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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 pelagic-based N cycling. While this study sheds new light on mechanisms of NO3– retention in the Curonian 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. [ABSTRACT FROM AUTHOR]

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 these differ between contrasting seasons. In this study, we measured pelagic and benthic assimilatory and dissimilatory NO3- processes in a large lagoon (Curonian Lagoon, SE Baltic Sea) to understand changes in NO3- cycling in relation to variation in riverine inputs and shifts in phytoplankton community composition. We show that in spring, benthic dissimilatory and assimilatory NO3- processes were important, while in summer, pelagic assimilatory processes dominated. During spring, diatom blooms promote greater delivery of nitrogen (N) and labile organic matter to the benthos resulting in greater denitrification in the sediments and a net flux of NO3- from the water column to the sediments. In summer, phytoplankton blooms dominated by buoyant cyanobacteria exhibited high rates of assimilatory uptake and greater particulate organic N export to the sea, but low rates of sediment--water exchange. Cyanobacteria blooms were associated with higher absolute rates of NO3- uptake, as well as higher mass-specific rates compared to spring. Given the low dissolved inorganic N in summer, high uptake indicates that the pelagic community possessed a nutritional strategy to efficiently utilize multiple N forms. Overall, our findings show that the seasonal succession from diatom to cyanobacteria-dominated communities is associated with a shift from strong benthic-pelagic coupling to predominantly pelagic-based N cycling. [ABSTRACT FROM AUTHOR]

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