Peter A. Raymond

Global carbon dioxide emissions from inland waters

Carbon dioxide (CO2) transfer from inland waters to the atmosphere, known as CO2 evasion, is a component of the global carbon cycle. Global estimates of CO2 evasion have been hampered, however, by the lack of a framework for estimating the inland water surface area and gas transfer velocity and by the absence of a global CO2 database. Here we report regional variations in global inland water surface area, dissolved CO2 and gas transfer velocity. We obtain global CO2 evasion rates of 1.8  petagrams of carbon (Pg C) per year from streams and rivers and 0.32  Pg C yr−1 from lakes and reservoirs, where the upper and lower limits are respectively the 5th and 95th confidence interval percentiles. The resulting global evasion rate of 2.1 Pg C yr−1 is higher than previous estimates owing to a larger stream and river evasion rate. Our analysis predicts global hotspots in stream and river evasion, with about 70 per cent of the flux occurring over just 20 per cent of the land surface. The source of inland water CO2 is still not known with certainty and new studies are needed to research the mechanisms controlling CO2 evasion globally.

Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere

Streams, rivers, lakes, and other inland waters are important agents in the coupling of biogeochemical cycles between continents, atmosphere, and oceans. The depiction of these roles in global-scale assessments of carbon (C) and other bioactive elements remains limited, yet recent findings suggest that C discharged to the oceans is only a fraction of that entering rivers from terrestrial ecosystems via soil respiration, leaching, chemical weathering, and physical erosion. Most of this C influx is returned to the atmosphere from inland waters as carbon dioxide (CO2) or buried in sedimentary deposits within impoundments, lakes, floodplains, and other wetlands. Carbon and mineral cycles are coupled by both erosion–deposition processes and chemical weathering, with the latter producing dissolved inorganic C and carbonate buffering capacity that strongly modulate downstream pH, biological production of calcium-carbonate shells, and CO2 outgassing in rivers, estuaries, and coastal zones. Human activities substantially affect all of these processes.

Riverine export of aged terrestrial organic matter to the North Atlantic Ocean.

Global riverine discharge of organic matter represents a substantial source of terrestrial dissolved and particulate organic carbon to the oceans. This input from rivers is, by itself, more than large enough to account for the apparent steady-state replacement times of 4,00–6,000 yr for oceanic dissolved organic carbon. But paradoxically, terrestrial organic matter, derived from land plants, is not detected in seawater and sediments in quantities that correspond to its inputs. Here we present natural 14C and 13C data from four rivers that discharge to the western North Atlantic Ocean and find that these rivers are sources of old (14C-depleted) and young (14C-enriched) terrestrial dissolved organic carbon, and of predominantly old terrestrial particulate organic carbon. These findings contrast with limited earlier data that suggested terrestrial organic matter transported by rivers might be generally enriched in 14C from nuclear testing, and hence newly produced. We also find that much of the young dissolved organic carbon can be selectively degraded over the residence times of river and coastal waters, leaving an even older and more refractory component for oceanic export. Thus, pre-ageing and degradation may alter significantly the structure, distributions and quantities of terrestrial organic matter before its delivery to the oceans.

The changing carbon cycle of the coastal ocean

The carbon cycle of the coastal ocean is a dynamic component of the global carbon budget. But the diverse sources and sinks of carbon and their complex interactions in these waters remain poorly understood. Here we discuss the sources, exchanges and fates of carbon in the coastal ocean and how anthropogenic activities have altered the carbon cycle. Recent evidence suggests that the coastal ocean may have become a net sink for atmospheric carbon dioxide during post-industrial times. Continued human pressures in coastal zones will probably have an important impact on the future evolution of the coastal oceans carbon budget.

