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Whole leachate and humic and fulvic acid fractions of dissolved organic matter (DOM) released from senescent littoral aquatic plants were exposed to varying spectra of ultraviolet radiation as well as natural UV of sunlight over different periods of time. Examination of the DOM by solid-state 13C nuclear magnetic resonance and pyrolytic gas chromatography-mass spectrometry before and after photolysis revealed only subtle changes to the bulk DOM. However, the DOM exposed to natural UV radiation showed immediate stimulation of and sustained bacterial growth. Chemical analyses by high performance liquid chromatography (HPLC) of the small organic fractions generated by photolysis of humic substances showed marked, pro-' gressively increasing release of numerous small fatty acids, particularly acetic, formic, citric, pyruvic, and levulinic, among others. Use of radiolabeled humic substances demonstrated that these small compounds photolyzed from the humic substances were readily metabolized by the bacteria. C. E. Williamson, and three anonymous reviewers, We thank M. Dedmon and A. Bell for assistance in the bacterial analyses and D. J. Clifford for pyrolytic analyses.These studies were supported by subventions of the National Science Foundation (OSR 91-0876 1 and DEB 92-20822) and the Department of Energy (NIGEC DE-FC03-90ER6 10 10).

High concentrations (20-75 pmol cm-3) of amorphous Fe(III) oxide were observed in unvegetated surface and Juncus eflusus rhizosphere sediments of a freshwater wetland in the southeastern United States. Incubation experiments demonstrated that microbial Fe(III) oxide reduction suppressed sulfate reduction and methanogenesis in surface scdimcnts and mediated 240% of depth-integrated (O-10 cm) unvegetated sediment carbon metabolism, compared to I 10% for sulfate reduction. In situ CO2 and CH, flux measurements verified that nonmethanogenic pathways accounted for -50% of unvegetated sediment carbon metabolism. Lower (-1 O-fold) rates of dark/anaerobic CH, flux from experimental vegetated cores relative to unvegetated controls suggested that methanogenesis was inhibited in the Juncus rhizosphere, in which active Fe(III) oxide reduction was indicated by the presence of low but readily detectable levels of dissolved and solid-phase Fe(II). Fe(III) oxide reduction accounted for 65% of total carbon metabolism in rhizosphere sediment incubations, compared to 22% for methanogenesis. In contrast, methanogenesis dominated carbon metabolism (72% of total) in experimental unvegetatcd sediment cores. The high Fe(III) oxide concentrations and reduction rates observed in unvegetated surface and Juncus rhizosphere sediments were perpetuated by rapid Fe(III) regeneration via oxidation of Fe(II) compounds coupled to 0, input from the overlying water and plant roots, respectively. The results indicate that Fe(III) oxide reduction could mediate a considerable amount of organic carbon oxidation and significantly suppress CH, production in freshwater wetlands situated within globally extensive iron-rich tropical and subtropical soil regimes.Natural and agricultural wetlands generate up to 50% of annual CH4 input to the atmosphere (Cicerone and Orcmland 19 8 8). Understanding factors responsible for regional variations in wetland CH4 emission is important for refining global atmospheric flux estimates, and hence assessing the current and projected contribution of CH4 to atmospheric warming (Bartlett and Harriss 1993). Recent studies indicate that a variety of factors such as wetland plant productivity (Whiting and Chanton 1993), microbial CH4 oxidation (King 1993), water table height (Freeman et al. 1993;Moore and Dalva 1993), and temperature (Bartlett and Harriss 1993) affect rates of wetland CH4 production and release. Another important factor is competition among methanogenic and other anaerobic respiratory bacteria for organic substrates in wetland sediments (Kiene 199 1). In sulfate-rich marine and brackish environments, sulfate-reducing bacteria effectively outcompete methanogens (Capone and Kiene 1988), and rates of wetland CH4 production and flux are uniformly low in such environments (Bartlett and Harriss 1993). In contrast, methanogenesis is considered to be the dominant anaerobic carbon oxidation process in sulfate-poor, orAcknowledgments

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1. Pelagic trophic structure and energy fluxes are evaluated predominantly on the basis of ingestion of particulate organic matter by living organisms and the effects of consumption on the population dynamics of trophic levels. 2. Population fluxes are not representative of the material and energy fluxes of either the composite pelagic region or the lake ecosystem. Metabolism of particulate and especially dissolved organic detritus from many pelagic and non-pelagic autochthonous and from allochthonous sources dominates both material and energy fluxes. Because of the very large magnitudes and relative chemical recalcitrance of these detrital sources, the large but slow metabolism of detritus provides an inherent ecosystem stability that energetically dampens the ephemeral, volatile fluctuations of higher trophic levels. 3. The annual time period is the only meaningful interval in comparative quantitative analyses of material and energy fluxes at population, community, and ecosystem levels. 4. Non-predatory death and metabolism by prokaryotic and protistian heterotrophs dominate. Continued application of animal-orientated relationships to the integrated, process-driven couplings of the aquatic ecosystems impedes understanding of quantitative ecosystem pathways and control mechanisms.

The emergent wetland and littoral components of the land-water zone are functionally coupled by the amounts and types of dissolved organic matter that are released, processed, transported to, and then further processed within the recipient waters. Operational couplings and integrations in freshwater ecosystems occur along physical and metabolic gradients of a number of scales from micrometer to kilometer dimensions. The operation and turnover of the microbial communities, largely associated with surfaces, generate the metabolic foundations for material fluxes along larger-scale gradients.Because of the predominance of small, shallow freshwater bodies, most dissolved organic carbon (DOC) of lacustrine and riverine ecosystems is derived from photosynthesis of higher plants and microflora associated with detritus, including sediments, and is only augmented by photosynthesis of phytoplankton. As the dissolved organic compounds generated in the wetland and littoral interface regions move toward the open-water regions of the ecosystems, partial utilization effects a selective increase in organic recalcitrance. Even though DOC from allochthonous and from interface sources is more recalcitrant than that produced by planktonic microflora, decomposition of the much larger interface quantities imported to the pelagic zone dominates ecosystem decomposition. The observed high sustained productivity of the land-water interface zone results from extensive recycling of essential resources (nutrients, inorganic carbon) and conservation mechanisms. On the average in lakes and streams, greater than 90 percent of the decomposition in the ecosystem is by bacteria utilizing DOM from non-pelagic sources of primary productivity. In addition to direct mineralization of DOC from non-pelagic sources, many of the organic compounds function indirectly to influence metabolism. New evidence is presented to demonstrate formation of complexes between humic and fulvic organic acids and extracellular enzymes. These complexes inhibit enzyme activity and can be transported within the ecosystem. The complex can be decoupled by mild ultraviolet photolysis with regeneration of enzyme activity in displaced locations.