Chronic food limitation of egg production in populations of copepods of the genusAcartia in the San Francisco estuary     

Wim J. Kimmerer  Nissa Ferm  Mary Helen Nicolini  Carolina Peñalva 《Estuaries and Coasts》2005,28(4):541-550

Egg production of planktonic copepods, is commonly measured as a proxy for secondary production in population dynamics studies and for quantifying food limitation. Although limitation of copepod egg production by food quantity or quality is common in natural waters, it appears less common or severe in estuaries where food concentrations are often high. San Francisco Estuary, California, has unusually low concentrations of chlorophyll compared to other estuaries. We measured egg production rates of three species ofAcartia, with dominate the zooplankton biomass at salinity above 15 psu, on 36 occasions during 1999–2002. Egg production was determined by incubating up to 40 freshly collected individual copepods for 24 h in 140 ml of ambient water. Egg production was less than 10 eggs female⁻¹ d⁻¹ most of the year, but as high as 52 eggs female⁻¹ d⁻¹ during month-long spring phytoplankton blooms. Egg production was a saturating function of total chlorophyll concentration with a mean of 30 eggs female⁻¹ d⁻¹ above a chlorophyll concentration of 12±6 mg chl m⁻³. We take chlorophyll to be a proxy for total food ofAcartia, known to feed on microzooplankton as well as phytoplankton. These findings, together with long-term records of chlorophyll, concentration and earlier studies of abundance of nauplius larvae in the estuary, imply chronic food limitation ofAcartia species, with sufficient food for maximum egg production <10% of the time over the last 25 yr. These results may show the most extreme example of food limitation of copepod reproduction in any temperate estuary. They further support the idea that estuaries may provide suitable habitat forAcartia species by virtue of other factors than high food concentration.  相似文献   

Gender and urban poverty: Single mothers in Amsterdam     

Joos Droogleever Fortuijn  Wim Ostendorf 《GeoJournal》2004,61(3):239-246

Urban poverty in Western countries is usually defined in terms of social exclusion and integration. Welfare states can be characterized according to three modes of integration, namely market exchange, redistribution and reciprocity. This paper compares the mechanisms of the social integration of unemployed single mothers with those of unemployed men. In two deprived neighbourhoods in Amsterdam, unemployed single mothers and unemployed native-born men were interviewed about their coping strategies with respect to the labour market, social security institutions and social networks. Although the single mothers have serious financial problems, they are better integrated into society at large than the unemployed men are. The single mothers are strongly oriented towards the labour market, successful in getting support from the social security institutions, have strong and supportive social networks, and are integrated into their neighbourhoods. In the conclusion we challenge the idea of vulnerable single mothers and demonstrate the relatively successful integration of this category of women. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Health, Safety and Environmental Risks of Underground Co2 Storage – Overview of Mechanisms and Current Knowledge     

Kay Damen  André Faaij  Wim Turkenburg 《Climatic change》2006,74(1-3):289-318

CO₂ capture and storage (CCS) in geological reservoirs may be part of a strategy to reduce global anthropogenic CO₂ emissions. Insight in the risks associated with underground CO₂ storage is needed to ensure that it can be applied as safe and effective greenhouse mitigation option. This paper aims to give an overview of the current (gaps in) knowledge of risks associated with underground CO₂ storage and research areas that need to be addressed to increase our understanding in those risks. Risks caused by a failure in surface installations are understood and can be minimised by risk abatement technologies and safety measures. The risks caused by underground CO₂ storage (CO₂ and CH₄ leakage, seismicity, ground movement and brine displacement) are less well understood. Main R&D objective is to determine the processes controlling leakage through/along wells, faults and fractures to assess leakage rates and to assess the effects on (marine) ecosystems. Although R&D activities currently being undertaken are working on these issues, it is expected that further demonstration projects and experimental work is needed to provide data for more thorough risk assessment.  相似文献   

Species diversity and ecology of Tonle Sap Great Lake, Cambodia     

Ian C. Campbell  Colin Poole  Wim Giesen  John Valbo-Jorgensen 《Aquatic Sciences - Research Across Boundaries》2006,68(3):355-373

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An evaluation of the role of daphnids in controlling phytoplankton biomass in clear water versus turbid shallow lakes     

Koenraad Muylaert  Steven Declerck  Jeroen Van Wichelen  Luc De Meester  Wim Vyverman 《Limnologica》2006,36(2):69-78

Phytoplankton and zooplankton were monitored during 2 years in four eutrophic shallow lakes (two turbid and two clear water) from two wetland reserves in Belgium. In each wetland, phytoplankton biomass was significantly higher in the turbid lake than in the clear water lake. Although total macrozooplankton biomass and the contribution of daphnids to total zooplankton biomass was comparable in the clear water and the turbid lakes, the grazing pressure of macrozooplankton on phytoplankton as estimated from zooplankton to phytoplankton biomass ratios was higher in the clear water lakes. Estimated grazing by daphnids in the clear water lakes was always high in spring. In summer, however, daphnid biomass was low or daphnids were even absent during prolonged periods. During those periods phytoplankton was probably controlled by smaller macrozooplankton or by submerged macrophytes through nutrient competition, allelopathic effects or increased sedimentation rates in the macrophyte vegetation.  相似文献   

Structural control on bulk melt properties: Single and double quantum Si NMR spectroscopy on alkali-silicate glasses     

