Ethylene and methane in the upper water column of the subtropical Atlantic

The vertical distributions of ethylene and methane in the upper water column ofthe subtropical Atlantic were measured along a transect from Madeira to the Caribbean andcompared with temperature, salinity, oxygen, nutrients, chlorophyll-a, and dissolved organiccarbon (DOC).Methane concentrations between 41.6 and 60.7 nL L−1 were found in the upper 20 m ofthe water column giving a calculated average flux of methane into the atmosphere of 0.82 µgm−2 h−1. Methane profiles reveal several distinct maxima in the upper 500 m of the watercolumn and short-time variations which are presumably partly related to the vertical migrationof zooplankton.Ethylene concentrations in near surface waters varied in the range of 1.8 to 8.2 nL L−1.Calculated flux rates for ethylene into the atmosphere were in the range of 0.41 to 1.35 µgm−2 h−1 with a mean of 0.83 µg m−2 h−1. Maximum concentrations of up to 39.2 nL L−1were detected directly below the pycnocline in the western Atlantic. The vertical distributionsof ethylene generally showed one maximum at the pycnocline (about 100 m depth) whereelevated concentrations of chlorophyll-a, dissolved oxygen, and nutrients were also found;no ethylene was detected below 270 m depth. This suggests that ethylene release is mainlyrelated to one, probably phytoplankton associated, source, while for methane, enhanced netproduction occurs at various depth horizons. For surface waters, a simple correlation betweenethylene and chlorophyll-a or DOC concentrations could not be observed. No considerablediurnal variation was observed for the distribution and concentration of ethylene in the upperwater column

Seasonal effect of zebra mussel colonies on benthic processes in the temperate mesotrophic Plateliai Lake, Lithuania

Sparse colonies of zebra mussel (Dreissena polymorpha) create net heterotrophic sediment patches via respiration, excretion, and biodeposition activities, but their effect as biogeochemical hotspots is scarcely investigated in nutrient-limited ecosystems. We analyzed the seasonal effect of zebra mussel colonies on benthic respiration (O2, TCO2, N2, and CH4) and nutrient fluxes (NH4+, NOx−, SRP, and SiO2) in a macrophyte-dominated mesotrophic temperate lake. Intact sediments with and without zebra mussel aggregates were collected in winter, summer, and autumn, and incubated to measure fluxes. The contribution of mussel colonies alone to benthic metabolism was also quantified. Sediments with mussels always had higher rates of respiration (O2 and TCO2) and nutrient recycling (NH4+ and SRP) as compared to bare sediments, while there was no effect on CH4, NO3−, and SiO2 fluxes. Mussel colonies stimulated nitrogen removal via denitrification, but only in the summer. The effect of colonies was particularly evident in warmer periods, due to mussel respiration and excretion and to biodeposits that increased microbial activity in sediments. In this mesotrophic lake, mussel aggregates contribute to alleviate nutrient (N and P) limitation, but their heterotrophic activity is likely buffered by nutrient uptake and oxygen production by submersed vegetation