Kattegat Area Research Articles

Spatial patterns in community structure of motile epibenthic fauna in coastal habitats along the Skagerrak – Baltic salinity gradient

Patterns in community structure and functioning of motile epibenthic fauna were investigated in shallow (0–1 m) sediment habitats along the Skagerrak–Baltic estuarine gradient (salinity range from 4 to 34). The study area was divided into five regions, reflecting different sea-basins along the 1260 km coastline, and fauna was collected at six sites within each region. Ten replicate samples of motile epibenthic fauna were taken randomly at each site with a portable drop trap (bottom area 1 m 2) in June and September in 2004. All together, 110 taxa were found, of which 45 had a marine and 65 a limnic origin. The marine species decreased along the salinity gradient while the limnic showed the opposite pattern. Number of species and abundance of epibenthic fauna exhibited considerable local and regional variation, with a trend of increase with decreasing salinity. Fauna biomass, on the other hand was significantly higher (six times) in the Skagerrak–Kattegat area compared to the Baltic. There was a significant difference in fauna composition among regions and season, but with high similarity within the five regions, which implies that management of such coastal habitats should preferably be based on scales of a region (ca. 100 km) or smaller. Predators were the dominant functional group in all coastal regions, with a species shift from Crustacea to Insecta along the salinity gradient and with gobid fish occurring in all regions. Grazers were the second most important group in the Skagerrak–Kattegat area, but planktovores were more important in two of the Baltic regions. The importance of shallow sediment bottoms as feeding and nursery grounds for coastal fish assemblages is discussed and compared throughout the investigated area.

Genetic variability in Atlantic herring (Clupea harengus harengus): description of protein loci and population data

In the search for genetic markers in the Atlantic herring (Clupea harengus harengus) samples of eye, heart, liver, and skeletal muscle from a total of 160 specimens from the Skagerrak area, the Kattegat area, and the Baltic Sea were analyzed by starch gel electrophoresis. Specific staining for 24 enzymes resolved 37 loci and 25 of these were considered usable in routine population surveys. The proportion of polymorphic loci varied between 0.125 and 0.167, by the 95 % criterion for polymorphism, and between 0.292 and 0.458, by the 99 % criterion; average hererozygosity ranged from 0.068 to 0.073. There were small but statistically significant allele frequency differences among sampling areas, indicating a potential discrepancy between actual genetic relationships and classical taxonomic status. Comparisons with previously published data revealed no significant heterogeneity among samples from both sides of the Atlantic ocean at any of the three polymorphic loci which could be used for such comparisons.

Sub-recent nitrogen-isotope trends in sediments from Skagerrak (North Sea) and Kattegat: Changes in N-budgets and N-sources?

We determined 15N/ 14N ratios of total nitrogen in surface sediments and dated sediment cores to reconstruct the history of N-loading of the North Sea. The isotopic N composition in modern surface sediments is equivalent to and reflects the isotopic mixture of oceanic nitrate on the one hand ( δ 15N = 5‰) and the imprint of river-borne nitrogen input into the SE North Sea ( δ 15N up to 12‰ in estuaries of the SE North Sea) on the other hand. We compare the results with δ 15N records from pre-industrial sediment intervals in cores from the Skagerrak and Kattegat areas, which both constitute significant depositional centres for N in the North Sea and the Baltic Sea/North Sea transition. As expected, isotopically enriched anthropogenic nitrogen was found in the two records from the Kattegat area, which is close to eutrophication sources on land. Enrichment of δ 15N in cores from the Skagerrak – the largest sediment sink for nitrogen in the entire North Sea – was not significant and values were similar to those found in sediment layers representing pre-industrial conditions. We interpret this isotopic uniformity as an indication that most riverine reactive nitrogen with its characteristic isotopic signature is removed by denitrification in shallow shallow-water sediments before reaching the main sedimentary basin of the North Sea.