Gas Exchange in Rivers and Estuaries: Choosing a Gas Transfer Velocity

We are writing this comment to call attention to large uncertainties in the estimates of CO2 flux from rivers and estuaries, a topic that has been receiving considerable attention recently (Raymond et al. 1997, 2000; Cai and Wang 1998; Frankignoulle et al. 1998). It is our view that there are too few direct measurements of the physical component of gas exchange (e.g., the gas transfer velocity, piston velocity, gas exchange coefficient, or k) for rivers and estuaries. While studies in streams, lakes, and marine systems have progressed to the point where the gas transfer velocity can be partially predicted from physical forcing functions (O’Connor and Dobbins 1958; Cole and Caraco 1998; Wanninkhof and McGillis 1999), this is not yet the case for estuaries and rivers. A comparison of gas transfer velocity measurements in estuaries and rivers reveals a general lack of agreement among studies and physically-based predictive models. Until we have a better understanding of the magnitude and causes of variation in estuarine gas transfer velocity estimates, it will be difficult to use gas exchange in rivers or estuaries to accurately mass-balance gases of interest. The exchange of CO2 between an aquatic ecosystem and the overlying atmosphere is an area of intense interest for several reasons: aquatic systems can be significant CO2 sources or sinks on a global or regional scale (Kling et al. 1991; Quay et al. 1992; Sarmiento and Sundquist 1992; Cole et al. 1994); the magnitude and direction of an ecosystems CO2 flux can provide important clues about metabolism in a given system (Depetris and Kempe 1993; Gattuso et al. 1993; Hamilton et al. 1995;

ZEBRA MUSSEL INVASION IN A LARGE, TURBID RIVER: PHYTOPLANKTON RESPONSE TO INCREASED GRAZING

Changes in the biomass of benthic bivalves can cause dramatic changes in total grazing pressure in aquatic systems, but few studies document ecosystem-level impacts of these changes. This study documents a massive decline in phytoplankton biomass con- current with the invasion of an exotic benthic bivalve, the zebra mussel ( Dreissena poly- morpha), and demonstrates that the zebra mussel actually caused this decline. In the fall of 1992 the zebra mussel became established at high biomass in the Hudson River Estuary, and biomass of mussels remained high during 1993 and 1994. During these 2 yr, grazing pressure on phytoplankton was over 10-fold greater than it had been prior to the zebra mussel invasion. This increased grazing was associated with an 85% decline in phyto- plankton biomass. Between 1987 and 1991 (pre-invasion), summertime chlorophyll aver- aged 30 mg/m 3 ; during 1993 and 1994 summertime concentrations were ,5 mg/m 3 . Over this same period, light availability increased, phosphate concentrations doubled, some planktonic grazers declined, and average flow was not different from the pre-invasion period. Thus, changes in these other factors were not responsible for phytoplankton declines. We developed a mechanistic model that reproduces the spatial and temporal dynamics of phytoplankton prior to the invasion of the zebra mussel (1987-1991). The model ac- curately predicts extreme declines in phytoplankton biomass after the invasion (1993-1994). The model demonstrates that zebra mussel grazing was sufficient to cause the observed phytoplankton decline. The model also allows us to test which features make the Hudson River sensitive to the impact of benthic grazers. The model suggests that the fate of light- scattering inorganic particles (turbidity) is a key feature determining the impact of benthic grazers in aquatic systems.

Anthropogenically enhanced fluxes of water and carbon from the Mississippi River.

The water and dissolved inorganic carbon exported by rivers are important net fluxes that connect terrestrial and oceanic water and carbon reservoirs. For most rivers, the majority of dissolved inorganic carbon is in the form of bicarbonate. The riverine bicarbonate flux originates mainly from the dissolution of rock minerals by soil water carbon dioxide, a process called chemical weathering, which controls the buffering capacity and mineral content of receiving streams and rivers. Here we introduce an unprecedented high-temporal-resolution, 100-year data set from the Mississippi River and couple it with sub-watershed and precipitation data to reveal that the large increase in bicarbonate flux that has occurred over the past 50 years (ref. 3) is clearly anthropogenically driven. We show that the increase in bicarbonate and water fluxes is caused mainly by an increase in discharge from agricultural watersheds that has not been balanced by a rise in precipitation, which is also relevant to nutrient and pesticide fluxes to the Gulf of Mexico. These findings demonstrate that alterations in chemical weathering are relevant to improving contemporary biogeochemical budgets. Furthermore, land use change and management were arguably more important than changes in climate and plant CO2 fertilization to increases in riverine water and carbon export from this large region over the past 50 years.

