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Ice algae are key contributors to primary production and carbon fixation in the Arctic, and light availability is assumed to limit their growth and productivity. We investigated photo-physiological responses in sea ice algae to increased irradiance during a spring bloom in West Greenland. During a 14 d field experiment, light transmittance through sea ice was manipulated to provide 3 under-ice irradiance regimes: low (0.04), medium (0.08), and high (0.16) transmittances. Chlorophyll a decreased with elevated light availability relative to the control. Maximum dark-adapted photosynthetic efficiency (ΦPSII_max) showed an initially healthy and productive ice algae community (ΦPSII_max > 0.6), with ΦPSII_max decreasing markedly under high-light treatments. This was accompanied by a decrease in the light utilization coefficient (α) and photosynthetic capacity (maximum relative electron transfer rate), and a decrease in the ratio of mono- to polyunsaturated fatty acids. This was partly explained by a corresponding increase of photoprotective pigments (diadinoxanthin and diatoxanthin), and a development of mycosporine-like amino acids as identified from a distinctive spectral absorption peak at 360 nm. After 14 d, in situ fluorescence imaging revealed significant differences in ΦPSII_max between treatments of dark-adapted cells (i.e. those sampled before sunrise and after sunset), during diel cycles, with clear chronic photoinhibition in high and medium treatments. Data demonstrate the high sensitivity of spring-blooming Arctic sea ice algae to elevated irradiance caused by loss of snow cover. The predicted loss of snow cover on landfast ice will negatively impact ice algae, their potential primary production, and nutritional quality for higher trophic levels.

Over the course of evolution for billions of years, bacteria that are capable of light-driven energy production have occupied every corner of surface Earth where sunlight can reach. Only two general biological systems have evolved in bacteria to be capable of net energy conservation via light harvesting: one is based on the pigment of (bacterio-)chlorophyll and the other is based on proton-pumping rhodopsin. There is emerging genomic evidence that these two rather different systems can coexist in a single bacterium to take advantage of their contrasting characteristics in the number of genes involved, biosynthesis cost, ease of expression control, and efficiency of energy production and thus enhance the capability of exploiting solar energy. Our data provide the first clear-cut evidence that such dual phototrophy potentially exists in glacial bacteria. Further public genome mining suggests this understudied dual phototrophic mechanism is possibly more common than our data alone suggested., Conserving additional energy from sunlight through bacteriochlorophyll (BChl)-based reaction center or proton-pumping rhodopsin is a highly successful life strategy in environmental bacteria. BChl and rhodopsin-based systems display contrasting characteristics in the size of coding operon, cost of biosynthesis, ease of expression control, and efficiency of energy production. This raises an intriguing question of whether a single bacterium has evolved the ability to perform these two types of phototrophy complementarily according to energy needs and environmental conditions. Here, we report four Tardiphaga sp. strains (Alphaproteobacteria) of monophyletic origin isolated from a high Arctic glacier in northeast Greenland (81.566° N, 16.363° W) that are at different evolutionary stages concerning phototrophy. Their >99.8% identical genomes contain footprints of horizontal operon transfer (HOT) of the complete gene clusters encoding BChl- and xanthorhodopsin (XR)-based dual phototrophy. Two strains possess only a complete XR operon, while the other two strains have both a photosynthesis gene cluster and an XR operon in their genomes. All XR operons are heavily surrounded by mobile genetic elements and are located close to a tRNA gene, strongly signaling that a HOT event of the XR operon has occurred recently. Mining public genome databases and our high Arctic glacial and soil metagenomes revealed that phylogenetically diverse bacteria have the metabolic potential of performing BChl- and rhodopsin-based dual phototrophy. Our data provide new insights on how bacteria cope with the harsh and energy-deficient environment in surface glacier, possibly by maximizing the capability of exploiting solar energy.

Internal hydraulic control, which occurs when stratified water masses are forced through an abrupt constriction, plays an enormous role in nature on both large and regional scales with respect to dynamics, circulation, and water mass formation. Despite a growing literature on this subject surprisingly few direct observations have been made that conclusively show the existence of and the circumstances related to internal hydraulic control in nature. In this study we present observations from the Little Belt, Denmark, one of three narrow straits connecting the Baltic Sea and the North Sea. The observations (comprised primarily of along-strait, detailed transects of salinity and temperature; continuous observations of flow velocity, salinity, and temperature at a permanent station; and numerous vertical profiles of salinity, temperature, fluorescence, and flow velocity in various locations) show that internal hydraulic control is a frequently occurring phenomenon in the Little Belt. The observations, which are limited to south-going flows of approximately two-layered water masses, show that internal hydraulic control may take either of two configurations, i.e. the lower or the upper layer being the active, accelerating one. This is connected to the depth of the pycnocline on the upstream side and the topography, which is both deepening and contracting toward the narrow part of the Little Belt. The existence of two possible flow configurations is known from theoretical and laboratory studies, but we believe that this has never been observed in nature and reported before. The water masses formed by the intense mixing, which is tightly connected with the presence of control, may be found far downstream of the point of control. The observations show that these particular water masses are associated with chlorophyll concentrations that are considerably higher than in adjacent water masses, showing that control has a considerable influence on the primary production and hence the ecosystem in the area.

