Southern German Bight Research Articles

Seasonal cycles and their spatial variability

Seasonal variations dominate many processes in continental shelf seas. A comprehensive coherent inter-disciplinary data set for one seasonal cycle was obtained by repeating the same cruise track in the southern North Sea at monthly intervals from August 1988 to October 1989. Measurements were made throughout the water column in vertically homogeneous and summer stratified regions and near the major estuaries. 97% of the surface temperature’s variance was in the seasonal cycle, driven by solar forcing; spatial variability was related to stratification and to contrasts between the waters off northeast England and the German Bight. The salinity seasonal cycle was small; spatial variability was governed by fresh water river inputs. Suspended sediment concentrations were largest near river mouths and coasts; material was transported eastward from East Anglia towards the German Bight in a distinct plume whose magnitude varied with seasonal wind patterns. There were large regional differences, with the greatest phytoplankton biomass and oxygen supersaturation developing in the Southern Bight and German Bight, those regions which experience extensive phytoplankton blooms in the spring. Annual primary productivity ranged from 79 gC m -2 a -1 for the English coastal region to 261 gC m -2 a -1 for the German Bight. Low oxygen concentrations were measured in late summer below the thermocline in regions on either side of the Dogger Bank. A budget of nutrient concentrations throughout the region suggests that nutrient supply to the phytoplankton in the winter is dominated by regeneration processes, rather than input from river run-off.

A tidal mixing front in an area influenced by land runoff

A potential energy model of stratification, adequate for thermal fronts, is extended to incorporate the added buoyancy input of river fresh water discharge. The model is discussed in the light of CTD data taken at a section in the southern German Bight which was repeatedly sampled throughout the year. A front observed in this area exhibits similarities with surface heat input induced fronts in the United Kingdom shelf despite the dominating contribution of salinity to the density field.

Litter pollution from ships in the German Bight

A 60 m length of beach at Helgoland was sampled approximately every third day for a year to determine the composition of litter deposited there. A total of 8473 items with a total weight of 1320 kg were identified as shipping wastes in the 106 samples. Plastics of all types composed 75% of the items, whereas wood represented 65% of the total weight. Ships' waste from along the main shipping routes in the southern German Bight may be a dominant source of the litter. The objects found were originally manufactured in 26 different nations. The study provides an estimation of the dimensions of litter pollution in the area of the inner German Bight.

Comparative fecundity of North Sea plaice ( Pleuronectes platessa L.)

Stage IV ovaries of plaice were obtained at sea from five spawning areas of the North Sea and English Channel (Eastern English Channel, Southern Bight, Central Southern North Sea, German Bight and Flamborough) in the years 1977, 1979 and 1980. Additional samples were provided from Dutch landings at IJmuiden for 1972. The numbers of eggs were counted from 790 fish. After a logarithmic transformation, analysis-of-covariance techniques were used to describe the relation between fecundity, given by the number of eggs, and fish length and age, and to examine fecundity differences between spawning area and year. After removing the effects of length and age, significant differences in fecundity were found between years. In the German Bight these were as much as 1.44 times. The data indicate that fecundities at a standard length and age were larger in 1980 than in 1979 but that the extent of the difference varied with spawning location. A gradual decrease in fecundity at a given length and age is shown from the eastern English Channel northwards to the German Bight. A significant decrease in fecundity with age, at a given length, is shown for all locations. Data from the Southern Bight and Flamborough were compared with those collected over the period 1947-9. The recent data from Flamborough show an increase in fecundity at length and age of 1.36 times and from the Southern Bight an increase of 1.6 times is found. The difference in the Southern Bight is reflected in the greater relative weight of the ovaries to body weight in 1980 compared with that in the 1940s. It is suggested that the modulation of fecundity could provide a potentially significant contribution to the natural regulation of the plaice population.

Field experiments on the fate of lead and chromium in an intertidal benthic mesocosm, the Bremerhaven Caisson

The Bremerhaven Caisson, a benthic mesocosm enclosing a 13 m2 sediment area and 13 m3 trapped water column at high tide, was used for field experiments on the fate of lead and chromium at the isle of Norderney, Southern German Bight. The caisson, made from seawater resistant aluminium, settled on fine sand (mean grain size: 0.2 mm) at an experimental site with about 0.9-m tidal height and 4.5 h of water coverage. During 38 consecutive tides a constant amount of lead was administered to the inflowing water, resulting in a nominal concentration of 0.1 mg l-1 at high tide. A second caisson was taken for chromium using the same procedure. The compartments analysed (water, seston, sediment, and infauna) showed distinct differences for lead and chromium. The particle-reactive element Pb adsorbed by 50–70% onto seston particles, the soluble element Cr-VI by less than 2%. Only a moderate enrichment was measured in the sediment, about 3–4% of the total Pb and Cr added accumulated in the top 3 cm. The fauna attained much higher accumulation factors than the sediment. The lead uptake rates of 15 species and 2 taxa can be placed in order according to the feeding type: filter feeder > deposit feeder (surface) > opportunistic feeder (deposit feeder or predator) > deposit feeder (depth). Chromium was accumulated about 10 times lower than lead, the uptake seems to be more related to the systematic position of the animal (calcareous shell versus chitinous cuticle). In the total mass balance only 0.2% of the lead added and 0.02% of the chromium added were fixed by the fauna.

Sources and sediment associations of heavy metals in polluted coastal regions

Vertical profiles and lateral distribution patterns of heavy metals in Recent sediments are increasingly employed in the assessment of the major sources of pollution in coastal regions, including estuaries and marginal seas. After a general review of studies from typical locations, examples from New Bedford Harbor, Mass./U.S.A., and from the southern German Bight in the North Sea are described in more detail. Chemical extraction procedures are used to evaluate different forms of metal associations in aquatic sediments: (i) heavy metals are transported and deposited as major, minor and trace constituents in the natural rock detritus, mostly in inert positions; (ii) slight alkalinity in sea water results in the formation and precipitation of hydroxides, carbonates, sulphides and phosphates of heavy metals; (iii) sorption and cation exchanges takes place in particular on fine-grained substances with large surface areas, e.g. on manganese dioxide, humic acids, hydrous iron oxides and clay minerals; (iv) organic substances play an important role in the transfer of metals between dissoved and solid phases, particularly by the process of flocculation; (v) under oxidizing conditions, the redox-sensitive Fe- and Mn-hydroxides and -oxides are significant “sinks” of heavy metals in aquatic systems. The characteristic rise in the residual content of lead, copper and cadmium in the polluted samples points toward the strong influence of atmospheric dust particles. Heavy metals immobilized in the bottom sediment constitute a potential hazard to water quality and aquatic life since they may be released as a result of chemical changes. In relation to the marine environment, the following mechanisms should be taken into account: (i) increased salinities at the inflow of rivers into the sea water will affect the partial exchange of metal cations sorbed onto the surface of suspended particles; (ii) oxygen deficiency in the bottom sediments may lead to the dissolution of hydrous iron and manganese oxides and to a release of co-precipitated heavy metals; (iii) complexing substances of both natural and synthetic origin increases the solubilization of metals; (iv) microbial activities can convert inorganic metal compounds to organic molecules and may thereby increase toxicity, as exemplified in organo-mercurial transformations.