Mindaugas Zilius ^1* Rūta Barisevičiūtė ² Stefano Bonaglia ³ Isabell Klawonn ⁴ Elise Lorre ¹ Tobia Politi ¹ Irma Vybernaite-Lubiene ¹ Maren Voss ⁴ Doanata Overlinge ¹ Paul Bukaveckas ⁵

• ¹ Klaipėda University, Klaipėda, Lithuania
• ² Center For Physical Sciences And Technology (CPST), Vilnius, Lithuania
• ³ University of Gothenburg, Gothenburg, Västergötland, Sweden
• ⁴ Leibniz Institute for Baltic Sea Research (LG), Warnemünde, Germany
• ⁵ Virginia Commonwealth University, Richmond, Virginia, United States

Estuarine systems, being situated at the interface between land and marine environments, are important sites for nitrate (NO3 -) retention and processing due to large inputs, long retention time, and high biogeochemical activity. However, it remains uncertain how pelagic and benthic processes control NO3 - cycling and how the relative importance of these processes is affected by seasonal changes in estuarine conditions. We measured the suite of processes governing NO3 -cycling in the Curonian Lagoon (Southeast Baltic Sea) during two time periods representing spring and summer conditions. We show that in spring, benthic dissimilatory and assimilatory NO3 -processes prevailed, while in summer, pelagic assimilatory processes dominated. During spring, warming temperatures and riverine nitrogen (N) inputs were associated with the onset of diatom blooms. N assimilation by diatoms resulted in the delivery of particulate organic N and organic matter to the benthos, resulting in greater denitrification in the sediments and a flux of NO3 -from the water column to the sediments. In summer, phytoplankton blooms of buoyant cyanobacteria and high rates of assimilatory uptake dominated, resulting in greater particulate organic N export from the lagoon into the sea. Given the low dissolved inorganic N concentrations in summer, high uptake indicates that the pelagic community possessed a nutritional strategy to efficiently utilize multiple N forms at high rates. Overall, our findings show that diatom-dominated communities foster strong benthic-pelagic coupling, whereas cyanobacteria dominance is associated with pelagicbased N cycling. While this study sheds new light on mechanisms of NO3 -retention in the Curnian Lagoon, further spatiotemporal resolution is recommended to better represent the variability in rates and to include other Baltic lagoons for a comprehensive understanding of N cycling in shallow estuarine systems.