Key processes in the coupled carbon, nitrogen, and phosphorus cycling of the Baltic Sea

Abstract

In this study we examine pools of carbon (C), nitrogen (N), and phosphorus (P) in the Baltic Sea, both simulated and reconstructed from observations. We further quantify key fluxes in the C, N, and P cycling. Our calculations include pelagic reservoirs as well as the storage in the active sediment layer, which allows a complete coverage of the overall C, N, and P cycling on a system-scale. A striking property of C versus N and P cycling is that while the external supplies of total N and P (TN and TP) are largely balanced by internal removal processes, the total carbon (TC) supply is mainly compensated by a net export out of the system. In other words, external inputs of TN and TP are, in contrast to TC, rather efficiently filtered within the Baltic Sea. Further, there is a net export of TN and TP out of the system, but a net import of dissolved inorganic N and P (DIN and DIP). There is on the contrary a net export of both the organic and inorganic fractions of TC. While the pelagic pools of TC and TP are dominated by inorganic compounds, TN largely consists of organic N because allochthonous organic N is poorly degradable. There are however large basin-wise differences in C, N, and P elemental ratios as well as in inorganic versus organic fractions. These differences reflect both the differing ratios in external loads and differing oxygen conditions determining the redox-dependent fluxes of DIN and DIP.

Large-Scale Nutrient Dynamics in the Baltic Sea, 1970–2016

“…Nevertheless, the empirical (Figure 8) and simulated net nutrient transports agree within a factor of two. However, in the budgets constructed with the threedimensional biogeochemical model, in which simulated nutrient concentrations are drastically corrected by the data assimilation procedure (Liu et al, 2017), the net nutrient exchanges are generally smaller by an order of magnitude and, in some straits, have the opposite direction to fluxes in the budgets presented here and simulated by Gustafsson et al (2017). As a result, in the simulation by Liu et al (2017) the entire Baltic Sea appears to be less interconnected and exhibits more nutrient retention.…”

“…As phosphorus sinks obtained by budgeting are pretty similar to estimates made by the simulation Gustafsson et al, 2017), the modeling results can also be used to deconstruct nitrogen sinks (cf. Figure 8A) into their components.…”

“…The procedure and peculiarities of implementation of these tools are discussed in detail by Savchuk (2010); some obtained results are presented, for instance, in the following studies (Conley et al, 2002;Savchuk, 2005;Savchuk and Wulff, 2009;Gustafsson et al, 2017;Lehtoranta et al, 2017).…”

“…The Simple As Necessary BALTic Sea model (SANBALTS, Wulff, 2007, 2009;Savchuk et al, 2008) highly aggregates variables (organic and inorganic forms of nutrients without explicit description of biota) resulting in correspondingly high spatial-temporal averaging (annual within seven homogeneous basins). The BAltic Long-Term large Scale Eutrophication Model (BALTSEM, Gustafsson et al, 2012Gustafsson et al, , 2017Savchuk et al, 2012a) describes long-term seasonal dynamics of the lower levels of marine ecosystem (from hydrography to zooplankton) within 13 sub-basins, considered horizontally homogeneous but having a high vertical resolution. Both models have been sufficiently validated and used for quantification of the eutrophication segment of the Baltic Sea Action Plan Backer et al, 2010;Anonymous, 2013).…”

“…Long-term (1970Long-term ( -2016 nutrient dynamics are studied, using basin-wide integrated nitrogen and phosphorus pools reconstructed from all available observations, instead of traditionally analyzed concentrations measured at fixed monitoring stations. Previously, these time series have been shown only in graphs for comparison with simulations (e.g., Savchuk and Wulff, 2009;Eilola et al, 2011;Gustafsson et al, 2017). Here, they are made available for the community in digital form for the first time.…”