Abstract. The aim of this study was to investigate the variability of the water exchanges in the Curonian Lagoon based on the hydraulic regime and the atmospheric forcings. A finite element hydrodynamic model has been applied to the Curonian Lagoon to simulate the circulation patterns for 10 years. With the help of a transport-diffusion model, the salinity distribution and the renewal times of the Curonian Lagoon have been investigated when forced by river runoff, wind, and Baltic Sea level fluctuations. The hydrodynamic model has been validated using in situ salinity measurements.Model results show that the variability depends mainly on seasonal changes in hydrographic forcing and on the dominant wind regimes that prevail over the Curonian Lagoon. Exchanges between the southern and the northern part of the lagoon mostly depend on the wind forcing and are much less influenced by the river discharge.However, when looking at the water renewal time, the most important factor is the river discharge into the lagoon. Other physical forcings only marginally determine the renewal time, and not even ice cover is able to influence it. Even if ice cover strongly inhibits the exchanges between the southern and northern lagoon, it is basically not able to change the absolute value of the renewal times.

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The large southern Baltic lagoons (Curonian, Vistula and Szczecin Lagoon) face a similar challenge, the need for economic, especially touristic development. This development is hampered by poor water quality, namely eutrophication with regular algae blooms and low water transparency and often insufficient bathing water quality. Therefore, beaches and bathing sites, one precondition for tourist development, are often lacking. Using the Curonian Lagoon as case study, we apply a Systems Approach Framework (SAF) and especially an Ecological-Social-Economic (ESE) assessment to analyse, whether it is reasonable to establish new beaches, against the background of an improved sewage treatment. In this systematic, stepwise and participatory Integrated Coastal Zone Management approach, we guide and support municipalities from the issue identification towards the final implementation by developing alternative scenarios, using model simulations and carrying out tourist surveys as well as supporting socio-economic studies. The present state of bathing water quality does allow opening a new bathing site with a beach at the lagoon and one town decided to do so. The socio-economic analyses did show that lagoon water quality and the possibility to go for a swim is not sufficiently important for tourists on the Curonian Spit, that opening a beach would make sense from an economic point of view. However, high lagoon water temperatures may help to extend the summer bathing season and a beach is regarded as additional attraction for tourists. A systematic involvement of stakeholders was imperative in this case study and we provide lessons learnt for a successful participatory process.

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The aim of this study was to investigate the sediment dynamics in the largest lagoon in Europe (Curonian Lagoon, Lithuania) through the analysis of in situ data and the application of a sediment transport model. This approach allowed to identify the propagation pathway of the riverine suspended sediments, to map erosion-accumulation zones in the lagoon and calculate the sediment budget over a 13-year-long simulation. Sampled suspended sediment concentration data are important for understanding the characteristics of the riverine and lagoon sediments, and show that the suspended organic matter plays a crucial role on the sediment dynamics for this coastal system. The numerical experiments carried out to study sediment dynamics gave satisfactory results and the possibility to get a holistic view of the system. The applied sediment transport model with a new formula for settling velocity was used to estimate the patterns of the suspended sediments and the seasonal and spatial sediment distribution in the whole river–lagoon–sea system. The numerical model also allowed understanding the sensitivity of the system to strong wind events and the presence of ice. The results reveal that during extreme storm events, more than 11.4 × 106 kg of sediments are washed out of the system. Scenarios without ice cover indicate that the lagoon would have much higher suspended sediment concentrations in the winter season comparing with the present situation with ice. The results of an analysis of a long-term (13 years) simulation demonstrate that on average, 62% of the riverine sediments are trapped inside the lagoon, with a marked spatially varying distribution of accumulation zones.