Wim J. Malfait  Werner E. Halter  Beat H. Meier 《Geochimica et cosmochimica acta》2007,71(24):6002-6018

The structure of 21 binary potassium, rubidium and cesium silicate glasses (in the range 15-50 mol% alkali oxide) was analyzed by ²⁹Si single quantum and double quantum MAS NMR spectroscopy. Their glass transition temperatures (T_g) were measured by calorimetry. The chemical shifts and the relative abundance of Qⁿ species correlate with the cationic field strength (Z/r) of the network modifier. A correlation is observed between T_g and the inverse of the entropy of mixing of the different Qⁿ species, which is explained in the framework of the Adam-Gibbs relaxation theory. At high alkali content, up to 44% of the SiO₄ tetrahedra are part of three-membered rings. At a given alkali content, the abundance of these rings increases with increasing cation size. The abundance of three-membered rings in K-silicate melts correlates with a temperature and a non-linear composition dependence of the heat capacity. It is also a possible cause for the anomalous volumetric behavior of potassium silicate glasses.  相似文献   

Using tree stems in multi-temporal terrestrial lidar scanning data to monitor landslides on vegetated slopes     

van Veen  Megan  Porter  Michael  Lato  Matt  Mitchell  Andrew  Fish  Colleen  Van Gassen  Wim 《Landslides》2022,19(4):829-840

Landslides - Terrestrial lidar scanning (TLS) has become a widely accepted expert tool for monitoring geohazards on bare or sparsely vegetated slopes through change detection. While trees can be an...  相似文献   

Contrasting Recolonization Strategies in Multi-Species Seagrass Meadows     

Rene N. Rollon  Erik D. De Ruyter Van Steveninck  Wim Van Vierssen  Miguel D. Fortes 《Marine pollution bulletin》1999,37(8-12):450-459

This study shows that in a multi-species seagrass meadow in a shallow and clear-water site, all the former seagrass species were able to recolonize in the artificially created gaps of 0.25 m² in size within ca. 2 yr. Extrapolation of the recolonization curves of the different species predicted a full recovery within 10 yr post-disturbance. Fitted curves for the dominant species Enhalus acoroides and Thalassia hemprichii showed contrasting strategies, the latter having a comparatively high intrinsic rate, achieving full recovery within ca. 2 yr post-disturbance. E. acoroides was the latest species to establish and the projected full-recovery time was among the longest (ca. 10 yr). The effect of timing of gap creation was generally not significant (except for Syringodium isoetifolium) neither was the temporal variation in density of most species outside the gaps. As recolonization by sexual propagules was found to be low, increasing the gap size would most probably require a much longer recovery period. A crude estimate for E. acoroides would be >10 yr for 1 m² of gap. Further, since the densities of most seagrass species vary significantly between sites, and colonization rates depend on adjacent seagrass densities, the recovery curves would also be different across sites.  相似文献   

Fast kinematic ray tracing of first- and later-arriving global seismic phases   总被引：1，自引：0，他引：1  

Harmen Bijwaard  & Wim Spakman 《Geophysical Journal International》1999,139(2):359-369

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Contrasting behaviour of trace metals in the Scheldt estuary in 1978 compared to recent years     

Rob F. Nolting  Wim Helder  Hein J. W. de Baar  Loes J. A. Gerringa 《Journal of Sea Research》1999,42(4):2393

Dissolved and particulate trace metals (Cu, Cd, Pb, Zn, Ni, Fe and Mn) measured at six stations along the Scheldt estuary in October/November 1978 are compared with more recent data. Based on Ca content in the suspended matter, three distinct geochemical regions could be distinguished: the upper estuary (salinity 1–7) dominated by fluvial mud, mid-estuary (salinity 7–17) where the composition of the suspended matter remained relatively constant, and the lower estuary where marine mud prevailed. Re-suspension of sediments is the major factor controlling the composition of the particles in the upstream region. Anoxic conditions prevailed in the upper part of the estuary extending to a salinity of 15 in 1978, while at present the seaward boundary of the anoxic water body is located at less saline waters. Furthermore, the present-day metal load is much lower than in 1978. As a consequence of the changed situation, maxima in dissolved concentrations of redox-sensitive metals in the mid/lower estuary have moved as well, which affects the trace metal re-distribution pattern. In the anoxic zone, exchange processes between dissolved and particulate metal fractions were strongly redox regulated, with Fe and Mn as excellent examples. Iron was removed from the dissolved phase in the early stages of mixing resulting in an increase in the suspended particulate matter of the leachable ‘non-residual' Fe fraction from 2 to 3.5%. Due to its slower kinetics, removal of Mn from solution occurred in mid-estuary where oxygen concentrations increased. Cu, Cd and Zn on the contrary were mobilised from the suspended particles during estuarine mixing. External inputs of Pb, and to a lesser extent of Cu, in the lower estuary resulted in the increase of their particulate and the dissolved concentrations. Calculated K_d (distribution coefficient) values were used to assess the redistribution between the dissolved and particulate phase of the investigated metals. Due to the existence of the anoxic water body in the upper estuary, the importance of redox processes in determining the K_d values could be demonstrated. The sequence of K_d values in the upper estuary (Fe, Cd, Zn, Pb > Cu > Ni, Mn) is significantly different from that in the lower estuary (Fe > Mn > Pb, Ni, Zn, Cu, Cd). Thus, in such a dynamic estuary single metal-specific K_d values cannot be used to describe redistribution processes.  相似文献