Abstract. This work is focused on the application of a modelling system to simulate 3-D interaction between the Curonian Lagoon and the Baltic Sea coastal waters and to reflect spatiotemporal dynamics of marine waters in the Curonian Lagoon. The model system is based on the finite element programme package SHYFEM which can be used to resolve the hydrodynamic equations in lagoons, coastal seas, estuaries and lakes. The results of a one year (2009) 3-D model simulation with real weather and hydrological forcing show that the saline water intrusions from the sea through Klaipėda Strait are gradually decreasing with distance from the sea and become negligible (average annual salinity about 0.5 ‰) at a distance of about 20 km to the south of Kiaulės Nugara island. Analyses of the simulation results also show this area to be highly heterogeneous according to the vertical salinity distribution. While in the deeper Klaipėda Strait (harbour waterway) differences in average salinity between near bottom and surface layers varies in the range 2-2.5 ‰, in the rest of the Curonian Lagoon it is less than 0.5 ‰. The exchange flow showed vertical structure, but was horizontally uniform with the presence of a two-directional flow that from time to time changes to either saline water one-directional flow to the Curonian Lagoon or fresh water one-directional flow to the sea. Two-directional flow duration decreases with a distance from sea entrance in Klaipėda Strait from around 180 days yr −1 close to the sea entrance to 50 days yr −1 just behind Kiaulės Nugara island. One-directional outflow duration is increasing with a distance from the sea entrance from 100 to 225 days yr −1 . One-directional inflow duration occurs in the range of 70-100 days yr −1 . The analysis of the ratio of buoyancy layer thickness to water depth (h b /H ) and the Wedderburn number identified the main importance of wind action on the flow structure. Strong winds from the North and NW determine a barotropic inflow which is mostly responsible for the salt water intrusion into the Curonian Lagoon. Absence of wind or cross-strait wind regimes allows the maintenance of a two-layer flow typical of estuarine dynamics.

The temporal variation and the spatial structure of marine populations strongly depend on the early life stages of the individuals and on their interaction with the environment. The physical dispersion of propagules (eggs and larvae) is a fundamental aspect, conditioning the successful recruitment of juveniles in the adult population. Coastal transitional ecosystems such as lagoons plays a role of nursery for species with economic relevance, such as demersal fishes, mollusks and crustaceans. Those environments promote the recruitment of the early stages for several organisms because they act as area of retention for propagules and concentration for resources. We applied in three different European coastal lagoon a lagrangian particle tracking model coupled with a hydrodynamic model, and developed a method to evaluate the connectivity inside a lagoon and between a lagoon and the sea. Each particle represents a generic passive pelagic larva with a duration of one month. The average connectivity has been estimated after two years of simulation under realistic tide and wind forcings. Our results allow to characterize the lagoons behavior considering different spatial scales and to compare the dynamics of different systems. This study is a first step toward a better knowledge of the factors influencing the ecological role of coastal lagoons.

The biogeochemical conditions at the sediment-water interface and along the water column near the discharge of the Santa Marta sewage outfall (SMSO) were studied during the non upwelling (NUPW) and upwelling (UPW) seasons by sedimentary properties and benthic metabolism measurements, as well as, by the implementation of a coupled 3D hydrodynamic-ecological model (AEM3D). Sediment properties (organic matter quantity, C, N and P pools and δ 13 C, δ 15 N and redox potential) and benthic metabolism (aerobic respiration, denitrification, nitrate ammonification and nutrient recycling) were analyzed in four stations located in the proximity and 100, 750 and 1800 m far from the untreated wastewater effluent discharge in both seasons in the Santa Marta Coastal Area (SMCA). From each site, sediment cores were collected between 20 and 30 m depth. Then, the nutrient fluxes were measured in the laboratory via dark incubations; sequentially to fluxes denitrification and dissimilative nitrate reduction to ammonium were measured via the r-IPT (Isotope Pairing Tecnnique). The results indicate that the sediments trace the impact of the outfall (at 750 m and 1800 m with a contribution of terrestrial organic carbon of ~ 40 and ~ 20%, respectively). The results suggest significantly higher sediment oxygen demands (SOD) in the outfall vicinity, as well as a suppression of denitrification and increments in the ammonia nitrogen release through disassimilatory reduction of nitrate to ammonium (DNRA), which was increased during the UPW season.On the other hand, AEM3D model was applied to analyze the seasonal variations of water physicochemical and biological parameters in SMAC under two different nutrient and organic matter loads from wastewater outfall (flow-rate of 1.0 m 3 s -1 and 2.5 m 3 s -1 ) and along the NUPW and UPW season. The model was set up, calibrated and validated based on benthic metabolic measurements carried out within the simulation period, satellite-derived chlorophyll-a (Chl-a) and sea surface temperature (SST) maps, HYCOM database and field and literature water quality data. The model was able to reproduce the magnitude and timing of complex dynamics and fast transitions of temperature, nutrients, and phytoplankton, including the time and duration of stratification and mixing periods during the NUPW and UPW seasons. The model was also able to capture the effect of fertilization from upwelling and from the outfall plume. The wind field was the main driver of nearshore hydrodynamics and the outfall plume dispersion. The shortest average residence times IV of the outfall plume (3.7 ± 0.4 days) corresponded to the period of highest upwelling intensity.Temperature, light intensity and nutrients were the factors that limited phytoplankton growth. The plume concentrations of TOC, TP and PO4 3increased slightly under two scenarios of different wastewater loading. The phytoplankton growth was limited in both NUPW and UPW seasons due to large changes in temperature and advection and mixing in the coastal area, resulting in lar...

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.

The Curonian Lagoon is Europe's largest lagoon and one of the most seriously impacted by harmful blooms of cyanobacteria. Intensive studies over the past 20 years have allowed us to identify the major drivers determining the composition and spatial extent of hyperblooms in this system. We summarize and discuss the main outcomes of these studies and provide an updated, conceptual scheme of the multiple interactions between climatic and hydrologic factors, and their influence on internal and external processes that promote cyanobacterial blooms. Retrospective analysis of remote sensed images demonstrated the variability of blooms in terms of timing, extension and intensity, suggesting that they occur only under specific circumstances. Monthly analysis of nutrient loads and stoichiometry from the principal tributary (Nemunas River) revealed large interannual differences in the delivery of key elements, but summer months were always characterized by a strong dissolved inorganic N (and Si) limitation, that depresses diatoms and favors the dominance of cyanobacteria. Cyanobacteria blooms occurred during high water temperatures, long water residence time and low-wind conditions. The blooms induce transient (night-time) hypoxia, which stimulates the release of iron-bound P, producing a positive feedback for blooms of N-fixing cyanobacteria. Consumermediated nutrient recycling by dreissenid mussels, chironomid larvae, cyprinids and large bird colonies, may also affect P availability, but their role as drivers of cyanobacteria blooms is understudied.