[1] In March 2005, an extensive mercury study was performed just before snowmelt at Col de Porte, an alpine site close to Grenoble, France. Total mercury concentration in the snowpack ranged from 80 ± 08 to 160 ± 15 ng l À1 , while reactive mercury was below detection limit (0.2 ng l À1 ). We observed simultaneously a production of gaseous elemental mercury (GEM) in the top layer of the snowpack and an emission flux from the snow surface to the atmosphere. Both phenomena were well correlated with solar irradiation, indicating photo-induced reactions in the snow interstitial air (SIA). The mean daily flux of GEM from the snowpack was estimated at $9 ng m À2 d À1 . No depletion of GEM concentrations was observed in the SIA, suggesting no occurrence of oxidation processes. The presence of liquid water in the snowpack clearly enhanced GEM production in the SIA. Laboratory flux chamber measurements enabled us to confirm that GEM production from this alpine snowpack was first driven by solar radiation (especially UVA and UVB radiation), and then by liquid water in the snowpack. Finally, a large GEM emission from the snow surface occurred during snowmelt, and we report total mercury concentrations in meltwater of about 72 ng l

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To elucidate the fate of river-borne nitrate in the estuarine environment, we measured nitrate concentrations and delta N-15 and delta O-18 of nitrate along the salinity gradient in the estuary of the river Elbe, one of the largest German rivers discharging into the North Sea. Nitrate concentrations in river waters ranged from 78 mu mol L-1 to 232 mu mol L-1; delta N-15 varied from 8.2% to 16.2%, and the delta O-18 values ranged from 20.1% to 3.2%. The nitrate concentrations in the German Bight were between 2 mu mol L-1 and 34 mu mol L-1, with delta N-15 between 8.0% and 12.2% and delta O-18 between 0.3% and 9.5%. Both riverine and marine end-member concentrations showed seasonal variations, with lower nitrate concentrations and more enriched isotope values during spring and summer compared to winter months. We found no indication in either concentrations or isotopic composition for a significant loss of nitrate within the estuary, but we found a significant increase of nitrate in the maximum turbidity zone in summer. We attribute this to nitrification reflected in a change in the oxygen isotopic composition. The entire riverine nitrate load is entrained into the North Sea by conservative mixing; this conflicts with both the presumed role of estuaries as effective N-sinks and with historical data from the Elbe estuary. Fundamental changes in the biogeochemical processes of the estuary have occurred over the past several decades due to extensive dredging and removal of sediment favorable for denitrification in the Elbe estuary that connects the port of Hamburg with the North Sea

Snow surfaces play an important role in the biogeochemical cycle of mercury in high-latitude regions. Snowpacks act both as sources and sinks for gaseous compounds. Surprisingly, the roles of each environmental parameter that can govern the air-surface exchange over snow are not well understood owing to the lack of systematic studies. A laboratory system called the laboratory flux measurement system was used to study the emission of gaseous elemental mercury from a natural snowpack under controlled conditions. The first results from three snowpacks originating from alpine, urban and polar areas are presented. Consistent with observations in the field, we were able to reproduce gaseous mercury emissions and showed that they are mainly driven by solar radiation and especially UV-B radiation. From these laboratory experiments, we derived kinetic constants which show that divalent mercury can have a short natural lifetime of about 4-6 h in snow.

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Bubbles rising through the water column are known to scavenge organic material and microorganisms, and transport them through the air–sea interface after bursting. This mechanism has important implications for air–sea exchange processes. However, little is known about how bubbles influence the chemical and biological properties of the sea‐surface microlayer (SML), a gelatinous film at the air–sea interface. We used floating mesocosms in the coastal Baltic Sea and a laboratory tank filled with seawater from the North Sea to study the effect of bubbling on the gelatinous nature of the SML. Bubbling was found to always increase concentrations of transparent exopolymer particles (TEP) in the SML. In the field, TEP in the SML already increased after 2 min (53% ± 63%) and 10 min (19% ± 12%) bubbling, respectively. During the tank experiment, TEP enriched in the SML by 312% (± 244%) after > 3 h of bubbling. Therefore, bubbling is a highly efficient mechanism for TEP enrichment in the SML. Bubbling caused enrichment and depletion of microbial abundances (prokaryotes, flagellates, eukaryotes) in the SML. However, the incorporation of 3H‐thymidine (i.e., bacterial carbon production) was consistently stimulated after 10 min of bubbling in the field experiment, indicating a bubble‐induced import of unstressed bacteria and fresh organic substrates into the SML. Overall, our results suggest that the gelatinous matrix of the SML is re‐formed within min after disruption by bursting bubbles, and, thus, highlights the importance of biogeochemical interactions within the air–sea interface.