Lars Chresten Lund-Hansen

Material transport from the nearshore to the basinal environment in the southern Baltic Sea I. Processes and mass estimates

Abstract Processes involved in erosion, transport and deposition of cohesive materials are studied in a transect from shallow (16 m) to deep (47 m) water of the SW Baltic Sea. The wave- and current-induced energy input to the seabed in shallow water is high with strong variability and suspended matter concentrations may double within a few hours. Primary settling fluxes (from sedimentation traps) are less than 10 g m −2 day −1 , whereas resuspension fluxes (evaluated from sedimentation flux gradients) are 15–20 times higher and the residence time for suspended matter in the water column is 1–2 days. Settling velocities of aggregates are on average six times higher than for individual particles resulting in an enhanced downward transport of organic matter. Wave-induced resuspension (four to six times per month) takes place with higher shear stresses on the bottom than current-induced resuspension (three to five times per month). The short residence time in the water column and the frequent resuspension events provide a fast operating benthic–pelagic coupling. Due to the high-energy input, the shallow water areas are nondepositional on time scales longer than 1–2 weeks. The sediment is sand partly covered by a thin fluff layer during low-energy periods. The presence of the fluff layer keeps the resuspension threshold very low ( −2 ) throughout the year. Evaluated from 3-D sediment transport modeling, transport from shallow to deep water is episodic. The net main directions are towards the Arkona Basin (5.5×10 5 t per year) and the Bornholm Basin (3.7×10 5 t per year). Energy input to the bottom in deep water is low and takes place much less frequently. Wave-induced resuspension occurs on average once per month. Residence time of particles (based on radioactive isotopes) in the water column is half a year and the sediment accumulation rate is 2.2 mm year −1 in the Arkona Basin.

Material transport from the near shore to the basinal environment in the southern Baltic Sea II: Synthesis of data on origin and properties of material

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

Enhanced erodibility of fine-grained marine sediments by Hydrobia ulvae

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 shortterm 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. D 2002 Elsevier Science B.V. All rights reserved.

Vertical sediment fluxes and wave induced sediment resuspension in a shallow water coastal lagoon

The present study describes variations in the vertical fluxes measured concurrently with sediment traps at both a shallow water (4 m) and a deeper water (7.5 m) position in a coastal lagoon in April 1995. A tripod equipped with five sediment traps (trap openings at 0.35 m, 0.75 m, 1.05 m, 1.40 m, and 1.80 m above the seabed) was placed at the shallow water position. This tripod was deployed three times during the study period and deployment periods varied between 2 d and 5 d. The second sediment trap, placed at the deep water position in the central part of the lagoon, measured vertical flux for intervals of 12 h at 1.4 m above the seabed. The horizontal distance between the sediment traps was 8 km. The average maximum vertical flux at the shallow water position reached 27.9 g m−2 d−1 during a period of high, westerly wind speeds, and a maximum vertical flux of 16.9 g m−2 d−1 was reached at the deep water position during a period of high, easterly wind speeds. Both strong resuspension events were closely related to increased wave shear stress derived from surface waves. Maximum wave-induced resuspension rate was 10 times higher at the shallow water position and 3.8 times higher at the deep water position compared with the net sedimentation rate in the lagoon. Small resuspension events occurred at the shallow water position during periods of increased current shear stress, Estimations of conditions for transport of sediment between shallow water and deep water showed that particles must be resuspended to a height between 3 m and 4 m and that current speeds must be higher than about 0.1 m s−1. An average sedimentation rate of 3.8 g m−2 d−1 was obtained at the shallow water position during a period without wave shear stress and low current shear stress. This rate measured by sediment traps is similar to a net sedimentation rate in the lagoon of 4.4 g m−2 d−1, which was determined by radiocarbon dating of a sediment core (Kristensen et al. 1995).

Resuspension studies in cylindrical microcosms: Effects of stirring velocity on the dynamics of redox sensitive elements in a coastal sediment

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.

Enhanced settling velocities and vertical transport of particulate matter by aggregation in the benthic boundary layer

