Pb levels. Analyses were performed at the Department of Radiation Physics, University of Lund, Sweden. Dating of the core was performed using the constant rate of supply (CRS) model (Appleby and Old®eld, 1978). The age of each sediment layer refers to the bottom part of the respective layers. Due to
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increasing errors when the Pb excess approaches zero the deeper sediment layers (below about 25 cm) were dated assuming the same sedimentation rate as in layers above 25 cm (~0.4 cm y 21), corrected for compaction. Datings have also been performed on two of the adjacent cores. The combined results from X-raying and Pb-datings indicate that the record of core K6A was the most extensive and it was therefore used as the main core for this study. The observed differences in the accumulation rate motivated the use of the CRS method.
in atmospheric pressure between the Azores and Iceland. This climatic oscillation has a strong in¯uence on the regime of westerly winds across the North Atlantic resulting in changes in winter temperatures and precipitation on both sides of the Atlantic Ocean (Kerr, 1997). Low-pressure systems and westerly winds in the North Sea region generally result in positive NAO-indices and vice versa. Normalised monthly NAO-indices were taken from Web site http://www.cgd.ucar.edu/cas/climind/ nao_monthly.html and as presented by Hurrell (1995).
2.3. Instrumental records and time series analysis
3. Results
The Swedish Meteorological and Hydrological Institute (SMHI) and the BohuslaÈn Water Conservation Association (in Swedish: GoÈteborgs och BohuslaÈns VattenvaÊrdsfoÈrbund) provided the hydrographical data including salinity, temperature, dissolved oxygen content, phosphate (PO4-P), dissolved inorganic nitrogen (DIN sum of NO2 NO3 and NH4) and chlorophyll a (Chl a). The position of their monitoring site in the fjord is: lat: 588 13 0 83 N long: 118 34 0 80 E. SMHI provided the air temperature data from Vinga Island located approximately 10 km west of GoÈteborg (Gbg in Fig. 1). Air temperature data were ®ltered using a 3-year moving average. Spectral analysis was used to determine if the salinity variations noted in KoljoÈ fjord's deep-water are cyclic. Since the data are unevenly distributed, we used the SPECTRUM program (Schulz and Stattegger, 1997). SPECTRUM is based on the Lomb-Scargle Fourier transformation of unevenly spaced time series together with a Welch-Overlapped-SegmentAveraging procedure. We have also calculated the 3-year moving average of the North Atlantic Oscillation indices (NAO), January±March, as an indication of the predominating winter weather conditions of the investigated time interval. Most of the deep-water exchange events occur during these three months. The North Atlantic Oscillation was discovered by Walker in the 1920s (Walker, 1924) and de®ned by Rogers (1984) as `a temporal ¯uctuation of the zonal wind strength across the Atlantic Ocean due to pressure variations in both the subtropical anticyclone belt and in the subpolar low near Iceland'. In other words, NAO can be described as the changes in the difference
The sediments in KoljoÈ Fjord, at water depths greater than approximately 25 m, are characterised by loose organic-rich mud and from time to time Beggiatoa bacterial mats (Gustafsson and Nordberg, 1999). Lamination within the sediments is best seen by using X-ray techniques as visual examination of the cores often underestimates the number of laminae present. The ®ve X-ray images presented here can easily be correlated and demonstrate three separate laminated sequences (Figs. 2 and 3). These are separated by homogeneous mud containing bioturbation structures and the remains of the macro/megafauna. Laminated sediments are widespread in the fjord system and become more widespread both temporally and spatially towards the west in KoljoÈ Fjord (Nordberg, unpubl. data). This is probably because most water renewal takes place across the eastern sill (S2, Fig. 1). However, a complete water renewal event registered around the New Year of 1993/1994 took place over the 8-m sill (S1) to the west (Fig. 1) (Gustafsson and Nordberg, 1999). 3.1. Sediment record The deepest 4 cm of the 53 cm long core K6A (49±53 cm) contains homogeneous sediments with bioturbation structures and mollusc shells. The oldest laminated sequence in the core (42±49 cm) represents the approximate time interval of 1830±1860, with a temporal break in the laminations at about 2 cm. A light lamina at 46 cm is surrounded by 10±12 distinct but thin laminae; these oldest laminae are best seen in Fig. 3, core 1995 and in core b, 1998. This sequence is
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Fig. 2. Composite diagram including time scale air temperature data from Vinga island (3-year moving average) NAO indices, winter values (average January±March and 3-year moving average), bottom-water salinity and oxygen from KoljoÈ Fjord, X-ray radiograph of core K6A, Corg from K6A.
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Fig. 3. X-ray radiographs from the investigated site in KoljoÈ Fjord. Core to the left was collected in 1995, and cores a, b, c and K6A were collected in 1998. The light lamina at 13 cm in core K6A is used as a level-peg for all cores in the ®gure.
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Table 1 210 Pb dating (C.R.S.) of core K6A from KoljoÈ Fjord Depth (cm)
Age (years)
Date
Depth (cm)
Age (years)
Date
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
1 3 6 9 12 15 17 20 22 24 26 28 32 34 37 39 42 45 47 50 52 55 58 62 65 68 71
1997 1995 1992 1989 1986 1984 1981 1978 1976 1974 1972 1970 1966 1964 1961 1959 1956 1953 1951 1948 1946 1943 1940 1936 1933 1930 1927
28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53
74 77 81 85 89 93 96 100 104 108 113 117 121 126 131 136 141 145 150 155 160 165 170 175 180 185
1924 1921 1917 1913 1909 1905 1902 1898 1894 1890 1885 1881 1877 1872 1867 1862 1857 1853 1848 1843 1838 1833 1828 1823 1818 1813
followed by approximately 17 cm of homogeneous sediments with bioturbating structures, macrofauna remnants and most often a few scattered gravel-to pebble-sized cinder particles. The most common shells found here are intact thin-shelled Macoma calcarea (Gmelin) with remarkably good preservation. The top 5 cm of this sequence is faintly banded and in places characterised by a few interbedded laminae. The second and most extensive and well-developed sequence of laminated sediments was formed between approximately 1930 and 1980 (25±7.5 cm). This sequence contains only a few interbedded thin bands of homogeneous sediments. The deepest part of the sequence is characterised by 3±4 cm of distinct laminations, including one thin, light lamina at 23 cm depth (Fig. 2). This light lamina is present eight `varves' above the initiation of this sequence. Light coloured sediments on the X-ray radiograph indicate minerogenic sediments, which absorb energy
from the X-rays to a greater extent. Minerogenic horizons most often result from storm events or other disturbances of the sea ¯oor. In recent times (since the 1970s and 1980s) dumping of dredged mud has become a common reason for anomalous sequences in the sediment records. Such events have been observed in several of the records from KoljoÈ Fjord. These sediments are easily recognised in the X-ray radiographs as well as by their content of `exotic' shallow water species of molluscs and foraminifera (Nordberg, unpubl. data). As no exotic species or other evidence of artefacts have been noted we believe that this light lamina was formed during a severe storm event. The 210 Pb analyses (Table 1) of this part of the core suggest that the lamination is annual. This part of the sequence with its distinct and conspicuous light lamina is easily recognised and can be used as a marker horizon in KoljoÈ Fjord. A second marker horizon is also a light, but not so distinct, layer at 13 cm that can be recognised over large areas of the fjord. This light