This chapter describes a period of low air temperatures, darkness or very little light, cold ice, minimum brine volumes, and strongly reduced ice permeability. The first section provides an overview of the physical and optical conditions of this winter ice (3.1). Both snowfall and snowdrift are frequent during winter and an overview of the effects of a thick snow cover on sea ice is provided (3.2). Increased colonization of the sea ice bottom by bacteria and ice algae is initiated in late winter around first light, and an example is given (3.3). Once these microorganisms have colonized the ice the important growth limitations of bacteria and algae are considered (3.4), along with how they are able to adapt to these extreme conditions, and their seasonal species succession (3.5). The purposes of EPS and the functioning of different pigments found in ice algae are described (3.6). This is combined with a description of low light conditions and related photosynthesis (3.7), and exemplified with a case study from Station North, NE Greenland (3.8).

This chapter is a description of the field and laboratory methods that are generally applied in sea ice ecological research. The first section describes sampling techniques and handling of sea ice samples (6.1). Calculations of brine and gas volumes are dealt with in (6.2), and how to measure and calculate optical parameters (6.3). Sampling, handling, and calculation of ice bacterial production are described (6.4), and methods for ice algae biomass spatial distributions are dealt with in (6.5). Measuring photosynthesis and ice algae primary production are described (6.6), and the determination of Chl a content is explained (6.7). The recently developed fluorescence imaging of ice algae distributions and derived photosynthetic parameters are described (6.8). This is followed by a description of the IP25, a substance synthesized by ice algae and applied for mapping sea ice and food web studies (6.9).

This chapter describes the period of development of new ice during autumn freeze-up, different ice types, and development of the brine channels (2.1). Optical properties of young and developing sea ice are described in terms of the transition from open water to bare sea ice, that later can be covered by snow (2.2). The incorporation and scavenging of ice algae, bacteria and meiofauna from the water column during ice growth and freeze-up are then described (2.3). Development of platelet ice, a prominent under-ice feature in Antarctica and recently observed in the Arctic, is described (2.4). The final section describes frost flowers and brine skim development, and the dynamics of the bacteria communities in the frost flowers and brine skim on top of newly formed sea ice (2.5).

This chapter describes the structure of the book and the diverse significance and importance of sea ice. The first section explains how and why the book is structured following the seasonal events governing the abiotic and biotic parameters in sea ice (1.1). The second section places sea ice into a physical and geopolitical perspective with the new and developing international borders in the Arctic Ocean (1.2). Sea ice ecosystems are then compared to other ecosystems (1.3), followed by a section describing the important and global ecological features of sea ice (1.4).

This chapter describes one of the most dynamic and ecological important seasons. The onset of ice melt initiates significant changes in the physical properties of the ice (4.1), and the related biotic processes in the spring/summer sea ice are addressed (4.2). Ice algal spring blooms are described and exemplified with two studies from contrasting Arctic sites (4.3). A Case Study 2 has a focus on effects and consequences for ice algae of increased irradiances at the ice bottom in terms of pigments, fatty acids, and MAAs (4.4). Ice algae bloom dynamics are investigated in a combined model and field study (4.5). Melt ponds termed “windows to the ocean” develop on the surface of the ice with increased light transmittance (4.6). Seeding of the ice or water column below with viable ice algae can initiate ice algae or pelagic blooms, and mechanisms are explored in model and field studies (4.7). Case study 3 show that species of meiofauna graze directly on the ice algae, and that the meiofauna establishes an important ecological component in the sea ice (4.8).

This chapter is a description of the role and importance of sea ice and sea ice biota on large scales and in relation to the effects of climate change. The decrease in summer sea ice extent and thickness are evident and described in (5.1). A Case Study 4 based on our observations in the Fram Strait and the Arctic Ocean illustrates some of the consequences and effects of increased inflow of warm Atlantic water (5.2). The question whether more light in an ice-free water column will increase pelagic primary production in the Arctic Ocean, is addressed with a model (5.3). Sea ice plays an important role in the exchange of CO2 between ocean and atmosphere and the sea ice CO2 pump is described in (5.4).

