991 resultados para 454


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The ice cover of the Arctic Ocean has been changing dramatically in the last decades and the consequences for the sea-ice associated ecosystem remain difficult to assess. Algal aggregates underneath sea ice have been described sporadically but the frequency and distribution of their occurrence is not well quantified. We used upward looking images obtained by a remotely operated vehicle (ROV) to derive estimates of ice algal aggregate biomass and to investigate their spatial distribution. During the IceArc expedition (ARK-XXVII/3) of RV Polarstern in late summer 2012, different types of algal aggregates were observed floating underneath various ice types in the Central Arctic basins. Our results show that the floe scale distribution of algal aggregates in late summer is very patchy and determined by the topography of the ice underside, with aggregates collecting in dome shaped structures and at the edges of pressure ridges. The buoyancy of the aggregates was also evident from analysis of the aggregate size distribution. Different approaches used to estimate aggregate biomass yield a wide range of results. This highlights that special care must be taken when upscaling observations and comparing results from surveys conducted using different methods or on different spatial scales.

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The amount of solar radiation transmitted through Arctic sea ice is determined by the thickness and physical properties of snow and sea ice. Light transmittance is highly variable in space and time since thickness and physical properties of snow and sea ice are highly heterogeneous on variable time and length scales. We present field measurements of under-ice irradiance along transects under undeformed land-fast sea ice at Barrow, Alaska (March, May, and June 2010). The measurements were performed with a spectral radiometer mounted on a floating under-ice sled. The objective was to quantify the spatial variability of light transmittance through snow and sea ice, and to compare this variability along its seasonal evolution. Along with optical measurements, snow depth, sea ice thickness, and freeboard were recorded, and ice cores were analyzed for chlorophyll a and particulate matter. Our results show that snow cover variability prior to onset of snow melt causes as much relative spatial variability of light transmittance as the contrast of ponded and white ice during summer. Both before and after melt onset, measured transmittances fell in a range from one third to three times the mean value. In addition, we found a twentyfold increase of light transmittance as a result of partial snowmelt, showing the seasonal evolution of transmittance through sea ice far exceeds the spatial variability. However, prior melt onset, light transmittance was time invariant and differences in under-ice irradiance were directly related to the spatial variability of the snow cover.

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The euphotic depth (Zeu) is a key parameter in modelling primary production (PP) using satellite ocean colour. However, evaluations of satellite Zeu products are scarce. The objective of this paper is to investigate existing approaches and sensors to estimate Zeu from satellite and to evaluate how different Zeu products might affect the estimation of PP in the Southern Ocean (SO). Euphotic depth was derived from MODIS and SeaWiFS products of (i) surface chlorophyll-a (Zeu-Chla) and (ii) inherent optical properties (Zeu-IOP). They were compared with in situ measurements of Zeu from different regions of the SO. Both approaches and sensors are robust to retrieve Zeu, although the best results were obtained using the IOP approach and SeaWiFS data, with an average percentage of error (E) of 25.43% and mean absolute error (MAE) of 0.10 m (log scale). Nevertheless, differences in the spatial distribution of Zeu-Chla and Zeu-IOP for both sensors were found as large as 30% over specific regions. These differences were also observed in PP. On average, PP based on Zeu-Chla was 8% higher than PP based on Zeu-IOP, but it was up to 30% higher south of 60°S. Satellite phytoplankton absorption coefficients (aph) derived by the Quasi-Analytical Algorithm at different wavelengths were also validated and the results showed that MODIS aph are generally more robust than SeaWiFS. Thus, MODIS aph should be preferred in PP models based on aph in the SO. Further, we reinforce the importance of investigating the spatial differences between satellite products, which might not be detected by the validation with in situ measurements due to the insufficient amount and uneven distribution of the data.

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Background: Studies of oyster microbiomes have revealed that a limited number of microbes, including pathogens, can dominate microbial communities in host tissues such as gills and gut. Much of the bacterial diversity however remains underexplored and unexplained, although environmental conditions and host genetics have been implicated. We used 454 next generation 16S rRNA amplicon sequencing of individually tagged PCR reactions to explore the diversity of bacterial communities in gill tissue of the invasive Pacific oyster Crassostrea gigas stemming from genetically differentiated beds under ambient outdoor conditions and after a multifaceted disturbance treatment imposing stress on the host. Results: While the gill associated microbial communities in oysters were dominated by few abundant taxa (i.e. Sphingomonas, Mycoplasma) the distribution of rare bacterial groups correlated to relatedness between the hosts under ambient conditions. Exposing the host to disturbance broke apart this relationship by removing rare phylotypes thereby reducing overall microbial diversity. Shifts in the microbiome composition in response to stress did not result in a net increase in genera known to contain potentially pathogenic strains. Conclusion: The decrease in microbial diversity and the disassociation between population genetic structure of the hosts and their associated microbiome suggest that disturbance (i.e. stress) may play a significant role for the assembly of the natural microbiome. Such community shifts may in turn also feed back on the course of disease and the occurrence of mass mortality events in oyster populations.

