A series of laboratory experiments was performed to measure dissolved organic carbon (DOC) production during herbivorous grazing by heterotrophic protists (ciliate Strombidinopsis acuminatum, dinoflagellate Oxyrrhis marina) and copepods (Calanus pacificus). DOC production by phytoplankton was 31~0 measured. Experiments were performed in artificial seawater to provide a low DOC background against which changes in DOC concentration could be measured directly. We found that DOC production during grazing was high, i.e. 16-37% of algal C content was released as DOC during an ingestion event. Bacterial growth rates were stimulated by grazer activity, most likely due to increased availability of labile DOC; breakage of fecal pellets by copepods may also have yielded DOC. In contrast, DOC production by phytoplankton was low, ranging from 3 to 7% of algal C content per day. Generalizing from these rates, a simple budget shows that grazer DOC production should ba: 4-6 times greater than phytoplankton DOC production in any region of the ocean where grazing is the dominant phytoplankton loss process. Both phytoplankton and grazer species influenced the carbohydrate composition of the DOC produced. Dissolved carbohydrates averaged 30 and 22% of total DOC in phytoplankton-only and grazer-containing treatments, respectively, and most variability in carbohydrate content was due to variations in polysaccharide levels. We conclude that planktonic grazers are potentially a major source of DOC in the marine envronment.Organic carbon molecules dissolved in seawater constitute one of the largest and most enigmatic carbon reservoirs on earth. The composition of seawater dissolved organic carbon (DOC) is poorly understood, because the bulk has not been characterized at the molecular level. Some 7-15% of the total DOC (depending on depth) is composed of amino acids and carbohydrates, with a minor lipid component (Williams and Druffel 1988). Recent improvements in analytical technique have led researchers to suggest a larger contribution from carbohydrates (17-34% of total DOC, predominantly polysaccharides) than was previously thought Pakulski and Benner 1992).Phytoplankton are often considered to supply a high percentage of marine DOC by direct exudation. Experimental evidence for phytoplankton DOC release is equivocal, however, and has been the subject of intense debate. A few phytoplankton species (e.g. Phaeocystis pouchetti, Chaetoceros Acknowledgments

The importance of terrestrial-derived organic matter for lake zooplankton communities remains debated, partly because little is known about the basic pathways by which allochthonous carbon is transferred to zooplankton, and whether these vary among the major taxonomic and functional groups. We quantified allochthony of three zooplankton groups (Cladocera, Calanoida, and Cyclopoida) across 18 lakes in Quebec, spanning broad gradients of dissolved organic matter (DOM) and lake trophy, using a multi-isotope (delta2H + delta13C), multi-source (terrestrial, phytoplanktonic, benthic) approach. All three zooplankton groups had significant levels of allochthony, but differed greatly in their respective patterns across lakes. Allochthony in Calanoida and Cyclopoida was linked to detrital food chains based on particulate organic matter (POM) and on DOM, respectively, whereas in Cladocera it appeared related to both pathways; not surprisingly this latter group had the highest mean allochthony (0.31; compared to 0.18 in Cyclopoida and 0.16 in Calanoida). This study highlights the complexity of the pathways of delivery and transfer of terrestrial organic matter in freshwaters, and underscores the role that microbial food webs play in this transfer.

The cycling of dissolved organic matter (DOM) and its significance to ecosystem metabolism was studled over a 16 mo period in a Thalassia testudjnum dominated meadow. The benthos was usually net autotrophic (annual gross primary production to respiration ratio [P:R] = 1.3) while water column respiration (R) exceeded gross primary production (annual P:R = 0.3). Net fluxes of dissolved organic carbon (DOC) from the benthos primarily occurred in the light (0 to 18 mm01 C m-2 d-') and from seagrass-dominated areas, suggesting that release of DOC was mainly due to seagrass exudation. Net benthic DOC fluxes measured in the light were significantly correlated (p < 0.0001, n = 61) with 'benthic net primary production (NPP). Average daily benthic NPP was significantly correlated to water column R (p < 0.002, n = 7) and appeared to explain about 88% of the variability in dally water column R. Estimates of bacterioplankton growth efficiencies ranged from 21 to 38 %, with peaks corresponding to maximal benthic DOC fluxes in spring and summer. Bacterioplankton were responsible for the remineralization of most (>50%) of the DOC released from the benthos on a daily basis. Annual estimates of bacterioplankton C demand, based on water column R (-8 m01 C m-' yr-l), represented >SO% of the benthic NPP (-14 m01 C m-' yr-'1. These measurements indicate a stronger linkage between benthic and water column processes than previously believed, and it appears that water column heterotrophic processes are largely dependent upon seagrass exudation.

The impact of anthropogenic CO2 emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO2 concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO2 concentrations, the Duke Forest Free‐Air CO2 Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO2 concentrations 200 μL L−1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO2 conditions, with significantly greater accumulations under the elevated CO2 treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ∼30 g C m−2 yr−1 was sequestered in the forest floor of the elevated CO2 treatment plots relative to the control plots maintained at ambient CO2 owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO2 effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO2 suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO2 (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO2.

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