Importance of Boreal Rivers in Providing Iron to Marine Waters

Abstract

This study reports increasing iron concentrations in rivers draining into the Baltic Sea. Given the decisive role of iron to the structure and biogeochemical function of aquatic ecosystems, this trend is likely one with far reaching consequences to the receiving system. What those consequences may be depends on the fate of the iron in estuarine mixing. We here assess the stability of riverine iron by mixing water from seven boreal rivers with artificial sea salts. The results show a gradual loss of iron from suspension with increasing salinity. However, the capacity of the different river waters to maintain iron in suspension varied greatly, i.e. between 1 and 54% of iron was in suspension at a salinity of 30. The variability was best explained by iron:organic carbon ratios in the riverine waters – the lower the ratio the more iron remained in suspension. Water with an initially low iron:organic carbon ratio could keep even higher than ambient concentrations of Fe in suspension across the salinity gradient, as shown in experiments with iron amendments. Moreover, there was a positive relationship between the molecular size of the riverine organic matter and the amount of iron in suspension. In all, the results point towards a remarkably high transport capacity of iron from boreal rivers, suggesting that increasing concentrations of iron in river mouths may result in higher concentrations of potentially bioavailable iron in the marine system.

Variations of total dissolved iron and its impacts during an extreme spring flooding event in the Songhua River

“…The concentration of TDFe in current study is comparable with that in the boreal blackwater rivers which are commonly characterized by a high TDFe content (Regina et al, 2010;Kritzberg et al, 2014). This may be attributed to the geological conditions of the Songhua River Basin surrounded by orogenic zones which are filled with iron-rich terrigenous sedimentary, volcanogenic, and intrusive rocks.…”

Sporadic hotspots for physico-chemical retention of aquatic organic carbon: from peatland headwater source to sea

“…Recent experiments also have shown a change in dissolved organic matter (DOM) quality in river water treated with saline solutions. Kritzberg et al (2014) found that high salinity caused a decrease in humic acids and aromatic DOM (as indicated by E4/E6 and SUVA 254 , respectively), and riverine average molecular weight has been observed to decrease even at low salinities (1-6; Asmala et al 2014b). Similarly in freshwaters it is the hydrophobic component of humic and fulvic acids that is the most reactive (Sharp et al 2006), and in coloured waters this fraction constitutes 60-80 % of the total DOC (Malcolm 1985), indicating significant potential for flocculation when DOC-rich waters enter estuaries.…”

Flocculation of dissolved organic matter controls the distribution of iron in boreal estuarine sediments

“…Also land use changes such as ditching and forest clearance have influenced the inputs of particulate material to the coastal zone (Yu et al, 2015). Moreover, the transport of riverine Fe and DOM into the Baltic has increased in recent decades as a consequence of brownification (Kritzberg et al, 2014), related primarily to the recovery of boreal freshwater systems from industrial acidification in the mid-20 th century (Monteith et al, 2007). These changes may be expected to influence both the sediment composition and diagenetic processes in the sediments at our study sites through time.…”

“…This component may be more substantial than previously thought and hence play a role in providing Fe as a micronutrient to the oceans (Kritzberg et al, 2014;Krachler et al, 2016). However, the majority of dissolved Fe in river water is associated with higher-molecular weight DOM, hence most riverine dissolved Fe is retained by flocculation in estuaries (Raiswell, 2011).…”

“…The role of flocculation in Fe sedimentation may be enhanced in boreal estuarine systems, where high DOM fluxes maintain a relatively large component of riverine Fe in the dissolved phase (Kritzberg et al, 2014). However, few studies have attempted to investigate the connection between flocculation and the Fe distribution in boreal estuarine sediments.…”