TY - JOUR AB - The rapid deglaciation of the Arctic increases the flux of terrestrial organic matter into the marine carbon cycle. The proportions, mixing, and turnover rates of organic matter from deglaciated terrestrial and glacial watershed sources are poorly understood in Arctic fjords. We characterized the mixing and flux of dissolved organic matter (DOM) in two Arctic fjords, Kongsfjorden and Scoresby Sund. DOM was sampled and analyzed along the natural mixing gradient from glacial meltwater to the marine waterbody to reveal chemical proxies for glacier-derived organic matter utilizing (ultra)high-resolution mass spectrometry of solid-phase extracted DOM. The molecular composition of DOM in Kongsfjorden was homogenous in elemental ratios of O/C, H/C, and N/C and aromaticity due to high mixing and low glacial run-off. In Scoresby Sund, a strong correlation between DOM composition and salinity was observed. The freshwater proportion of DOM was characterized by a low N/C ratio, unsaturated molecules, and a high terrestrial index. The surface stratification in Scoresby Sund prevented vertical mixing of glacier-derived and marine DOM with denser water masses. Glacial meltwater DOM in Scoresby Sund was similar to terrestrial DOM, suggesting either an unknown source of additional DOM or selective DOM removal processes. In summary, the high proportion of terrestrial DOM proxies suggests a strong imprint of deglaciation on the Arctic carbon cycle. AU - Moye, F.* AU - Geuer, J.K.* AU - Burau, C.* AU - Harir, M. AU - Schmitt-Kopplin, P. AU - Koch, B.* AU - Harder, T.* AU - Tebben, J.* C1 - 75200 C2 - 57845 CY - 111 River St, Hoboken 07030-5774, Nj Usa TI - Chemical proxies of glacier‐derived and marine organic matter reveal low mixing in summer‐stratified Arctic fjords. JO - Limnol. Oceanogr. PB - Wiley PY - 2025 SN - 0024-3590 ER - TY - JOUR AB - In aquatic ecosystems, nitrogen (N) loading is mitigated in redox transition zones principally through the processes of denitrification and anaerobic oxidation of ammonium (anammox). Here, we investigate the N cycling processes in the water column of a seasonal stratified lake influenced by benthic processes in Southern Germany (Fohnsee) during the development of the vertical redox stratification between April and September. Concentration profiles and stable isotope compositions of NO3− and NH4+ together with numerical modeling and quantification of the hydrazine synthase gene (hzsB) and nitrite reductase (nirK and nirS) genes were used to identify the predominant nitrogen-transformation processes at lake Fohnsee throughout the spring and summer periods. Water chemistry data, quantitative polymerase chain reaction analysis and increases of δ15N and δ18O values of nitrate from 7.0‰ to 41.0‰ and 2.0‰ to 28.0‰, respectively, showed that nitrate reduction to nitrite and NO occurs in an upward moving zone of the water column from June to September following the displacement of the oxycline caused by thermal stratification. We also observed an increase in δ15N of ammonium from 15‰ to 28‰ in the anoxic water column. Modeling results suggest that this shift in δ15N-NH4+ is predominantly controlled by mixing between ammonium stemming from the oxic water column with δ15N values of 25‰ and ammonium that is likely formed in the lake sediments by oxidation of organic matter with δ15N values of 11‰. Observed gene abundances (hzsB, nirK, and nirS) in lake water samples collected in June and July indicated the co-occurrence of nitrate reduction and low rates of anammox, while the presence of sulfide in August and September may have inhibited the activity of anammox bacteria near the sulfate-reduction zone at the lake bottom. This study revealed temporal and spatial (e.g., depth dependent) variations in the dominant N-transformation processes in the investigated lake. AU - Peña Sanchez, A.* AU - Duffner, C. AU - Wunderlich, A.* AU - Mayer, B.* AU - Schulz, S. AU - Schloter, M. AU - Einsiedl, F.