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We continue to improve our understanding of the carbon cycle.
Microorganisms in rivers and streams play a crucial role in the global carbon cycle that has not previously been considered. . .This would also have implication for the nitrogen cycle and other nutrients too since the mineralization of organic matter by bacteria etc. frees those nutrients. I wonder how this would affect current thinking about the sources of such nutrients which can have effects on coastal waters where they accumulate?Microorganisms such as bacteria and single celled algae in rivers and streams decompose organic matter as it flows downstream. They convert the carbon it contains into carbon dioxide, which is then released to the atmosphere. . .
Recent estimates by Battin's team and others conclude there is a net flux, or outgassing, of carbon dioxide from the world's rivers and streams to the atmosphere of at least two-thirds to three-quarters of a gigatonne (Gt) of carbon per year. This flux has not been taken into account in the models of the global carbon cycle used to predict climate change. . ." they are missing from all global carbon cycling, even from the IPCC (Intergovernmental Panel on Climate Change) reports. Rivers are just considered as inert pipelines, receiving organic carbon from Earth and transporting it to the ocean." . . .
He argues that the latest estimates of how much carbon is transferred to the atmosphere from rivers and streams are very conservative. "The actual outgassing of carbon dioxide is probably closer to 2 Gt of carbon per year," says Battin. "Our surface area estimates only consider larger streams and rivers, because it is very hard to estimate accurately the surface area of small streams. So small streams are excluded, although in terms of microbial activity, they are the most reactive in the network."
Two gigatonnes of carbon per year is close to half the estimated net primary production of the world's vegetation each year. Realising that this quantity of carbon may be delivered straight back to the atmosphere, rather than being taken to the ocean where some of it is removed by marine organisms and ends up in sediment, could have profound consequences for our understanding of the system.
FWIW, here's the original research:
http://www.nature.com/ngeo/journal/v1/n2/full/ngeo101.html
Abstract and intro:
Nature Geoscience 1, 95 - 100 (2008)
Published online: 20 January 2008 | doi:10.1038/ngeo101
Subject Categories: Biogeochemistry | Hydrology, hydrogeology and limnology
Biophysical controls on organic carbon fluxes in fluvial networks
Tom J. Battin1,2, Louis A. Kaplan3, Stuart Findlay4, Charles S. Hopkinson5, Eugenia Marti6, Aaron I. Packman7, J. Denis Newbold3 & Francesc Sabater8
Abstract
Metabolism of terrestrial organic carbon in freshwater ecosystems is responsible for a large amount of carbon dioxide outgassing to the atmosphere, in contradiction to the conventional wisdom that terrestrial organic carbon is recalcitrant and contributes little to the support of aquatic metabolism. Here, we combine recent findings from geophysics, microbial ecology and organic geochemistry to show geophysical opportunity and microbial capacity to enhance the net heterotrophy in streams, rivers and estuaries. We identify hydrological storage and retention zones that extend the residence time of organic carbon during downstream transport as geophysical opportunities for microorganisms to develop as attached biofilms or suspended aggregates, and to metabolize organic carbon for energy and growth. We consider fluvial networks as meta-ecosystems to include the acclimation of microbial communities in downstream ecosystems that enable them to exploit energy that escapes from upstream ecosystems, thereby increasing the overall energy utilization at the network level.
Introduction
Fluvial networks link multiple components of the landscape, including soils and groundwater, with the atmosphere and the oceans. Each year streams and rivers of the world transport, transform or store nearly 2 Pg of terrestrial organic carbon (Box 1), a quantity that represents a large fraction of the global annual terrestrial net ecosystem production1, 2, 3, 4 (see Box 2 for a glossary of terms). Most of the respired carbon originates from terrestrial vegetation and is initially stored in soils where turnover times can range from years to centuries5. This has led to the conventional wisdom that fluvial carbon is processed and refractory. Recent estimates of CO2 outgassing from streams and rivers3, 4, 6, 7, 8 contradict this perception, and suggest that land-derived organic carbon is an important integrator of terrestrial and aquatic ecosystem processes that fuel the net heterotrophy of fluvial ecosystems. This raises the question: how can organic carbon be oxidized during its route from continents to oceans given transit times in fluvial ecosystems of days to weeks relative to extended residence times in soils?
To address this question we have developed a conceptual model that integrates recent progress from geosciences9, microbial ecology10 and biogeochemistry7, 8, 11, 12, 13, 14, 15, 16 and describes how microorganisms adapt to the structure and dynamics of the geophysical world. The ensuing microbial processes exert control over net heterotrophy in and CO2 outgassing from fluvial networks, and ultimately influence global carbon fluxes. Our core concept is that the efficiency with which streams, rivers and estuaries retain and oxidize organic carbon rests on the evolution of microbial physiological capacities in response to geophysical opportunities that involve extending the residence time of organic molecules in transport. Of course, direct contact between microbes and their substrates is essential, so geophysical opportunities involve transport rates, and microbial capacities involve reaction rates. Transport rates primarily reflect physical or geophysical constraints (for example, geomorphology, hydrologic connectivity), whereas reaction rates reflect the evolution of microbial pathways (for example, metabolic diversity). Further, because our focus is on microbial metabolism of dissolved organic carbon (DOC) (Box 1) from headwater streams to estuaries, our concept also draws on advances in fluvial network theory, DOC molecular-level chemistry and microbial biogeography.