Abstract 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-depositional area transect in the southwest Baltic Sea. Water depths ranged between 15 and 46 m and stations were visited during eight cruises between October 1996 and December 1998. A specially designed bottom water sampler equipped with a particle camera system was used to investigate in situ aggregate characteristics: settling velocity, size, abundance. POC, PON, and chlorophyll-A contents. BBL characteristics including particulate matter concentrations, residence times, Kolmogoroff length scale, fluid flow, and shear stress were studied as well. Results showed no clear seasonal variation in either chlorophyll-A or particulate organic carbon (POC) content in the bottom water. The sediment surface was permanently covered with a layer of fluff material consisting of settled organic rich aggregates. Bottom shear stresses ranged between 0.001 and 0.10N m−2 whereas experimentally determined critical shear stress varied over a more narrow range (0.013–0.024 Nm−2). The calculated size of the turbulent eddies was always larger than the mean aggregate size so no physical fragmentation of aggregates occurred in the BBL. The mean settling velocity of aggregates (> 50 μm) was calculated to be between 0.06 0.3 cm s−1 and for suspended material (< 50 μm) 0.023 0.054 cm s−1. Accordingly, residence times in the BBL ranged between 1.4 and 5.6 hours for aggregates and between 7.7 and 18.1 hours for suspended material. A seasonal variation was detected for aggregates, which comprised up to 90% of total particulate matter in March 1997 but only 6% in October 1997. Material transported towards the sediment surface was enhanced by aggregation between 5948% (March 1997) and 18% (October 1997). The vertical flux of POC by aggregated material was calculated to be between 0.06–4.11 g Cm−2 d−1 and for the POC in suspension this figure was between 0.04–1.27 g Cm−2 d−1.

The LABEREX chamber for studying the critical shear stress for fine-grained sediment

Abstract Geografisk Tidsskrift, Danish Journal of Geography 99: 1–7, 1999 A new portable instrument, the Laberex chamber—LAB oratory ERosional EXperiments—for studies of the “fluff layer” and sediment critical shear stress is described. The chamber consists of a cylindric plexiglass tube with an inner diameter of 84 mm. A four-bladed impeller connected to a motor is placed in the centre of the chamber. Rotation of the impeller induces resuspension. Light attenuation is measured across the centre line in the chamber at a wave length of 633 nm (red light). A PC automatically controls both the stirring voltage that is increased at predetermined time-intervals and collects data from the light attenuation meter every second. Velocity distributions in the chamber and the relation between stirring voltages and critical shear stress are known from laser doppler measurements in the chamber.

Effects of benthic diatoms, fluff layer, and sediment conditions on critical shear stress in a non-tidal coastal environment

Sixteen sediment samples were collected from a square grid (4x4) with a horizontal distance of about 150 m between positions in Arhus Bay in the south-west Kattegat (14 to 15 m water depth). Critical shear stress (τ c ) 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) τ c . group and a high (B) τ c group in relation to an erosion rate. A significant (P<0.001) difference in median τ c 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 (r 2 : 0.7, P<0.0001) between τ c 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 τ c . 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 τ c ). Benthic diatoms were dominated by Haslea crucigeroides, Pleurosigma strigosum, and Bacillaris paxillifer. Spatial variability of sediment parameters was high and variability of a stabilit/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 Arhus Bay was supposedly related to a measured increase in Secci depth in the bay and thereby increased light penetration depth.

Effects of sea-ice light attenuation and CDOM absorption in the water below the Eurasian sector of central Arctic Ocean (>88°N)

This is a study of the optical, physical and biological parameters of sea ice and the water below it at stations (n=25) in the central (>88°N) Eurasian sector of the Arctic Ocean during the summer 2012 record low sea-ice minimum extent. Results show that photosynthetically active radiation (PAR) transmittance of the ice was low (0.09) and apparently related to a high degree of backscattering by air-filled brine channels left by brine draining. The under-ice PAR was also low (8.4±4.5 SD µmol photons m−2 s−1) and partly related to the low transmittance. There were no significant differences in multi-year and first-year PAR transmittances. In spite of this low under-ice PAR, only 3% of the transmitted PAR through the ice was absorbed by phytoplankton in the water. On average, chlorophyll-a concentrations were low (0.34±0.69 SD mg chl-a m−3) in the water compared to the high (a 375=0.52 m−1) coloured dissolved organic matter (CDOM) absorption coefficient with a strong terrestrial optical signature. Two distinct clusters of stations with waters of Pacific and North Atlantic origin were identified based on significant differences in temperature, salinity and CDOM absorption coefficient between water masses. The under-ice light field for bare ice was parameterized as follows: I z=I o(1−0.55)*(0.09)*exp(−0.17*z).

Advection-induced oxygen variability in the North Sea-Baltic Sea transition

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.The events consist of both a rapid emergence and a rapid disappearance of oxygen-depletion. Changes in oxygen concentration amounted to more than 20 g m−2 d−1 for the total water columns. Such high rates of change can not be explained by net local bottom oxygen consumption (0.6 g m−2 d−1) or net local water oxygen consumption (1.6 g m−2 d−1). The oxygen variations were influenced by the local and regional meteorological conditions. The observed instance of shallow water oxygen-depletion was connected to upward movement of the pycnocline and associated advective transport of oxygen-depleted Kattegat bottom waters to a shallow water area. Similarly, rapid disappearance of the bottom water oxygen deficit in a shallow water area was found to depend more on pycnocline lowering in connection with advective transport, than on the effect of local wind driven mixing.