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.