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minerogenic layer varies between 1 and 10 mm in thickness. The 210Pb dating suggests the year 1966 and if the laminae are counted from the 1930 level, assuming the varves are annual, the light band was deposited at the end of the 1960s almost consistent with the 210Pb date. In September 1969 there was a severe westerly hurricane on the Swedish west coast, the most severe recorded this century. We believe that this lamina was formed during that event. Seven or eight laminae above this light 1969-layer at 13 cm, there is another distinct light but thin lamina. The lamina is present two laminae below the uppermost varve, which is 210Pb dated at approximately 1980 suggesting that this light lamina was deposited in the late 1970s. The laminations within this part of the sequence together with the 210Pb dating suggest that the lamination is annual or almost annual not unlike classic varve sedimentation (De Geer, 1912; Anderson, 1996; Kemp, 1996). Finally, a sequence of homogeneous sediments is present between 7 and 1±2 cm depth. In this part of the core, mollusc shells are rare but polychaetes were found alive in 1993±1994 (Gustafsson and Nordberg, 1999). On top of the cores collected in September 1998, 1±2 cm of laminated sediments were also preserved. In contrast, in the autumn of 1995 and spring of 1996, no laminae were seen in the X-ray radiographs so these two to three new laminae have been formed since 1996 (Figs. 2 and 3). 3.2. Organic carbon The organic carbon content of core K6A shows a clear pattern. Generally the sediments are organic-rich varying between 4.4 and 7.7%. The lowest concentrations were noted between approximately 8 and 24 cm depth and at 0±1 cm. These lower values coincide with the two most recent laminated sequences. High concentrations, generally more than 7%, are seen in the uppermost homogeneous sequence between 1±2 and 8 cm depth. The oldest laminated sequence (42±49 cm) differs in character from the two most recent sequences with the carbon content exceeding 6%. In the homogeneous sequence between 25 and 42 cm the carbon content varies between 5.5 and 6.5%, whereas in the deepest homogeneous sediments the carbon content exceeds 7%, which is of the same magnitude as the
most recent homogeneous sediments. Another core (K40, not presented here) sampled at the same location in August 1993 shows exactly the same pattern although it is more condensed than K6A. 3.3. Instrumental data hydrography A compilation of existing deep-water salinity data from the KoljoÈ Fjord (average 30±43 m) results in a salinity record from 1946 to 1998. Two shorter periods, between 1934 and 1938 and between 1909 and 1911, were also available (Fig. 2). One of the most striking features of the record is that there have been at least two periods characterised by different salinities. Relatively low salinities (26±28.5) have dominated since the beginning of the 1980s and the higher salinities (28.5±31) dominated between 1946 and 1980. Salinity data collected between 1934 and 1937 are also characterised by higher salinities. There may have been another low salinity period before the 1930s as suggested by the low salinities between 1909 and 1911 (BjoÈrk, 1913). The higher salinity regimes, from the 1930s to 1980, appear to coincide with the formation and preservation of the laminated sediments. In the late 1960s, however, low salinities coincide with homogeneous sediments. During the period 1980±1996 salinity decreased below approximately 28.5, which also coincides with the formation of homogeneous sediments in the fjord. This was also the case for the homogeneous sediments deposited when measurements were taken between 1909 and 1911. This latter data set is very short, so caution is advised in the interpretation of these results. During the spring and autumn phytoplankton blooms of 1994 and 1995, thin laminae were formed and preserved temporarily. After some time, however, the thin algal mat was consumed by the meiofauna (Gustafsson and Nordberg, 1999). During core collection in 1998 new laminae had formed since the collection of cores in 1995 (Fig. 3). The salinity curve shows that there was a bottom-water renewal event in January±March 1996 when the salinity increased to almost 29 and passed the here determined empirical salt limit for the formation and preservation of lamina in KoljoÈ Fjord. Another prominent feature in the deep-water salinity record is the ¯uctuations, which appear to
K. Nordberg et al. / Journal of Sea Research 46 (2001) 187±200
Fig. 4. Spectral analysis of salinity data from KoljoÈ Fjord. The two peaks at 12.9 and 5.4 y are indicated. The dashed lines mark the upper and lower con®dence intervals. Settings: OFAC 4.0; HIFAC 1.0; Hanning window and three segments with 50% overlap. For detailed description of the parameters see Schulz and Stattegger (1997). A 6 db bandwidth of 0.077 y 21 and an 80% con®dence interval were used.
be cyclical. Cycles extend over several years between major salinity increases with only partial or minor water exchange in between. The typical curve shows a distinct increase in salinity with a subsequent gentle salinity decrease of 1.5±3 over several years. This possible cyclicity is also characteristic of the period of relatively low salinities initiated in approximately 1980. To test for true cyclicity we performed a spectral analysis. The lowest reliable frequency in this power spectrum is 0.077 y 21 (i.e. 12.9 years), assuming that at least two full cycles are observed within each WOSA segment. Peaks with lower frequency than this limit should be discarded. There are two clear peaks in the spectrum (Fig. 4), one peak around 12.9 years and a second peak around 5.4 years. Documented variations in oxygen concentrations in the bottom water (Fig. 5) reveal a connection between salinities and oxygen levels. High salinities are related to evolving low oxygen conditions and low salinities are related to more frequent water renewals. The oxygen and phosphate concentrations illustrate the well-known co-variation where low oxygen conditions and high concentrations of PO4 are related and vice versa. During low oxygen conditions, phosphate is trapped below the pycnocline and large amounts of phosphate are released from the sediments. Since the
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start of measurements in the late 1950s there appear to be no trends over time other than the close relationship to the oxygen concentrations, which in turn is related to salinity concentrations. This also appears to be the case for nitrogen; no temporal trend is documented in the bottom water. Similar features have been documented by Andersson (1996). He presented data from Skagerrak coastal waters outside KoljoÈ Fjord over the 1971±1990 period; these data show no signi®cant trends in the concentrations of DIN and PO4 for the important winter values available for the spring bloom. Instrumental data from the surface water (average 0, 5 and 10 m depth) started in the late 1950s show no dramatic change over time in any of the variables (Fig. 6). Before 1980, however, salinities were generally slightly higher and the amplitudes of salinity variations were somewhat higher. Phosphate nitrogen and chlorophyll a, however, show no variations over time except for the normal annual ¯uctuations. Phosphate and DIN are macro-nutrients essential for the size of primary production.
4. Discussion and conclusions 4.1. Salinity variations A comparison of the instrumental measurements and sediment records from KoljoÈ Fjord shows a close connection between lamination and deep-water salinity. Fig. 2 illustrates that laminae are formed and preserved when salinities exceed an empirical value of approximately 28±28.5. This value is characteristic of KoljoÈ Fjord, but other fjords may have their own intrinsic salinity limits for the formation and preservation of laminae. Generally, higher bottom-water salinity results in the development of a stronger pycnocline, which in turn leads to a decrease in both the diffusion of oxygen across the pycnocline and in entrainment. In addition, the probability, and hence the frequencies, of bottomwater renewal events decreases due to the higher densities of the more saline bottom water. The oxygen de®ciency develops as a result of oxygen consumption and a stable water column. The lack of water renewal, sometimes in combination with an in¯ow of partly oxygen-depleted water, leads to a continuous and
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Fig. 5. NAO indices (average January±March) and hydrography data from KoljoÈ Fjord monitoring site (average 30±43 m). For location see Fig. 1.
relatively fast decline in the oxygen content during the decay of oxygen-consuming material on the sea¯oor. The diffusion rates of oxygen across the pycnocline is far too slow to compensate for this oxygen consumption (Stigebrandt, 1991). During periods of lower salinities (lower than approximately 28±28.5) laminations do not develop or are not preserved. A weaker strati®cation of the water column allows a more ef®cient exchange of oxygen across the pycnocline and more importantly
results in more frequent and extensive water renewals. These frequent and extensive oxygenation events lead to a more sustainable benthic community and faster re-colonisations of bioturbating fauna after episodes of oxygen de®ciency (Josefson and Widbom, 1988; Alve, 1995). The salinity decrease is the combined effect of continuous diffusion, wind-induced mixing and lack of large in¯ows of water with substantially higher salinity than the existing bottom water. The longer the time over which the diffusion and mixing
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Fig. 6. Hydrography data and chlorophyll a (Chl a) content from KoljoÈ Fjord (average 0±10 m) monitoring site (see Fig. 1).