Colonization by ice algae at the bottom of the sea ice is important ecologically as these algae are the major carbon source for zooplankton and the Polar food chains from early spring and until ice break-up several months later. At that time, ice algae can seed the water column promoting the pelagic production. We collected a time-series of ice growth, surface roughness, ice algae biomass (Chl-a), photosynthetic performance, primary production, irradiance, species composition, under ice ADCP data, nutrients, and ice algae spatial distribution in Kangerlussuaq, Greenland. A significant increase in biomass, primary production, and photosynthetic performance was observed at the bottom of the first-year sea ice. This colonization was not related to any change in light or nutrient availability, but instead occurred over a few days after which all parameters remained stable and constant. Colonization was concurrent with active ice growth associated with a period of very cold air temperatures as low as minus 35 °C. This new ice growth featured ice crystals and dendrites that significantly increased the surface roughness of the bottom, which facilitated the colonization. The dendrites are about 0.5 mm high and with a similar distance between the crests. The species composition of ice algae was similar in the water below and at the ice bottom, though biomass remained significantly lower in the water, and was dominated by Fragilariopsis oceanica and Navicula vanhoeefi. It is concluded that the colonization of the bottom of the sea ice is solely related to the increase in surface roughness initiated in the growing ice.

Sea level rise and more frequent storm and precipitation events associated with climate change are predicted to increase salinity fluctuations in estuarine and inshore areas, where foundation species such as eelgrass (Zostera marina L.) will be exposed to more frequent salinity changes. Effects of acute hyposalinity exposure on seagrasses remain poorly understood compared to the effects of more prolonged, constant salinity. Here, we examined growth and photo-physiological responses of Z. marina to 5 levels of stable salinity (5, 12 19, 25, 33) and compared effects of prolonged (16 days) versus acute (24 and 48 hours) exposure to hyposalinity (salinity 5 and 12) using fluorescence imaging. We also examined if fluorescence kinetics were affected by age differences across leaves. Growth reached an optimum at salinity 19 and was more affected by hyposalinity than hypersalinity. Rapid reduction from salinity 25 to 5 decreased the maximum quantum yield (Fv/Fm) after just 48 h. In contrast with prolonged exposure, non-photochemical quenching processes were not increased at salinity 5 after 48 h. Young leaves were more susceptible to extreme hyposalinity than older leaves (e.g., lower photosynthetic quantum yield), which emphasizes the importance of considering shoot-scale and within-shoot variations in studies of stress response patterns. Differences between hyposalinity and hypersalinity responses were generally replicated in the literature, but we were not able to detect any differences across studies. Overall, these results suggest that eelgrass is tolerant to large fluctuations in salinity, but sudden extreme reductions may act as a severe co-stressor, and contribute to accumulated stress-exposure effects (chronic or lasting effects).

Data on optical properties such as diffuse attenuation coefficient K d(PAR), beam attenuation coefficient (c p) and the optically active constituents (OACs) CDOM, Chl-a and suspended particulate matter were obtained in a Danish temperate coastal plain estuary (56°N) and a Vietnamese tropical ria (12°N) at high discharges. The major difference was the spatial distribution of the optical properties against distance, best described by significant power functions in the ria, compared to significant linear functions in the coastal plain. It was hypothesized that estuarine morphometry could explain this spatial distribution. Partition and multiple regression analyses showed that Chl-a governed K d(PAR) and beam attenuation coefficient in both estuaries. Significant, high correlations were obtained by multiple regression analyses in the estimation of K d(PAR) and beam attenuation coefficients in the two estuaries using OACs as input parameters. It is concluded that there are no large differences in OAC concentrations between the two estuaries. The spatial distributions of OACs and optical properties were significantly different and governed by the estuary morphometry, i.e. a power distribution in the tropical ria and a linear function in the temperate coastal plain estuary, and applicable to similar estuary types.

The aims of the study were to analyse the relations between the physics of a water column and the location of the subsurface chlorophyll maximum (SCM) peaks in a strongly stratified estuary. Could extension and depth location of the SCM be explained by the physical conditions in terms of water column stratification and density interface? Questions were addressed by obtaining data on water column density (CTD), chlorophyll a (Chl a), nutrients, (F v/F m), σPSII and K d(PAR) at 15 positions along a 575 km transect in the Kattegat estuary. Results showed that the estuary was strongly stratified with mixed surface and bottom layers intercepted by a layer where density increased with depth. The SCM occurred only in this density interface, and widths of SCM and density interface were highly correlated. The surface waters were nearly depleted of inorganic nitrogen, phosphate and silicate though with significant higher concentrations in the waters below the interface. The Chl a concentration was comparatively higher in the SCM peak as well as maximum quantum efficiency (F v/F m) and functional cross sectional area (σPSII). The SCM was maintained at very low light levels and by a diapycnal nitrogen flux, with a stratified water column and nutrient depleted surface waters as predecessors. It was concluded that the depth location and vertical extension of the SCM in the estuary were closely linked to the physical structure of the water column in terms of density interface and stratification.

