Frontiers in Marine Science (Jul 2014)
Carbon budgets for two Portuguese estuaries: implications for the management and conservation of coastal waters
Abstract
Abstract: Carbon budgets vary markedly within and between Tagus and Sado estuaries reflecting the human pressure. These estuaries seem to generate carbon within a total of 0.13 MtC yr-1 for Tagus and 0.02 MtC yr-1 for Sado. Anthropogenic inputs, autochthonous carbon production and primary production are indicated as the main responsible for the carbon production within the estuaries. Carbon exported to the ocean and to the atmosphere by both estuarine systems is higher during the non-productive period. Emissions for the atmosphere represent, in terms of cost, about €375,000 per year. This study is important for an efficient protection and preservation of Tagus and Sado estuarine ecosystems and is a contribution to the integration of coastal and global carbon cycles. 1. INTRODUCTION The coastal environment presents high levels of complexity, diverse habitats and sustains a high level of biodiversity. These provide goods and services that support different uses which should be undertaken in a sustainable way. However, these coastal environments are facing increasing and significant impacts, driven by economic and social pressures. In order to resolve these problems, policy-makers world-wide seek to develop strategies to protect, conserve and manage the coastal environment. To accomplish that is imperative a scientific knowledge of such environments, particularly their role in the carbon and nutrients cycles. Human activities associated with coastal environments, such as land-use change, bulkhead construction, wastewater discharge, wetland removal and dredging, are increasingly important factors that affect carbon sources, cycling and budgets. Climate warming and hydrologic “events” have also resulted in changes to continental shelf carbon cycling (Maher and Eyre, 2012; Bauer et al., 2013). Although future climate change is predicted to lead to an increase in river carbon fluxes, it is also likely to lead to increased uncertainties in predicting these fluxes. Therefore, in management and conservation of coastal waters, the detection and assessment of carbon budgets is an important environmental issue since they can optimize interventions by state agencies and local communities. For a detailed carbon budget calculation it is essential to quantify the air-sea exchange of carbon and also the fluxes of carbon across the land/ocean boundaries. An example of such land/ocean boundaries are estuaries, which are effective filters for terrestrial/riverine organic inputs and impose a by-pass of carbon towards the atmosphere for the global carbon cycle. Furthermore, recent research (Chen and Borges, 2009; Oliveira et al., 2012) points out that estuaries influence strongly how global coastal ocean acts as a source or sink for atmospheric CO2. In order to characterise the role of two of the greatest Portuguese estuaries (Tagus and Sado) in terms of carbon impacts on the ocean, carbon budgets, integrating simultaneously both inorganic and organic carbon components and CO2 air-water fluxes, were estimated over an annual cycle. 2. ESTUARIES The Portuguese Tagus and Sado estuaries (Figure 1) are important ecological systems subjected to considerable urban, industrial and agricultural pressures and considered to be moderately productive. These anthropogenic pressures, expressed as chemical oxygen demand (COD), reach values of 131,743 t yr-1 and 30,385 t yr-1, respectively for Tagus and Sado river basins (APA, 2014). Figure 1. Map of Portugal showing Tagus and Sado estuaries. The two well mixed mesotidal estuaries were selected as they are close geographically, vary in freshwater inputs, span a range of river to marine dominated systems and are geomorphically distinct (Table 1). 3. METHODOLOGY 3.1. Data Carbon budgets are based predominantly on direct measured of carbon flows. Carbon seasonal measurements were conducted from 1999 to 2004 on Tagus and Sado estuaries. Additionally, physical, chemical and biological parameters were also determined, generating a multiparameter data base contributing to a better knowledge of water quality changes in the systems and therefore respond to the Water Framework Directive. 