Plain Language Summary
This study focuses on the source of dissolved iron (dFe) in the Amur River Basin because dFe is known to contribute greatly to phytoplankton growth in the Sea of Okhotsk. Nevertheless, there has been little research on the dFe source of rivers, especially in the Amur-Mid Basin which is situated in a sporadic permafrost area. To gain a better understanding of dFe source in the Amur-Mid Basin, we made a landcover map with high resolution of 30 m using Landsat-8 data and a machine learning technique (decision tree analysis). Then we investigated the coverages of permafrost wetland in the sampled watersheds and found that the larger the coverage of permafrost wetlands, the higher was dFe concentration in the rivers. This is the first study to demonstrate the direct evidence about the importance of permafrost wetlands to supply dFe and DOC to rivers in the Amur-Mid Basin.
1 Introduction
Iron is an essential trace element for the growth of all organisms. It plays an important role in in vivo metabolic processes, including photosynthesis, electron transfer, nitrate reduction, and nitrogen fixation (Crichton, 2001; Sunda, 2012). Martin and Fitzwater (1988) first showed the possibility that iron limits phytoplankton growth in a high-nutrient low-chlorophyll (HNLC) region where the phytoplankton productivity is low despite the abundance of nutrients. Subsequent studies confirmed that dissolved iron (dFe) concentrations in many ocean areas are too low for phytoplankton to fully utilize nutrients (Bruland & Lohan, 2003; Martin et al., 1989, 1990, 1994; Price et al., 1994; Takeda & Obata, 1995). In the past, the main iron source for the ocean was thought to be aeolian dust (Martin & Fitzwater, 1988). However, recent studies have suggested that riverine dFe is also an important source to support primary production in coastal areas and the ocean (Laglera & Vandenberg, 2009; Matsunaga et al., 1998; Moore & Braucher, 2008; Nishioka et al., 2014), stimulating interest in iron dynamics in terrestrial environments.
The Sea of Okhotsk is known as one of the oceans with the richest marine resources in the world. Some studies indicated that abundant dFe derived from forests and wetlands in the Amur River Basin contributes to the creation of high biological productivity in the Sea of Okhotsk (Nishioka et al., 2014; Shiraiwa, 2012; Suzuki et al., 2014). For example, Sanjiang Plain, a large wetland in northeastern China, is an important dFe source for the Amur River, and relatively high dFe concentrations were observed in the Songhua River and the Ussuri River that pass through the Sanjiang Plain (Wang et al., 2012). In addition, wetlands in the Amur-Lower Basin from Khabarovsk to Nikolaevsk-na-Amure also greatly contribute to supply dFe to the Amur River through tributaries (Nagao et al., 2007). Therefore, wide areas of Amur-Lower Basin including the Sanjiang Plain are believed to be a particularly important dFe source for the Amur River. Some studies reported, however, that relatively high dFe concentration was also observed in the Bureya River and the Zeya River which are the representative large rivers in the Amur-Mid Basin where permafrost is sporadically distributed (Nagao et al., 2007; Shamov et al., 2014) (Figure 1a). Based on the authors’ recent field survey on permafrost distribution in the Bureya River Basin, permafrost existence was generally confirmed under wetlands (peat bogs) in flat valleys (Tashiro et al., 2020). Accordingly, these permafrost wetlands can be the most important dFe source for permafrost-affected rivers in the Amur-Mid Basin.
Since permafrost constrains the path of water to the surface of the active layer (a soil layer that thaws during summer) rich in organic matter, permafrost coverage in a watershed is associated with river water chemistry (Olefeldt et al., 2014; Petrone et al., 2006). In the study in Caribou–Poker Creeks of Alaska, Petrone et al. (2006) found that the permafrost-dominated watershed transports more dissolved organic carbon (DOC) and fewer mineral components (Ca2+, Mg2+, K+, and Na+) than the permafrost-poor watershed. It is thus important to determine the permafrost coverage to understand biogeochemical cycles from land to river in the arctic regions and to predict the influence of permafrost degradation on river water chemistry under a warming climate.
Examining the permafrost distribution over a wide area, however, is not at all easy. Instead, latitude has often been used to explain the regional difference in chemical compositions of rivers and soil pore waters (Kawahigashi et al., 2004; Pokrovsky et al., 2015, 2016; Raudina et al., 2017, 2018). Pokrovsky et al. (2016), for example, investigated DOC concentration in tributaries of the Ob River and revealed that riverine DOC concentration decreased northward with change in permafrost regime from discontinuous (50–90% coverage) to continuous (>90%) permafrost. However, even within the same regions where the permafrost regime is equal, riverine DOC concentration has a large variation that cannot be explained by permafrost regime or latitude (Olefeldt et al., 2014; Pokrovsky et al., 2015). To better understand the biogeochemical cycles in permafrost regions, grasping the detailed information about permafrost distribution in a given study area is also needed in addition to water sampling.
The objectives of this study are: (1) to understand the permafrost distribution on a regional scale utilizing a landcover map based on local survey and remote sensing technique, and (2) to clarify the importance of permafrost wetlands as a dFe source for rivers by investigating the relationship between the coverage of permafrost wetland and dFe concentration. This study provides not only river water quality data in the Amur-Mid Basin, but also shows the usefulness of applying a remote sensing technique to understanding biogeochemical processes in arctic and subarctic regions.
2 Materials and Methods
2.1 Study site
A field survey was conducted in the Tyrma region, which is approximately 270 km northwest of Khabarovsk in the Russian Far East (Figure 1a). Mean annual air temperature is −1.96℃ and annual precipitation is 654.6 mm. The Tyrma region is located just north of the permafrost boundary and is situated in a sporadic permafrost area (Obu et al., 2019; Shamov et al., 2014). The sporadic permafrost area mostly covers the Bureya River Basin and the Zeya River Basin which are the large tributaries of the Amur River (Figure 1a). In this paper, Amur-Mid Basin indicates the Bureya River Basin and the Zeya River Basin. In the Tyrma region, four large rivers, namely, the Yaurin River, the Gujik River, the Gujal River, and the Sutyri River join the Tyrma River and eventually join the Bureya River (Figure 1b).
Vegetation in the Tyrma region is roughly divided into two types: wetlands in the flat valleys are characterized by shrubs, such as bog blueberry (Vaccinium uliginosum ), cowberry (Vaccinium vitis-idaea L. ), and ledum (Ledum decumbens ) and scattered larches (Larix gmelinii var. gmelinii ) (Figure 2a & Figure S1); and the forest on the ridges and hillslopes are characterized by spruces (Picea ajanensis ) and white birches (Betula platyphylla ) (Figure 2b). Topsoil layers in the wetlands and the forests are composed of peat soils. In particular, thick peat soil layers are formed in the wetlands due to long-term accumulation of sphagnum biomass. This type of wetland, called “Mari ” is a typical landscape in the flat valleys of the Tyrma region. Even more important is the fact that permafrost generally exists underneath a thick peat soil layer in wetlands (Tashiro et al., 2020). In this paper, the word “wetland” or “permafrost wetland” indicates Mari. In addition to the forests and wetlands, grasslands are often found along large rivers (Figure 3c). Grasslands cover flat areas of land (floodplain) near large rivers and are characterized by herbaceous plants spreading over sediment deposits.