Factors influencing the dissolved iron input by river water to the open ocean

Abstract

Abstract. The influence of natural metal chelators on the bio-available iron input to the ocean by river water was studied. Ferrous and ferric ions present as suspended colloidal particles maintaining the semblance of a dissolved load are coagulated and settled as their freshwater carrier is mixed with seawater at the continental boundary. However, we might argue that different iron-binding colloids become sequentially destabilized in meeting progressively increasing salinities. By use of a 59 Fe tracer method, the partitioning of the iron load from the suspended and dissolved mobile fraction to storage in the sediments was measured with high accuracy in mixtures of natural river water with artificial sea water. The results show a characteristic sequence of sedimentation. Various colloids of different stability are removed from a water of increasing salinity, such as it is the case in the transition from a river water to the open sea. However, the iron transport capacities of the investigated river waters differed greatly. A mountainous river in the Austrian Alps would add only about 5% of its dissolved Fe load, that is about 2.0 µg L −1 Fe, to coastal waters. A small tributary draining a sphagnum peat-bog, which acts as a source of refractory low-molecular-weight fulvic acids to the river water, would add approximately 20% of its original Fe load, that is up to 480 µg L −1 Fe to the ocean's bio-available iron pool. This points to a natural mechanism of ocean iron fertilization by terrigenous fulvic-iron complexes originating from weathering processes occurring in the soils upstream.

Iron biogeochemistry across marine systems – progress from the past decade

“…Much of this fulvic phase is present as small colloids and in the truly dissolved fraction (<1 kD). When these phases reach the saline coastal water substantial aggregation of the Fe-oxyhydroxide fraction with associated CDOM is observed, whereas iron associated to the fulvic fraction show little aggregation (Stolpe and Hassellöv, 2007) and survives the sequential sequestration from the water column during gradual mixing with seawater (Krachler et al, 2005). It is possible that this land-derived fraction can reach the open ocean, as indicated by recent data (Laglera and van den Berg, 2009).…”

“…The stability constants of these strong L 1 ligands are very similar to those reported by Laglera and van den Berg (2009) for Fe bound to fulvic acids, indicating the importance of these ligands in controlling the solubility of dissolved iron in riverine and coastal systems. Clearly, Fe speciation in estuarine and near-shore waters can not be addressed in a generalized manner and systems may differ depending on watershed characteristics (e.g., pristine versus anthropogenically impacted) as well as the level and type of riverine input (Öztürk and Bizsel, 2003;Krachler et al, 2005) (see Sect. 5 for colloidal matter).…”

Characteristics and behavior of dissolved organic matter in the Kumaki River, Noto Peninsula, Japan

“…Humic substances composed of humic and fulvic acid account for 40-80% of DOM in river water (Malcolm 1985;Mostofa et al 2010). Thus, they play an important role in biogeochemical processes, such as buffering pH and aiding complexation of trace elements in aquatic environments (Matsunaga et al 1998;Krachler et al 2005;Nakagawa et al 2008). Moreover, these substances exhibit various structural features because of differences in their source 6 materials and formation environment (Malcolm 1985;Kalbitz 2001).…”

Comparison of controlling mechanisms of flocculation processes in estuaries

“…Production of autochthonous DOC in estuaries may also occurs through several processes (Sharp, 1991). Aquatic fulvic acids accounts for 50 to 80 % of the total amount of DOC in coastal waters (Krachler, et al, 2005). The reasons for such increase in DOC concentrations from rivers to the Caspian Sea in North of Iran, which may be related to exudation by microalgae require further studies.…”