Results from simultaneous velocity, salinity and nutrient measurements within the Southern Jutland Current and in the Skagerrak are presented. The measurements were undertaken during the 1990 and 1994 period along three sections perpendicular to the northwest coast of Denmark with the main purpose of estimating the importance of the Southern Jutland Current as a carrier of nutrients and contaminants from the continental rivers. Based on a total of 64 transects, we estimated the average cyclonic circulation in the outer Skagerrak at 0.8 Sv (1 Sv = 106·m3·s−1), half of which occurs in the upper 50–100 m (S<35 psu). The northward flow along the west coast of Denmark, at Tyborön, averaged 0.045 Sv within 100 km from the coast. Although this average flow is almost negligible, the northward flux of nutrients (mainly as DIN, inorganic N or TN, total N) along the coast is not a negligible contribution to the fluxes into the Skagerrak. Tidal currents and strong longshore nutrient gradients give rise to significant diffusive fluxes. Including some intermittent outflows of low-salinity water from the southern North Sea, we find an average northward DIN (TN) flux of 350 (600)·106 kg per year. While the diffusive component dominates, it seems that large outflows of southern North Sea water during winter or early spring, directly related to high freshwater discharge, is the only way in which the Kattegat nitrogen budget may be influenced. Such large outflows were recorded in the spring of 1988, 1989 and 1994. The last period is included in our measurements.
A total of 680 seawater samples were collected and analysed for volatile halogenated organic compounds, and 280 seawater samples were analysed for chlorinated phenols in the Skagerrak. The sampling was done along three transects along the Danish west coast on five occasions during the years 1991 to 1993. Pentachlorophenol (PCP) was the only chlorophenol detected on all occasions, which implies that it is transported as a dissolved species rather than particle bound. The results indicate that the origin of PCP in the Skagerrak is the Baltic and the coastal areas of Sweden and Norway. The biogenic volatile halocarbons constitute the largest fraction of the halocarbons in the area. The data support the findings that volatile chloroethenes are naturally produced. Therefore, the Skagerrak acts as a source for these compounds. The flux of the compounds investigated is directed from the sea to the atmosphere except for carbon tetrachloride.
The Benthos 373 plankton silhouette camera was one of the first instruments used to study the in situ particle size spectra of flocculated material. Based on a concept originally developed by H. Edgerton for the study of live zooplankton samples, the relatively non-invasive nature of the camera has made it well suited for the study of flocculated suspensions in the coastal zone. Results from studies using the camera showed clearly the dynamic nature of particle flocculation in both fresh and salt water environments. Originally limited by the time required to carry out image analysis of the photographs obtained, recent advances in the digitization of photo negatives and image analysis software have increased the usefulness of this camera for studies of particulate material in the coastal zone.
Distribution and growth dynamics of ephemeral macroalgae were investigated in some shallow (0–1 m) bays on the Swedish west coast during the period 1992 to 1994. Variation in cover and biomass was assessed in nine bays, and in one of them the seasonal dynamics of these algae was followed intensively over three years. Frequent measurements were taken of algal biomass, degree of cover, in situ growth, variable fluorescence and C/N-ratios. Irradiance and water nutrient concentrations were measured concurrently with the growth measurements. Ephemeral macroalgae were dominated by Cladophora and Enteromorpha species and occurred in all sampled bays, except one, covering 10 to 100% of the bottom sediment. Generally, a rapid biomass increase was recorded from mid-May, which peaked after six weeks at 400–600 g dwt·m−2. Later in the season, strong variations in biomass, cover and species composition were observed, suggesting that these opportunistic algae form a highly dynamic community. Initial growth rates estimated from biomass samples were similar to those recorded from in situ cage experiments, and also agreed with growth rates calculated from a model. For all species studied growth rate was within the range 10 to 30 g dwt·m−2·d−1, irrespective of method used. Low algal C/N-ratios (mean = 12.7) in 1993 (cold and rainy summer) indicated that growth was not limited by nutrients, but rather by light. In 1994 (warm and sunny summer), mean C/N-ratios were 20, reflecting the opposite situation. The appearance of these opportunistic algae in shallow bays which historically had been without macroalgal communities has changed the characteristics of these areas by altering habitat complexity. This could have important consequences for trophic interactions involving many species, thereby altering community structure and function.
The pelagic primary production and the secondary production of four species of benthic suspension feeders were frequently measured over a period of approximately one year in four different habitats. Simplified energy budgets over one-year periods are presented including ingestion, absorption, faeces production, respiration and secondary production. The production of a Mytilus edulis culture exceeded primary production by about 6.25 times, whereas semi-exposed shallow-water populations of Cerastoderma edule and Mya arenaria had a production roughly equal to primary production. In an exposed area, the latter species had a secondary production:primary production ratio of 0.15:1, whereas for a deeper-living (40 m) Amphiura filiformis population this relation was 0.0034:1. M. edulis followed by C. edule and M. arenaria in the semi-exposed habitat had the comparatively lowest Respiration:Absorption ratios and the highest Production:Absorption ratios. This study emphasizes the ecological importance of horizontal advective processes for energy transfer from the pelagic to the benthic system.
New information is presented on activity, abundance, and size spectra of bacteria as well as on concentration and composition of particles in the benthic boundary layer (BBL) and in the lower water column. The distribution of these parameters is related to the hydrodynamic regime close to the seafloor. Particular focus is on modelling the potential increase in nutrient access of particle-associated bacteria due to fluid shear. Microbial activities and concentrations of various parameters displayed distinct distribution patterns in the BBL and were up to a factor of 7.5 higher than in the adjacent water column. Utilization of 14C-amino acids appeared to be independent of POC and PON concentration, but was related to the contribution of phytogenic material to the POC. The higher microbial activities in the turbulent BBL than in the comparatively stagnant water column could be accounted for by the potential enhancement of nutrient flux due to fluid shear, assuming tight particle association of the bacterial community in the BBL.
A version of the Owen tube with reduced flow disturbance and wider operating concentration range —UWB QUISSET—is described. Quasi in situ settling velocity distributions of suspended particulate matter (SPM) were determined using these during an intercalibration exercise in the Elbe estuary in June 1993. Systematic variation was observed, in response to tidal resuspension, advection and redeposition. The observed variation in median settling velocity was in good agreement with that obtained by other workers using different settling tubes. Systematic differences in absolute values of up to an order of magnitude between the various techniques may have been caused by differences in tube design, sample handling or raw data interpretation.
The population structure of Mya arenaria has been investigated in the sublittoral zone of the Pomeranian Bay (southern Baltic Sea). Box-corer samples were collected during a 1.5-year period in 1993/94 to follow changes in size and age structure of the clam populations in different parts of the study area. Large spatial differences in population structure were found between the sheltered southwest of the Bay and the shallow and exposed Oder Bank in the centre. The stock of the Oder Bank consisted of two different clam types. A slow-growing cohort was assumed to be autochtonous on the Oder Bank, whereas a fast-growing one was assumed to have immigrated from the surrounding area. The contribution of the two cohorts to the total density varied seasonally. Because of bedload transport of clams, the contribution to the reduction of the clam stock by wintering sea ducks could not be quantified. In the southwest of the Pomeranian Bay erosion was of minor importance. High mortality rates during the first two years of life were assumed to be caused by predation. Mortality rates of older cohorts remained stable until old age. Variations in cohort strength were related to interannual differences in the reproductive success. A mild winter presumably lowers the reproductive success in the subsequent summer. Severe oxygen depletion in summer 1994 caused a strong reduction in the clam stock at stations deeper than 10 m.