processes take place, the lower the resulting salinity, and the lower the salinity the greater the possibility of an extensive water exchange event. A convincing con®rmation of the importance of salinity is the formation of homogeneous sediments during the low salinity event of the late 1960s and the formation of three new laminae following the salinity increase in 1996 (Fig. 2). The salinity record suggests a cyclicity with spectral analysis giving peaks at 5.4 and 12.9 years. At present it is dif®cult to explain this cyclicity fully and its relationship to the well-known weather cycles of 7±11 years, the NAO and/or variations in the tidal force acting directly on the oceans and the atmosphere (Lamb, 1995). The salinity is subordinate to this cyclical variation, which has a similar patterns during low- and high-salinity periods. A lower initial salinity
may facilitate the exchange of bottom water, but as there seems to be no difference in the cyclical pattern between low- and high-salinity periods, it appears that this cyclic phenomenon is of a larger-scale character. Similar long-term cyclic salinity variations are observed in the deep-water of Gullmar Fjord (Nordberg et al., 2000; Nordberg, unpubl. data). However, the hydrography of the Gullmar Fjord is in contrast to KoljoÈ Fjord strongly determined by the open and adjacent Skagerrak (Svansson, 1984). 4.2. Organic carbon and primary production Laminated sediments in non-glaciated marine areas are generally evidence of oxygen de®ciency and a lack of macro- and megafauna (Kemp, 1996 and references therein). Increased accumulation of organic
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material leads to increased oxygen consumption and consequently increased organic content within the sediments. The lack of fauna that feed upon the sedimented planktonic organisms and the lack of bioturbation further increase the accumulation the preservation potential and the concentration of organic material in the sediments (Aller and Aller, 1998; Van der Weijden et al., 1999). It is also accepted that the decay of organic matter is generally a slower process or at least not faster in oxygendepleted environments (Can®eld, 1994; Hulthe et al., 1998; Van der Weijden et al., 1999). Consequently the organic carbon record from KoljoÈ Fjord has the potential to reveal the environmental history of the area if most is derived from surface export production and not derived as terrestrial material from the immediate hinterland. In Fig. 2, the Corg curve is interesting in that the carbon content is signi®cantly lower in the laminated sequences. Note that this is also the case in the top laminated sample (0±1 cm) where accumulation is recent and where consequently diagenetic processes have had little time to proceed. The results suggest that primary production may have been lower during the deposition of the recent laminated sequences. Unfortunately Chl a measurements were not started until 1986, so it has not proved possible to compare chlorophyll concentrations before and after the extensive lamination period deposited before 1980. The data set from 1986 shows no clear trends, but a few Chl a peaks can be seen within a relatively constant annual pattern (Fig. 6). 4.3. Correlation between sediments salinity and weather conditions Temperature data from the region and the NAO index (January±March) together with the sediment records provide interesting insights into the mechanisms controlling the fjord environment (Nordberg et al., 2000). There is a clear correlation between winter temperatures and the NAO index (winter values). As the NAO index is an expression of air-pressure and the wind systems in the north-east Atlantic, it also reveals the weather conditions in Scandinavia. Hence mainly negative NAO values, translated into weather conditions, lead to prevailing cold and dry winters with frequently occurring easterly and north-easterly
winds. These conditions enhance the out¯ow of lowsaline surface water from the Kattegat-Skagerrak and from the fjords and subsequent upwelling along the coast increases the frequency of water renewal events in the fjords. In contrast, positive NAO values lead to prevailing westerly winds with mild and humid winters. Westerly winds transport low-saline Baltic water towards the coast and trap low-saline surface water along the coast (Gustafsson and Stigebrandt, 1996; Gustafsson, 1999) and in fjords, which in turn prevents deep-water exchange. This mechanism is typical of the Gullmar Fjord (Nordberg et al., 2000), but in KoljoÈ Fjord with its shallow sills the situation is different. Here the saline and dense bottom water of the fjord is an obstacle for bottom water exchange, as evidenced by the deposition of laminated sequences. Comparisons with the NAO index show that the laminations were formed and preserved during the negative phase between 1930 and the early 1970s. The homogeneous sequences before and after this temporal interval are characterised by positive index values. The discrepancy between the termination of the laminated sequence in 1980 and the change from negative NAO values towards positive values in the early 1970s can be explained by the in¯ow of highly saline water in 1974, which prevented any major water exchange event for several years. In that year a similar saline water mass occurred at 25±30 m depth at the BornoÈ station in Gullmar Fjord (Nordberg, unpubl. data). About 1980±1981 the salinity had decreased below 28.5 in KoljoÈ Fjord and the lamination ceased. During the negative phase of the NAO index, winters were cold and summer temperatures were generally high, which also enhanced the thermohaline strati®cation. In addition, cold winters lead to sea ice formation in the fjords, which is present until late spring and can partly obstruct water exchange. In KoljoÈ Fjord a positive NAO index and low salinity conditions in the coastal area also lead to relatively lower salinities in the bottom water. Lower bottomwater salinities result in more frequently occurring water exchange and the more ef®cient diffusion of oxygen across a relatively weaker pycnocline. The mild, humid winters with limited ice cover and cold summers result in a relatively weaker thermohaline strati®cation. The effects of all these weather conditions are clearly observed in the sediment record. Above and below the laminated sequence formed
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between approximately 1930 and 1980 the NAO index is mainly positive and here the sediments are homogeneous. 4.4. Salinity and trophic level In Scandinavian fjords the deep water is usually enriched with nutrients trapped below the pycnocline (Aure and Stigebrandt, 1989; Stigebrandt et al., 1996). These nutrients are consequently not available to the phytoplankton when the pycnocline is intact. When the strati®cation is very weak or destroyed during periods of advective exchange of basin water, nutrients come close to the surface. In western Scandinavia this generally occurs during the spring phytoplankton bloom (Aure and Stigebrandt, 1989). During periods of higher bottom water salinities in KoljoÈ Fjord when laminated sediments are formed and preserved, the bottom water renewal events occur less often, resulting in fewer overturns of the water column. This phenomenon may have led to lower primary production and lower accumulation of organic material on the sea ¯oor. If an overturn takes place in the winter or early spring, well before the spring bloom, much of the nutrients will have left the fjord system before the phytoplankton is able to utilise it. In addition, if water exchange events take place when the spring bloom peaks, most of the primary production may be transported out of the fjord system and accumulate elsewhere, which may also lead to apparent lower primary production. The hydrographic data do not, however, allow us to determine whether it is fewer deep-water overturns, the timing of the renewals, or a combination of the two that has resulted in lower primary production and/or lower accumulation of organic carbon in the laminated sediments in KoljoÈ Fjord. 5. Concluding remarks KoljoÈ Fjord on the Swedish west coast is one of several coastal areas which are considered strongly in¯uenced by human-induced eutrophication. Here, however, we present data that clearly demonstrate that natural causes can have a signi®cant impact on the marine environment. In KoljoÈ Fjord, which is not subjected to signi®cant human pollution, natural conditions may even be the main causes of oxygen
