Reviews and Syntheses: Carbon biogeochemistry of Indian estuaries
Abstract. The goal of this review is to provide a comprehensive overview of the magnitude and drivers of carbon cycling dynamics in the major estuaries of India. Data from a total of 32 estuaries along the Bay of Bengal (BB) and the Arabian Sea (AS) were compiled from the literature and re-analysed based on changes in season (wet vs. dry) and marine end-members (e.g., BB vs. AS). The estuaries are generally undersaturated in dissolved oxygen relative to the atmosphere and strongly influenced by local and regional precipitation patterns. Speciation of the dissolved inorganic carbon (DIC) pool is dominated by bicarbonate and primarily variability in DIC is controlled by a combination of carbonate weathering, the degree of precipitation, the length of the estuaries, in situ respiration, and mixing. Carbonate dissolution had the largest influence on DIC during the wet season, while respiration was the primary control of DIC variability in the estuaries connected with BB during the dry season. Interestingly, the influence of anaerobic metabolism on DIC is observed in the oxygenated mangrove dominated estuaries, which we hypothesize is driven by porewater exchange in intertidal sediments. Dissolved organic carbon (DOC) generally behaves non-conservatively in the studied estuaries. The DOC-particulate organic carbon (POC) inter-conversion and DOC mineralization are evident in the BB during the dry season and AS estuaries, respectively. The wet season δ13CPOC shows dominance of freshwater algae, C3 plant material, as well as marine organic matter in POC. However, anthropogenic inputs are evident in some estuaries in eastern India during the dry season. POC respiration was identified in the AS; however, a link between POC and CH4 is identified throughout both the regions. pCO2 is controlled principally by respiration with freshwater discharge only playing a marginal important role in the BB. The AS estuaries act as a CO2 source to the atmosphere; however, the BB estuaries vary between a source and sink. POC together with methanotrophy and dam abundance appear to control CH4 concentrations, and all of the studied estuaries act as a CH4 source to the atmosphere. Additionally, anthropogenic inputs and groundwater exchange also show potential influences in some cases. The Indian estuaries contribute 2.62 % and 1.09 % to the global riverine DIC and DOC exports to the ocean, respectively. The total CO2 and CH4 fluxes from Indian estuaries are estimated as ~9718 Gg yr-1 and 3.27 Gg yr-1, which contributes ~0.67 % and ~0.12 %, respectively, to global estimates of estuarine greenhouse gas emissions. While a qualitative idea on the major factors controlling the carbon biogeochemistry in India is presented through this work, a more thorough investigation including rate quantification of the above-mentioned mechanisms is essential for precise accounting of the C budget of Indian estuaries.
Manab Kumar Dutta et al.
Status: final response (author comments only)
- RC1: 'Comment on bg-2022-200', Anonymous Referee #1, 09 Nov 2022
- RC2: 'Comment on bg-2022-200', Anonymous Referee #2, 25 Nov 2022
Manab Kumar Dutta et al.
Manab Kumar Dutta et al.
Viewed (geographical distribution)
The authors reviewed carbon biogeochemistry in the India Coast, presented its spatiotemporal variability and discussed the potential drivers. As a continuum between land and ocean, estuarine system is important while suffering a great spatiotemporal heterogeneity. Such study could help constrain this variation and better understand its essential role on global carbon budget. However, this manuscript of its current version still saves a large space for improvement, particularly from its data interpretation and content structure perspectives. I am afraid this manuscript will need a thorough revision to fit for the journal.
(1) Traceable data is vital for a review article. Unfortunately, there is no clear pathway(s) for data sources in this manuscript. For example, how many sampling stations, how many observations for each estuary, sampling time, etc.? We even do not know if the authors are presenting annual average data or just one-time surveyed data. Why there is no standard deviation for each estuary data in figures? A proper conclusive data table (can be supplementary material) is badly needed to show its rigor and reliability.
