forests. The three large Siberian rivers, Lena, Yenisei, and Ob, which also have the highest 58 proportion of forests within their watersheds, contribute about 90% of the total lignin discharge 59 to the Arctic Ocean. The composition of river DOC is also characterized by elevated levels of p-60 hydroxybenzenes, particularly during the low flow season, which indicates a larger contribution 61 from mosses and peat bogs. The lignin composition was strongly related to the average 14 C-age 62 of DOC supporting the abundance of young, boreal-vegetation-derived leachates during spring 63 flood, and older, soil-, peat-, and wetland-derived DOC during groundwater dominated low flow 64 conditions, particularly in the Ob and Yukon Rivers. We observed significant differences in 65 DOC concentration and composition between the rivers over the seasonal cycles with the 66 Mackenzie River being the most unique, the Lena River being similar to the Yenisei, and the 67 Yukon being most similar to the Ob. The observed relationship between the lignin phenol 68 composition and watershed characteristics suggests that DOC discharge from these rivers could 69 increase in a warmer climate under otherwise undisturbed conditions. 70 71 4

The optical properties of chromophoric dissolved organic matter (CDOM) were investigated in the Canadian Archipelago and coastal Beaufort Sea surface waters using fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC). Environmental dynamics of individual components were evaluated and compared to salinity, in situ fluorescence, absorption at 312 nm (a312), dissolved organic carbon, and lignin phenol concentrations. A positive linear relationship between four fluorescent components and lignin phenols suggests a terrestrial origin, whereas two components were unrelated to a river source, suggesting an autochthonous source. Elevated concentrations of terrestrial components were observed in the Mackenzie River plume near the coast of Alaska and decreased as water was transported to the Canadian Archipelago. The two nonterrestrial components exhibited only background levels in concentrations along the transect, suggesting minimal productivity within plume and archipelago surface waters. The relative abundance of terrestrial components in relation to nonterrestrial components allowed us to distinguish water masses including Atlantic, Archipelago, and Mackenzie River plume, respectively. This study illustrates the usefulness of PARAFAC to fingerprint water masses based on the optical characteristics of CDOM and shows promise to improve our understanding of upper Arctic Ocean ventilation.

It has been documented that the global proliferation of dam construction on the major river has reduced nutrient and sediment loading to coastal environments. As a consequence, dams can impact marine ecological systems by changing nutrient concentrations and ratios in the coastal zone. From 1998–2004, we conducted a high resolution oceanographic investigation of the East China Sea (ECS) before and after the first filling phase (June 2003) of the Three‐Gorges Dam (TGD). We found that the Si:N ratio in the River affected region changed from 1.5 in 1998 to 0.4 in 2004 with sediment loading significantly reducing (about 55%) at the Datong station after June 2003. Most importantly, we found that the PP had declined by 86% between 1998 and 2003, both measured during the high flood season. The results suggest that the ECS ecosystem may respond sensitively to changes in the nutrient supply arising from the TGD project.

Nonpoint source pollution from agriculture and urbanization is increasing globally at the same time climate extremes have increased in frequency and intensity. We review >200 studies of hydrologic and gaseous fluxes and show how the interaction between land use and climate variability alters magnitude and frequency of carbon, nutrient, and greenhouse gas pulses in watersheds. Agricultural and urban watersheds respond similarly to climate variability due to headwater alteration and loss of ecosystem services to buffer runoff and temperature changes. Organic carbon concentrations/exports increase and organic carbon quality changes with runoff. Nitrogen and phosphorus exports increase during floods (sometimes by an order of magnitude) and decrease during droughts. Relationships between annual runoff and nitrogen and phosphorus exports differ across land use. CH 4 and N 2 O pulses in riparian zones/floodplains predominantly increase with: flooding, warming, low oxygen, nutrient enrichment, and organic carbon. CH 4 , N 2 O, and CO 2 pulses in streams/rivers increase due to similar factors but effects of floods are less known compared to base flow/droughts. Emerging questions include: (1) What factors influence lag times of contaminant pulses in response to extreme events? (2) What drives resistance/resilience to hydrologic and gaseous pulses? We conclude with eight recommendations for managing watershed pulses in response to interactive effects of land use and climate change.