Natural and Anthropogenic Sediment Mixing Processes in the South-Western Baltic Sea

Abstract

Natural and anthropogenic sediment mixing can significantly impact the fidelity of sedimentary records of climate and environmental variability and human impact. This can lead to incorrect interpretations of the previous state(s) of a given ecosystem, its forcing mechanisms, and its future development. Here, natural and anthropogenic sediment mixing processes (i.e., bioturbation, hydroturbation and direct anthropogenic impact) are investigated in the southwestern Baltic Sea by sedimentological, ichnological, geochemical, and radionuclide analyses to assess their impact on timemarker profiles and sediment deposition. Depth profiles of mercury and caesium-137 display a varyingly strong disturbance down to 5-25 cm. The deviations from undisturbed profiles can be used to estimate the relative degree of sediment mixing. Sedimentary fabric analysis of high-resolution X-radiographs provides further insight into bioturbation. Ichnofossils identified in short sediment cores suggest that the primary sedimentary structure is partly overprinted by burrowing organisms living in the upper 5-7 cm of sediment. Meteorological and hydrological data from automated measuring stations combined with results from resuspension experiments show that hydrodynamic events, such as storms and saline water inflows from the North Sea, have the potential to resuspend and laterally transport sediment particles in the southwestern Baltic Sea. Partially graded layers in the scale of mm to cm in the investigated cores are likely associated with such hydrodynamic events. Multiple generations of linear traces on the seafloor are made visible through sidescan-sonar mapping and document a strong anthroturbation in the muddy sediments of the Mecklenburg Bight and the Arkona Basin, e.g., by bottom trawling. Depending on the core location and assumed mixing depth, determined mean net linear sedimentation rates range between 0 and 3.5 mm/a and reconstructed net mass accumulation rates range between 0 and 1.86 kg/m 2 /a in the mud basins. The calculable inventory of anthropogenic contaminants in the sediments illustrates the important, at least temporary storage function of the mud basins and of the adjacent sandy areas over industrial times. The findings of this study help identify the

Physical Disturbance by Bottom Trawling Suspends Particulate Matter and Alters Biogeochemical Processes on and Near the Seafloor

“…These trawl-induced changes to surface seabed physical properties have the potential to affect the distribution and survival of benthic organisms that require particular sediment properties (e.g., water or organic carbon content) as well as altering biogeochemical gradients in the sediments. The persistence of the track is in agreement with other authors who have seen that, especially in areas with low background disturbance and on muddy seabeds, tracks may persist from months to years (Krost et al, 1990;Schwinghamer et al, 1998;Tuck et al, 1998;Smith et al, 2007;Palanques et al, 2014;Oberle et al, 2018;Bunke et al, 2019).…”

“…Physical disturbance of the seabed can either lead to increased heterogeneity/roughness, e.g., in areas of low fishing intensity and on flat sedimentary bottoms, or decreased roughness, e.g., in areas of high fishing intensity and/or where seafloor structures are leveled (Martín et al, 2014). The parallel furrows left by trawl door tracks on soft sediments are particularly easy to detect with acoustic mapping methods, e.g., side-scan sonar and multibeam, and such tracks can remain for many years (Krost et al, 1990;Oberle et al, 2018;Bunke et al, 2019).…”

“…More recently, van Denderen et al (2020) estimated that a combination of trawling disturbance and hypoxia reduced the benthic community biomass by at least 50% in 14% of the Baltic Sea. Bunke et al (2019) saw evidence of extensive disruption of sedimentary fabrics by trawling in the south-west Baltic Sea, although bioturbation, storms and marine water inflows also contributed to the disruption. Given that many areas of the Baltic Proper comprise soft sediment seabeds, physical disturbance by trawling can be expected to cause sediment suspension and affect biogeochemical processes across the sediment-water interface; however, data is lacking.…”

Quantifying Contemporary Organic Carbon Stocks of the Baltic Sea Ecosystem

“…We select particulate and dissolved organic carbon in the water column (POC, DOC) and particulate organic carbon in the upper sediment as non-living pools. The sediment carbon stock is calculated for the top layer (10 cm) that is above the average mixing depth of 5-10 cm of the Baltic Sea (Bunke et al, 2019) and thus still involved in the carbon cycling via natural and anthropogenic mixing processes. This restriction follows the methods of former studies estimating marine sediment carbon stocks (Burrows et al, 2017;Diesing et al, 2017;Luisetti et al, 2019).…”

Geohazard assessment of the coastal zone – the case of the southern Baltic Sea

“…Cae sium 137 is an ar ti fi cial radionuclide, which en tered the en vi ron ment af ter 1945 as a re sult of nu clear weap ons test ing and ac ci dents at nuclear power plants. Its pres ence in de pos its shows the thick ness of the layer that has un der gone redeposition dur ing the last few de cades Bunke et al, 2019).…”