Bengt Liljebladh

Observations of the deepwater flow into the Baltic Sea

It has earlier been hypothesized that the deep water flowing into the Baltic Sea forms a bottom pool in the Arkona Sea, just inside the entrance sills. The flow from the pool farther into the Baltic Sea was assumed to be baroclinic geostrophic and controlled by the pool stratification. This was supported by a realistic statistical description of the deepwater flow into the Baltic Sea, computed using historical vertical density profiles from a hydrographical station in the Arkona Sea. However, basin-wide synoptic measurements of density and currents were not available for a direct verification of the pool model. A hydrographic survey was undertaken in the Arkona Sea shortly after a major inflow event in the beginning of 1993. The conductivity-temperature-depth sections reveal a thick bottom pool of deep water separated by a halocline from the surface water of Baltic origin. Approaching the northern coastal boundary from the central parts of the pool, the halocline sank by about 20 m before hitting the bottom. The ship-mounted acoustic Doppler current profiler recorded a complicated current field. However, subtraction of the assumed barotropic part gives a current field quite similar to the baroclinic geostrophic current field computed from the density distribution. Thus, a dense bottom pool, including a baroclinic geostrophic boundary current along the northern flank of the pool, is quite evident from our measurements.

The Internal Seiches in Gullmar Fjord. Part I: Dynamics

Abstract Internal seiche motions with period 1–3 days in Gullmar Fjord are investigated based on mooring data with high vertical and time resolution. The period and structure of the internal seiches are well described by a simple analytical three-layer model taking into account blocking of the basin water at the sill. Energy budgets for forcing and damping of the seiches are estimated. The internal seiches are forced by direct wind stress on the surface and by internal coastal waves of frequency close to the seiches, the latter contribution being a little larger. The e-folding timescale for the internal seiche damping is approximately equal to the seiche periods, which means that the seiches are effectively damped, but also that the forcing must be in near resonance in order to maintain the observed seiche motions. In the basin water the seiches manifest themselves with large vertical motions and clear upward phase propagation. The phase propagation is related with a continuous, rather than strictly layer...

Krill behaviour as recorded by acoustic doppler current profilers in the Gullmarsfjord

Abstract Vertical distribution of sound scattering layers were observed using bottom deployed acoustic doppler current profilers (ADCP) during early spring of 1996 and autumn of 1997 in the Gullmarsfjord on the Swedish west coast. Variations in relative backscatter were interpreted in relation to horizontal water velocities, oxygen saturation as well as differences in the light, salinity and temperature regimes. Net catches revealed that much of the backscatter below 20-m depth was associated with the presence of krill, principally Meganyctiphanes norvegica . Horizontal currents seemed to influence the migration and distribution of krill, which showed weak vertical migration patterns with low abundance during periods of strong intermediate in- and outflows, while during periods with weaker currents, a more regular diel migration occurred. Horizontal water velocities >5 cm s −1 seemed to have the potential to decrease the peak in the backscatter profile. Mean vertical migration rates of krill was 1 cm s −1 , while maximum vertical migration rates were estimated to be 2.5–3 cm s −1 . The range of the vertical migration was different in 1997 due to severe oxygen deficiency in the bottom water, which prevented the krill from descending >80 m. The commencement of vertical migration correlated closely to the seasonal light conditions. The descent was immediately triggered by sunrise, while ascent occurred with a delay of about 1 h at sunset.

Flow regime in the Irbe Strait

Abstract: The current structure in a strait connecting a semi-enclosed bay and the Baltic Sea is studied on the basis of data obtained during the Gulf of Riga Project in 1993-1995. The observations comprised hydrographic snapshots and a 10-day intense campaign IRBEX-95 of CTD, current, sea-level and meteorological measurements. The baroclinic forcing due to the density difference, the barotropic forcing due to the sea-level difference, and the wind forcing are considered as factors driving the water flow through the Irbe Strait. A regular flow scheme (outflow in the northern part and inflow near the southern slope of the strait) which is related to the quasi-permanent salinity front was shown to prevail on average. Current oscillations having inertial and diurnal periods and forcing-dependent current fluctuations are frequently observed to be superimposed on the mean structure of the currents. A relatively quick response of the hydrographic fields to almost periodic (2-day) changes of the local wind and the sea level is stated. The current is preferably contra-directional to the wind stress, but well correlated with the sea level difference between the open sea and the strait. However, the described regular current scheme seems to contribute the most to the water, salt and nutrient exchange through the Irbe Strait.

The Internal Seiches in Gullmar Fjord. Part II: Contribution to Basin Water Mixing

The mixing in the basin water (the water below sill level) of Gullmar Fjord has been investigated with the main focus on the contribution from internal seiches. A companion paper reports evidence for dissipation of internal seiche energy in the basin water after near-critical reflection from the bottom. In the present paper the magnitude and variation of basin water mixing is investigated, using the budget method. The results are related to variations of three energy sources, namely (i) the internal seiches, (ii) the internal tides, and (iii) the internal waves generated by the external seiche. The mixing efficiency, defined as the irreversible work against buoyancy forces due to mixing divided by the total mechanical energy loss from the sources mentioned above, is about 7%, similar to results obtained for other fjords. Large variations in mixing are shown to be related to large variations of the energy sources. The internal seiches are found to be important for the mixing, with a contribution that is 144% of the internal tide contribution during the most energetic period and 92% on average over the investigated periods. Including contributions from the external seiche, the wind forcing is responsible for 61% of the basin water mixing, while tidal forcing is responsible for 39%.