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depletion. Both KoljoÈ Fjord and Gullmar Fjord (Nordberg et al., 2000) pinpoint the importance of distinguishing natural causes from human-induced effects on the marine environment. This is essential for the understanding of the marine environment and for taking adequate actions against human-induced hazards in the sea. Acknowledgements We thank the crews of r/v Arne Tiselius and r/v Skagerak for their assistance at sea. We are grateful for the ®nancial support from the Swedish Natural Science Research Council (NFR grants no. G-AA/GU 09874-307 and G-AA/GU 09874-309 K. Nordberg), Futura Foundation, Oscar and Lili Lamm Foundation, Carl Trygger Foundation and GoÈteborg University Marine Research Centre (GMF). References Anderson, R.Y., 1996. Seasonal sedimentation: a framework for reconstructing climatic and environmental change. In: Kemp, A.E.S. (Ed.). Paleoclimatology and Paleoceanography from Laminated Sediments. Geol. Soc. Spec. Publ. 116, pp. 1±15. Andersson, L., 1996. Trends in nutrient and oxygen concentrations in the Skagerrak-Kattegat. J. Sea Res. 35, 63±71. Aller, R.C., Aller, J.Y., 1998. The effect of biogenic irrigation intensity and solute exchange on diagenetic reaction rates in marine sediments. J. Mar. Res. 56, 905±936. Alve, E., 1995. Benthic foraminiferal distribution and recolonization of formerly anoxic environments in Drammensfjord, southern Norway. Mar. Micropal. 25, 169±187. Appleby, P.G., Old®eld, F., 1978. The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210Pb to the sediment. Catena 5, 1±8. Aure, J., Stigebrandt, A., 1989. On the in¯uence of topographic factors upon the oxygen consumption rate in sill basins of Fjords. Est. Coast. Shelf Sci. 28, 59±69. Aure, J., Danielssen, D., Sñtre, R., 1996. Assessment of eutrophication in Skagerrak coastal waters using oxygen consumption in fjordic basins. ICES J. Mar. Sci. 53, 589±595. Baden, S.P., Loo, L., Pihl, L., Rosenberg, R., 1990. Effects of eutrophication on benthic communities including ®sh: Swedish west coast. Ambio 19, 113±122. Barnett, P.R., Watson, O.J., Connelly, D., 1984. A multiple corer for taking virtually undisturbed samples from shelf, bathyal and abyssal sediments. Oceanol. 7, 399±408. BjoÈrk, G., Liungman, O., Rydberg, L., 2000. Net circulation and salinity variations in an open-ended Swedish fjord system. Estuaries 23, 367±380. BjoÈrk, W., 1913. Bidrag till kaÈnnedomen om nordhafsraÈkans
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(Pandalus borealis Kr.) utbredning och biologi i Kattegatt och Skagerack. Svenska Hydrogra®sk-Biologiska Kommissionens Skrifter, 1±11. Can®eld, D.E., 1994. Factors in¯uencing organic carbon preservation in marine sediments. Chem. Geol. 114, 315±329. De Geer, G., 1912. A geochronology of the last 12,000 years. Compte Rendus du XI Congres GeÂologique International (Stockholm, 1910), 241±253. Diaz, R.J., Rosenberg, R., 1995. Marine benthic hypoxia: A review of its ecological effects and the behavioural responses of benthic macrofauna. Oceanogr. Mar. Biol. Ann. Rev. 33, 245±303. Edebo, L., Haamer, J., Lindahl, O., Loo, L.O., Piriz, L., 2000. Recycling of macronutrients from sea to land using mussel cultivation. Int. J. Environ. Pollut. 13, 190±207. Gustafsson, B., 1999. High frequency variability of the surface layers in the Skagerrak during SKAGEX. Cont. Shelf Res. 19, 1021±1047. Gustafsson, B., Stigebrandt, A., 1996. Dynamics of the freshwaterin¯uenced surface layers in the Skagerrak. J. Sea Res. 35, 39±53. Gustafsson, M., Nordberg, K., 1999. Benthic foraminifera and their response to hydrography, periodic hypoxic conditions and primary production in the KoljoÈ fjord on the Swedish west coast. J. Sea Res. 41, 163±178. Haamer, J., 1995. Phycotoxin and oceanographic studies in the development of the Swedish mussel farming industry. PhD Thesis, Dept. Earth Sciences, Oceanography, GoÈteborg University Ser. A4. Hulthe, G., Hulth, S., Hall, P., 1998. Effect of oxygen on degradation rate of refractory and labile organic matter in continental margin sediments. Geochim. Cosmochim. Acta 62, 1319±1328. Hurrell, J.W., 1995. Decadal trends in the North Atlantic Oscillation: regional temperatures and precipitation. Science 269, 676±679. Josefson, A.B., Rosenberg, R., 1988. Long-term soft-bottom faunal changes in three shallow fjords, west Sweden. Neth. J. Sea Res. 22, 149±159. Josefson, A.B., Widbom, B., 1988. Differential response of benthic macrofauna and meiofauna to hypoxia in the Gullmar Fjord basin. Mar. Biol. 100, 31±40. Kemp, A.E.S., 1996. Laminated sediments as paleo-indicators. In: Kemp, A.E.S. (Ed.), Paleoclimatology and Paleoceanography from Laminated Sediments. Geol. Soc. Spec. Publ. 116, vii±xii. Kerr, R.A., 1997. A new driver for the Atlantic's moods and Europe weather? Science 275, 754±755.
Lamb, H.H., 1995. Climate, History and the Modern World. 2nd ed. Routledge, London, 433 pp. Lindahl, O., Belgrano, A., Davidsson, L., Hernroth, B., 1998. Primary production, climatic oscillations, and physico-chemical processes: the Gullmar Fjord time-series data set (1985±1996). ICES J. Mar. Sci. 55, 723±729. Nixon, S.W., 1995. Coastal marine eutrophication: A de®nition, social causes, and future concerns. Ophelia 41, 199±219. Nordberg, K., Gustafsson, M., Krantz, A.-L., 2000. Decreasing oxygen concentrations in the Gullmar Fjord, Sweden, as con®rmed by benthic foraminifera, and the possible association with NAO. J. Mar. Syst. 23, 303±316. Rogers, J.C., 1984. The association between the North Atlantic Oscillation and the Southern Oscillation in the Northern Hemisphere. Mon. Weather Rev. 112, 1999±2015. Rosenberg, R., 1985. EutrophicationÐthe future marine coastal nuisance? Mar. Pollut. Bull. 16, 227±231. Rosenberg, R., 1990. Negative oxygen trends in Swedish coastal bottom waters. Mar. Pollut. Bull. 21, 335±339. Rosenberg, R., Loo, L.O., 1988. Marine eutrophication induced oxygen de®ciency: effects on soft bottom fauna, Western Sweden. Ophelia 29, 213±225. Rosenberg, R., Hellman, B., Johansson, B., 1991. Hypoxic tolerance of benthic fauna. Mar. Ecol. Prog. Ser. 79, 127±131. Rosenberg, R., Cato, I., FoÈrlin, L., Grip, K., Rodhe, J., 1996. Marine environment quality assessment of the Skagerrak-Kattegat. J. Sea Res. 35, 1±8. Schulz, M., Stattegger, K., 1997. Spectrum: Spectral analysis of unevenly spaced paleoclimatic time series. Comp. Geosc. 23, 929±945. Stigebrandt, A., 1991. Computations of oxygen ¯uxes through the sea surface and the net production of organic matter with application to the Baltic and adjacent seas. Limnol. Oceanogr. 36, 444±454. Stigebrandt, A., Aure, J., Molvaer, J., 1996. Oxygen budget methods to determine the vertical ¯ux of particulate organic matter with application to coastal waters of western Scandinavia. Deep-Sea Res. II 43, 7±21. Svansson, A., 1984. Hydrography of the Gullmar fjord. Medd. Havs®skelab., Lysekil. No 297, 22 pp. Walker, G.T., 1924. Correlations in seasonal variations of weather: IX. Mem. Ind. Meteor. Dept. 24, 273±332. Van der Weijden, C.H., Reichart, G.J., Visser, H.J., 1999. Enhanced preservation of organic matter in sediments deposited within the oxygen minimum zone in the northeastern Arabian Sea. Deep-Sea Res. I 46, 807±830.
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Climate, hydrographic variations and marine benthic hypoxia in KoljoÈ Fjord, Sweden Kjell Nordberg a,*, Helena L. Filipsson a, Mikael Gustafsson a, Rex Harland b, Per Roos c a
Department of Earth Sciences, Oceanography, GoÈteborg University, PO Box 460, SE-405 30 GoÈteborg, Sweden b Centre for Palynology, University of Shef®eld, Dainton Building, Brook Hill, Shef®eld S3 7HF, and DinoData Services, 50 Long Acre, Bingham, Nottingham NG13 8AH, UK c Department of Radiation Physics, University of Lund, SE-221 85 Lund, Sweden Received 27 July 2000; accepted 5 July 2001
Abstract Since the late 1970s, Scandinavian waters have been extensively investigated for human-induced marine pollution, especially marine eutrophication, oxygen de®ciency in bottom waters and subsequent benthic mortality. The most serious oxygen de®ciencies are noted in the sill fjords along the Swedish west coast, in southern Norway and in large areas of the southern Kattegat and Baltic Sea. One of these sill fjords, KoljoÈ Fjord, is located on the Swedish west coast. This fjord is characterised by frequently occurring episodes of hypoxia/anoxia which last for months or even years. Sediments are laminated and the fjord is generally regarded as seriously affected by human-induced eutrophication. We detail the environmental development of a welldocumented fjord by combining high resolution sediment records with long hydrographical and meteorological instrumental data, and we present ultra high-resolution sediment information together with long-term instrumental records of air-temperatures, NAO indices and hydrography from KoljoÈ Fjord. These data show, in contrast to the current opinion focusing on anthropogenic eutrophication, that natural causes are the most important factors controlling the marine environment in this sparsely populated area. Natural variables concerned are fjord physiography, weather and hydrography (including the macro-nutrients DIN and PO4-P), sediment laminations and organic carbon. Interactions between fjord physiography, weather and hydrography regulate the possibility for water exchange and deep-water renewals. The present study points to the importance of natural causes for the environmental status of sill basins and semi-enclosed areas along the west coast of Sweden. q 2001 Elsevier Science Ltd All rights reserved. Keywords: Sill fjord; Sweden; Skagerrak; Hypoxia; Laminated sediments; Hydrography; Climate; Eutrophication
1. Introduction Marine offshore eutrophication in Scandinavian waters has been the focus of research among a large number of marine scientists for about 20 years (Baden et al., 1990; Diaz and Rosenberg, 1995; Nixon, 1995 and references in these). The consensus is that there * Corresponding author. E-mail address: [email protected] (K. Nordberg).