(2) Following the first major concern, then the data interpretation is problematic. First, the data visualization needs improvement, why only list Sundarbans and Hooghly Estuary sampling stations in Fig.1? Differences on estuaries or dry/wet cycle cannot be well distinguished in both figures and supplementary figures. Second, the way of data processing is also unclear, for example, how do the authors conduct statistical analysis, t-test? two-way ANOVA? any process to meet the assumptions? In supplementary figures, several estuaries are excluded to meet a high p-value relationship seems arbitrary and misleading, same as the threshold 6800 μatm for pCO2. Is there any reason/accordance to do so?
Also, I have a feeling that the authors messed with riverine and estuarine data. For example, in Line 815 the “~10.30 Tg C yr-1” belongs to riverine export fluxes (Krishna et al., 2019) rather than “export fluxes from Indian estuaries”, and the following discussion (Lines 819–831) is all about riverine C exports. Accordingly, in Fig. 7 export flux values may put in wrong place. Similarly, I do not think there are so many dams built in coastal estuaries list in Table 1. This is the reason why readers are curious about the data details, if so, I would suggest the authors clarify each estuary area/coordinates and further check about the data.
(3) The manuscript structure is organized in a research article format instead of a review. In addition, the separated discussions on DIC, DOC, POC, CO2, CH4 read super repetitive and distracting. In fact, carbon biogeochemistry is comprehensive and synthesized, drivers (e.g. hydrologic, biochemical, etc.) on any single carbon species would further impact on other carbon interactivities and then the entire carbon budget. Re-organization of manuscript structure to look at the drivers more synthetically is highly recommended.
(4) Many important information are missing, such as temperature gradient, wind speeds, net ecosystem productions, submarine groundwater discharge rates, two end-members values, etc., these are decisive to estuarine carbon biogeochemistry. Also, I am curious about the anthropogenic impact on estuarine carbon biogeochemistry. It seems the anthropogenic discharges in this study are mostly referred as sewage discharges to upper rivers, then how to identify the anthropogenic carbon in lower estuarine area proportionally?
Line 210: more details on “statistical analysis”.
Line 347: references.
Line 348: “DIC addition/removal” details.
Line 399: should be “riverine DIC” instead of “estuarine DIC”
Line 483: the difference between “Terrestrial DOC” and “Riverine DOC” ?
Line 492: where is “Fig. 12A”?
Line 540-543: the purpose for comparing regional DOC/DON to POC/PON? or DOC fraction in global coastal ocean?
Line 552-553: you cannot say this unless the data about POC/DOC from two end-members.
Line 570: why 6800 μatm threshold?
Line 579: further explain “a decrease of aerobic bacterial activity with increasing DOC”
Line 616: further explain “freshwater mixing is not the major driver of POC”, as it shows lower salinity with higher POC and 13C values.
Line 668-671: more direct evidence is needed to evaluate anthropogenic impact on pCO2 rather than population density. For example, anthropogenic pCO2 is 100 μatm out of total pCO2 400 μatm in Estuary A, where as anthropogenic pCO2 is 200 μatm out of total pCO2 1000 μatm in Estuary B.
Line 686: where is “Fig. 21”?
Line 695-696: wrong statement “nitrification plays crucial role in increasing pH”
Line 699: “unlikely”
Line 712-713: details for FCO2
Link 862: where is “Table 6”
Tables and Figures
Table 1: add coordinates, references
Table 2: are they annual averaged numbers? Standard deviation?
Table 5: confusing table, please improve
Fig. 1: why only zoom in two estuaries? Instead display C3 and C4 plants area, population density is more important to be visualized.
Fig. 2 - Fig. 6: cannot distinguish that data between dry and wet, standard deviation needed.
Fig. 6: estuarine export fluxes values should be river-borne C, the figure is unnecessary if only two components are evaluated.
For all supplementary figures: there is no spatial information, reason why exclude several estuarine data, the number of observations are too small, standard deviations? Data interpretation seems unconvincing due to potential data manipulation.