Riverine transport of biogenic elements to the Baltic Sea – past and possible future perspectives

Abstract

Abstract. The paper reviews critical processes for the landsea fluxes of biogenic elements (C, N, P, Si) in the Baltic Sea catchment and discusses possible future scenarios as a consequence of improved sewage treatment, agricultural practices and increased hydropower demand (for N, P and Si) and of global warming, i.e., changes in hydrological patterns (for C). These most significant drivers will not only change the total amount of nutrient inputs and fluxes of organic and inorganic forms of carbon to the Baltic Sea, their ratio (C:N:P:Si) will alter as well with consequences for phytoplankton species composition in the Baltic Sea. In summary, we propose that N fluxes may increase due to higher livestock densities in those countries recently acceded to the EU, whereas P and Si fluxes may decrease due to an improved sewage treatment in these new EU member states and with further damming and still eutrophic states of many lakes in the entire Baltic Sea catchment. This might eventually decrease cyanobacteria blooms in the Baltic but increase the potential for other nuisance blooms. Dinoflagellates could eventually substitute diatoms that even today grow below their optimal growth conditions due to low Si concentrations in some regions of the Baltic Sea. C fluxes will probably increase from the boreal part of the Baltic Sea catchment due to the expected higher temperatures and heavier rainfall. However, it is not clear whether dissolved organic carbon and alkalinity, which have opposite feedbacks to global warming, will increase in similar amounts, because the spring flow peak will be smoothed out in time due to higher temperatures that cause less snow cover and deeper soil infiltration.

Arctic River Dissolved and Biogenic Silicon Exports—Current Conditions and Future Changes With Warming

“…DSi yields are also generally low in Arctic rivers (Dittmar & Kattner, ), with average DSi flux of the six largest rivers draining into the Arctic ranging from 32 to 82 mol DSi day −1 km −2 (average 54.4) (Holmes et al, ). The DSi concentrations and yields observed (Table ) from the most northern rivers in our study fall within that range expected of Arctic rivers, while the more southern rivers in this study display values typical of rivers draining boreal landscapes (Dürr et al, ; Humborg et al, ; Phillips & Cowling, ). The majority of DSi was exported during the freshet or summer period, while the least amount was exported during the winter, a seasonal pattern typical of high latitude rivers (Guo et al, ; Holmes et al, ).…”

Nutrient Fluxes from Land to Sea: Consequences of Future Scenarios on the Oder River Basin – Lagoon – Coastal Sea System

“…P fluxes will decrease due to improved WWT, which is a very likely process as Poland has to follow up the UWWTD. The same tendency of increasing N and decreasing P fluxes was also found by HUMBORG et al (2007). Among the wildcards (extreme scenarios), the adjustment of nutrient surpluses in Poland to the level of Mecklenburg-Western Pomerania would cause the strongest increase in N emissions.…”

“…3) The largest potential for reductions of nutrients from point sources is by im proving waste water treatment (WWT) in the southern and eastern parts of the Baltic Sea catchment (ERIKS- SON et al, 2007;HUMBORG et al, 2007;SCHERNEWSKI et al, 2008).…”

Scenarios for reduction of nutrient load from point sources in Estonia

“…The decrease will be more pronounced when the efficiency of wastewater treatment is higher by introducing high-grade (tertiary) treatment. This measure together with the increased number of people who are connected to wastewater treatment plants will provide an almost immediate response (Humborg et al 2007;Bryhn 2009 Recently the ministers of the environment of the member states of HELCOM adopted an action plan to considerably reduce the anthropogenic nutrient load to the Baltic Sea and restore a good ecological status by 2021 doi: 10.2166/nh.2012.136 (HELCOM 2007a). To achieve this, country-specific annual nutrient input reduction targets were proposed, based on the principle of maximum allowable nutrient inputs to the sea that were set at the level of about 21,000 tons of phosphorus and 600,000 tons of nitrogen.…”