Estuaries are known to act as ʻfiltersʼ of riverine nutrient load by permanently removing and temporary retaining the load on its way to the open sea. The estuarine turnover of N, key element in coastal eutrophication, is of particular interest for the coastal zone of the Baltic Sea, receiving ~ 680 kt riverine total N each year. The study by Bartl & Hellemann et al. investigates how the estuarine N filter function is affected by varying environmental settings, for which the authors compared microbial N removal (denitrification, anammox) and N retention (nitrification, ammonium assimilation) processes in the benthic system of two contrasting Baltic Sea estuaries: the small, oligotrophic, northern Baltic Öre Estuary with long residence time and non-permeable sediments, and the large, eutrophied, southern Baltic Vistula Estuary with short residence time and a large share of permeable sediments. Surprisingly, rates of nitrification and ammonium assimilation in the benthic boundary layer did not significantly differ between the two estuaries. Meanwhile, as expected, rates of denitrification in the sediment were twice as high in the eutrophied as in the oligotrophic estuary. All microbial rates were connected to the availability of phytoplankton-derived particulate organic matter (POM). The study suggests that in stratified estuaries, independent of their specific environmental settings, POM links riverine N load and benthic N turnover, and functions as a temporary N reservoir that is microbially processed over time, thus allowing for effective coastal filtering even at low process rates. The study calls attention to including geomorphological and hydrological features when assessing the coastal N filter function. The study was carried out in the framework of the BONUS COCOA project and involved also former ABRU members Susanna Hietanen and Petra Tallberg. The publication in the journal Biogeosciences is open access and can be accessed here.
In a cross-Baltic collaboration between the University of Helsinki (HU) and the German Leibniz Institute for Baltic Sea Research Warnemuende (IOW), first-authors Ines Bartl (IOW) and Dana Hellemann (HU) investigated how the nitrogen (N) filter function of estuaries in the Baltic Sea is affected by varying environmental settings, such as riverine N load, trophic status, geomorphology, sediment composition, and water and particle residence time.