* C1 - 64841 C2 - 52071 SP - 1194-1210 TI - Seasonal dynamics of anaerobic oxidation of ammonium and denitrification in a dimictic lake during the stratified spring-summer period. JO - Limnol. Oceanogr. VL - 67 IS - 5 PY - 2022 SN - 0024-3590 ER - TY - JOUR AB - We present a comparison of the dissolved stable isotope composition of silicate (δ30Si(OH)4) and nitrate (δ15 NO3-) to investigate the biogeochemical processes controlling nutrient cycling in the upwelling area off Peru, where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Besides strong upwelling of nutrient rich waters mainly favoring diatom growth, an anticyclonic eddy influenced the study area. We observe a tight coupling between the silicon (Si) and nitrogen (N) cycles in the study area. Waters on the shelf showed high Si(OH)4 concentrations accompanied by diminished NO3- concentration as a consequence of intense remineralization, high Si fluxes from the shelf sediments, and N-loss processes such as anammox/denitrification within the OMZ. Correspondingly, the surface waters show low δ30Si(OH)4 values (+2‰) due to low Si utilization but relatively high δ15 NO3- (+13‰) values due to upwelling of waters influenced by N-loss processes. In contrast, as a consequence of the deepening of the thermocline in the eddy center, a pronounced Si(OH)4 depletion led to the highest δ30Si(OH)4 values (+3.7‰) accompanied by high δ15 NO3- values (+16‰). In the eddy center, high NO3-: Si(OH)4 ratios favored the growth of non-siliceous organisms (Synechococcus). Our data show that upwelling processes and the presence of eddies play important roles controlling the nutrient cycles and therefore also exert a major influence on the phytoplankton communities in the Peruvian Upwelling. Our findings also show that the combined approach of δ30Si(OH)4 and δ15 NO3- can improve our understanding of paleo records as it can help to disentangle utilization and N-loss processes. AU - Grasse, P.* AU - Ryabenko, E. AU - Ehlert, C.* AU - Altabet, M.A.* AU - Frank, M.* C1 - 49120 C2 - 41660 CY - Hoboken SP - 1661-1676 TI - Silicon and nitrogen cycling in the upwelling area off Peru: A dual isotope approach. JO - Limnol. Oceanogr. VL - 61 IS - 5 PB - Wiley-blackwell PY - 2016 SN - 0024-3590 ER - TY - JOUR AB - We followed the viral response to Fe-enrichment over 3 weeks during the second iron-enrichment experiment (EisenEx) in the Southern Ocean. Although viral abundance increased in a rather constant way during the course of the experiment inside the patch, the production of bacterial viruses and the fraction of infected cells in bacterioplankton exhibited pronounced maxima on days 7 and 21. In contrast, these parameters remained fairly constant outside the Fe-fertilized patch. Viral production was also stimulated directly after the Fe-enrichment. Within the Fe-replete patch, viral abundance and production was on average 2- and 3-fold, respectively, higher than outside the patch. On average 40% of the bacteria contained viruses in a lytic stage inside the Fe-replete patch compared to 21% outside the patch. About 11-12% of the bacteria contained a viral genome inducible with mitomycin C in both inside and outside the patch. Randomly amplified polymorphic desoxyribonucleic acid polymerase chain reaction analysis suggested the development of a specific viral community in the patch. Although short-term stimulation of viral production was likely due to increased bacterial activity, the long-term stimulation was likely also influenced by an increased encounter rate between bacterial viruses and hosts. Viral lysis was responsible for most of the bacterioplankton mortality in the patch. Overall, most of the bacterial carbon production entered the detrital or dissolved organic matter pool via the viral shunt in the patch. AU - Weinbauer, M.G.* AU - Arrieta, J.M.* AU - Griebler, C. AU - Herndl, G.J.* C1 - 1636 C2 - 26300 SP - 774-784 TI - Enhanced viral production and infection of bacterioplankton during an iron-induced phytoplankton bloom in the Southern Ocean. JO - Limnol. Oceanogr. VL - 54 IS - 3 PB - Amer Soc Limnology Oceanography PY - 2009 SN - 0024-3590 ER -