Abstract. Rapid Arctic warming accelerates permafrost thaw, causing an additional release of terrestrial organic matter (OM) into rivers, and ultimately, after transport via deltas and estuaries, to the Arctic Ocean nearshore. The majority of our understanding of nearshore OM dynamics and fate has been developed from freshwater rivers, despite the likely impact of highly dynamic estuarine and deltaic environments on transformation, storage, and age of OM delivered to coastal waters. Here, we studied OM dynamics within the Lena River main stem and Lena Delta along an approximately ∼1600 km long transect from Yakutsk, downstream to the delta disembogue into the Laptev Sea. We measured particulate organic carbon (POC), total suspended matter (TSM), and carbon isotopes (δ13C and ∆14C) in POC to compare riverine and deltaic OM composition and changes in OM source and fate during transport offshore. We found that TSM and POC concentrations decreased by 55 and 70 %, respectively, during transit from the main stem to the delta and Arctic Ocean. We found deltaic POC to be strongly depleted in 13C relative to fluvial POC, indicating a significant phytoplankton contribution to deltaic POC (∼68 ±6 %). Dual-carbon (∆14C and δ13C) isotope mixing model analyses suggested an additional input of permafrost-derived OM into deltaic waters (∼18 ±4 % of deltaic POC originates from Pleistocene deposits vs ∼ 5 ±4 % in the river main stem). Despite the lower concentration of POC in the delta than in the main stem (0.41 ±0.10 vs. 0.79 ±0.30 mg L-1, respectively ), the amount of POC derived from Pleistocene deposits in deltaic waters was almost twice as large as POC of Yedoma origin in the main stem (0.07 ±0.02 and 0.04 ±0.02 mg L-1, respectively). We assert that estuarine and deltaic processes require consideration in order to correctly understand OM dynamics throughout Arctic nearshore coastal zones and how these processes may evolve under future climate-driven change.
Review of the manuscript “Particulate organic matter in the Lena River...” by Ogneva et al.
The paper addresses a fundamental question of riverine fluxes of particulate organic carbon in still poorly studied permafrost regions, and its potential impact on surrounding marine environments, and as such it fits the scope and potentially can make a good addition to the journal.
Major critical comments are listed below.
The main conclusions of the authors – that estimation of river OM discharge to the coastal zone cannot be based solely on the data of the main stem far from the deltaic region – is certainly useful for modelers, although not novel and generally agrees with large body of evidences collected for example, by Shirshov’s Institute of RAS in Arctic rivers and coastal zones (A.P. Lisitsyn’s marginal filters concept suggested more than 30 years ago).
The Introduction is well written but it is way too general. Former studies on the POC were not discussed (Semiletov, Kutscher, E. Karlsson). As a result, specific objectives and novelty of this work are unlear; and no new nypothesis is proposed to be tested ( a degree of pOC lost in the deltaic zone or the age and origin of POC could be such hypotheses). In anyway, the authors should clearly position this work with respect to former studied of the Lena River to prove its real novelty.
The Discussion is very much driven by postulated overwhelming role of phytoplankton in POC, d13C, D14C control in the main stem vs. delta. Without Chl a analysis, or any information on the phytoplankton, such a discussion is not substantiated and suggested explanations have low novelty and probably unwarranted. As a minimal research efforts, the authors could examine their TSM samples by SEM to show the presence of higher amount of diatoms in their deltaic samples vs main stem samples
Examination of C/N ratio could aslo help a lot in distinguishing different sources of POM
The discussion and data treatment (Fig 4) also ignore that part of POM may be represented by contemporary vegetation debris (i.e., lignin), especially from larch trees, dominating the Lena catchment
Specific comments:
L117-120 This might be true; however, di not the former works of Semiletov, Kutscher etc address the transformation of C between Zhigansk/Yakutsk and the delta?
L176-177 Provide some numbers on the magnitude of Delta14C between “old” and “modern” for non-experienced reader
L185-187 Neglecting the beginning of spring flood may underestimate sizable amount of riverine C, transported to the delta (which is not the case for the winter time). Justification ere is needed.
L197-198 Former studies already shown this; why additional efforts are needed?
L203-205 Unclear. If there is no difference in deltaic region (L197-198), why there should be any in the river main stem? More likely explanation is due to seasonal variations in C concentrations in the Arctic GRO dataset.
Fig. 2 is well presented. However, the data of former researchers, obtained at these transects (at least, the Yakutsk – Kusur one) should be also presented
Section 4.1.1 can be strongly shortened; the novelty of these findings is low. Summarize in one paragraph. Some relevant information can be shifted to the caption of Fig. S1.
L314-320 This is site description; re-arrange
L353 Present the numbers of velocities in thee regions
L420 There should be some data for the man stem
L439 d13C of POC?
L558-563 The novelty of the present study seems to be low
We characterized organic matter along the Lena River over a transect from Yakutsk north to the Lena Delta (approximately 1640 km). We identified the distribution, main sources, and transformation of organic carbon on its way from the permafrost catchment to the Arctic Ocean. We show that the Lena Delta as the interface between the Lena River and the Arctic Ocean plays a crucial role in determining the qualitative and quantitative composition of organic matter discharged into the Arctic Ocean.
We characterized organic matter along the Lena River over a transect from Yakutsk north to the...
