Modeling pathways of riverine nitrogen and phosphorus in the Baltic Sea

Abstract

[1] A better understanding of the fate of nutrients entering the Baltic Sea ecosystem is an important issue with implications for environmental management. There are two sources of nitrogen and phosphorus: riverine input and atmospheric deposition. In the case of nitrogen, the fixation of dinitrogen by diazotrophic bacteria represents a third source. From an analysis of stable nitrogen isotope ratios it was suggested that most of the riverine nitrogen is sequestered in the coastal rim, specifically along the southern Baltic Sea coast with its coarse sediments, whereas nitrogen from fixation dominates the central basins. However, pathways of nutrients and timescales between the input of the nutrients and their arrival in different basins are difficult to obtain from direct measurements. To elucidate this problem, we use a source attribution technique in a three-dimensional ecosystem model, ERGOM, to track nutrients originating from various rivers. An "age" variable is attributed to the marked elements to indicate their propagation speeds and residence times. In this paper, we specifically investigate the spreading of nitrogen and phosphorus from the riverine discharges of the Oder, Vistula, Neman and Daugava. We demonstrate which regions they are transported to and for how long they remain in the ecosystem. The model results show good agreement with source estimations from observed d15 N values in sediments. The model results suggest that 95% of nitrogen is lost by denitrification in sediments, after an average time of 1.4 years for riverine nitrogen. The residence time of riverine phosphorus is much longer and exceeds our simulated period of 35 years.

A Novel Modeling Approach to Quantify the Influence of Nitrogen Inputs on the Oxygen Dynamics of the North Sea

“…Compared to previous TBNT studies (e.g., Ménesguen et al, 2006;Radtke et al, 2012) this implies a different treatment of the diffusive transport. In our study its calculation bases on the spatial gradients of the bulk variable concentration.…”

“…This is done by introducing additional model state variables and processes for each property. This concept dates back to the work by Ménesguen and Hoch (1997), who introduced the age as a property, and later on has been applied to various models and ecosystems (e.g., Wijsman et al, 2004;Blauw et al, 2006;Ménesguen et al, 2006;Lacroix et al, 2007;Neumann, 2007;Timmermann et al, 2010;Radtke et al, 2012;Painting et al, 2013;Troost et al, 2013;Perrot et al, 2014;Radtke and Maar, 2016).…”

“…In fact, it is difficult to state which approach is more correct than the other as the diffusive transport according to Fick's first law is a net transport, i.e., the actual gross transports are not known. However, it has to be noted that the diffusive transports of the individual fraction variables X i according to Ménesguen et al (2006) and Radtke et al (2012) can be opposed, while those following Equation (1) are unidirectional. Despite this different treatment the resulting differences in the fraction variable distributions are expected to be generally small and noticeable differences may only occur in the case of large differences between the spatial gradients of the fraction and bulk variables.…”

“…This method has been applied to various European marine ecosystems, including the Baltic Sea (e.g., Neumann, 2007;Radtke et al, 2012) and the North Sea (e.g., Blauw et al, 2006;Lacroix et al, 2007;Painting et al, 2013;Troost et al, 2013). Troost et al (2013) demonstrated the importance of atmospheric N deposition for the off-shore regions of the southern North Sea, where it contributed 10-20% to overall TN and even 20-30% to NPP in 2002.…”

Nutrient transports in the Baltic Sea – results from a 30-year physical–biogeochemical reanalysis

“…The magnitude of DIN imports and exports in areas with greater water depths are much smaller. Obviously, DIN supplied from land is already consumed in the coastal zone (Voss et al, 2005;Almroth-Rosell et al, 2011), and, consequently, only a minor fraction of the nitrogen supplied to the shallow area can continuously reach regions deeper than 100 m Radtke et al, 2012).…”