Optical constituents as suspended particulate matter (SPM), chlorophyll (Chl-a), colored dissolved organic matter (CDOM), and grain sizes were obtained on a transect in the arctic fjord-type estuary Kangerlussuaq (66°) in August 2007 along with optical properties. These comprised diffuse attenuation coefficient of downwelling PAR (K d(PAR)), upwelling PAR (K u(PAR)), particle beam attenuation coefficient (c p), and irradiance reflectance R(−0, PAR). PAR is white light between 400 and 700 nm. The estuary receives melt water from the Greenland Inland Ice and stations covered a transect from the very high turbid melt water outlet to clear marine waters. Results showed a strong spatial variation with high values as for suspended matter concentrations, CDOM, diffuse attenuation coefficient K d(PAR), particle beam attenuation coefficients (c p), and reflectance R(−0, PAR) at the melt water outlet. Values of optical constituents and properties decreased with distance from the melt water outlet to a more or less constant level in central and outer part of the estuary. There was a strong correlation between inorganic suspended matter (SPMI) and diffuse attenuation coefficient K d(PAR) (r 2 = 0.92) and also for particle beam attenuation coefficient (c p; r 2 = 0.93). The obtained SPMI specific attenuation—K d * (PAR) = 0.13 m2 g−1 SPMI—and the SPMI specific particle beam attenuation—c p * = 0.72 m2 g−1—coefficients were about two times higher than average literature values. Irradiance reflectance R(−0, PAR) was comparatively high (0.09−0.20) and showed a high (r 2 = 0.80) correlation with K u(PAR). Scattering dominated relative to absorption—b(PAR)/a(PAR) = 12.3. Results strongly indicated that the high values in the optical properties were related to the very fine particle sizes (mean = 2–6 μm) of the suspended sediment. Data and results are discussed and compared to similar studies from both temperate and tropical estuaries.

We have compared 14 different sediment incubation chambers, most of them were used on bottom landers. Measurements of mixing time, pressure gradients at the bottom and Diffusive Boundary Layer thickness (DBL) were used to describe the hydrodynamic properties of the chambers and sediment–water solute fluxes of silicate (34 replicates) and oxygen (23 replicates) during three subsequently repeated incubation experiments on a homogenized, macrofauna-free sediment. The silicate fluxes ranged from 0.24 to 1.01 mmol m−2 day−1 and the oxygen fluxes from 9.3 to 22.6 mmol m−2 day−1. There was no statistically significant correlation between measured fluxes and the chamber design or between measured fluxes and hydrodynamic settings suggesting that type of chamber was not important in these flux measurements. For verification of sediment homogeneity, 61 samples of meiofauna were taken and identified to major taxa. In addition, 13 sediment cores were collected, sectioned into 5–10-mm slices and separated into pore water and solid phase. The pore water profiles of dissolved silicate were used to calculate diffusive fluxes of silicate. These fluxes ranged from 0.63 to 0.87 mmol m−2 day−1. All of the collected sediment parameters indicated that the sediment homogenization process had been satisfactorily accomplished. Hydrodynamic variations inside and between chambers are a reflection of the chamber design and the stirring device. In general, pump stirrers with diffusers give a more even distribution of bottom currents and DBL thicknesses than paddle wheel-type stirrers. Most chambers display no or low static differential pressures when the water is mixed at rates of normal use. Consequently, there is a low risk of creating stirrer induced pressure effects on the measured fluxes. Centrally placed stirrers are preferable to off-center placed stirrers which are more difficult to map and do not seem to give any hydrodynamic advantages. A vertically rotating stirrer gives about five times lower static differential pressures at the same stirring speed as the same stirrer mounted horizontally. If the aim is to simulate or mimic resuspension at high flow velocities, it cannot be satisfactorily done in a chamber using a horizontal (standing) rotating impeller (as is the case for most chambers in use) due to the creation of unnatural conditions, i.e. large static differential pressures and pre-mature resuspension at certain locations in the chamber.

The study is based on a 1 year record (n=46) of the diffuse attenuation coefficient—Kd(PAR) (m−1)—in combination with CTD-casts at one position in Arhus Bay, Kattegat, the North Sea–Baltic Sea estuarine transition. Water samples were collected occasionally for determination of suspended particulate matter (SPM) and chromomorphic dissolved organic matter (CDOM). The Kd(PAR) varied between 0.152 and 0.557 (m−1) with an average of 0.293 (m−1). The depth of the photic zone (1% light level) varied between 8.3 and 15.7 m at maximum and average Kd(PAR). Maximum Kd(PAR) values occurred in periods of high chl-a concentrations. The average CDOM absorption coefficient was 0.232 (m−1) whereas the average chl-a and inorganic SPM concentrations equalled 3.3 (mg m−3) and 4.5 (g m−3). Light attenuation partitioning in percent was: water (9), CDOM (17), SPM (42), and phytoplankton (32) at average conditions. Attenuation by phytoplankton reached up to 74% at high chl-a concentrations whereas SPM light attenuation reached 45% at high SPM concentrations. Results show that light attenuation in the bay is governed by phytoplankton and SPM concentrations whereas CDOM is of less significance. Light absorption in the PAR range—a(PAR)—ranged between 0.262 (m−1) at average conditions and 0.49 (m−1) at maximum phytoplankton concentrations. Scattering was governed by SPM at average conditions whereas phytoplankton scattering was higher than SPM scattering at chl-a concentrations >4.5 (mg m−3).

