The Helgoland Roads time series is one of the richest temporal marine data sets available. Running since 1962, it documents changes for phytoplankton, salinity, Secchi disc depths and macronutrients. Uniquely, the data have been carefully quality controlled and linked to relevant meta-data, and the pelagic time series is further augmented by zooplankton, intertidal macroalgae, macrozoobenthos and bacterioplankton data. Data analyses have shown changes in hydrography and biota around Helgoland. In the late 1970s, water inflows from the south-west to the German Bight increased with a corresponding increase in flushing rates. Salinity and annual mean temperature have also increased since 1962 and the latter by an average of 1.67°C. This has influenced seasonal phytoplankton growth causing significant shifts in diatom densities and the numbers of large diatoms (e. g. Coscinodiscus wailesii). Changes in zooplankton diversity have included the appearance of the ctenophore Mnemiopsis leidyi. The macroalgal community also showed an increase in green algal and a decrease in brown algal species after 1959. Over 30 benthic macrofaunal species have been newly recorded at Helgoland over the last 20 years, with a distinct shift towards southern species. These detailed data provide the basis for long-term analyses of changes on many trophic levels at Helgoland Roads.
The development of the bed bathymetry of an experimental dumping area was followed over three and a half years by means of multibeam echosounder techniques. Two types of material were discharged in the bight of Mecklenburg in the Baltic Sea in approximately 20 m of water depth. One set of the discharges was 2900 m³ of glacial till and the other set was a 2400 m³ mixture of glacial till, sand and minor amounts of cohesive matter. Only approximately 2500 m³ (86 %) of the glacial till and 1500 m³ (63 %) of the mixed soil materials were deposited on the seabed. This means that already during the dumping process a considerable part of the sediment material drifted away. The glacial till formed crater-like rings of 30 m diameter with peaks up to 1.4 m above seabed, whereas the spatial structure of the mixed soil material was somewhat more diffuse, but with similar magnitudes in the peaks and troughs.The morphological changes were small and their quantification required a high measuring precision in the order of few cm in the vertical. The dominant processes of surface deformation was flattening of peaks and filling of troughs. The speed of this process decreased with horizontal 2 scale: structures of less than 4 m horizontal extension had a trend to disappear within less than five years, whereas structures of larger than 8 m extension showed little change and are estimated to remain detectable for many decades. In contrast to the reworking of the matter inside the dumping structures, no net transport of material out the dumping area could be detected.Extrapolating the observed morphological changes into the future it is estimated that without significant decrease in internal shear strength of the disposed till the structures will persist for at least 70 years. This can be attributed to the high internal stability of the dumped glacial till and the low hydrodynamic forces present at the seabed in this region.
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