has been an increase in human sewage and agricultural fertilisers in the North Sea, including the Skagerrak Kattegat and Baltic Sea since the 1960s and 1970s (Aure et al., 1996; Rosenberg et al., 1996). This has in some places, especially in areas subjected to local outlets, resulted in enhanced primary production, altered algal composition, decreased Secchi depth increased accumulation of oxygen-consuming organic material on the sea ¯oor, increased severity of oxygen de®ciency in bottom water and subsequent benthic
1385-1101/01/$ - see front matter q 2001 Elsevier Science Ltd All rights reserved. PII: S 1385-110 1(01)00084-3
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Fig. 1. Map of the investigation area with the core location (black dot), monitoring station (cross) and the sill areas (S1 S2 and S3) indicated. Core location: 588 13 0 62 N/118 34 0 25 E; Monitoring site: 588 13 0 83 N/118 34 0 80 E.
mortality. In spite of ambitious sewage treatment, no clear improvements have been noted in the fjords or along the Swedish west coast. Despite a lack of signi®cant local sources of pollutants in the fjords, the human-induced eutrophication is considered to be ongoing, and even worsening the so-called largescale eutrophication (Rosenberg, 1990; Lindahl et al., 1998). Twenty years of studies focusing on human-induced marine eutrophication have partly resulted in a reduced awareness of the importance of the physiography of the fjord basins, the sill depth and variations in weather conditions in¯uencing exchange of basin water. The aim of this study was to detail the environmental development of a well-documented fjord by combining ultra-high resolution sediment records with instrumental data on hydrography and meteorology. The periodic hypoxia/anoxia and the
laminated sediments in the fjord have created a natural high-resolution environmental archive in the sediments. Scandinavian sill fjords with a limited oxygen supply represent a unique environmental archive in which the continuous accumulation of ®ne sediments and the minimal tidal activity and bioturbation create a natural laboratory for high spatial and temporal resolution studies. KoljoÈ Fjord on the Swedish west coast (Fig. 1) is one of several fjords that are considered to be heavily affected by marine eutrophication (Rosenberg, 1985, 1990; Josefson and Rosenberg, 1988; Haamer, 1995; Edebo et al., 2000). The region is characterised by crystalline bedrock, clay-®lled valleys and mixed forests. The KoljoÈ Fjord area has few local sewage outlets, since, like so many other Scandinavian fjords, it is located in a sparsely populated area. Most settlements in the KoljoÈ Fjord area
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and in the nearby Havstens fjord area are summerhouses with no signi®cant in¯uence on the marine environment. In addition, the hinterland is little farmed with only scattered small holdings. KoljoÈ Fjord is part of an open-ended fjord system (Fig. 1) and has a maximum depth of 56 m. To the west the fjord is connected with the Skagerrak by a sill of 8 m depth (S1) while to the east, between the KoljoÈ Fjord and Havstens Fjord, the sill depth is 12 m (S2). The Havstens Fjord, in turn, is separated from the sea to the south by a sill at 20 m depth (S3). KoljoÈ Fjord is characterised by brackish conditions (salinities: 20±30), a strati®ed water column, temporal oxygen de®ciency in the deep-water, laminated sediments, Beggiatoa bacterial mats covering the sea-¯oor and a lack of a benthic macrofauna (Gustafsson and Nordberg, 1999). A strong pycnocline at between 15 and 25 m separates the deep-water from the surface water. Tidal activity is very low (0.15±0.2 m) and of minor importance for the exchange of deep-water in the fjord (BjoÈrk et al., 2000). Deep-water renewal usually takes place during the winter and early spring at intervals of one to several years (Gustafsson and Nordberg, 1999). This renewal occurs when the thermocline is weak and episodes of upwelling along the Skagerrak coast lead to more saline water spilling over the fjord sills and ®lling the deeper parts of the fjords with oxygenated water. Prevailing winter winds on the Swedish west coast are westerly but deep-water exchange only takes place after periods of strong offshore winds between north and east, when the surface water is forced from the coast allowing upwelling from depth. Following a renewal event, the oxygen content decreases and when it is below 2 cm 3 dm 23 the benthic macrofauna starts to show signs of hypoxia (Rosenberg and Loo, 1988); below 0.5 cm 3 dm 23 most of the benthic macrofauna dies (Rosenberg et al., 1991). The lack of benthic fauna and consequent bioturbation is a prerequisite for the formation and preservation of laminated sediments. An unusually long record of hydrographic measurements is available for KoljoÈ Fjord, dating back to the mid-1930s with a downtime for World War II. These data are used here for the ®rst time to compare directly our proxy information with direct measurements.
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2. Material and methods Twelve short high-quality sediment cores were collected in September 1998 at a single site in 43 m of water from KoljoÈ Fjord (Fig. 1) at lat. 588 13 0 62 N long. 118 34 0 25 E. In a previous investigation (1993± 1994) on living benthic foraminifera, the same core location had been sampled (Gustafsson and Nordberg, 1999). We used a Multiple Corer Mark III-400 (Barnett et al., 1984) which gives a virtually undisturbed sediment surface. After collection the sediment cores were X-rayed on board ship using an Andrex BV (155 140 kV/10 mA) portable X-ray machine. A core collected in September 1995, at the same core location, is presented here in comparison with the cores collected in September 1998. 2.1. Sediment analyses After X-raying, one of the 1998 cores was sliced at 1-cm intervals and analysed for organic carbon (Corg) using a Carlo Erba NA 1500 analyser and dated by the 210 Pb method. At the same site we had previously dated two cores and investigated one core for its organic carbon content. The samples analysed for organic carbon were pre-treated with hydrochloric acid to remove any calcium carbonate. 2.2.
210
Pb dating
Sediment samples of 0.5±1 g were completely dissolved using hot HF/HNO3/HCl in the presence of 209Po acting as a yield determinant for the 210Pb daughter isotope 210Po. The polonium isotopes were plated on polished nickel discs from dilute hydrochloric acid in the presence of ascorbic acid to complex the iron. The discs were analysed for 210Po and 209Po using alpha spectrometry counting over at least 2 days. The mass of each sample was corrected for its salt content by using known porosity and assuming a salinity of 28. Radium-226 was measured by gamma spectrometry in order to determine the supported 210Pb levels. Analyses were performed at the Department of Radiation Physics, University of Lund, Sweden. Dating of the core was performed using the constant rate of supply (CRS) model (Appleby and Old®eld, 1978). The age of each sediment layer refers to the bottom part of the respective layers. Due to
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increasing errors when the Pb excess approaches zero the deeper sediment layers (below about 25 cm) were dated assuming the same sedimentation rate as in layers above 25 cm (~0.4 cm y 21), corrected for compaction. Datings have also been performed on two of the adjacent cores. The combined results from X-raying and Pb-datings indicate that the record of core K6A was the most extensive and it was therefore used as the main core for this study. The observed differences in the accumulation rate motivated the use of the CRS method.
in atmospheric pressure between the Azores and Iceland. This climatic oscillation has a strong in¯uence on the regime of westerly winds across the North Atlantic resulting in changes in winter temperatures and precipitation on both sides of the Atlantic Ocean (Kerr, 1997). Low-pressure systems and westerly winds in the North Sea region generally result in positive NAO-indices and vice versa. Normalised monthly NAO-indices were taken from Web site http://www.cgd.ucar.edu/cas/climind/ nao_monthly.html and as presented by Hurrell (1995).