Primary production by macroalgae in Kattegat, estimated from monitoring data, seafloor properties, and model simulations

The aim of the study was to estimate yearly macroalgal production in the Kattegat. The estimate was calculated from the abundance and distribution of nine of the most dominant macroalgal species, and from factors important for abundance, distribution and growth (e.g. bottom topography and sediment composition, irradiance, nitrogen concentrations and seawater temperature). The result showed that 6.6% of the Kattegat area is suitable for macroalgal growth. The estimated production was 4–514 g C m −2 year −1 depending on depth and sub-area. The total yearly production was estimated to 119×10 6 kg C y −1.

Stochastic modelling of dissolved inorganic nitrogen in space and time

Environmental monitoring datasets often contain a large amount of missing values, and are characterized as being sampled over time on a distinct number of locations in the area of interest. This paper proposes a stochastic approach for modelling such data in space and time, by taking the spatial and temporal correlations in data into account. It has been applied to observations of dissolved inorganic nitrogen in the Kattegat during the period 1993–1997. Modelling results are shown as maps of the spatial distribution of dissolved inorganic nitrogen (DIN) in 4 weeks, representing the four seasons, and as time series of DIN at three different locations. However, the model approach could be applied to any space–time point given by a location in the Kattegat area and a week in the 5-year period 1993–1997. The results can be interpreted from a biological and physical point of view. Thus for the specific application the approach seems to perform very well. The results obtained could be used to improve status reporting of the environment, or as forcing functions for time series models and deterministic, hydrodynamic ecosystem models.

Modelling recruitment and abundance of Atlantic cod, Gadus morhua, in the eastern Skagerrak–Kattegat (North Sea): evidence of severe depletion due to a prolonged period of high fishing pressure

In exploited fish populations, most of the variance in stock size is attributed to highly variable recruitment. Understanding the recruitment process and how it relates to stock size (i.e. the often sought for stock-recruitment functions) and to environmental stochasticity is of central importance to fisheries science and management. However, in many, if not most, exploited marine fish populations, fishing mortality is the other major force affecting stock size. Here we explored the effects of several biotic and abiotic variables on Atlantic cod ( Gadus morhua ) recruitment and abundance in the eastern Skagerrak–Kattegat area by deploying a GAM (general additive model) type of modelling. It was recognised that fishing pressure and not variable environment via recruitment was the pivotal variable explaining the dynamic of the cod stocks. Due to a prolonged period of high fishing intensity, local cod stocks in the study area, such as the one in the Kattegat may be considered as severely depleted, or possibly, on the verge of total extinction. It was further observed that population biomass and recruitment were substantially uncoupled, which may be regarded as an effect of continued transportation of recruits from spawning aggregations in adjacent areas such as the North Sea.

Seismic evidence for Late Proterozoic orogenic structures below the Phanerozoic sedimentary cover in the Kattegat area, SW Scandinavia

Commercial seismic reflection profiles from SW Scandinavia image Sveconorwegian orogenic structures in the late Precambrian basement beneath the thin Phanerozoic cover. We interpret (1) pronounced west to WNW dipping reflections as Sveconorwegian localized ductile shear which overprint; (2) weaker east dipping reflections, interpreted as Gothian or early Sveconorwegian compressional structures. Folded and laminar seismic fabrics may represent F and C type crystalline thrust sheets emplaced during the Gothian and Sveconorwegian events. Some of the dipping reflections are observed to sole into (3) an interval of subhorizontal reflections at 2.5–4.5 s two‐way travel time, which we interpret as a “fossil” ductile to brittle transition that was unroofed during the Gothian Orogeny. (4) NW‐SE striking thrusts cutting the Sveconorwegian structures may indicate a shift in the late Sveconorwegian stress field. They may represent an early stage in the subsequent Phanerozoic basin evolution in the Danish area.

