Seasonal contribution of terrestrial organic matter and biological oxygen demand to the Baltic Sea from three contrasting river catchments

Abstract

Abstract. To examine the potential influence of terrestrially derived DOM on the Baltic Sea, a year-long study of dissolved organic matter (DOM) was performed in three river catchments in Sweden. One catchment drains into the Bothnian Sea, while two southern catchments drain into the Baltic proper. Dissolved organic carbon (DOC) concentrations were positively correlated with discharge from forested catchments over the year. While the overall concentrations of DOC were several times higher in the southern two catchments, higher discharge in the northern catchment resulted in the annual loadings of DOC being on the same order of magnitude for all three catchments. Biological oxygen demand (BOD) was used as a proxy for the lability of carbon in the system. The range of BOD values was similar for all three catchments, however, the ratio of BOD to DOC (an indication of the labile fraction) in Ume river was four times higher than in the southern two catchments. Total annual BOD loading to the Baltic Sea was twice as high in the northern catchment than in the two southern catchments. Lower winter temperatures and preservation of organic matter in the northern catchment combined with an intense spring flood help to explain the higher concentrations of labile carbon in the northern catchment. Lower lability of DOM as well as higher colour in the southern catchments suggest that wetlands (i.e. peat bogs) may be the dominant source of DOM in these catchments, particularly in periods of low flow. With climate change expected to increase precipitation events and temperatures across the region, the supply and quality of DOM delivered to the Baltic Sea can also be expected to change. Our results indicate that DOM supply to the Baltic Sea from boreal rivers will be more stable throughout the year, and potentially have a lower bioavailability.

Mass and UV-visible spectral fingerprints of dissolved organic matter: sources and reactivity

“…In PC2, low flow conditions lead to an increase in the relative contribution of peat and wetlands, as well as drainage through deeper layers of forest soil. The importance of the boreal forest and associated peat and wetlands has been seen before in bulk DOC studies (Bishop et al, 1993;Cole et al, 2007;Asmala et al, 2013), and specifically in these catchments (Reader et al, 2014). Here we can see the expected shift in the absorbance spectra as these sources become dominant under low flow conditions, with a strong increase in mid-to-low UV absorption and a smaller decrease from 350 nm into the visible, effectively an increase in the spectral slope ratio (S R ; Helms et al, 2008).…”

“…The larger Ume catchment is divided into two large subcatchments, Ume river, which is high altitude and also highly regulated with several hydropower dams and Vindeln catchment, which is unregulated and largely forested. The forested parts of the catchment have been previously shown to drive seasonal DOM dynamics (Reader et al, 2014), thus characteristics such as flow and temperature have been calculated for the Krycklan subcatchment, which is a forested, low altitude subcatchment of Vindeln catchment (Laudon et al, 2013). The two smaller rivers, Emån river, and Lyckeby river, are located in southern Sweden.…”

“…Samples for microbial degradation were filtered only through GF/F filters to remove large particles and plankton, whilst keeping the microbial community largely intact. Reader et al, 2014 (Goldman et al, 1987). Samples were then placed in the dark at ambient temperature (mean 20.6 • C) for 4 months.…”

Resource partitioning confirmed by isotopic signatures allows small mammals to share seasonally flooded meadows

“…In the future, with respect to climate change and the resultant expected increases in extreme flood events in northern Europe (Reader et al, ) and the arrival of new species due to changes in distribution ranges and consequent changes to communities and food webs (Baltensperger et al, ), knowledge of the formation of small mammal communities may help in the prognosis of ecosystem changes and predicting at‐risk species.…”

“…A greater number of species (Barnosky, Hadly, Maurer, & Christie, 2001;Hallett, 1991) or functional diversity of these species (Wood, McKinney, & Loftin, 2017) enhances the stability of a community, increasing the potential to withstand negative influences (Scheffer et al, 2012). Under conditions of increased and more frequent floods (prognosis by Reader, Stedmon, & Kritzberg, 2014), ecosystems may reorganize (Brown, Whitham, Ernest, & Gehring, 2001). The arrival of new species and resulting changes in food webs (Baltensperger et al, 2015) may be buffered by compensation from complementary species.…”