Recent benthic foraminifera have been investigated in surface samples (0–8 cm) from thirteen sites along a profile in the Skagerrak, between Norway and Denmark. The investigated transect is sampled in an oceanographically well investigated area. Principal component analysis based on the 32 most abundant taxa is used to recognize similarities within the large data set of benthic foraminifera (a total of 93 samples), which is shown to cluster into four assemblage groups (A–D). The spatial distribution of these foraminiferal groups indicates that they inhabit areas that correspond with the delimitations of different water masses. We have identified areas where it is possible to investigate effects on the sea floor of different tracers, natural or anthropogenic contaminants, related to specific water masses.
The Skagerrak and Kattegat form a transition zone between the Baltic and the North Sea. Both areas are subject to increased nutrient loads. In this paper a non-parametric method is used to calculate the changes in nutrient and oxygen concentrations in the area. The period chosen was 1971 to 1990 and the parameters were dissolved inorganic nitrogen, phosphorus, silicat, total nitrogen and total phosphorus together with oxygen and oxygen saturation. Data have been sorted after salinity and the analyses have been carried out for different water masses and subareas. The results demonstrate that both surface and deep water in the Kattegat show increasing trends during winter for all nutrients except silicate. During summer there is an increase in total nitrogen and total phosphorus while silicate shows a decrease. In the Skagerrak the picture is more variable; in the eastern part, however, there is a clear increase for all inorganic nutrients during winter in the coastal water. Oxygen shows a declining trend in most areas.
Measurements of persistent organic pollutants (POP) in air and deposition have been carried out at sea-based and coastal sites on the Swedish west coast. The aim was to obtain knowledge concerning the input of these contaminants to the Skagerrak via atmospheric transport and deposition. The measurements showed a continuous deposition of POP, here represented by PAH, PCB and HCH, to the sea surface of the Skagerrak and an important episodic input in connection with long-range air transport especially together with precipitation. However, the deposition fluxes may also be affected by local sources. A seasonal variation was observed in the concentrations of the different compounds in both air and deposition. The deposited amounts of the selected compounds were within the same order of magnitude at the coastal and sea-based stations but there were differences in the deposition on the individual sampling occasions. There were minor geographical variations with a somewhat greater deposition of HCH and PAH in the southern part of Skagerrak.
The food choice of juvenile flounder (Platichthys flesus) and turbot (Scophthalmus maximus) was studied in the northern Baltic Sea during the years 1988, 1989, 1994 and 1995, The diet included organisms from 30 species/taxa in flounder (n=306) and 10 species/taxa in turbot (n=41). Flounder less than or equal to 45 mm mainly consumed meiofauna (dominating taxon: Harpacticoida, Copepoda) and larger fish (46-101 mm) consumed macrofauna (dominating taxa: Oligochaeta, Amphipoda and Chironomidae). In terms of biomass, macrofauna dominated for all sizes of flounders, and meiofauna was important only for the smallest fish. A strong seasonal variation could be detected in the diet. In spring, macrofauna dominated for all size classes of fish (only fish >30 mm were caught in spring), while in summer and autumn meiofauna dominated the diets for fish less than or equal to 45 mm in size. Juvenile turbot (22-88 mm) consumed macrofauna and small fish. Turbot less than or equal to 30 mm consumed mainly amphipods, while >30 mm turbot consumed mysid shrimps, amphipods and fish.The ontogenetic shift from meio- to macrofauna-sized prey in flounders occurs at a larger fish size in the northern Baltic Sea than reported in other areas, possibly depending on the increased relative importance of meiofauna in the northern Baltic. The seasonal variation in the diet could be due to seasonally changing abundances in the zoobenthos, or for the small fish (1-group, spring), to switching from meio- to macrofauna in order to optimize their energy gain. The 0-group flounders consumed meiofauna for a long period, possibly due to a learning-process or simply due to easy availability of meiofauna. Turbot has a much larger mouth gap than flounders, thus allowing them to consume macrofauna from the beginning of their benthic life.