Vertical particulate matter fluxes were measured by sediment traps at 1.4 m (seabed) and 8 m (pelagic) above the seabed, along with fluorescence (CTD casts), current conditions and salinity, in a semi-enclosed bay in southwest Kattegat covering a close to three-month period. The Kattegat forms the estuarine transitional zone between the high-saline North Sea and low-saline Baltic Sea, as shown by measured maximum and minimum bottom salinities of 32.5 and 17.0. There were no indications of suspended matter transport into the bay that affected the vertical flux. There was a significant and close correlation (r 2 = 0.24) between current shear stress and seabed vertical flux. Average vertical flux was 11.8 g m −2 d −1 at the seabed with a wide range (0.4–57.6 g m −2 d −1 ), which is 6.9 times the average flux of 1.7 g m −2 d −1 at 8 m above the seabed. The flux at this height was generally governed by the production of organic material in the water and showed little variation. The C:N ratio of material collected in the 8 m trap was on average 6.9, close to the Redfield ratio (6.7) and implying flux of fresh material. The average C:N seabed ratio was 8.8. Periods of low C:N seabed ratios close to the Redfield ratio were observed before enhanced organic production in the water column, i.e. spring bloom and sub-surface peak. There was a positive and significant correlation (r 2 = 0.13) between seabed fluxes and C:N ratio. This shows that more decomposed material with a higher C:N ratio is resuspended at higher fluxes. Organic carbon content (percentage) in collected material was high with a maximum of 18% following the spring bloom at the 8 m trap, and an average of 11.4%. Average seabed organic carbon was 8.6%. There was a positive correlation (r 2 = 0.42) between vertical flux and seabed organic carbon content. This shows that the inorganic part of the collected material increases at higher current shear stress. It was concluded that the seabed flux was not affected by changes in the flux related to a spring bloom in terms of amounts, but that the related changes in C:N ratio and organic carbon were discernible, particularly in periods of low current shear stress.

Water and sediment transport were measured and calculated during a two months period on three locations at a 3 km long tidal divide in the Danish Wadden Sea. The water transport was flood dominated in the largest channel, almost neutral in the smaller channel and ebb dominated on the tidal flat. The tidal currents were short and strong during flood tide and long lasting but weak during ebb tide at all three stations. This tidal asymmetry caused the net sand transport across the tidal divide to be flood directed on all three stations. During a storm surge the currents were ebb-directed persistently for 21 hours despite large variations in water level. Grain-size distributions infer that the net sand transport converges to the northeast, indicating a recent north-eastward (flood directed) migration of the tidal divide. The suspended sediment transport during a tidal cycle is generally directed towards the north but on few special occasions the suspended sediment transport is southward directed beca...

Sixteen sediment samples were collected from a square grid (4×4) with a horizontal distance of about 150 m between positions in Århus Bay in the south-west Kattegat (14 to 15 m water depth). Critical shear stress (τc) was measured in all samples and related to sediment parameters: grain-sizes, organic matter, water content, porosity, and chlorophyll-a (chl-a) content, in upper layers. Samples were divided into a low (A) τc group and a high (B) τc group in relation to an erosion rate. A significant (Pc was found between groups A (0·0284 N m−2) and B (0·0380 N m−2). Average chl-a concentrations in groups A (1·4 μg g−1) and B (1·8 μg g−1) were not significantly different (P=0·47) but there was a significant and positive correlation (r2: 0·7, Pc and diatom film abundance. Sediment organic matter and water content were significantly higher in group B compared with A, which contradicts that watery and organic rich sediments generally exhibit low τc. This was explained by the presence of a diatom film cover on the fluff layer that inhibits the action of erosive forces. A fluff layer is characterized by a high water and organic content. The fluff layer was present in the majority of the samples but the highest average chl-a content and a significant (P=0·020) higher abundance of diatom film was observed in group B (high τc). Benthic diatoms were dominated by Haslea crucigeroides, Pleurosigma strigosum, and Bacillaris paxillifer. Spatial variability of sediment parameters was high and variability of a stability/erodibility parameter even exceeded those recorded for highly heterogeneous tidal flats. The occurrence of benthic diatoms at 14–15 m of water depth in the eutrophic Århus Bay was supposedly related to a measured increase in Secci depth in the bay and thereby increased light penetration depth.

