Controls on carbon cycling in two contrasting temperate zone estuaries: The Tyne and Tweed, UK
Introduction
Estuaries are generally considered to be net heterotrophic systems and often act as sources of CO2 to the Earth's atmosphere (Smith and Hollibaugh, 1993, Frankignoulle et al., 1998). In Europe alone, estimates of annual estuarine CO2 emissions range between 30 and 60 million tons of carbon, which corresponds to 5–10% of the anthropogenic emissions from Western Europe (Frankignoulle et al., 1998). The magnitude of this flux depends mainly on the balance between uptake of photosynthetic CO2 and its release via respiration. In turn, the relative intensity of these two processes depends on a wide variety of factors, including enhanced allochthonous input of labile organic matter, water residence time, sunlight availability, rates of community metabolism, temperature and nutrient load (Smith and Hollibaugh, 1993, Howland et al., 2000, Wang and Veizer, 2000, Abril et al., 2002, Ram et al., 2003). In addition, several studies have shown that seasonal variations can lead to estuaries shifting from net autotrophic to net heterotrophic functioning (Kemp et al., 1992, Smith and Hollibaugh, 1997, Howland et al., 2000, Ram et al., 2003).
Understanding the relative importance of respiration versus photosynthesis within an estuary can therefore provide a valuable insight into carbon cycling dynamics (i.e., production versus consumption) in these highly complex systems. Two parameters that can be used as natural labels to evaluate these processes include isotope ratios of dissolved inorganic carbon (δ13CDIC) and dissolved oxygen (δ18ODO). DIC concentrations in an estuarine environment are mainly controlled by
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the degree of mixing between marine and freshwater,
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atmospheric efflux,
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the relative intensities of photosynthetic and oxidative processes,
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carbonate and/or atmospheric CO2 dissolution, and
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sediment re-suspension of organic matter and its subsequent turnover (Wang and Veizer, 2000, Abril et al., 2003, Abril et al., 2004).
In a system where soil CO2 is primarily derived from decomposition of C3 plant organic matter (δ13C ca. −27‰; Wang et al., 1998), the CO2 produced has the same δ13CDIC value as the initial substrate as the carbon undergoes little or no fractionation during turnover. The partial diffusion of this CO2 gas can then result in a 13C-enrichment of up to +4.4‰ (Cerling et al., 1991). The dissolution of CO2 and its subsequent reaction with the DIC pool at temperatures between 10 and 20 °C and pH between 7.0 and 9.0 results in a fractionation of about +8‰ (Wigley et al., 1978). Therefore, DIC in river water during baseflow conditions typically has δ13C values ranging around −15‰ for systems dominated by silicate weathering. If carbonates are dissolved the δ13CDIC values are further enriched since most marine carbonates generally range around 0‰ (Faure, 1986). δ13CDIC values around 0‰ can also be expected in atmospherically equilibrated waters, since the isotopic composition of atmospheric CO2 is around −8‰ and equilibration with aquatic DIC causes the above-mentioned fractionation of +8‰.
Photosynthesis and respiration can further alter DIC concentrations and its isotopic composition. Photosynthesis causes a 13C-enrichment due to preferential removal of the lighter isotope during DIC (or CO2) uptake whereby respiration causes a negative isotopic shift due to the oxidation of organic matter with a more negative δ13C signature (Mook and Tan, 1991). This is particularly the case when more 13C-depleted organic matter from C3 (the most common plant species in terrestrial environments) is converted to DIC. The above shows that several factors can influence the isotopic composition of the DIC. Therefore, the isotopic composition of dissolved oxygen (δ18ODO) becomes a highly useful tool to isolate the influence of photosynthesis versus respiration.