The transport of fine suspended matter (SPM) at a number of stations arranged along three sections along the northwestern coast of Jutland is investigated by means of repeated simultaneous observations of SPM and velocity with high horizontal and vertical resolution. The amount of SPM transported into the Skagerrak with the inflowing water agrees well with earlier estimates of deposites in the area. Most of the SPM originates from the central and northern parts of the North Sea. Only about one fourth of the SPM entering the Skagerrak originates from the southeastern North Sea. The observations indicate a settling of SPM through the deep water which can account for the high deposition rates observed in the eastern part of the Skagerrak.
The grazing impact by a dense population of filter-feeding ascidians Ciona intestinalis on horizontally flowing water (driven by density circulation) in a shallow cove (Kertinge Nor, Denmark) has been described and quantified by means of a simple one-dimensional numerical model. The agreement between observations and modelled predictions was satisfactory. The applied numerical model has the following analytical solution in the idealized case: Cx = C0e−(fx/Y2), where Cx = algal concentration at a downstream distance x, C0 = initial concentration, f = F/vc; F = area specific population filtration rate; vc = current velocity; Y2 = depth of mixed layer below halocline. The numerical model quantifies the actual grazing impact while the analytical model illustrates the governing physics in well-known terms. To describe situations with no current (i.e. stagnant water), we performed simulation studies in the laboratory and measured vertical profiles of algal cells over filter-feeding C. intestinalis. The results showed that phytoplankton became reduced in a near-bottom water layer of 20–30 cm thickness. Such water layers may develop in stagnant water (calm days and no advective currents), thus uncoupling the pelagic food and the filter feeders which within a short time will experience extremely meagre food conditions.
Variation in the sensitivity to stress of Macoma balthica was measured in several French and Dutch estuaries. For adult and juvenile Macoma balthica exposed to copper under conditions of starvation, differences in mortality rate, condition, glycogen, burrowing rate and copper content were assessed. No significant differences were observed between adults and juveniles; the influence of treatment and origin was always evident. Animals from the most southern estuaries, Loire and Gironde, near to the species's southern limit of distribution, showed, in the field, the strongest deviations for the ecophysiological traits measured, and were in the experiments the most sensitive to stress.
Seasonal dynamics of Zostera noltii was studied during 1984 in Arcachon Bay, France. In this Bay, Z. noltii colonizes 70 km2, i.e. approximately 50% of the total area, while Z. marina occupies only 4 km2. Densities and length of vegetative and generative shoots and above-ground and below-ground biomasses were monitored in four meadows which differed according to their location in the Bay, tidal level and sediment composition. Three of these meadows were homogeneous, well-established beds whilst the fourth was under colonization and patchy. Shoot densities and maximal below-ground biomass were lower in the inner silty seagrass bed than in the sandy meadows located in the centre of the Bay. Maximal above-ground biomasses were similar in the two population types. In the well-established beds, vegetative shoot densities, above-ground and below-ground biomasses showed a unimodal pattern with minima in winter (4000 to 9000 shoots·m−2, 40 to 80 g DW·m−2, and 40 to 60 g DW·m−2, respectively) and maxima in summer (11000 to 22000 shoots·m−2, 110 to 150 g DW·m−2, and 140 to 200 g DW·m−2, respectively). Reproductive shoots were observed from the beginning of June until the end of September, except in the colonizing bed where they persisted until December. Furthermore, in the latter, maximal reproductive shoot density was higher (2600 shoots·m−2) than in the established beds (650 to 960 shoots·m−2). The total production of Z. noltii in Arcachon Bay was estimated to approximately 35.6·106 kg DW·y−1 (19.4·106 kg DW·y−1 for above-ground parts and 16.2·106 kg DW·y−1 for below-ground parts).