Relative influence of 129I sources in a sediment core from the Kattegat area

The depth profiles of the 129I concentration and the 129I/ 127I ratio in a surface sediment core from the Kattegat area have been analyzed in order to obtain information about the different sources of 129I in that core. Therefore, a mathematical model that relates the measured values to the available emission data from the nuclear fuel reprocessing plants and nuclear weapons tests has been applied. Results show that the reprocessing plants at La Hague and Sellafield are the main sources of 129I in the sediment. Results about the transfer from the release points at the reprocessing plants to the sampling zone agree with other literature data. The model calculates quite fast the sedimentation of 129I in the sampling place, probably attached to organic matter. Finally, an estimation of approximately 89 kg of 129I released by Sellafield between 1952 and 1968 has been obtained from the model.

The Lower–Middle Jurassic of the Anholt borehole: implications for the geological evolution of the eastern margin of the Danish Basin

This study of Upper Pliensbachian – Bajocian/Bathonian deposits in a borehole drilled on the island of Anholt, Denmark incorporates sedimentology, biostratigraphy (palynomorphs and foraminifera), palaeomagnetism and coal petrology. The studied succession records a gradual change from marine inner shelf storm-influenced clays to mainly terrestrial sands, clays, and lignite containing a flora of mainly freshwater algae and pollen. The regression was initiated at the Pliensbachian–Toarcian boundary and marine influence ceased during Bajocian–Bathonian times; the regression thus took place earlier at Anholt than in the centre of the Danish Basin. The sediments in the Anholt borehole are referred to the Fjerritslev and Haldager Sand Formations. Although the Lower–Middle Jurassic boundary is commonly placed at the boundary between the two formations, our data indicate that at Anholt the upper Fjerritslev Formation (member F-IV) is of Aalenian age. The Lower–Middle Jurassic boundary occurs close to the boundary between members F-III and F-IV of the Fjerritslev Formation. In contrast to other Lower–Middle Jurassic successions in the North Sea region, smectites of inferred volcanic origin are preserved in the Anholt section, suggesting limited burial and hence less intense diagenetic illitisation or chloritisation of smectites. A down-hole increase in diagenetic influence is reflected by the increase down-section both in the thermal stability of kaolinite and in the vitrinite reflectance. Kaolinite of inferred authigenic origin forms a white powder in the quartz-dominated sands of the Haldager Sand Formation; this kaolinite is thermally very unstable and is interpreted to be of late diagenetic, post-uplift origin. The vitrinite reflectance data indicate that the Jurassic formations have been exposed to thermal maturation corresponding to burial to a depth of 1000–1200 m below their present depth. Post-maturation uplift of the order of 1 km probably occurred partly during Late Cretaceous – Paleocene inversion in the Kattegat area and partly during Oligocene–Recent regional uplift, the latter being the most important of the two uplift phases. Palaeomagnetic data indicate that the main carrier of magnetic remanence is fine-grained magnetite. The stable remanence shows a pronounced inclination shallowing, which is attributed to post-depositional compaction.

Triassic and Jurassic transtension along part of the Sorgenfrei–Tornquist Zone in the Danish Kattegat

In the Kattegat area, Denmark, the Sorgenfrei–Tornquist Zone, an old crustal weakness zone, was repeatedly reactivated during Triassic, Jurassic and Early Cretaceous times with dextral transtensional movements along the major boundary faults. These tectonic events were minor compared to the tectonic events of the Late Carboniferous – Early Permian and the Late Cretaceous – Early Tertiary, although a dynamic structural and stratigraphic analysis indicates that the Sorgenfrei–Tornquist Zone was active compared to the surrounding areas. At the end of the Palaeozoic, the area was a peneplain. Regional Triassic subsidence caused onlap towards the north-east, where the youngest Triassic sediments overlie Precambrian crystalline basement. During the Early Triassic, several of the major Early Permian faults were reactivated, probably with dextral strike-slip along the Børglum Fault. Jurassic – Early Cretaceous subsidence was restricted primarily to the area between the two main faults in the Sorgenfrei–Tornquist Zone, the Grenå–Helsingborg Fault and the Børglum Fault. This restriction of basin development indicates a change in the regional stress field at the Triassic–Jurassic transition. Middle Jurassic and Late Jurassic – Early Cretaceous subsidence followed the Early Jurassic pattern with local subsidence in the Sorgenfrei–Tornquist Zone, but now even more restricted to within the zone. The subsidence showed a decrease in the Middle Jurassic, and increased again during Late Jurassic – Early Cretaceous times. Small faults were generated internally in the Sorgenfrei–Tornquist Zone during the Mesozoic with a pattern that indicates a broad transfer of strike-slip/oblique-slip motion from the Grenå–Helsingborg Fault to the Børglum Fault.

