Carbon in catchments: connecting terrestrial carbon losses with aquatic metabolism

For a majority of aquatic ecosystems, respiration ( R ) exceeds autochthonous gross primary production (GPP). These systems have negative net ecosystem production ([NEP] = [GPP] – R ) and ratios of [GPP]/ R of <1. This net heterotrophy can be sustained only if aquatic respiration is subsidized by organic inputs from the catchment. Such subsidies imply that organic materials that escaped decomposition in the terrestrial environment must become susceptible to decomposition in the linked aquatic environment. Using a moderate-sized catchment in North America, the Hudson River (catchment area 33 500 km 2 ), evidence is presented for the magnitude of net heterotrophy. All approaches (CO 2 gas flux; O 2 gas flux; budget and gradient of dissolved organic C; and the summed components of primary production and respiration within the ecosystem) indicate that system respiration exceeds gross primary production by ~200 g C m –2 year –1 . Highly 14 C-depleted C of ancient terrestrial origin (1000–5000 years old) may be an important source of labile organic matter to this riverine system and support this excess respiration. The mechanisms by which organic matter is preserved for centuries to millennia in terrestrial soils and decomposed in a matter of weeks in a river connect modern riverine metabolism to historical terrestrial conditions. Extra keywords : river, watershed, p CO 2 Introduction In most terrestrial ecosystems the amount of organic C produced by photosynthesis (Gross Primary Production; GPP) is largely consumed by the combined respiration ( R ) of the plants themselves and heterotrophic consumer organisms. The difference between GPP and R , or net ecosystem production ([NEP] = [GPP] – R) is small compared with either GPP or R . There are only three fates for this terrestrial NEP: long term storage, burning in fires, and export. The export of organic C from terrestrial ecosystems is an import of organic C to aquatic ecosystems. This export can be a substantial fate for terrestrial NEP compared with burial. For example, in the small (15–20 ha) catchments at the Hubbard Brook Experimental Forest (Likens et al . 1977), hydrologic export of organic C and burial in forest soils are co-equal (Fig. 1). These catchments are relatively young, ~12 000 years old, and are still storing some organic C in soils (Likens et al. 1977). It is conceivable that older catchments are closer to steady state with respect to organic C storage; in these cases, export would be the dominant fate of NEP. Since export is a large term for terrestrial NEP, it is reasonable to ask what is the fate of the exported terrestrial organic matter. The amount of organic C that is buried on the continental shelves, one of the largest depositional environments on the planet, is about equal to the amount of terrestrial particulate organic C (POC) transported by rivers (Mulholland and Watts 1982; Berner and Berner 1987; Hedges et al. 1997). The amount of dissolved organic carbon (DOC) transported by rivers is large enough to support turnover of the marine DOC pool (Williams and Druffel 1987). It is tempting to equate riverine delivery with sedimentary burial or DOC accumulation in the ocean but, intriguingly, new chemical and isotopic evidence suggests that very little of this terrestrial C actually accumulates in Fig. 1. Loss and net accumulation of organic C for an undisturbed watershed at the Hubbard Brook Experimental Forest in New Hampshire, USA. Data from Gosz et al. (1978), Likens et al. (1977), Bormann and Likens (1979) and McDowell and Likens (1988). DOC is not the only form of organic matter lost from the forest; thus, the loss term is an underestimate of true loss. 102 Jonathan J. Cole and Nina F. Caraco the ocean (Prahl et al . 1994; Hedges et al. 1997; Opsahl and Benner 1997; Druffel et al. 1998). The inference is that it must decompose on the continental margins, in river deltas or perhaps in the lower reaches of rivers themselves. The decomposition of terrestrial organic C in rivers is the subject of this review. The hydrologic export of organic C from terrestrial systems represents an import into the receiving aquatic system. This input of allochthonous organic C can be quite large in comparison to autochthonous primary production within the aquatic system itself. The import can be visualized as the product of water load and the concentration of organic C in the water. Water load is the volume of water (m 3 ) per unit area of the receiving aquatic system (m 2 ) per year. Since the terrestrial catchment is generally much larger than the aquatic receiving system, the water load (m year –1 ) often greatly exceeds the precipitation input of water, especially for rivers, and especially in non-arid regions. For example, consider conditions representative of the north-eastern USA and Canada. At representative riverine DOC concentrations of 830 m M (10 mg C L –1 ) and a water load of 100 m year –1 (precipitation – evapotranspiration = 0.5 m year –1 ; catchment area 200 times larger than the area of the river), the input of terrestrial DOC is 1000 g C m –2 year –1 , which is considerably larger than primary production in all but the most productive riverine environments. The input of terrestrial POC would make this import term larger still. Clearly, terrestrial POC and DOC enter riverine systems, and this input is large in comparison to autochthonous primary production, and this allochthonous organic matter may simply flow through the system without being metabolized. If some of it were metabolized we would expect to see an effect on the fluxes of CO 2 or O 2 into or out of the river. Obviously, the net effect would depend on the magnitudes of autochthonous GPP, the fraction of GPP that was respired v. exported, and the amount of terrestrial organic matter that was oxidized. For example, suppose that the allochthonous load was 1000 g C m –2 year –1 (83 mol C m –2 year –1 ) and 25% of this were respired in the river. This riverine respiration of terrestrial organic C generates a CO 2 source, or O 2 sink, of ~57 mmol m –2 day –1 . At zero autochthonous GPP this source would be seen as a net source of CO 2 of this magnitude. As riverine GPP increases, this net heterotrophy decreases. Similarly, when the fraction of autochthonous GPP that is respired approaches 100%, net heterotrophy approaches the full 57 mmol m –2 day –1 (Fig. 2). The net flux could be manifested either as gas exchange with the atmosphere or as the export of water which was elevated (compared with water inputs) in dissolved inorganic C (DIC) or depleted in oxygen. The net gas exchange of CO 2 or O 2 between the river and the atmosphere is, thus, a minimal estimate of net heterotrophy and an underestimate of the amount of terrestrial organic matter respired in the river. This paper demonstrates that a majority of large rivers for which we can obtain data are net sources of CO 2 to the atmosphere, consistent with the idea that they are net heterotrophic. It considers in detail the C budget of one river, the Hudson River, (USA) and suggests that terrestrial organic matter that was produced 1000–5000 years ago fuels a substantial portion of metabolism in the Hudson.