3.2. Carbon Analytical Procedures Particulate organic (POC) and inorganic (PIC) carbon determinations were made using a CHN Fissons NA 1500 Analyser. The calibration standard used was acetanilide. Dissolved organic carbon (DOC) analysis were performed by high temperature catalytic oxidation (HTCO), using a commercial Shimadzu TOC-5000A analyser. Samples were acidified to pH3PO4 acid and immediately frozen until analyses. Dissolved inorganic carbon (DIC) and the CO2 partial pressure (pCO2) in seawater were calculated from in situ temperature, total alkalinity (TA) and corrected pH using the carbonic acid dissociation constants given by Millero et al. (2006) and the CO2 solubility coefficient of Weiss (1974). Errors associated with DIC and pCO2 calculations were estimated to be, respectively, ±6 µmol kg-1 and ±10 µatm (accumulated errors on TA and pH). The air-water CO2 fluxes were computed according to the equation CO2 Flux = kspCO2, where k is the gas transfer velocity (given by Wanninkhof, 1992), s is the solubility coefficient of CO2 and pCO2 is the air-water gradient of pCO2. 4. RESULTS Results revealed significant differences between both estuaries in terms of carbon inputs and outputs. Carbon fluxes are always higher in Tagus than in Sado. Carbon exported to the ocean by both estuarine systems is higher during the non-productive period, with values reaching 2258 tC d-1 for Tagus and 41 tC d-1 for Sado. Emissions for the atmosphere, are also higher during the non-productive period, attaining values of 220 tC d-1 and 60 tC d-1, respectively for Tagus and Sado estuaries. Pricing these emissions at the current tax (May 7, 2014), they represent in terms of cost about € 375,000. Extrapolating for an annual basis, Tagus exports a total of 0.52 MtC yr-1, of which 90% goes to the ocean and 10% is emitted to the atmosphere, while Sado estuary exports 0.04 MtC yr-1, of which 40% is exported to the ocean and 60% is emitted to the atmosphere (Figure 2). Carbon balance indicates that 0.13 MtC yr-1 for Tagus and 0.02 MtC yr-1 for Sado (Figure 2) are generated inside the system suggesting other sources of carbon, besides the river inputs. Figure 2. Annual carbon inputs and outputs in Tagus and Sado estuaries. Values in MtC yr-1. According to APA (2014), and converting COD to TOC, the total anthropogenic loading for Tagus and Sado river basins is of 0.04 MtC yr-1 and 0.01 MtC yr-1, respectively. Therefore, in both estuaries anthropogenic sources inputs of organic carbon are extremely high, although this fraction comprises only 15% and 8% of the total carbon pool, respectively for Tagus and Sado estuaries. The major contribution of carbon is both estuaries is in the inorganic form. Considering the population within these two river basins, it can be infrared that every year each inhabitant roughly releases 12.6 and 29.3 kg of carbon to Tagus and Sado superficial waters, respectively. 5. CONCLUSIONS The results presented illustrate that Tagus and Sado estuaries represent an important land/ocean boundary for carbon transformation and emission, and confirm the anthropogenic pressure that these estuaries are subject to. Carbon budgets vary markedly within and between these two estuaries reflecting the human pressure. Anthropogenic inputs, autochthonous carbon production and primary production are indicated as the main responsible for the carbon production within the estuaries. Both estuaries export carbon to the ocean and to the atmosphere. The inorganic carbon faction has a major role in the carbon budget, enriching the ocean in carbon dioxide, contributing this for the greenhouse effect. Our understanding of organic and inorganic carbon fluxes in Tagus and Sado estuaries is vital for an efficient protection and preservation of such ecosystems being helpful in limit human-caused damage and in restoring damaged estuarine/coastal ecosystems. In addition, the economic impact of the carbon fluxes to the atmosphere, estimated as €375,000 per year, creates the appropriate incentives to reduce emissions and shift them to higher-value uses. Suggesting, therefore, a coastal management re-oriented towards a more adaptive approach through the use of carbon market-based policies. This study is a contribution to the integration of coastal and global carbon cycles. However, additional efforts are required to fully merge other components subsystems, such as salt marshes, with these budgets. Moreover, a fully comprehension of the community metabolism in these estuaries will greatly improve this integration.
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