Natural dissolution of modern shallow water benthic foraminifera: taphonomic effects on the palaeoecological record

Comparison of the living and dead assemblages of benthic foraminifera from the coastal areas of the Skagerrak–Kattegat reveals that, although there are considerable similarities in terms of the presence/absence of species, there are major differences in relative abundance at species level. Furthermore, the relative abundance of calcareous tests in the living assemblages is considerably higher than that of the dead assemblages (average values of 67 and 30%, respectively). Since the environments are microtidal, there is little transport of foraminiferal tests due to tidal currents so there are no introduced exotic species although there is local transport caused by waves and wave-induced currents. However, the major taphonomic process appears to be dissolution, of calcareous tests. This is manifested through etching and breakage of test walls, leading to complete decalcification which leaves a residue of organic linings of certain taxa (e.g., Ammonia beccarii), and a marked increase in abundance of agglutinated tests in the dead assemblages. Although dissolution is normal in marsh settings throughout the world, the Skagerrak–Kattegat area is unusual in the intensity of dissolution in the subtidal zone. Whereas most of the calcareous species occur throughout the investigated depth range (except marsh), two of the subtidal agglutinate species (Eggerelloides scaber and Ammotium cassis) show upper depth limits. Consequently, the agglutinated foraminifera are better indicators than calcareous foraminifera of upper depth limits in these shallow subtidal environments.

Marginal marine environments of the Skagerrak and Kattegat: a baseline study of living (stained) benthic foraminiferal ecology

This is the first detailed investigation of the distribution and ecology of living (stained) shallow water (0–6 m) foraminifera along the Skagerrak–Kattegat coast, eastern North Sea. A total of 25 species (13 agglutinated; 12 calcareous) are common in the 169 sediment surface samples which were collected from 27 geographic areas. The sediment grain size and total organic carbon (TOC) content are strongly variable and the salinity and temperature ranges were 10–31‰ and 9–30°C, respectively, at the time of sampling (July to October) but temperatures down to freezing occur during the winter. The species are divided into six environmental categories of which the first five comprise euryhaline and the sixth essentially stenohaline taxa: (1) species associated only with marsh plants, (2) species basically, but not entirely, associated with marsh plants, (3) species basically, but not entirely, restricted to non-marsh areas, (4) species solely recorded in non-marsh intertidal to subtidal environments, (5) species restricted to subtidal areas, (6) species basically living in the most open marine areas. In this region, marshes have a patchy distribution and they are small and compressed due to low tidal ranges (<40 cm). Balticammina pseudomacrescens (not reported here before) lives in the most elevated, landward, terrestrial parts of marshes and thus defines the uppermost limit of the influence of marine water. However, the marshes are generally dominated by Jadammina macrescens and Miliammina fusca at the landward and seaward sides, respectively. Jadammina macrescens is observed living epiphytically on decaying Carex leaf debris. The most widely distributed euryhaline species are Elphidium williamsoni, Miliammina fusca, Ammonia beccarii, and Haynesina germanica. The former two are common only in sediments with a mud content less than about 60%, whereas the latter two are common even in sediments with >80% mud. Ammoscalaria runiana is common only in coarse-grained sediments (<20% mud) with low TOC (≤0.7%). There are no marked biogeographic boundaries within the Skagerrak–Kattegat area but 10 of the 25 commonly occurring species have not been reported from the adjacent Baltic Sea, probably partly due to the brackish character of the water there. The southern limits of distribution of the northern species, Elphidium albiumbilicatum, Ammotium cassis, and Ophthalmina kilianensis, are in the Kattegat–Baltic Sea.

