Search

The quality and quantity of organic matter (OM) in a river system directly affects ecosystem health; thus, managers benefit from an in‐depth understanding of the drivers and sources of OM. In the Snake River, a highly altered river‐reservoir system in the semi‐arid western United States, OM production and loading are key drivers of reservoir anoxia, which leads to several deleterious processes such as mercury methylation. However, sources and quantities of OM to the Snake River, and the effects of impoundment on OM moving through the river‐reservoir system, are not well understood. Particulate organic carbon (POC), dissolved organic carbon (DOC), particulate nitrogen (PN), chlorophyll a (chl‐a), and δ15N–PN and δ13C–POC isotopic ratios were measured bi‐weekly for over 2 years at four locations through the Snake River Hells Canyon Reservoir Complex to determine spatial and temporal patterns of OM quantities and sources. POC concentrations increased through the riverine zone upstream of the reservoirs, likely due to in situ primary production and/or inputs from tributaries and agricultural drains; then decreased through the most upstream reservoir likely due to particle settling. Isotopic ratios and other OM source indicators (δ15N–PN, δ13C–POC, POC:PN, chl‐a:POC) show that the dominant source of particulate OM was phytoplankton with seasonal terrestrial/macrophytic inputs. Results highlight the effects of major tributary and agricultural drain inputs, primary production, and impoundment on OM composition and concentration through a large river‐reservoir system and may inform water quality management efforts in this and similar systems. Plain Language Summary: The character and quantity of organic matter impact the biogeochemistry, plant growth, and food webs of aquatic systems. Common sources of organic matter in aquatic systems include phytoplankton, macrophytes, benthic algae, and terrestrial soil and leaves. In reservoirs, particulate organic matter may settle out and decompose as flow velocities decrease, releasing nutrients, fueling primary production, and depleting oxygen in the water column. Understanding organic matter sources, forms, and seasonality can therefore inform watershed management efforts to improve water quality. In this study, we analyzed organic matter and its source signatures in the Snake River through the Hells Canyon Reservoir Complex on the Idaho‐Oregon border (USA) over a 2‐year period. We highlight the effects of impoundment and seasonal runoff on dissolved and particulate organic carbon and show that particulate organic carbon sources were primarily associated with phytoplankton with seasonal contributions from macrophytes and (or) terrestrial soil and leaves. Key Points: Riverine zone tributary influx and primary production increased organic carbon and chlorophyll a concentrationsSettling associated with the transition from the lotic to lentic decreased particulate organic carbon and chlorophyll a concentrationsParticulate organic matter was primarily phytoplankton sourced, with seasonal terrestrial and/or macrophytic contributions [ABSTRACT FROM AUTHOR]

Annual medics and clovers have distinct properties in terms of usage as cover crops in Mediterranean orchards, but little is known of their capacity to adapt to the level of shading encountered on an orchard floor. A 2-year field experiment was conducted in South–Eastern Sicily to investigate the effects of withholding 0·50 of sunlight on the phenology and growth pattern of four medic and five clover accessions, focusing on traits known to be important for cover cropping. Shading delayed both seedling emergence and the onset of flowering (by up to 5 and 9 days, respectively), while it extended both the growth period and the overall life-cycle duration (by up to 5 and 11 days, respectively). It also induced an increase in cover crop height (from 34 to 38 cm) and crop light use (from 0·60 to 0·94 g DW/m2/MJ), but a reduction in soil coverage, above-ground dry biomass, maximum growth rate and maximum relative growth rate (by up to 13, 18, 21 and 7%, respectively), so compromising the competitiveness of cover crops against weeds. The responses to shading varied between genotypes. Medicago polymorpha ecotype S. Rosalia, Medicago rugosa ecotype Piano Lauro and ecotype Zappulla were the strongest competitors against weeds, whereas Trifolium tomentosum ecotype Bucampello was interesting in terms of its biomass yield and crop light use. The performance was unstable over seasons, so any future attempt to improve the species’ performances under shade by breeding will need to focus on reseeding capacity. [ABSTRACT FROM PUBLISHER]

Mercury and organic carbon concentrations vary dynamically in streamwater at the Sleepers River Research Watershed in Vermont, USA. Total mercury (THg) concentrations ranged from 0.53 to 93.8 ng/L during a 3-year period of study. The highest mercury (Hg) concentrations occurred slightly before peak flows and were associated with the highest organic carbon (OC) concentrations. Dissolved Hg (DHg) was the dominant form in the upland catchments; particulate Hg (PHg) dominated in the lowland catchments. The concentration of hydrophobic acid (HPOA), the major component of dissolved organic carbon (DOC), explained 41–98% of the variability of DHg concentration while DOC flux explained 68–85% of the variability in DHg flux, indicating both quality and quantity of the DOC substantially influenced the transport and fate of DHg. Particulate organic carbon (POC) concentrations explained 50% of the PHg variability, indicating that POC is an important transport mechanism for PHg. Despite available sources of DHg and wetlands in the upland catchments, dissolved methylmercury (DmeHg) concentrations in streamwaters were below detection limit (0.04 ng/L). PHg and particulate methylmercury (PmeHg) had a strong positive correlation ( r 2 = 0.84, p < 0.0001), suggesting a common source; likely in-stream or near-stream POC eroded or re-suspended during spring snowmelt and summer storms. Ratios of PmeHg to THg were low and fairly constant despite an apparent higher methylmercury (meHg) production potential in the summer. Methylmercury production in soils and stream sediments was below detection during snowmelt in April and highest in stream sediments (compared to forest and wetland soils) sampled in July. Using the watershed approach, the correlation of the percent of wetland cover to TmeHg concentrations in streamwater indicates that poorly drained wetland soils are a source of meHg and the relatively high concentrations found in stream surface sediments in July indicate these zones are a meHg sink. [ABSTRACT FROM AUTHOR]

San Pablo Bay is an estuary, within northern San Francisco Bay, containing elevated sediment mercury (Hg) levels because of historic loading of hydraulic mining debris during the California gold-rush of the late 1800s. A preliminary investigation of benthic microbial Hg cycling was conducted in surface sediment (0–4 cm) collected from one salt-marsh and three open-water sites. A deeper profile (0–26 cm) was evaluated at one of the open-water locations. Radiolabeled model Hg-compounds were used to measure rates of both methylmercury (MeHg) production and degradation by bacteria. While all sites and depths had similar total-Hg concentrations (0.3–0.6 ppm), and geochemical signatures of mining debris (as εNd, range: –3.08 to –4.37), in-situ MeHg was highest in the marsh (5.4±3.5 ppb) and ≤0.7 ppb in all open-water sites. Microbial MeHg production (potential rate) in 0–4 surface sediments was also highest in the marsh (3.1 ng g–1 wet sediment day–1) and below detection (<0.06 ng g–1 wet sediment day–1) in open-water locations. The marsh exhibited a methylation/demethylation (M/D) ratio more than 25× that of all open-water locations. Only below the surface 0–4-cm horizon was significant MeHg production potential evident in the open-water sediment profile (0.2–1.1 ng g–1 wet sediment day–1). In-situ Hg methylation rates, calculated from radiotracer rate constants, and in-situ inorganic Hg(II) concentrations compared well with potential rates. However, similarly calculated in-situ rates of MeHg degradation were much lower than potential rates. These preliminary data indicate that wetlands surrounding San Pablo Bay represent important zones of MeHg production, more so than similarly Hg-contaminated adjacent open-water areas. This has significant implications for this and other Hg-impacted systems, where wetland expansion is currently planned. [ABSTRACT FROM AUTHOR]