The effects of variable riverine inputs and seasonal shifts in phytoplankton communities on nitrate cycling in a coastal lagoon

Estuarine systems, being situated at the interface between land and marine environments, are important sites for nitrate (NO 3 – ) retention and processing due to large inputs, long retention time, and high biogeochemical activity. However, it remains uncertain how pelagic and benthic processes control NO 3 – cycling and how the relative importance of these processes is affected by seasonal changes in estuarine conditions. We measured the suite of processes governing NO 3 – 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 NO 3 – 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 NO 3 – 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 NO 3 – 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.