Oceanographic variability in the Skagerrak and Northern Kattegat, May–June, 1990

The Skagerrak Experiment (SKAGEX), was a large, international, ICES-supported joint venture, carried out in the Skagerrak–Kattegat area on four different surveys in the period 1990–1991. It involved some 20 institutes and, at times, up to 17 research vessels. The main aim of the Experiment was to identify and quantify the different water masses entering and leaving the Skagerrak area and their variation over time. It also aimed to investigate the mechanisms that drive the circulation and to study their effects on biological processes. The aim was to be attained mostly through extensive synoptic observations. This paper focuses on the variability in physical, chemical and biological parameters during the first part of SKAGEX, 24 May–20 June 1990. During the first half of the period of investigation, the main outflow from the Skagerrak, represented by the Norwegian Coastal Current, was barotropic with daily mean velocities varying from 10–40 cm s−1. During the second half a clear baroclinic current component developed, giving rise to near surface velocities of up to 100 cm s−1. A pronounced feature in the Skagerrak during the study was the counter-clockwise circulation of the Norwegian Coastal Current at times of strong northwesterly winds. During such conditions this surface water reached as far as the Danish coast south of 57°N and upwelling along the Norwegian coast was also found. During northerly winds upwelling also occurred along the Swedish coast. The nutrient-rich Jutland Coastal Water, originating from the German Bight, was never found to reach the inner part of the Skagerrak during this first part of SKAGEX. It was partly blocked or diluted by other water-masses. A large “ridge” of nutrient-rich Atlantic water was found in the central Skagerrak throughout the investigation. It is shown that this elongated “ridge” was associated with the deepest (>500 m) area of the Skagerrak. Within this area, high subsurface chlorophyll concentrations were always found and, due to the persistence of the supply of nutrients, it is concluded that this phenomenon could be one of the main reasons for the high productivity of the Skagerrak.

Eemian Climatic and Hydrographical Instability on a Marine Shelf in Northern Denmark

Benthic foraminifera and stable isotope data from the last interglaciation (Eemian, substage 5e) from a borehole at Skagen, Denmark, provide evidence for major environmental and hydrographic changes during this period. During the first millennium of the Eemian, water masses covering northern Denmark became gradually warmer. Temperate conditions prevailed during most of the interglaciation, but these were interrupted by two periods with decreased water temperatures. The first cooling (Event S-1) was not very distinct at Skagen, but the second (Event S-2), seen in both the foraminiferal and oxygen isotope record, represents a large shift to subarctic conditions. Carbon isotopes indicate a change in ocean circulation during both events. No comparable climate variations are seen within the Holocene record at the site. The final cooling of the water masses associated with the substage 5e/5d boundary occurred within a few hundred years. These last interglacial climatic changes were probably caused by variations in strength and/or position of the North Atlantic Drift, possibly as a result of varying vigor of the Atlantic conveyor. In addition, minor variations in the fossil assemblages also indicate fluctuations in the inflow of Atlantic water to the Skagerrak–Kattegat area during the warm intervals of substage 5e.