δ18ODO signatures have been employed in only a few studies to examine respiration–photosynthesis dynamics in aquatic systems (Quay et al., 1995, Wang and Veizer, 2000). The air–water equilibrium value for dissolved O2 (DO) is +24.5‰. It is the result of a +0.7‰ shift during dissolution (Benson and Krause, 1984) from the atmospheric oxygen that has a universal value of +23.8‰ (Coplen et al., 2002). If photosynthesis dominates, DO becomes supersaturated and the resulting δ18ODO decreases to values less than +24.5‰, due to the production of isotopically lighter O2. During aquatic photosynthesis the latter originates from the source water that is always more enriched in 16O compared to the original atmospheric O2 (Wang and Veizer, 2000). On the other hand, respiration leads to DO under-saturation, and the resulting δ18ODO becomes more positive than the air equilibrium value of +24.5‰ due to the preferential removal of 16O from the dissolved oxygen pool (Kiddon et al., 1993, Wang and Veizer, 2000). The isotopic composition of δ18ODO is not influenced by carbonate or silicate weathering and generally moves into the opposite direction (compared to δ13CDIC) when influenced by either photosynthesis or respiration. Since this isotope system also has a distinct value for atmospheric equilibration it provides a powerful tool combined with δ13CDIC to differentiate processes.
In this study we have measured pH, temperature, alkalinity, chlorophyll-a (chl-a), δ13CDIC and δ18ODO in summer low-flow conditions (July 2003) in the Tyne and Tweed estuaries and winter high flow-conditions (December 2003) in the Tweed estuary. The objectives of this study were as follows:
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to outline the dominant processes of carbon cycling, and
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to understand the dominance of secondary photosynthetic and respiration processes that in turn control the emission or uptake of CO2.
This setting is ideal to compare two contrasting estuaries in terms of anthropogenic influence under almost identical climatic conditions. While both rivers drain catchments containing large areas of peat moorland, the Tyne estuary is characterised by a high degree of urbanisation whereas agricultural activity is the dominant anthropogenic influence in the Tweed. To our knowledge it is also the first European study to apply combined stable isotopes of DIC and DO.
Section snippets
Study sites
The River Tyne flows through the city of Newcastle and its suburbs with an approximate population of 1,100,000. It has a total drainage area of ∼2900 km2 and an average freshwater flow of 48 m3 s−1. Its two main tributaries are the North Tyne, which receives humic-rich waters from blanket peat areas, and the South Tyne, which drains relatively pristine moorland. The geology in the upper Tyne basin consists of Carboniferous limestones and the Namurian Millstone Grit Series, which is characterised
Chlorophyll-a
Chl-a concentrations during all surveys in both estuaries were relatively low and ranged from 0.2 to 1.7 μg L−1 (July 2003) and from 2.1 to 4.6 μg L−1 (December 2003) in the Tweed estuary and from 0.7 to 2.8 μg L−1 (July 2003) in the Tyne estuary. Although comparisons in the literature are not available for the Tyne, similarly low chl-a concentrations (<2 μg L−1) were previously reported for the Tweed and were attributed to a rapid flushing, which hampers repeated algal cell division within the tidal
Isotopic mixing curves for DIC
In order to verify whether δ13CDIC signatures obey conservative mixing between marine and riverine sources, a two end-member mass balance equation using both isotopic and concentration data was utilised (Mook and Tan, 1991):where SS, SF and SM and DICS, DICF and DICM refer to the salinities and total DIC of the sample, the freshwater and the marine components, respectively, and δ13CDIC-pred refers to the
Conclusion
Terrigenous organic matter delivered to the world's oceans can undergo significant modification during estuarine transport, even in systems with relatively short freshwater residence times such as the Tyne and Tweed. These conclusions are of relevance for the understanding of estuarine ecosystem dynamics as well as net balance transportation of carbon from continents to oceans. Therefore, new tools that help elucidate seasonal and site-specific variability in net heterotrophy and autotrophy are
Acknowledgements
We thank A. Pike, J. Smith, A. Anestis, S. Mowbray, R. Dwyer and A. Mennim for technical assistance during sample collection and processing and S. Waldron for help with δ13CDIC analyses. We also thank G. Cowie and R. Upstill-Goddard for providing constructive discussions, and the UK Environment Agency and Scottish Environment Protection Agency for providing us with river flow data. This research was funded by the UK Natural Environmental Research Council (NERC) under grants NER/T/S/2000/00186
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Present address: Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA.