Simulating transport and distribution of marine macro-plastic in the Baltic Sea.

We simulated the spatial distribution and dynamics of macro plastic in the Baltic Sea, using a new Lagrangian approach called the dynamical renormalization resampling scheme (DRRS). This approach extends the super-individual simulation technique, so the weight-per-individual is dynamic rather than fixed. The simulations were based on a mapping of the macro plastic sources along the Baltic coast line, and a five year time series of realistic wind, wave and current data to resolve time-variability in the transport and spatial distribution of macro plastics in the Baltic Sea. The model setup has been validated against beach litter observations and was able to reproduce some major spatial trends in macroplastic distributions. We also simulated plastic dispersal using Green's functions (pollution plumes) for individual sources. e.g. rivers, and found a significant variation in the spatial range of Green's functions corresponding to different pollution sources. We determined a significant temporal variability (up to 7 times the average) in the plastic concentration locally, which needs to be taken into account when assessing the ecological impact of marine litter. Accumulation patterns and litter wave formation were observed to be driven by an interplay between positive buoyancy, coastal boundaries and varying directions of physical forcing. Finally we determined the range of wind drag coefficients for floating plastic, where the dynamics is mostly directly wind driven, as opposed to indirectly by surface currents and waves. This study suggests that patterns of litter sorting by transport processes should be observable in many coastal and off-shore environments.

Long-term environmental risks of the Baltic Sea's "memory effect" revealed by ocean modeling and observation of reprocessing-derived radiotracers.

Although previous research indicated that the Baltic Sea has a strong "memory effect" for trapping pollutants/nutrients, the associated environmental risks are not well understood due to the knowledge gaps in the long-term hydrodynamics-driven exchange of pollutants/nutrients between the North Sea and the Baltic Sea. In this work, we exploited 99Tc and 129I released from the two European nuclear reprocessing plants as oceanic tracers and pollutant proxies, and performed a five-decade hindcast simulation to quantitatively estimate the fluxes and timescales of marine transport of pollutants/nutrients in the North-Baltic Sea. Modeling results underline two potential environmental risks of the Baltic Sea's "memory effect": (1) ∼26 years of environmental half-life for any existing water-soluble pollutants/nutrients in the Baltic Sea driven by its hydrodynamics; (2) the Baltic Sea as a pollutant reservoir continuously exporting 3 % of contaminations per year to the downstream areas after any pollution event. Our findings provide fundamental knowledge for understanding the long-term hydrodynamics-driven pollutant/nutrient transport in the North-Baltic Sea, facilitating the future regional management of the marine environment.

Anthropogenic 236U and 233U in the Baltic Sea: Distributions, source terms, and budgets.

The Baltic Sea receives substantial amounts of hazardous substances and nutrients, which accumulate for decades and persistently impair the Baltic ecosystems. With long half-lives and high solubility, anthropogenic uranium isotopes (236U and 233U) are ideal tracers to depict the ocean dynamics in the Baltic Sea and the associated impacts on the fates of contaminants. However, their applications in the Baltic Sea are hampered by the inadequate source-term information. This study reports the first three-dimensional distributions of 236U and 233U in the Baltic Sea (2018-2019) and the first long-term hindcast simulation for reprocessing-derived 236U dispersion in the North-Baltic Sea (1971-2018). Using 233U/236U fingerprints, we distinguish 236U from the nuclear weapon testing and civil nuclear industries, which have comparable contributions (142 ± 13 and 174 ± 40 g) to the 236U inventory in modern Baltic seawater. Budget calculations for 236U inputs since the 1950s indicate that, the major 236U sources in the Baltic Sea are the atmospheric fallouts (∼1.35 kg) and discharges from nuclear reprocessing plants (> 211 g), and there is a continuous sink of 236U to the anoxic sediments (589 ± 43 g). Our findings also indicate that the limited water renewal endows the Baltic Sea a strong "memory effect" retaining aged 236U signals, and the previously unknown 236U in the Baltic Sea is likely attributed to the retention of the mid-1990s' discharges from the nuclear reprocessing plants. Our preliminary results demonstrate the power of 236U-129I dual-tracer in investigating water-mass mixing and estimating water age in the Baltic Sea, and this work provides fundamental knowledge for future 236U tracer studies in the Baltic Sea.

Methodology for defining homogeneous water bodies for management purposes.

European legislation requires monitoring of toxic algae in marine areas where shellfish are harvested for consumption. Monitoring assumes the existence of homogeneous water bodies, the definition of which have important implications for stakeholders and consumers. Yet, the definition of homogeneous water bodies remains unclear. Here we present a methodology to divide coastal and estuarine waters into homogeneous water bodies to monitor toxic algae. The proposed method is mainly based on water transport, and secondarily on oceanographic characteristics; salinity and sea surface height. We apply the methodology to the Limfjord in Denmark and demonstrate its usefulness in areas with a complicated coastal morphology. The oceanographic descriptors applied in the method are standard outputs from coastal hydrodynamical models. Provided that validated and high resolution model output is available for a given area, the technique is thus adaptable to other morphologically and oceanographically complicated estuarine and coastal areas where toxic algae monitoring is necessary.

The potential of current- and wind-driven transport for environmental management of the Baltic Sea.

The ever increasing impact of the marine industry and transport on vulnerable sea areas puts the marine environment under exceptional pressure and calls for inspired methods for mitigating the impact of the related risks. We describe a method for preventive reduction of remote environmental risks caused by the shipping and maritime industry that are transported by surface currents and wind impact to the coasts. This method is based on characterizing systematically the damaging potential of the offshore areas in terms of potential transport to vulnerable regions of an oil spill or other pollution that has occurred in a particular area. The resulting maps of probabilities of pollution to be transported to the nearshore and the time it takes for the pollution to reach the nearshore are used to design environmentally optimized fairways for the Gulf of Finland, Baltic Proper, and south-western Baltic Sea.