Toxic dinoflagellate “blooms” in the Kattegat, North Sea, during the Holocene

Data are presented on the temporal distribution of the toxic dinoflagellate Gymnodinium catenatum in the Kattegat region of the North Sea. Based on dinoflagellate cyst and pollen analyses and a high resolution chronology, a relationship between climatic and oceanographic variations and the blooming of G. catenatum and other dinoflagellates is demonstrated. Gymnodinium catenatum first migrated into the Kattegat about 6200 yr B.P. and had its first occurrence of high numbers at about 4300–4500 yr B.P. At about 4000 yr B.P., in connection with an oceanographic change and climatic deterioration, the species decreased abruptly and subsequently disappeared. It became re-established again at about 2000 yr B.P. and occurred in massive “blooms” during the so-called mediaeval warm epoch round about 700–800 yr B.P. (1200–1400 A.D.). At the time of the so-called Little Ice Age, approximately 300 yr B.P. (1500–1600 A.D.), G. catenatum again became extinct in the Kattegat area. This study clearly demonstrates that massive “blooms” of toxic dinoflagellates occur in the historical record before any anthropogenic impact on the environment.

Epizootiology of Ichthyophonus hoferi in herring populations off the Swedish west coast

DAO Diseases of Aquatic Organisms Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials DAO 27:187-195 (1996) - doi:10.3354/dao027187 Epizootiology of Ichthyophonus hoferi in herring populations off the Swedish west coast Rahimian H, Thulin J During August-September 1991, Ichthyophonus hoferi caused a mass mortality among the herring Clupea harengus population of the Skagerrak-Kattegat. This was the first record of a mass mortality of herring due to I. hoferi along the eastern coast of the North Atlantic and the first record of I. hoferi from the Skagerrak and Kattegat area. From August 1991 to December 1994 herring were sampled and examined both macro- and microscopically for signs of ichthyophonosis. Macroscopically visible signs of ichthyophonosis were found in 269 of the 25156 herring examined. Microscopic examination of different organs showed more than 4 times the prevalence than did macroscopic examinations. The ratio between the results of the micro- and macroscopical examinations, i.e. signs of passive/signs of active phase of infection, varied with the age of the fish as well as with the season. The prevalence of infection fluctuated seasonally, probably due in part to the migration pattern of herring; the spring spawning population was assumed to be the most heavily infected population of the area. The number of herring killed as a result of infection is estimated to be more than 300 million, i.e. more than 10% of the population. Differences between the recent epizootic among herring stocks due to I. hoferi and those reported from the western North Atlantic are also discussed in the 'Discussion' section. Epizootiology · Herring · Clupea harengus · Ichthyophonus hoferi · Skagerrak · Kattegat Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in DAO Vol. 27, No. 3. Publication date: January 12, 1996 Print ISSN:0177-5103; Online ISSN:1616-1580 Copyright © 1996 Inter-Research.

Foraging behaviour of larval cod (Gadus morhua) influenced by prey density and hunger

Fish larvae meet diverse environmental conditions at sea, and larval growth and chance of survival depend on a flexible response to environmental variability. The present study focuses on the flexibility of the foraging behaviour of larval cod in a series of laboratory experiments on larval search activity, prey selectivity, and hunger in a variable prey environment. Gadus morhua eggs were collected in March 1992 and 1993 from the Kattegat area, Denmark, fertilised and incubated in the laboratory. After hatching, the larvae were transferred to rearing tanks of 172 litres. The behaviour of larvae (6 to 7 mm long) was observed visually, and prey attacks, swimming activity and gut contents were registered across a range of 1 to 120 copepod nauplii l-1. When prey density decreased, larvae increased their swimming activity, increased their responsiveness to prey (distance of reaction) and decreased their prey size selectivity. Behavioural response was to a large degree determined by the level of hunger, represented by the number of newly ingested prey in the gut. The findings show that cod larvae have a flexible response to changes in feeding conditions and imply that larvae can grow and survive even in the lower range of (mean) prey densities measured at sea.