The common mud snail Hydrobia ulvae is a widespread and dominant deposit feeder on fine-grained substrata along the European Atlantic coastline. Previous studies have shown that mud snail activities such as grazing, faecal pellet production and mucous production may influence physical properties of the surface sediment layer and thus depositional and erosional processes. To quantify the influence of Hydrobia density on key parameters such as erosion threshold and erosion rate, a short-term laboratory experiment was conducted. Snails were placed on fine-grained sediment at densities of 10000 and 50000 ind m-2 and erosion experiments were carried out one, three and five days after establishment of the sediment beds. Controls without H. ulvae were treated the same way. The presence of H. ulvae significantly increased the erosion rate and decreased the erosion threshold compared to snail-free control plots. The erosion rate was increased by a factor of 2 to 4 when H. ulvae were present and showed stronger influence by the snail than the erosion threshold. Snail density did not affect the erosion threshold, but the erosion rate doubled with an increase in density from 10000 to 50000 ind m-2. The erosion rate was only marginally different after one day but the difference increased over time and the erosion rate was significantly different for all treatments after five days. No significant time dependence was observed for the erosion threshold. The results generally confirm results obtained in situ and differences can be related to different hydrodynamic conditions under field and laboratory conditions.

The Pomeranian Bight (southern Baltic Sea) is a mixing zone between waters of the Baltic Proper and the river Oder, which drains a densely populated and highly industrialised catchment of central Europe. The bight is a nondepositional area, and all material produced in its water column, from erosion of strata at the seafloor and cliffs, and delivered by rivers, is transported near the seafloor to the depositional areas of the Arkona, Bornholm and Gdansk basins. In this contribution, we assess the origin, transformation and mass fluxes of material through the bight based on an integrated field study conducted in the period 1996–1998. The transport mechanism is by wave- and current-induced resuspension and settling cycles, which effectively enrich organic-rich material and associated substances (organic pollutants, heavy metals) in deeper water; the estimated transport time is less than 6 months. The phases in which the material is transported are suspended matter in the water column, a particle- and aggregate-rich benthic boundary layer of 20 m water depth), the fluffy layer is not readily distinguished from the underlying soft, organic-rich sediment and the change in physical and chemical properties is gradual. The organic matter passing through the coastal zone in the southern Baltic is unaffected by biological or chemical modifications in composition. We find no evidence for a preferential removal of nitrogen or phosphorus, even if the speciation of phosphorus changes from biological compounds to minerals. The compositional changes which we see, i.e., in the nitrogen isotopic composition and in trace metal concentrations, are mainly caused by dilution of the river signal. In the case of

The importance of aggregation processes in the benthic boundary layer (BBL) with regards to settling velocities and flux of organic carbon was investigated at four stations on a coastal-depositiona...

A new box-corer is described. Placement and penetration of the box-corer is video-assisted to secure a sample representative of the bottom. The box itself is constructed of fully transparent polycarbonate which allows for video control observation of the sediment/water interface during sampling. The box is open during the descent towards the seabed but closes when the release is triggered. The box-corer is hydraulically damped so that the box after landing is very gently pushed into the seabed to a predetermined depth. A sediment sample with a total length of about 35 cm and with a surface area of 400 cm2 (20.0x20.0 cm) is collected in the box together with the near bottom water to a height of about 10 cm above the bed. The box-corer height is 180 cm and the total weight about 200 kg. The legs can be dismounted which makes transportation easy. The box-corer has been operated with success both in coarse sand and in soft silt and clay dominated sediments.

Effects of resuspension on the release of dissolved, redox sensitive elements (Fe, Mn) was studied in cylindrical microcosms. Effects from changing water stirring velocity in sediment pools were evaluated through measurements of pore water profiles of dissolved Mn, Fe and redox potential. Mn was a good natural marker to follow such effects. At current velocities below the threshold velocity for resuspension (37 cm s-1), Mn release rates to overlying water were 100 times higher compared to steady-state values. Pulse increases in Mn concentration were the result of convective currents inside flow chambers. These results were strongly supported by measurements of Eh profiles in the sediment pore water. Furthermore, impacts from increasing stirring velocity were found down to 1.9 cm depth below the resuspended layer of sediment.

Geografisk Tidsskrift, Danish Journal of Geography 98: 10–19. A stainless steel tripod equipped with five sediment traps placed at 0.35 m intervals above the seabed was deployed both in wave and current dominated shallow water environments. The length of tripod deployments varied between three and seven days. Sedimentation fluxes during resuspension in the wave-dominated environment ranged from 24.1 g m−2 day−1 at 1.75 m to 36.4 g m−2 day−1 at 0.35 m. In the current-dominated environment the fluxes ranged from 5.2 g m −2 day−1 at 1.75 m to 573.0 g m −2 day−1 at 0.35 m. Maximum shear stresses during the resuspension events in each of the two environments were of similar size, 0.042 and 0.06 N m −2, respectively. Additional deployments of the tripod during periods with low current speeds and no surface wave activity showed that the primary sedimentation varied between 3.0 g m −2 day−1 and 10 g m −2 day−1 in both environments. Sedimentation flux variation with height above the seabed was very small ...