An Experiment with Forced Oxygenation of the Deepwater of the Anoxic By Fjord, Western Sweden

In a 2.5-year-long environmental engineering experiment in the By Fjord, surface water was pumped into the deepwater where the frequency of deepwater renewals increased by a factor of 10. During the experiment, the deepwater became long-term oxic, and nitrate became the dominating dissolved inorganic nitrogen component. The amount of phosphate in the water column decreased by a factor of 5 due to the increase in flushing and reduction in the leakage of phosphate from the sediments when the sediment surface became oxidized. Oxygenation of the sediments did not increase the leakage of toxic metals and organic pollutants. The bacterial community was the first to show changes after the oxygenation, with aerobic bacteria also thriving in the deepwater. The earlier azoic deepwater bottom sediments were colonized by animals. No structural difference between the phytoplankton communities in the By Fjord and the adjacent Havsten Fjord, with oxygenated deepwater, could be detected during the experiment.

Spatial Variability of Diapycnal Mixing and Turbulent Dissipation Rates in a Stagnant Fjord Basin

Two microstructure profilers, two ships, and four moorings with acoustic Doppler current profilers and conductivity‐temperature loggers were used in an intensive effort to map the spatial and temporal variations of vertical mixing in the stagnant deep basin of Gullmar Fjord, Sweden. During three days in the beginning of August 2001 a continuous time series of turbulent kinetic energy dissipation profiles was obtained with one microstructure profiler at a fixed position near the deepest part of the fjord. During the same period the other microstructure profiler was used to obtain six sections of dissipation through the length of the basin. Two moorings were deployed in the fjord basin for one month from the end of July to the end of August. The mapping of dissipation rates reveals that the dissipation in the deep basin is confined to areas just inside the sill. More than 77% of the dissipation in the fjord basin happens above the sloping bottoms closest to the sill.

Observations of Turbulence Caused by a Combination of Tides and Mean Baroclinic Flow over a Fjord Sill

Thisstudyinvestigates thedissipation ratesandflow conditionsattheDrobakSillinthe Oslofjord.Thearea was transected 13 times with a free-falling microstructure shear probe during 4 days in June 2011. At the same time, an ADCP was deployed inside the sill. During most tidal cycles, internal hydraulic jumps with high dissipation rates were foundon the downstreamside of the sill. However, the internal responsevariedstrongly betweendifferenttidal cycleswith similar barotropicforcing.In the beginning of the observational period,ebb tides had no hydraulic jumps, and in the end one of the flood tides did not have a hydraulic jump. During the same period, the mean baroclinic exchange flow changed from inflow to outflow in the bottom layer. The authors conclude that the conditions at the sill are on the edge of forming hydraulic jumps and that the mean baroclinic exchange may push the flow above or below the limit of a hydraulic jump depending on the situation. This conclusion is supported by two-layer hydraulic theory. The volume-integrated dissipation rates within 500m from the sill crest compare well with estimates of energy loss in the lower layer calculated from the Bernoulli drop under the assumption of no energy loss in the upper layer. Finally, the mean dissipation rateatthesillwascomparedwiththeradiationofinternaltidalenergyawayfromthesill,anditwasfoundthat about 60%‐90% of the total energy loss was dissipated locally.

Overturning and Dissipation Caused by Baroclinic Tidal Flow near the Sill of a Fjord Basin

Abstract Dissipation time series and moored velocity and density time series on the inner slopes of the Gullmar Fjord sill showed that the internal tides generated at the sill radiated to the head of the fjord, were reflected, and then radiated back to the sill, where they dissipated their energy mainly below sill level. A large amount of the dissipation was caused by a transitional flow at a particular phase of the internal tide, when the bottom layer descended down the sill slope and had to pass a constriction set up by a submarine hill. The inward, baroclinic bottom-layer flow transformed into a supercritical bottom jet, which separated from the bottom just downstream of the constriction. A large fraction of the dissipation took place in the successive rebounding region (the hydraulic jump) above the bottom jet, where overturns of the same size as the vertical extent of the rebounding region were observed. More than half of the dissipation was happening in the bottom boundary layer below the jet. Durin...

Effects of ecological engineered oxygenation on the bacterial community structure in an anoxic fjord in western Sweden

Oxygen-depleted bodies of water are becoming increasingly common in marine ecosystems. Solutions to reverse this trend are needed and under development, for example, by the Baltic deep-water OXygenation (BOX) project. In the framework of this project, the Swedish Byfjord was chosen for a pilot study, investigating the effects of an engineered oxygenation on long-term anoxic bottom waters. The strong stratification of the water column of the Byfjord was broken up by pumping surface water into the deeper layers, triggering several inflows of oxygen-rich water and increasing oxygen levels in the lower water column and the benthic zone up to 110 μmol l−1.We used molecular ecologic methods to study changes in bacterial community structure in response to the oxygenation in the Byfjord. Water column samples from before, during and after the oxygenation as well as from two nearby control fjords were analyzed. Our results showed a strong shift in bacterial community composition when the bottom water in the Byfjord became oxic. Initially dominant indicator species for oxygen minimum zones such as members of the SUP05 clade declined in abundance during the oxygenation event and nearly vanished after the oxygenation was accomplished. In contrast, aerobic species like SAR11 that initially were restricted to surface waters could later be detected deep into the water column. Overall, the bacterial community in the formerly anoxic bottom waters changed to a community structure similar to those found in oxic waters, showing that an engineered oxygenation of a large body of anoxic marine water is possible and emulates that of a natural oxygenation event.