Palaeozoic structural development along the Tornquist Zone, Kattegat area, Denmark

The interpretation of newly released commercial 2D reflection seismic data in the Kattegat area, Denmark, has provided us with a better understanding of the Palaeozoic tectonic processes along the Tornquist Fault Zone. A Base Palaeozoic time structure map, a Lower Palaeozoic TWT isopach map, a “true” Lower Palaeozoic TWT isopach map, an Upper Carboniferous/Lower Permian syn-rift TWT isopach map, a Top pre-Zechstein time structure map and a Zechstein combined TWT isopach and Palaeogeography map have been generated. The uniform Lower Palaeozoic sequence thickness in the Kattegat, both inside and outside the Tornquist Zone indicates only minor lateral movements if any, whereas the extensive Upper Silurian sequence, increasing in thickness to the north, indicates a relatively fast regional subsidence. The Base Palaeozoic time structure map and the Late Palaeozoic syn-rift isopach map show a clear Late Palaeozoic extension in the area. The syn-rift isopach map, in combination with the time-equivalent opening of the Skagerrak graben at right angles to the Tornquist Zone in the Kattegat, indicates that this extensional tectonic event had a dextral slip component. Measurements on internal extensional faults in the Tornquist Zone, give a minimum right-lateral displacement of 10.4 km. The footwall blocks were deeply eroded during the Early Permian rifting, and at Zechstein times the area became a peneplane. The Tornquist Zone was later exposed to several tectonic phases, where dextral slip played a role, indicated by the “push up” and “pull down” structures caused by restraining and releasing bends of the Børglum Fault. The dextral displacement along the Børglum Fault since the beginning of the Permian is in the order of 5–7 km based on the displacement of a Lower Palaeozoic local depocentre. Early Permian depocentres and faults, which gives a total amount of right-lateral displacement since the Early Palaeozoic in the order of 15–20 km. The continuously repeated tectonic episodes along the Tornquist Zone throughout most of the Phanerozoic, show that the zone was easily reactivated, implying deep-seated basement faults. The Tornquist Zone can be seen as a “buffer zone”, between continental blocks, whenever changes in the regional stress field are induced.

Do toxic algal blooms affect subtidal soft-bottom communities?

An extensive bloom of the toxic flagellate Chrysochromuha polylepjs occurred over much of the Skagerrak and Kattegat area during May-June 1988 with severe effects on biota in shallow marine habitats. Results reported here are from annual grab-sampling of subtidal soft-bottom communities off the southwest coast of Norway during the period 1986 to 1991 at depths between 69 and 184 m, and describe changes In fauna associated with the toxic algal bloom. In the 3 years prior to the bloom the fauna at each station was persistent and only modest year-to-year changes in the number of species, abundance and species composition were observed. Samples collected 1 yr after the bloom event revealed a significant reduction in both number of species and individuals. The most obvious changes were found at sites situated in accumulation areas characterised by fine sediments indicating a possible connection between the quantity of sinking toxic material and the degree of faunal perturbation. A DCA ordination demonstrated a clear switch in faunal composition in the study area subsequent to the bloom, and a tendency of the fauna to return to the pre-bloom communities 2 yr after the bloom event. Principal species affected were moderately abundant oligochaetes, polychaetes, crustaceans, molluscs and sipunculids. Allocation of each species into the categories 'gaining' and 'losing' according to annual changes in abundance gave a good description of the community disturbance probably induced by the toxic alga. Other possible causes for the observed faunal perturbation may be changes in water temperature, food input, predation interference and changes in sediment composition, but none of these were found to give reliable, alternative explanations for the substantial changes observed in the soft-bottom communities subsequent to the toxic bloom.

Dating of marine sediments by an incomplete mixing model

A marine sediment core from the Kattegat area (Sweden) has been studied for dating purposes. The 137Cs and 210Pb profiles have been measured and described by a model of incomplete activity mixing. A time-dependent 137Cs flux function for the sediment has been constructed from atmospheric data and the 137Cs contents in water from the Sellafield nuclear reprocessing plant. It has been used as the 137Cs input function for our model. The results show that the model clearly reproduces the measured profile.