Strong sources of CO2 in upper estuaries become sinks of CO2 in large river plumes

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An extensive search of the literature and data banks identified studies of water-to-air CO2 exchange in 106 estuaries. Generally, pCO2 in upper estuaries is highly supersaturated with respect to the atmospheric CO2, and so a large amount of CO2 is released to the atmosphere per unit surface area. Wider mid and lower estuaries are associated with slower river flow and lower turbidity, and therefore greater biological productivity. Further, mixing with low-pCO2 seawater reduces pCO2 and, thereby, the water-to-air CO2 flux on the ocean side. All of the globe's estuaries release 0.26 Pg C/y to the atmosphere. However, nutrients that are provided by large rivers, such as the Amazon and Changjiang (Yangtze), and those entrained by the river plumes promote photosynthesis to such an extent that the water becomes undersaturated. Accordingly, the large river plumes become a CO2 sink even many hundred kilometers beyond the river mouth.

Highlights

► The water-to-air exchange of CO2 from 106 estuaries worldwide was examined. ► Upper estuaries are strong sources of CO2, whose fluxes fall oceanward. ► Water-to-air fluxes are highest in fall and lowest in winter. ► Globally, estuaries release around 0.26 Pg C/y to the atmosphere. ► Large-river plumes are commonly sinks of CO2.

Introduction

Rivers are the major conduits of water, nutrients, minerals and carbon from land to the oceans. Within river basins, atmospheric CO2-saturated rainwater falls on rocks and soils, and CO2 is converted into plant tissue by photosynthesis. Yet, most CO2 enters the terrestrial carbon cycle when rainwater percolates through carbonates and silicates. Particularly in soils, bacterial oxidation decomposes photosynthetically generated organic carbon. Respiration in roots is an equally important source of CO2 in soil. Soil water, groundwater, and runoff slowly find their way into rivers, and carbon is thereafter transported by rivers to the oceans in the form of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), particulate inorganic carbon (PIC) and particulate organic carbon (POC). Denudation and mud slides are another source of ancient carbon in river water [1, 2, 3].

In most aquatic systems, respiration exceeds autochthonous gross primary production, with net heterotrophy sustained by the input of organic carbon from the catchment. Mostly owing to the decomposition of organic matter, the partial pressure of CO2 (pCO2) in soil water and river water is supersaturated with respect to CO2 in the atmosphere. Moreover, remineralization of DOC and POC that are carried by rivers typically makes river and estuary ecosystems highly heterotrophic. Consequently, most rivers, and therefore estuaries, are sources of CO2 to the atmosphere [2, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13••, 14••, 15, 16, 17]. Since most studies of the riverine transport of carbon to oceans have really considered only the amount transported by rivers, and have ignored the amount released from estuaries [18], the amount that actually enters the oceans must be smaller than they have determined.

Although some individual estuaries have been analyzed in detail, general patterns of the global distribution of the water-to-air CO2 fluxes in estuaries are still not well understood, because of a lack of data. Fortunately, a huge number of studies have recently been published around the globe. For example, the summary of Chen and Borges [13••] concerned 32 estuaries, while that of Laruelle et al. [14••] covered 60. The present investigation considers 106 estuaries for which pCO2 data are available in either the literature or various data banks. For the first time, the water-to-air fluxes of CO2 in the upper, mid and lower estuaries worldwide are systematically analyzed.

Following Elliott and McLusky [19] and Jiang et al. [20], this work adopts perhaps the most widely applied definition of an estuary, which was originally offered by Cameron and Pritchard [21], as ‘a semi-enclosed coastal body of water, which has a free connection with the open sea and within which seawater is measurably diluted with freshwater derived from land drainage’. However, large bodies of semi-enclosed coastal seas such as the Baltic or the Bohai, are not regarded as estuaries. Upper/mid/lower estuaries are operationally defined as those areas of estuaries with salinities below 2, between 2 and 25, and above 25, respectively, whenever salinity data are available. Otherwise, divisions are artificially made based approximately on one-thirds of the distance from the point where the river starts to widen to the river mouth.

Section snippets

General patterns

Although relationships between pCO2 and river hydrography, changes in land use, fertilizer and waste water discharges, atmospheric deposition, water-to-air CO2 exchange, the turbidity and biological productivity of estuarine water and tidal motions, or the interaction between the water and the bottom sediments is not always evident, some general patterns of the saturation state of CO2 in estuarine waters seem to exist. The narrowness of upper estuaries causes the flow rate to be high, such that

Large river plumes

Major rivers provide a disproportionately important link between terrestrial and marine materials: the world's 10 largest rivers transport around 40% of the fresh water and the particulate materials that enter the oceans. Furthermore, around 40% of the organic matter in the global oceans is buried in the deltas of large rivers [40]. A case in point is the Changjiang (Yangtze) River, which is the longest and largest river in China in terms of discharge. The Changjiang River is the third longest

Geographical variations

Unlike continental shelves in temperate (23.5–50°) and high-latitude (>50°) regions where seas typically act as sinks for atmospheric CO2 [13••], the estuaries in these regions mostly release CO2 to the atmosphere (Figure 2). This contrast reflects the fact that open oceanic waters are generally undersaturated in CO2 in temperate and high-latitude regions [49], and, the continental shelves are highly productive, promoting the open-ocean CO2 undersaturation. Incoming river water to the estuaries

Seasonal variations

Although relevant data vary greatly, most continental shelves are generally sinks for CO2 during most of the year, with the possible exception of summer (June–August in the northern hemisphere) because the warming of seawater thermodynamically increases pCO2 [13••]. Most data reveal that, generally, the water in estuaries releases CO2 in all seasons although the flux seems to be highest in autumn (September–November; 73.2 ± 93.4 mmol C/m2/d) and lowest (53.4 ± 65.1 mmol C/m2/d) in winter

Conclusions

Rivers are the major conduits for transporting terrestrial material toward the oceans, but not all transported material reaches the oceans. Upper estuaries are found usually to be highly supersaturated in terms of pCO2, while mid estuaries are less so and lower estuaries are the least supersaturated. They together release some of the carbon that is transported by the rivers to the atmosphere. However, large river plumes are often found to be sinks of CO2 because of a high rate of photosynthesis.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

This research was partially supported by the National Science Council of Taiwan (NSC 98-2621-M-110-001-MY3; 99-2611-M-110-002-MY3) and Aim for the Top University Plan. The manuscript was completed while C.T.A. Chen was on leave at the Second Institute of Oceanography, Hangzhou. Y.C. Chang of IMGC, NSYSU helped with preparing the manuscript. W.J. Cai, A. Borges and two anonymous reviewers provided valuable comments which helped strengthening the manuscript.

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