External nutrient loads to the Baltic Sea, 1970-2006

Any research related to nutrient biogeochemistry of the Baltic Sea, especially studies of eutrophication requires knowledge of the long-term dynamics of external nutrient inputs. Information accumulated in the HELCOM’s pollution load compilations is too aggregated and, until recently, covered only specific years. On the other hand, national datasets with higher resolutions are often difficult to access. Therefore, over the years considerable efforts have been put into a compilation of consistent estimates of nutrient input to the entire Baltic Sea.Excluding exchange with the Skagerrak from this report, the external nutrient input is considered here as consisting of three component parts: waterborne land loads, direct point sources at the coasts, and atmospheric depositions. The present reconstruction of time series of these three components is based on three major sources. The compilation of the land loads database has started within the project “Large-scale Environmental Effects and Ecological Processes in the Baltic Sea, 1990-1995” (Wulff et al., 2001c), continued during the MARE (“Marine Research on Eutrophication, 1999-2006”) project (Wulff et al., 2001a, Eriksson Hägg et al., 2010), and was most recently updated and expanded in connection with the latest HELCOM’s activities on the pollution load compilation (HELCOM, 2011) and the revision of the Baltic Sea Action Plan (BSAP, Wulff et al., 2009). However, there is an important difference between information contained in the HELCOM publications and the data presented here. By its very international nature HELCOM has to deal with whatever data are officially provided by the contracting parties, ending up with certain gaps and inconsistencies in the data sets (e.g. see discussion in HELCOM, 2011). On the other hand, considering eutrophication as an imbalance in the large-scale nutrient cycles, whereby more nutrients come into the system than leave it (e.g. Savchuk and Wulff, 2009 and references therein), we need to know the total amounts of external input as close to the reality as possible. Therefore, in our reconstructions we have been trying to both fill such gaps in and correct possible sources of inconsistencies. The reconstructed data sets have extensively been used by ourselves for various nutrient budget estimates (e.g. Wulff et al., 2001b, Savchuk, 2005) and as the boundary conditions for biogeochemical models (e.g. Savchuk and Wulff, 2007, 2009) including development of the eutrophication segment of BSAP (Wulff, 2007), as well as by many researchers around the Baltic Sea, for instance, within several projects of the BONUS+ research programme (e.g.Eilola et al., 2011; Eriksson Hägg et al., 2011; Meier et al., 2011). The implemented nutrient inputs have naturally been described in these publications, but briefly. In order to facilitate afurther distribution of reconstructed inputs and their usage, here we describe the process of reconstruction in more detail and make available the full data sets in digital form.The data used for the reconstruction have kindly been provided by several institutions and agencies around the Baltic Sea (see below) as well as directly by HELCOM during preparation of PLC-4 and PLC-5 (see HELCOM, 2004, 2011). Since not all data providershad given the permission to distribute the original raw measurements, we have here 3 aggregated all the inputs according to the spatial segmentation of the Baltic Sea (Fig.1) currently implemented in the biogeochemical model BALTSEM (BAltic sea Long-Term large Scale Eutrophication Model). Note also that some part of riverine inputs is available in a decision support system Baltic Nest with a much higher spatial resolution, as is further explained below