The Skagerrak receives large amounts of freshwater, both in the form of rather low-saline waters from the Baltic Sea and the southern North Sea and in pure form from local rivers. These waters, mixed with the underlying Atlantic Water, participate in a variable, mainly wind-driven, cyclonic surface circulation in the Skagerrak.This paper presents a thorough analysis of hydrographic data to provide insight into the distribution and circulation of freshwater in the Skagerrak. From measured salinity and temperature profiles, we compute vertically integrated variables such as the freshwater content, the potential energy and the steric sea level. Furthermore, the density variance is calculated from the density fields. The calculations show that freshwater is accumulated mainly along the Swedish and Norwegian coasts. The computed topography of the steric sea level agrees well with published results for the coast as determined by geodetic methods, This shows that the steric effect gives the dominating contribution to the topography of the mean sea level in the Skagerrak. The computed distribution of freshwater along the coasts is thus largely verified by the geodetically determined mean sea level. We find strong indications of an extensive recirculation of freshwater within the Skagerrak, due to advection/dispersion from the storage along the Norwegian coast. The horizontal distribution of density variance in the upper 10 m mirrors the spreading of low-saline water from the Kattegat, Deeper down there are maxima in density variance along the coasts, reflecting occasional downwelling and strengthening of the baroclinic coastal currents due to time-varying winds. The mean potential energy along the coasts increases monotonously in the cyclonic direction, from the inflow region in the southwest to the outflow region off the western part of the Norwegian Skagerrak coast. The corresponding flow increase is mainly due to incorporation of Atlantic Water into the surface layer. Using a rough energy budget for the Skagerrak, we find that the potential energy of the coastal current system cannot be maintained by wind-driven diapycnal mixing. We suggest that isopycnal downwelling along the coasts provides the necessary energy, The considerable annual variation found in potential energy in the coastal currents covaries with the variation of the wind stress amplitude.
This quality assessment of the Skagerrak-Kattegat is mainly based on recent results obtained within the framework of the Swedish multidisciplinary research projekt 'Large-scale environmental effects and ecological processes in the Skagerrak-Kattegat' completed with relevant data from other research publications. The results show that the North Sea has a significant impact on the marine ecosystem in the Skagerrak and the northern Kattegat. Among environmental changes recently documented for some of these areas are: increased nutrient concentrations, increased occurrence of fast-growing filamentous algae in coastal areas affecting nursery and feeding conditions for fish, declining bottom water oxygen concentrations with negative effects on benthic fauna, and sediment toxicity to invertebrates also causing physiological responses in fish. It is concluded that, due to eutrophication and toxic substances, large-scale environmental changes and effects occur in the Skagerrak-Kattegat area.
An Intercomparison Experiment carried out in the turbidity maximum of the Elbe estuary aimed to determine the relative performance of a number of methods of measuring the settling velocity of estuarine flocs. These comprised several Owen Tubes, with different sampling protocols, side withdrawal tubes, a settling box, and two in situ video systems. There were significant differences between the results which may partly relate to small-scale spatial and temporal patchiness in the turbidity field. Owen Tubes generally give settling velocities an order of magnitude smaller than the direct video measurements, which may indicate that the tubes disrupt flocs on sampling. Between Owen Tubes different methods of calculating the results may lead to different median settling velocities, especially at low concentrations. However, a well-controlled sampling protocol with the settling tubes gave consistent results. The direct video methods appear to give comparable results, and need to be evaluated further.
Characteristic features of the large-scale hydrography of the Skagerrak are reviewed. A dominant overall feature is the deep reaching cyclonic circulation. The modelling of the circulation in the Skagerrak, as part of the current pattern in the North Sea is discussed. Simplified mechanistic models are derived to elucidate the governing dynamics. Qualitative and quantitative implications of these models are compared to what is known from observations. It is concluded that the basic cyclonic circulation in the area is to a large extent forced by the mixing of high-saline deep water into the outflowing low-saline surface water of Baltic origin. The time dependent part of the motion, on the other hand, is probably an effect of the local wind as well as of the wind over the rest of the North Sea. In both cases the topography, viz. the existence of the Norwegian Trench, plays a crucial role.
