Lagoons, being situated at the interface between land and marine environments, are potentially important sites for nitrate (NO₃^–) retention due to massive nutrient loads, long retention time and high biogeochemical transformation rates. Although nitrogen (N) cycling is of considerable importance for eutrophication, only a few studies have quantified the multiple processes responsible for NO₃^– turnover (i.e. denitrification, DNRA, and NO₃^– assimilation) in lagoons. Simultaneous measurements of pelagic assimilatory and benthic dissimilatory processes in combination with quantifying riverine loads allowed us to depict a complete budget of seasonal NO₃^– cycling in the largest European lagoon (Curonian Lagoon, SE Baltic Sea). The specific NO₃^– transformation pathway was driven by the seasonal availability of NO₃^– in the water column. In spring, NO₃^– was in excess and the dominant N form in the water column fueling both pelagic and benthic processes. Deep light penetration illuminated half of the lagoon water column and sediment, and sustained photosynthetic activity, thus affecting benthic processes. In summer, a decrease in riverine loads resulted in lower NO₃^– concentrations, which affected only benthic processes. Whereas pelagic assimilation rates were maintained by active organic mineralization coupled with nitrification. Together, NO₃^– processes can retain up to ~70 % of the total N load. Our results show that under present climate conditions eutrophic lagoons are effective NO₃^– filters, but with ongoing changes and extreme events, their filter capacity may decrease.

Primary Presenter: Mindaugas Zilius, Klaipeda University (mindaugas.zilius@jmtc.ku.lt)

Authors:

Paul Bukaveckas, Virginia Commonwealth University Center for Environmental Studies  ()

Isabell Klawonn, Leibniz Institute for Baltic Sea Research  ()

Maren Voss, Leibniz Institute for Baltic Sea Research, Warnemünde  ()

Irma Vybernaite-Lubiene, Klaipeda University  ()

Elise Lorre, Klaipeda University  ()

Tobia Politi, Klaipeda University  ()

Stefano Bonaglia, University of Gothenburg  (stefano.bonaglia@gu.se)