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Precisely determined refractive indices of glass shards from 32 ash-rich, volcaniclastic sediments, mostly turbidites interbedded with nonvolcanic sediments in the Mariana Trough, range from 1.480 to 1.585 (corresponding to SiO2 ca. 75 to 49%), with most in the range 1.500 to 1.540 (SiO2 ca. 70-62%) and a second, smaller mode between ca. 1.560 and 1.585 (57 to 49% SiO2). Shards are almost exclusively colorless from 1.480 to ca. 1.530, light brown with minor colorless and green tones between 1.530 and 1.560, and dominantly brown at higher refractive indices. Tubular pumice shards are more common at higher silica percentages and non- to poorly-vesicular cuniform shards at low SiO2 values, but there is no clear correlation between shape and composition of shards. About half of the samples have bimodal shard populations with silica differences ranging up to 20 percent; unimodal layers have a range of up to about 7 percent SiO2. Of 21 samples in which one type of shard dominates, seven have the main mode in the rhyolitic composition (>69% SiO2), eight in the intermediate range (56 to 69% SiO2), and five in mafic composition (SiO2 <53%). These unusually abundant mafic shards occur mainly in site survey piston cores, SP-IA and 4E, and in Holes 454, 456, 458, and 459B. These are the sites closest to the present arc. Hole 453, containing by far the most vitric tuff turbidites, shows a gradual increase in silica content of ash layers upward to the hole from Cores 36 to 19 (about 4.6 to 3.0 Ma). A drastic decrease in ash-rich beds in the younger (Pleistocene) part of this hole was noted by the shipboard party (see site chapter, Site 453) and was interpreted by them as indicating increasing distance from the arc volcanoes as the trough opened. The increase in silica in ashes from the early to the late Pliocene at Site 453 could be interpreted in the same way and might indicate that the trough started to open in early Pliocene time.

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Actividades desarrolladas durante el período: Revisión de la producción sociológica y antropológica referente a procesos de desarrollo rural, (los modelos y estilos de desarrollo, las metodologías de extensión), la agricultura familiar, la etnografía de la ciencia y las técnicas, las formas de conocer científico-tecnológico y local), los procesos de comunicación entre los mediadores sociales y políticos del desarrollo y los productores; trabajo de campo; recopilación de información secundaria.

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The microzooplankton grazing dilution experiments were conducted at stations 126, 127, 131 and 133-137, following Landry & Hassett (1982). Seawater samples (whole seawater - WSW) were taken via Niskin bottles mounted on to a CTD Rosette out of the chlorophyll maximum at each station. Four different dilution levels were prepared with WSW and GF/F filtered seawater - 100% WSW, 75% WSW, 50% WSW and 25% WSW. The diluted WSW was filled in 2.4 L polycarbonate bottles (two replicates for every dilution level). Three subsamples (250 - 500 mL depending on in situ chlorophyll) of the 100% WSW were filtered on to GF/F filters (25 mm diameter) and chlorophyll was extracted in 5 mL 96% ethanol for 12-24 hours. Afterwards it was measured fluorometrically before and after the addition of HCl with a Turner fluorometer according to Jespersen and Christoffersen (1987) on board of the ship. In addition, one 250 mL subsample of the 100% WSW was fixed in 2% Lugol (final concentration), to determine the microzooplankton community when back at the Institute for Hydrobiology and Fisheries Science in Hamburg. Also, one 50 mL subsample of the 100% WSW was fixed in 1 mL glutaraldehyde, to quantify bacteria abundance. The 2.4 L bottles were put in black mesh-bags, which reduced incoming radiation to approximately 50% (to minimize chlorophyll bleaching). The bottles were incubated for 24 hours in a tank on deck with flow-through water, to maintain in situ temperature. An additional experiment was carried out to test the effect of temperature on microzooplankton grazing in darkness. Therefore, 100% WSW was incubated in the deck tank and in two temperature control rooms of 5 and 15°C in darkness (two bottles each). The same was done with bottles where copepods were added (five copepods of Calanus finmarchicus in each bottle; males and females were randomly picked and divided onto the bottles). In addition, two 100% WSW bottles with five copepods each were incubated at in situ temperature at 100% light level (without mesh-bags). All experiments were incubated for 24 hours and afterwards two subsamples of each bottle were filtered on to GF/F filters (25 mm diameter); 500 - 1000 mL depending on in situ chlorophyll. One 250 mL subsample of one of the two replicates of each dilution level and each additional experiment (temperature and temperature/copepods) was fixed in 5 mL lugol for microzooplankton determination. One 50 mL subsample of one of the two 100% WSW bottles as well as of one of the additional experiments without copepods was fixed in 1 mL glutaraldehyde for bacteria determination later on. Copepods were fixed in 4% formaldehyde for length measurements and sex determination.