Sediment fluxes measured by sediment traps placed at different heights above the sea-bed at two near-coastal sites and in a sheltered bay are analysed in relation to wave and current conditions. Traps were deployed for 13, 15 (coastal sites) and 6 months and recovered with a mean period of 16·6, 14·0 and 30 days, respectively. The traps with an aspect ratio of 5 were placed at 0·3, 0·5, 0·8, 1·0, 2·0, 4·0, 6·0, 8·0 and 10·0 m above the sea-bed at one location, whereas an aspect ratio of 2·3 was used at the second coastal location with similar trap intervals. The trap in the sheltered bay was placed at 1·0 m above the sea-bed. Results showed that sediment fluxes varied between 505·0 and 38·9 g m −2 day −1 at 0·3 m above the sea-bed at a coastal site, whereas only small flux variations occurred in the sheltered bay. Accumulation rates were measured by 210 Pb at the three sites and was 2·4 g m −2 day −1 at the same site where fluxes varied between 505·0 and 38·9 g m −2 day −1 at 0·3 m above the sea-bed. This shows that re-suspension here is high and frequent. A similar pattern was found at the other coastal site, whereas accumulation rate and sediment flux were very similar in the sheltered bay. Analyses showed that re-suspension was induced both by surface wave activity related to periods of high wind speeds, and by current shear stress in more calm periods. Re-suspension is quantified as a time series by applying the introduced depth-integrated re-suspension flux (DIRF). High DIRF occurred during periods of high surface wave activity, and one re-suspension event accounted for about 25% of the total re-suspension flux at the site, although the event had a very low frequency of occurrence, less than 5% of the time. About 50% of the total re-suspension flux occurred in the interval between 0·0 and 20·0 g m −2 day −1 . Comparison of different time scales for measuring sediment fluxes and re-suspension using sediment traps, shows that a time scale of 24–12 h gives an adequate resolution.

The yearly nutrient supply from land and atmosphere to the study area in SW Kattegat is 10 900 tons of N and 365 tons of P. This is only few percent of the supply from adjacent marine areas, as the yearly transport through the study area is 218 000 tons of N and 18 250 tons of P. Yearly net deposition makes up 1340 tons of N (on average 2.5 g m–2 yr–1) and 477 ton of P (on average 0.9 g m–2 yr–1). Shallow-water parts of the study area have no net deposition because of frequent (>35% of the year) resuspension. Resuspension frequency in deep water is

Conductivity, temperature and depth (CTD) measurements were recorded weekly at a fixed position in the shallow water of Arhus Bay over a period of 1 year. Variations in stratification and salinity are described in this study. Outflow of low salinity water from the Baltic Sea, and an estuarine return flow of high salinity bottom waters originating in the North Sea were observed. Strong stratification occurs during spring, summer and late autumn, whereas stratification is weak or absent during winter. A stratification model in terms of the potential energy of the water column (φ) has been applied including wind, solar insolation, estuarine circulation and current terms. Observations of stratification are very well described by this model, although considerable differences were found to occur during a period of very strong stratification. The interface is placed at a water depth of about 8 m during spring and early summer, and at a depth of about 12 m during autumn. A comparison of frequently used stratification parameters shows a highly positive correlation between φ and the density difference between surface and bottom water. A positive correlation between φand Brunt–Vaisala frequency was also found in periods of stable oceanographic and meteorological conditions, whereas no correlation was found in periods of unstable conditions.

Danish Journal of Geography 95: 1–11, 1995. This paper describes some preliminary results from a field experiment on suspended sediment transport and cross-shore profile evolution in an environment subjected to storm surges. The experiment was conducted during the fall of 1994 at the barrier beach of Skallingen, SW Denmark. Data were collected over several tidal cycles comprising low through moderately-high offshore energy conditions. During the experimental period, a swash bar developed on the foreshore and migrated onshore, accreting significantly in the process. Surf zone sediment transport was heavily influenced by a rip current crossing the second bar. Topographic changes in the nearshore consisted of a progressive lowering of vertical bar relief as a result of a balance between offshore directed sediment transport due to the rip during moderate/high energy conditions, and onshore wave-induced transport during low energy conditions when the waves were not breaking over the second bar.

Instances of strong oxygen variations are described for two shallow water stations in the Kattegat, situated at the fluctuating frontal zone between outflowing surface water from the Baltic and inflowing bottom water from the Skagerrak/North Sea.

Sieving and settling analyses of 29 samples from different sedimentary environments have been compared with the use of invariant log-hyperbolic parameters. Settling velocities were not converted into millimetre size. The comparison was based on kurtosis, skewness and sorting of the distributions. The comparison of the parameter values have been evaluated by a two-tailed t-test. The log-hyperbolic parameters values for kurtosis between the two methods differed significantly for the beach sediments. Skewness differed significantly in all three environments and sorting only differed significantly for the beach sediments. (q-N) a measure of fit between observations and the estimated log-hyperbolic distribution differed significantly in the beach and eolian sediments. The study shows that settling velocities give a better fit to the log-hyperbolic distribution opposed to sieve analysis. The study shows that the differences in the log-hyperbolic sediment parameter values differ significantly in nearly half of the comparisons. It is concluded, that great attention must be paid to the method of grain-size analysis.

