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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]

In lake food webs, planktonic bacteria and algae represent the greatest bioconcentration step for Hg2+ and monomethyl-Hg (MeHg). As they are the most abundant organisms in planktonic trophic webs and also the main food resource for herbivorous plankton, they can mobilize large amounts of Hg to higher trophic levels. In Andean Patagonian lakes (Argentina), dissolved organic matter (DOM) concentration and character, coupled with photo-reactions, play a central role in the complexation of Hg2+ in the water column and can even regulate the uptake of Hg2+ by planktonic algae. In this investigation we evaluated the DOM character of natural waters (NW) from four Andean lakes and studied its influence on the uptake of 197Hg2+ in a strain of the picocyanobacteria Synechococcus by using Hg2+ labeled with 197Hg2+. The uptake of radiolabeled Hg2+ by Synechococcus showed different magnitude in NW of lakes Moreno, El Trébol, Morenito and Escondido. Increasing lake DOM concentration reduced the bioavailability of Hg2+ as indicated by the lower uptakes rates found in NW with higher complexity and concentration of the DOM pool. Uptakes of Hg2+ by this picocyanobacteria contrasted among NW from pelagic (surface and bottom) and littoral compartments of Lake Escondido which suggest that the entry of this metal may be highly variable even in the same environment. The study of the uptake of radiolabeled Hg2+ in a set of dilutions of NW from Lake Escondido demonstrated that the bioavailability of Hg2+ decrease with increasing DOM concentration.

The species composition and the size structure of natural planktonic food webs may provide essential information to understand the fate of mercury and, in particular, the bioaccumulation pattern of Hg2+ in the water column of lake ecosystems. Heterotrophic and autotrophic picoplankton and phytoplankton are the most important entry points for Hg in aquatic ecosystems since they concentrate Hg2+ and MeHg from ambient water, making them available to planktonic consumers at higher trophic levels of lake food webs. In this investigation we studied the uptake of 197Hg2+ in natural plankton assemblages from four Andean lakes (Nahuel Huapi National Park, Patagonia, Argentina), comprised in the size fractions 0.2-2.7 μm (picoplankton), 0.2-20 μm (pico and nanoplankton) and 20-50 μm (microplankton) through experiments using Hg2+ labeled with 197Hg2+. The experimental results showed that the uptake of Hg2+ was highest in the smallest plankton fractions (0.2-2.7 μm and 0.2-20 μm) compared to the larger fraction comprising microplankton (20-50 um). This pattern was consistent in all lakes, reinforcing the idea that among pelagic organisms, heterotrophic and autotrophic bacteria with the contribution of nanoflagellates and dinoflagellates constitute the main entry point of Hg2+ to the pelagic food web. Moreover, a significant direct relationship was found between the Hg2+ uptake and surface index of the planktonic fractions (SIf). Thus, the smaller planktonic fractions which bore the higher SI were the major contributors to the Hg2+ passing from the abiotic to the biotic pelagic compartments of these Andean lakes.