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Climate-related shifts in forest composition and chemistry will likely affect the quantity and quality of dissolved organic matter (DOM) entering inland waters, and consequently, carbon dioxide (CO 2 ) evasion. We examined the photodegradation of DOM derived from 2 common riparian plant species ( Populus tremuloides and Salix alba ) grown at ambient (360 ppm) and elevated (720 ppm) atmospheric CO 2 concentrations. Rates and total photolytic CO 2 production were determined for sterilized leachates ranging from 5 to 100 mg L −1 dissolved organic carbon (DOC). Based on multiple regression analysis, DOC concentration, followed by plant species, best predicted the rate and total flux of CO 2 . Photolytic CO2 production increased linearly with DOC concentration; however, the 5 mg L −1 treatment had the greatest rate per unit carbon, suggesting a self-shading effect of increasing DOC. The atmospheric CO2 conditions under which the plants were grown had no statistically significant effect, despite observed differences in CO 2 fluxes between ambient and elevated Populus leachates. Fluorescence data suggest differences in photolytic CO 2 production among treatments are related to differences in plant chemistry within the humic fraction. Thus, the magnitude of the photolytic CO 2 flux from fresh waters in the future will depend primarily on climate-related changes in the quantity of terrestrial DOM inputs and secondarily by DOM source.

Organic matter fluxes and food web interactions in lakes depend on the abilities of heterotrophic microbial communities to access and degrade organic matter, a process that begins with extracellular hydrolysis of high molecular weight substrates. In order to determine whether patterns of enzymatic hydrolysis vary among shallow lakes of different trophic status, we investigated the hydrolysis of six specific organic macromolecules (polysaccharides) in the spring and late summer in four adjacent shallow lakes of eutrophic, oligotrophic, and dystrophic status in coastal North Carolina, USA. The spectrum of enzyme activities detected differed strongly between lakes, with all six polysaccharides hydrolyzed in West Mattamuskeet in May, while only two substrates were hydrolyzed in Lake Phelps in August/September. Differences in the spectrum of enzyme activities, and therefore the capabilities of heterotrophic microbial communities, were likely driven by variations among lakes in primary productivity patterns, sediment–water interactions, and/or water chemistry. Our data represent a first step towards a better understanding of carbon substrate availability and differences in carbon cycling pathways in shallow lakes of different trophic status.

Previous studies indicate that leachates derived from plants grown under elevated CO 2 are more recalcitrant and consequently reduce bacterial productivity. Here bacteria were cultured on organic leachates derived from trembling aspen (Populus tremuloides) and white willow (Salix alba) grown under ambient (360 ppm) and elevated (720 ppm) CO 2 concentrations and exposed and unexposed to simulated solar radiation. In treatments not exposed to light, determined numbers of colony forming units (CFU mL -1 ) were higher on ambient CO 2 than elevated CO 2 leachates in both plant species. After exposure to simulated sunlight, the number of CFU mL -1 decreased on the ambient CO 2 leachates and increased on the elevated leachates, particularly the aspen. These results demonstrate that environmental parameters, mediated by climate change, may have both positive and negative effects on the growth of bacteria.

The Sacramento-San Joaquin Delta is the major drinking water source for 23 million California residents. Consequently, many studies have examined disinfection byproduct (DBP) formation in relation to Delta dissolved organic carbon (DOC) concentration. However, DOC characteristics within the Delta are not the same as those entering downstream water treatment facilities. As water is transferred to Southern California through the California Aqueduct, a 714.5 km-open channel, it is exposed to sunlight, potentially altering DBP precursors. We collected water from three sites within the Delta and one nearthe California Aqueduct, representing different DOC sources, and irradiated them in a solar simulator at a dose equivalent to that received during four days conveyance in the aqueduct. Photolytic changes in DOC were assessed by measuring CO2 and organic acid production, fluorescence, and ultraviolet absorbance over time. Trihalomethane (THM) and haloacetic acid (HAA) formation potentials, as well as the distribution of hydrophobic, transphilic, and hydrophilic acid fractions were determined at exposures equivalent to one and four days. Solar irradiation significantly decreased ultraviolet absorbance and fluorescence intensity, produced organic acids, and increased the hydrophilic fraction of waters. These changes in DOC caused a shift in bromine incorporation among the THM and HAA species. Our results are the first to demonstrate the importance of sunlight in altering DOC with respect to DBP formation.

[1] We generated suspended sediment solutions using river sediments and river water at concentrations similar to those observed during 1.5 year floods (Q1.5) and a dam removal (∼325 mg L−1) on the Deep River, North Carolina. Suspended sediment solutions were exposed to simulated solar radiation, equivalent to one clear, summer day at the study site (35°N). Concentrations of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), soluble reactive phosphorus (SRP) and total dissolved iron (Fed) were measured before and after exposure. Total dissolved carbon (TDC) budgets for each experiment were produced using DOC and DIC data. Sediment suspensions in the presence of simulated solar radiation were significant sources of dissolved C (119 ± 11 μmol C L−1 d−1; ± values indicate 1 standard error) and DON (1.7 ± 0.5 μmol N L−1 d−1) but not DIN or SRP. Extrapolations through the Deep River water column suggest that suspended sediments in the presence of light represent dissolved organic matter fluxes of 3.92 mmol C m−2 d−1 and 40 μmol N m−2 d−1. Additionally, sediment suspensions lowered river water Fed concentrations immediately (∼24%) and progressively (∼40–90%) in both light and dark treatments. Our research suggests suspended sediments in river ecosystems are potential sources of dissolved organic C and dissolved organic N while effectively removing Fed from the water column.

Nanomolar concentrations of steroid hormones such as 17beta-estradiol can influence the reproductive development and sex ratios of invertebrate and vertebrate populations. Thus their release into surface and ground waters from wastewater facilities and agricultural applications of animal waste is of environmental concern. Many of these compounds are chromophoric and susceptible to photolytic degradation. High intensity UV-C radiation has been demonstrated to degrade some of these compounds in engineered systems. However, the degradation efficacy of natural solar radiation in shallow fresh waters is less understood. Here photolytic experiments with 17beta-estradiol demonstrated modest photodegradation (~26%) when exposed to simulated sunlight between 290 and 720 nm. Photodegradation significantly increased (~40-50%) in the presence of 2.0-15.0 mg/l of dissolved organic carbon (DOC) derived from humic acids of the Suwannee River, GA. However, rates of photodegradation reached a threshold at approximately 5.0 mg/l DOC. Observed suppression of photolysis in the presence of a radical inhibitor (i.e. 2-propanol) indicated that a significant proportion of the degradation was due to radicals formed from the photolysis of DOC. Although photodegradation was greatest in full sunlight containing UV-B (290-320 nm), degradation was also detected with UV-A (320-400 nm) and visible light (400-720 nm) alone.