2.3. Instrumental records and time series analysis
3. Results
The Swedish Meteorological and Hydrological Institute (SMHI) and the BohuslaÈn Water Conservation Association (in Swedish: GoÈteborgs och BohuslaÈns VattenvaÊrdsfoÈrbund) provided the hydrographical data including salinity, temperature, dissolved oxygen content, phosphate (PO4-P), dissolved inorganic nitrogen (DIN sum of NO2 NO3 and NH4) and chlorophyll a (Chl a). The position of their monitoring site in the fjord is: lat: 588 13 0 83 N long: 118 34 0 80 E. SMHI provided the air temperature data from Vinga Island located approximately 10 km west of GoÈteborg (Gbg in Fig. 1). Air temperature data were ®ltered using a 3-year moving average. Spectral analysis was used to determine if the salinity variations noted in KoljoÈ fjord's deep-water are cyclic. Since the data are unevenly distributed, we used the SPECTRUM program (Schulz and Stattegger, 1997). SPECTRUM is based on the Lomb-Scargle Fourier transformation of unevenly spaced time series together with a Welch-Overlapped-SegmentAveraging procedure. We have also calculated the 3-year moving average of the North Atlantic Oscillation indices (NAO), January±March, as an indication of the predominating winter weather conditions of the investigated time interval. Most of the deep-water exchange events occur during these three months. The North Atlantic Oscillation was discovered by Walker in the 1920s (Walker, 1924) and de®ned by Rogers (1984) as `a temporal ¯uctuation of the zonal wind strength across the Atlantic Ocean due to pressure variations in both the subtropical anticyclone belt and in the subpolar low near Iceland'. In other words, NAO can be described as the changes in the difference
The sediments in KoljoÈ Fjord, at water depths greater than approximately 25 m, are characterised by loose organic-rich mud and from time to time Beggiatoa bacterial mats (Gustafsson and Nordberg, 1999). Lamination within the sediments is best seen by using X-ray techniques as visual examination of the cores often underestimates the number of laminae present. The ®ve X-ray images presented here can easily be correlated and demonstrate three separate laminated sequences (Figs. 2 and 3). These are separated by homogeneous mud containing bioturbation structures and the remains of the macro/megafauna. Laminated sediments are widespread in the fjord system and become more widespread both temporally and spatially towards the west in KoljoÈ Fjord (Nordberg, unpubl. data). This is probably because most water renewal takes place across the eastern sill (S2, Fig. 1). However, a complete water renewal event registered around the New Year of 1993/1994 took place over the 8-m sill (S1) to the west (Fig. 1) (Gustafsson and Nordberg, 1999). 3.1. Sediment record The deepest 4 cm of the 53 cm long core K6A (49±53 cm) contains homogeneous sediments with bioturbation structures and mollusc shells. The oldest laminated sequence in the core (42±49 cm) represents the approximate time interval of 1830±1860, with a temporal break in the laminations at about 2 cm. A light lamina at 46 cm is surrounded by 10±12 distinct but thin laminae; these oldest laminae are best seen in Fig. 3, core 1995 and in core b, 1998. This sequence is
K. Nordberg et al. / Journal of Sea Research 46 (2001) 187±200
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Fig. 2. Composite diagram including time scale air temperature data from Vinga island (3-year moving average) NAO indices, winter values (average January±March and 3-year moving average), bottom-water salinity and oxygen from KoljoÈ Fjord, X-ray radiograph of core K6A, Corg from K6A.
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Fig. 3. X-ray radiographs from the investigated site in KoljoÈ Fjord. Core to the left was collected in 1995, and cores a, b, c and K6A were collected in 1998. The light lamina at 13 cm in core K6A is used as a level-peg for all cores in the ®gure.
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Table 1 210 Pb dating (C.R.S.) of core K6A from KoljoÈ Fjord Depth (cm)
Age (years)
Date
Depth (cm)
Age (years)
Date
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
1 3 6 9 12 15 17 20 22 24 26 28 32 34 37 39 42 45 47 50 52 55 58 62 65 68 71
1997 1995 1992 1989 1986 1984 1981 1978 1976 1974 1972 1970 1966 1964 1961 1959 1956 1953 1951 1948 1946 1943 1940 1936 1933 1930 1927
28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53
74 77 81 85 89 93 96 100 104 108 113 117 121 126 131 136 141 145 150 155 160 165 170 175 180 185
1924 1921 1917 1913 1909 1905 1902 1898 1894 1890 1885 1881 1877 1872 1867 1862 1857 1853 1848 1843 1838 1833 1828 1823 1818 1813
followed by approximately 17 cm of homogeneous sediments with bioturbating structures, macrofauna remnants and most often a few scattered gravel-to pebble-sized cinder particles. The most common shells found here are intact thin-shelled Macoma calcarea (Gmelin) with remarkably good preservation. The top 5 cm of this sequence is faintly banded and in places characterised by a few interbedded laminae. The second and most extensive and well-developed sequence of laminated sediments was formed between approximately 1930 and 1980 (25±7.5 cm). This sequence contains only a few interbedded thin bands of homogeneous sediments. The deepest part of the sequence is characterised by 3±4 cm of distinct laminations, including one thin, light lamina at 23 cm depth (Fig. 2). This light lamina is present eight `varves' above the initiation of this sequence. Light coloured sediments on the X-ray radiograph indicate minerogenic sediments, which absorb energy
from the X-rays to a greater extent. Minerogenic horizons most often result from storm events or other disturbances of the sea ¯oor. In recent times (since the 1970s and 1980s) dumping of dredged mud has become a common reason for anomalous sequences in the sediment records. Such events have been observed in several of the records from KoljoÈ Fjord. These sediments are easily recognised in the X-ray radiographs as well as by their content of `exotic' shallow water species of molluscs and foraminifera (Nordberg, unpubl. data). As no exotic species or other evidence of artefacts have been noted we believe that this light lamina was formed during a severe storm event. The 210 Pb analyses (Table 1) of this part of the core suggest that the lamination is annual. This part of the sequence with its distinct and conspicuous light lamina is easily recognised and can be used as a marker horizon in KoljoÈ Fjord. A second marker horizon is also a light, but not so distinct, layer at 13 cm that can be recognised over large areas of the fjord. This light
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minerogenic layer varies between 1 and 10 mm in thickness. The 210Pb dating suggests the year 1966 and if the laminae are counted from the 1930 level, assuming the varves are annual, the light band was deposited at the end of the 1960s almost consistent with the 210Pb date. In September 1969 there was a severe westerly hurricane on the Swedish west coast, the most severe recorded this century. We believe that this lamina was formed during that event. Seven or eight laminae above this light 1969-layer at 13 cm, there is another distinct light but thin lamina. The lamina is present two laminae below the uppermost varve, which is 210Pb dated at approximately 1980 suggesting that this light lamina was deposited in the late 1970s. The laminations within this part of the sequence together with the 210Pb dating suggest that the lamination is annual or almost annual not unlike classic varve sedimentation (De Geer, 1912; Anderson, 1996; Kemp, 1996). Finally, a sequence of homogeneous sediments is present between 7 and 1±2 cm depth. In this part of the core, mollusc shells are rare but polychaetes were found alive in 1993±1994 (Gustafsson and Nordberg, 1999). On top of the cores collected in September 1998, 1±2 cm of laminated sediments were also preserved. In contrast, in the autumn of 1995 and spring of 1996, no laminae were seen in the X-ray radiographs so these two to three new laminae have been formed since 1996 (Figs. 2 and 3). 3.2. Organic carbon The organic carbon content of core K6A shows a clear pattern. Generally the sediments are organic-rich varying between 4.4 and 7.7%. The lowest concentrations were noted between approximately 8 and 24 cm depth and at 0±1 cm. These lower values coincide with the two most recent laminated sequences. High concentrations, generally more than 7%, are seen in the uppermost homogeneous sequence between 1±2 and 8 cm depth. The oldest laminated sequence (42±49 cm) differs in character from the two most recent sequences with the carbon content exceeding 6%. In the homogeneous sequence between 25 and 42 cm the carbon content varies between 5.5 and 6.5%, whereas in the deepest homogeneous sediments the carbon content exceeds 7%, which is of the same magnitude as the