Shipboard measurements from late 2006 made by the Danish Galathea 3 Expedition and satellite sea surface temperature images revealed a chain of cool and warm ‘mushroom’ dipole vortices that mixed warm, salty, oxygen-poor waters on and near the continental shelf of the Great Australian Bight (GAB) with cooler, fresher, oxygen-rich waters offshore. The alternating ‘jets’ flowing into the mushrooms were directed mainly northwards and southwards and differed in temperature by only 1.5°C; however, the salinity difference was as much as 0.5, and therefore quite large. The GAB waters were slightly denser than the cooler offshore waters. The field of dipoles evolved and distorted, but appeared to drift westwards at 5km day–1 over two weeks, and one new mushroom carried GAB water southwards at 7km day–1. Other features encountered between Cape Leeuwin and Tasmania included the Leeuwin Current, the South Australian Current, the Flinders Current and the waters of Bass Strait.

Bottom-mounted acoustic Doppler current profiler measurementsindicate that the net transport of water (844 m3 s-1) in theLittle Belt makes up only 6% of the total transport betweenthe Baltic Sea and the North Sea. This is a smaller percentagethan the 9% commonly found in the literature. Owing to barotropicand tidal currents the gross transport is 5 times larger. Thenet transport is directed towards the North Sea mainly in thetop 32 m of the water column but towards the Baltic Sea it occursin the lower 5 m of the water column. The resulting transportof phosphorus is strongly affected by vertical mixing in an areaof hydraulic control in the narrow part of the Little Belt. Comparisonsof phosphorus profiles in stratified waters and in the mixingarea indicate a yearly entrainment of 15 tonnes P from the bottomwater to the surface layer. This vertical transport of P formspart of an internal loop in the general transport between theBaltic Sea and the North Sea. Compared to the transport observed15-16 years ago, the present net phosphorus transport of 163tonnes yr-1 from the Baltic Sea through the Little Belt is substantiallylower.

A combined inflow and intrusion event was studied in the Little Belt region in the southwest Kattegat during autumn 1999. A persistent period of outflow from the Baltic Sea towards the North Sea was replaced by a strong southward flow in the opposite direction. This reverse of flow direction was related to a change in dominant wind direction and prevailed for about 14 days as demonstrated by current measurements. The study is based on CTD-casts at seven stations, ADCP current data, water sampling for nutrient analyses ((NH3+NH4), (NO2+NO3), PO4 and SiO2), and phytoplankton species composition. The reversal caused a considerable increase in bottom water salinity reaching up to 10 at several stations. The inflow water reached into a region of strong mixing where surface and bottom water became nearly fully mixed in the Little Belt constriction where high current speeds prevail. An intrusion of mixed water developed when current again turned into a flow out of the Little Belt and the intrusion was clearly recognised as a band of high fluorescence. The increase in fluorescence was related to a strong bloom of the potential toxic diatom Pseudo-nitzschia pseudodelicatissima. Cell concentrations in the intrusion reached up to 3249 cells ml−1. Nutrient concentrations were consistently higher in the inflow bottom water as compared to surface concentrations. The bloom was supposedly fuelled by the mixing of nutrient rich bottom water into the photic zone. The occurrence and frequency of these intrusion events are discussed in relation to general hydrographical conditions in the region, as well as the intrusions contribution to the primary production. A conceptual model for the development of the intrusion is proposed.

Effekten af strømbetinget opblanding på fytoplankton og zooplankton sammensætningen i en lagdelt vandsøjle blev undersøgt i Lillebælt i april 2015. De hydrografiske forhold blev kortlagt gennem kontinuerlige målinger med ADCP og undulerende CTD (Scanfish III) langs et transekt fra Tragten nord for Fredericia til det centrale Lillebælt. Vandprøver til analyse af fytoplankton blev taget fra tre strata med CTD forsynet med en rosette med Niskin-vandhentere og zooplankton blev indsamlet med et Multinet-maxi (HydroBios, Kiel) fra 9 strata i vandsøjlen. Under kraftig sydgående strøm viste hydrografien forekomst af hydraulisk kontrol, med et kontrolpunkt ved indgangen til Snævringen, og en total opblanding af den før kontrolpunktet stærkt lagdelte vandsøjle til følge. Fluorescensen steg som følge heraf i de nedre vandlag. Fytoplankton-forekomsten i det nedre stratum i Tragten adskilte sig makant fra de to øverste strata, mens prøverne fra det centrale Lillebælt viste en mere ligelig fordeling mellem de tre strata. Også for zooplanktons vedkommende viste opblandingen ændringer i fordelingen af arterne i vandsøjlen. Den kraftige opblanding forårsaget af hydraulisk kontrol kan føre til forøget biologisk produktion i og omkring de danske stræder, hvilket kan have stor betydning for økosystemerne i de indre danske farvande.