Use of 14C and 13C natural abundances for evaluating riverine, estuarine, and coastal DOC and POC sources and cycling : a review and synthesis

Though not typically regarded as “biomarkers” in the traditional sense of the word, the radioactive and stable isotopes of carbon (14C and 13C, respectively) can serve as powerful tools for identifying sources and estimating turnover times of organic matter in aquatic systems. Paired 14C and 13C measurements of carbon pools can provide an additional degree of specificity for studies of organic matter cycling as a result of: (1) the lower susceptibility of natural isotopes to diagenetic effects that can alter organic biomolecules; (2) the “dual” isotopic nature of the approach; (3) the unique input functions for each isotope; and (4) the greater dynamic range in Δ14C (−1000 to ∼+200‰) compared to δ13C (∼−32 to −12‰). Relatively few geochemical studies in rivers, estuaries and the coastal ocean waters have employed 14C and 13C analyses of organic matter. In this paper we summarize the available data on 14C and 13C measurements in dissolved and particulate organic carbon (DOC and POC, respectively) in these systems. A brief review is presented of current methods for the separation and oxidation of DOC and POC from water samples, for subsequent Δ14C and δ13C analyses. We also compile the existing datasets on paired 14C and 13C measurements across the riverine to coastal marine continuum in order to elucidate sources, ages, and transformations of organic matter within each system, and during transport from rivers to the coastal ocean. The natural range in the Δ14C values of both DOC and POC across similar system types was 500 and 1000‰, respectively. In general, riverine DOC was enriched in 14C relative to POC in rivers and estuaries, but the opposite generally held for coastal marine waters. This is indicative of the different sources and transport mechanisms for DOC and POC within and across these three general types of systems. During river and estuarine transport, DOC generally becomes enriched in 13C and depleted in 14C due to simultaneous additions from autochthonous production and removals from heterotrophic bacteria and abiotic processes. Bacterial utilization experiments indicate that bacteria preferentially utilize a 14C enriched (i.e. young) DOC fraction and, therefore, DOC utilization is a partial explanation for the 14C-depeleted riverine and estuarine DOC. It is concluded that through the use of paired 14C and 13C measurements in DOC and POC, a more robust interpretation of sources, sinks, and residence times of organic matter may be attained than by using either isotope separately.

Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers

Scaling is an integral component of ecology and earth science. To date, the ability to determine the importance of air – water gas exchange across large spatial scales is hampered partly by our ability to scale the gas transfer velocity and stream hydraulics. Here we report on a metadata analysis of 563 direct gas tracer release experiments that examines scaling laws for the gas transfer velocity. We found that the gas transfer velocity scales with the product of stream slope and velocity, which is in alignment with theory on stream energy dissipation. In addition to providing equations that predict the gas transfer velocity based on stream hydraulics, we used our hydraulic data set to report a new set of hydraulic exponents and coefficients that allow the prediction of stream width, depth, and velocity based on discharge. Finally, we report a new table of gas Schmidt number dependencies to allow researchers to estimate a gas transfer velocity using our equation for many gasses of interest.

Carbon dioxide concentration and atmospheric flux in the Hudson River

We made direct measurements of the partial pressure of CO2 (PCO2) in the tidal-freshwater portion of the Hudson River Estuary over a 3.5-yr period. At all times the Hudson was supersaturated in CO2 with respect to the atmosphere. PCO2 in surface water averaged 1125±403 (SD) μatm while the atmosphere averaged 416±68 μatm. Weekly samples at a single, mid-river station showed a pronounced and reproducible seasonal cycle with highest values (∼2000 μatm) in mid-to-late summer, and lowest values (∼500 μatm) generally in late winter. Samples taken along the length of the 190-km section of river showed a general decline in CO2 from north to south. This decline was most pronounced in summer and very slight in spring. Diel and vertical variation were small relative to the standing stock of CO2. Over six diel cycles, all taken during the algal growing season, the mean range was 300±114 μatm. CO2 tended to increase slightly with depth, but the gradient was small, about 0.5 μmol m−1, or an increase of 190 μatm from top to within 1 m of the bottom. For a large subset of the samples (n=452) we also calculated CO2 from measurements of pH and total DIC. Calculated and measured values of CO2 were in reasonably good agreement and a regression of calculated versus measured values had a slope of 0.85±0.04 and an r2 of 0.60. Combining our measurements with recent experimental studies of gas exchange in the Hudson, we estimate that the Hudson releases CO2 at a rate of 70–162 g C m−2 yr−1 from the river to the atmosphere.