most recent homogeneous sediments. Another core (K40, not presented here) sampled at the same location in August 1993 shows exactly the same pattern although it is more condensed than K6A. 3.3. Instrumental data hydrography A compilation of existing deep-water salinity data from the KoljoÈ Fjord (average 30±43 m) results in a salinity record from 1946 to 1998. Two shorter periods, between 1934 and 1938 and between 1909 and 1911, were also available (Fig. 2). One of the most striking features of the record is that there have been at least two periods characterised by different salinities. Relatively low salinities (26±28.5) have dominated since the beginning of the 1980s and the higher salinities (28.5±31) dominated between 1946 and 1980. Salinity data collected between 1934 and 1937 are also characterised by higher salinities. There may have been another low salinity period before the 1930s as suggested by the low salinities between 1909 and 1911 (BjoÈrk, 1913). The higher salinity regimes, from the 1930s to 1980, appear to coincide with the formation and preservation of the laminated sediments. In the late 1960s, however, low salinities coincide with homogeneous sediments. During the period 1980±1996 salinity decreased below approximately 28.5, which also coincides with the formation of homogeneous sediments in the fjord. This was also the case for the homogeneous sediments deposited when measurements were taken between 1909 and 1911. This latter data set is very short, so caution is advised in the interpretation of these results. During the spring and autumn phytoplankton blooms of 1994 and 1995, thin laminae were formed and preserved temporarily. After some time, however, the thin algal mat was consumed by the meiofauna (Gustafsson and Nordberg, 1999). During core collection in 1998 new laminae had formed since the collection of cores in 1995 (Fig. 3). The salinity curve shows that there was a bottom-water renewal event in January±March 1996 when the salinity increased to almost 29 and passed the here determined empirical salt limit for the formation and preservation of lamina in KoljoÈ Fjord. Another prominent feature in the deep-water salinity record is the ¯uctuations, which appear to
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Fig. 4. Spectral analysis of salinity data from KoljoÈ Fjord. The two peaks at 12.9 and 5.4 y are indicated. The dashed lines mark the upper and lower con®dence intervals. Settings: OFAC 4.0; HIFAC 1.0; Hanning window and three segments with 50% overlap. For detailed description of the parameters see Schulz and Stattegger (1997). A 6 db bandwidth of 0.077 y 21 and an 80% con®dence interval were used.
be cyclical. Cycles extend over several years between major salinity increases with only partial or minor water exchange in between. The typical curve shows a distinct increase in salinity with a subsequent gentle salinity decrease of 1.5±3 over several years. This possible cyclicity is also characteristic of the period of relatively low salinities initiated in approximately 1980. To test for true cyclicity we performed a spectral analysis. The lowest reliable frequency in this power spectrum is 0.077 y 21 (i.e. 12.9 years), assuming that at least two full cycles are observed within each WOSA segment. Peaks with lower frequency than this limit should be discarded. There are two clear peaks in the spectrum (Fig. 4), one peak around 12.9 years and a second peak around 5.4 years. Documented variations in oxygen concentrations in the bottom water (Fig. 5) reveal a connection between salinities and oxygen levels. High salinities are related to evolving low oxygen conditions and low salinities are related to more frequent water renewals. The oxygen and phosphate concentrations illustrate the well-known co-variation where low oxygen conditions and high concentrations of PO4 are related and vice versa. During low oxygen conditions, phosphate is trapped below the pycnocline and large amounts of phosphate are released from the sediments. Since the
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start of measurements in the late 1950s there appear to be no trends over time other than the close relationship to the oxygen concentrations, which in turn is related to salinity concentrations. This also appears to be the case for nitrogen; no temporal trend is documented in the bottom water. Similar features have been documented by Andersson (1996). He presented data from Skagerrak coastal waters outside KoljoÈ Fjord over the 1971±1990 period; these data show no signi®cant trends in the concentrations of DIN and PO4 for the important winter values available for the spring bloom. Instrumental data from the surface water (average 0, 5 and 10 m depth) started in the late 1950s show no dramatic change over time in any of the variables (Fig. 6). Before 1980, however, salinities were generally slightly higher and the amplitudes of salinity variations were somewhat higher. Phosphate nitrogen and chlorophyll a, however, show no variations over time except for the normal annual ¯uctuations. Phosphate and DIN are macro-nutrients essential for the size of primary production.
4. Discussion and conclusions 4.1. Salinity variations A comparison of the instrumental measurements and sediment records from KoljoÈ Fjord shows a close connection between lamination and deep-water salinity. Fig. 2 illustrates that laminae are formed and preserved when salinities exceed an empirical value of approximately 28±28.5. This value is characteristic of KoljoÈ Fjord, but other fjords may have their own intrinsic salinity limits for the formation and preservation of laminae. Generally, higher bottom-water salinity results in the development of a stronger pycnocline, which in turn leads to a decrease in both the diffusion of oxygen across the pycnocline and in entrainment. In addition, the probability, and hence the frequencies, of bottomwater renewal events decreases due to the higher densities of the more saline bottom water. The oxygen de®ciency develops as a result of oxygen consumption and a stable water column. The lack of water renewal, sometimes in combination with an in¯ow of partly oxygen-depleted water, leads to a continuous and
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Fig. 5. NAO indices (average January±March) and hydrography data from KoljoÈ Fjord monitoring site (average 30±43 m). For location see Fig. 1.
relatively fast decline in the oxygen content during the decay of oxygen-consuming material on the sea¯oor. The diffusion rates of oxygen across the pycnocline is far too slow to compensate for this oxygen consumption (Stigebrandt, 1991). During periods of lower salinities (lower than approximately 28±28.5) laminations do not develop or are not preserved. A weaker strati®cation of the water column allows a more ef®cient exchange of oxygen across the pycnocline and more importantly
results in more frequent and extensive water renewals. These frequent and extensive oxygenation events lead to a more sustainable benthic community and faster re-colonisations of bioturbating fauna after episodes of oxygen de®ciency (Josefson and Widbom, 1988; Alve, 1995). The salinity decrease is the combined effect of continuous diffusion, wind-induced mixing and lack of large in¯ows of water with substantially higher salinity than the existing bottom water. The longer the time over which the diffusion and mixing
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Fig. 6. Hydrography data and chlorophyll a (Chl a) content from KoljoÈ Fjord (average 0±10 m) monitoring site (see Fig. 1).
processes take place, the lower the resulting salinity, and the lower the salinity the greater the possibility of an extensive water exchange event. A convincing con®rmation of the importance of salinity is the formation of homogeneous sediments during the low salinity event of the late 1960s and the formation of three new laminae following the salinity increase in 1996 (Fig. 2). The salinity record suggests a cyclicity with spectral analysis giving peaks at 5.4 and 12.9 years. At present it is dif®cult to explain this cyclicity fully and its relationship to the well-known weather cycles of 7±11 years, the NAO and/or variations in the tidal force acting directly on the oceans and the atmosphere (Lamb, 1995). The salinity is subordinate to this cyclical variation, which has a similar patterns during low- and high-salinity periods. A lower initial salinity
may facilitate the exchange of bottom water, but as there seems to be no difference in the cyclical pattern between low- and high-salinity periods, it appears that this cyclic phenomenon is of a larger-scale character. Similar long-term cyclic salinity variations are observed in the deep-water of Gullmar Fjord (Nordberg et al., 2000; Nordberg, unpubl. data). However, the hydrography of the Gullmar Fjord is in contrast to KoljoÈ Fjord strongly determined by the open and adjacent Skagerrak (Svansson, 1984). 4.2. Organic carbon and primary production Laminated sediments in non-glaciated marine areas are generally evidence of oxygen de®ciency and a lack of macro- and megafauna (Kemp, 1996 and references therein). Increased accumulation of organic
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material leads to increased oxygen consumption and consequently increased organic content within the sediments. The lack of fauna that feed upon the sedimented planktonic organisms and the lack of bioturbation further increase the accumulation the preservation potential and the concentration of organic material in the sediments (Aller and Aller, 1998; Van der Weijden et al., 1999). It is also accepted that the decay of organic matter is generally a slower process or at least not faster in oxygendepleted environments (Can®eld, 1994; Hulthe et al., 1998; Van der Weijden et al., 1999). Consequently the organic carbon record from KoljoÈ Fjord has the potential to reveal the environmental history of the area if most is derived from surface export production and not derived as terrestrial material from the immediate hinterland. In Fig. 2, the Corg curve is interesting in that the carbon content is signi®cantly lower in the laminated sequences. Note that this is also the case in the top laminated sample (0±1 cm) where accumulation is recent and where consequently diagenetic processes have had little time to proceed. The results suggest that primary production may have been lower during the deposition of the recent laminated sequences. Unfortunately Chl a measurements were not started until 1986, so it has not proved possible to compare chlorophyll concentrations before and after the extensive lamination period deposited before 1980. The data set from 1986 shows no clear trends, but a few Chl a peaks can be seen within a relatively constant annual pattern (Fig. 6). 4.3. Correlation between sediments salinity and weather conditions Temperature data from the region and the NAO index (January±March) together with the sediment records provide interesting insights into the mechanisms controlling the fjord environment (Nordberg et al., 2000). There is a clear correlation between winter temperatures and the NAO index (winter values). As the NAO index is an expression of air-pressure and the wind systems in the north-east Atlantic, it also reveals the weather conditions in Scandinavia. Hence mainly negative NAO values, translated into weather conditions, lead to prevailing cold and dry winters with frequently occurring easterly and north-easterly
winds. These conditions enhance the out¯ow of lowsaline surface water from the Kattegat-Skagerrak and from the fjords and subsequent upwelling along the coast increases the frequency of water renewal events in the fjords. In contrast, positive NAO values lead to prevailing westerly winds with mild and humid winters. Westerly winds transport low-saline Baltic water towards the coast and trap low-saline surface water along the coast (Gustafsson and Stigebrandt, 1996; Gustafsson, 1999) and in fjords, which in turn prevents deep-water exchange. This mechanism is typical of the Gullmar Fjord (Nordberg et al., 2000), but in KoljoÈ Fjord with its shallow sills the situation is different. Here the saline and dense bottom water of the fjord is an obstacle for bottom water exchange, as evidenced by the deposition of laminated sequences. Comparisons with the NAO index show that the laminations were formed and preserved during the negative phase between 1930 and the early 1970s. The homogeneous sequences before and after this temporal interval are characterised by positive index values. The discrepancy between the termination of the laminated sequence in 1980 and the change from negative NAO values towards positive values in the early 1970s can be explained by the in¯ow of highly saline water in 1974, which prevented any major water exchange event for several years. In that year a similar saline water mass occurred at 25±30 m depth at the BornoÈ station in Gullmar Fjord (Nordberg, unpubl. data). About 1980±1981 the salinity had decreased below 28.5 in KoljoÈ Fjord and the lamination ceased. During the negative phase of the NAO index, winters were cold and summer temperatures were generally high, which also enhanced the thermohaline strati®cation. In addition, cold winters lead to sea ice formation in the fjords, which is present until late spring and can partly obstruct water exchange. In KoljoÈ Fjord a positive NAO index and low salinity conditions in the coastal area also lead to relatively lower salinities in the bottom water. Lower bottomwater salinities result in more frequently occurring water exchange and the more ef®cient diffusion of oxygen across a relatively weaker pycnocline. The mild, humid winters with limited ice cover and cold summers result in a relatively weaker thermohaline strati®cation. The effects of all these weather conditions are clearly observed in the sediment record. Above and below the laminated sequence formed
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between approximately 1930 and 1980 the NAO index is mainly positive and here the sediments are homogeneous. 4.4. Salinity and trophic level In Scandinavian fjords the deep water is usually enriched with nutrients trapped below the pycnocline (Aure and Stigebrandt, 1989; Stigebrandt et al., 1996). These nutrients are consequently not available to the phytoplankton when the pycnocline is intact. When the strati®cation is very weak or destroyed during periods of advective exchange of basin water, nutrients come close to the surface. In western Scandinavia this generally occurs during the spring phytoplankton bloom (Aure and Stigebrandt, 1989). During periods of higher bottom water salinities in KoljoÈ Fjord when laminated sediments are formed and preserved, the bottom water renewal events occur less often, resulting in fewer overturns of the water column. This phenomenon may have led to lower primary production and lower accumulation of organic material on the sea ¯oor. If an overturn takes place in the winter or early spring, well before the spring bloom, much of the nutrients will have left the fjord system before the phytoplankton is able to utilise it. In addition, if water exchange events take place when the spring bloom peaks, most of the primary production may be transported out of the fjord system and accumulate elsewhere, which may also lead to apparent lower primary production. The hydrographic data do not, however, allow us to determine whether it is fewer deep-water overturns, the timing of the renewals, or a combination of the two that has resulted in lower primary production and/or lower accumulation of organic carbon in the laminated sediments in KoljoÈ Fjord. 5. Concluding remarks KoljoÈ Fjord on the Swedish west coast is one of several coastal areas which are considered strongly in¯uenced by human-induced eutrophication. Here, however, we present data that clearly demonstrate that natural causes can have a signi®cant impact on the marine environment. In KoljoÈ Fjord, which is not subjected to signi®cant human pollution, natural conditions may even be the main causes of oxygen
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depletion. Both KoljoÈ Fjord and Gullmar Fjord (Nordberg et al., 2000) pinpoint the importance of distinguishing natural causes from human-induced effects on the marine environment. This is essential for the understanding of the marine environment and for taking adequate actions against human-induced hazards in the sea. Acknowledgements We thank the crews of r/v Arne Tiselius and r/v Skagerak for their assistance at sea. We are grateful for the ®nancial support from the Swedish Natural Science Research Council (NFR grants no. G-AA/GU 09874-307 and G-AA/GU 09874-309 K. Nordberg), Futura Foundation, Oscar and Lili Lamm Foundation, Carl Trygger Foundation and GoÈteborg University Marine Research Centre (GMF). References Anderson, R.Y., 1996. Seasonal sedimentation: a framework for reconstructing climatic and environmental change. In: Kemp, A.E.S. (Ed.). Paleoclimatology and Paleoceanography from Laminated Sediments. Geol. Soc. Spec. Publ. 116, pp. 1±15. Andersson, L., 1996. Trends in nutrient and oxygen concentrations in the Skagerrak-Kattegat. J. Sea Res. 35, 63±71. Aller, R.C., Aller, J.Y., 1998. The effect of biogenic irrigation intensity and solute exchange on diagenetic reaction rates in marine sediments. J. Mar. Res. 56, 905±936. Alve, E., 1995. Benthic foraminiferal distribution and recolonization of formerly anoxic environments in Drammensfjord, southern Norway. Mar. Micropal. 25, 169±187. Appleby, P.G., Old®eld, F., 1978. The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210Pb to the sediment. Catena 5, 1±8. Aure, J., Stigebrandt, A., 1989. On the in¯uence of topographic factors upon the oxygen consumption rate in sill basins of Fjords. Est. Coast. Shelf Sci. 28, 59±69. Aure, J., Danielssen, D., Sñtre, R., 1996. Assessment of eutrophication in Skagerrak coastal waters using oxygen consumption in fjordic basins. ICES J. Mar. Sci. 53, 589±595. Baden, S.P., Loo, L., Pihl, L., Rosenberg, R., 1990. Effects of eutrophication on benthic communities including ®sh: Swedish west coast. Ambio 19, 113±122. Barnett, P.R., Watson, O.J., Connelly, D., 1984. A multiple corer for taking virtually undisturbed samples from shelf, bathyal and abyssal sediments. Oceanol. 7, 399±408. BjoÈrk, G., Liungman, O., Rydberg, L., 2000. Net circulation and salinity variations in an open-ended Swedish fjord system. Estuaries 23, 367±380. BjoÈrk, W., 1913. Bidrag till kaÈnnedomen om nordhafsraÈkans
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