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11.
Catherine Lalande Jacqueline M. Grebmeier Paul Wassmann Lee W. Cooper Mikhail V. Flint Valentina M. Sergeeva 《Continental Shelf Research》2007
Drifting sediment traps were deployed at 9 stations in May-June (ice-covered conditions) and July-August (ice-free conditions) 2004 in the Chukchi Sea to investigate the variability in export fluxes of biogenic matter in the presence and absence of sea ice cover. Measurements of chlorophyll-a (Chl-a), particulate organic carbon (POC), particulate nitrogen (PN), phytoplankton, zooplankton fecal pellets, and the stable carbon isotope composition (δ13C) of the sinking material were performed along Barrow Canyon (BC) and a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. POC export fluxes were similarly high in the presence (378±106 mg C m−2 d−1) and in the absence of ice cover (442±203 mg C m−2 d−1) at the BC stations, while fluxes were significantly higher in the absence (129±98 mg C m−2 d−1) than in the presence of ice cover (44±29 mg C m−2 d−1) at the EHS stations. The C/N ratios and δ13C values of sinking organic particles indicated that POC export fluxes on the Chukchi continental shelf were mostly composed of freshly produced labile material, except at the EHS stations under ice cover where the exported matter was mostly composed of refractory material probably advected into the EHS region. Chl-a fluxes were higher under ice cover than in ice-free water, however, relatively low daily loss rates of Chl-a and similar phytoplankton carbon fluxes in ice-covered and ice-free water suggest the retention of phytoplankton in the upper water column. An increase in fecal pellet carbon fluxes in ice-free water reflected higher grazing pressure in the absence of ice cover. Elevated daily loss rates of POC at the BC stations confirmed other indications that Barrow Canyon is an important area of carbon export to the basin and/or benthos. These results support the conclusion that there are large spatial and temporal variations in export fluxes of biogenic matter on the Chukchi continental shelf, although export fluxes may be similar in the presence and in the absence of ice cover in highly productive regions. 相似文献
12.
The influence of socio-cultural factors on the adaptive capacity, resilience and trade-offs in decision-making of households and communities is receiving growing scholarly attention. In many partly transformed societies, where the market economy is not well developed, livelihood practices are heavily structured by kinship and indigenous social and economic values. Farm investment decisions and incentives to produce agricultural commodities are shaped by a host of considerations in addition to market imperatives like profit. In one such partly transformed society in East New Britain Province, Papua New Guinea, we examine the adaptation decisions of smallholders in response to the drastic drop of yield in their cocoa plots caused by the sudden outbreak of Cocoa Pod Borer. To explain why the impact of the pest has been so great we examine the interconnections between household responses, the local socio-cultural and economic context of smallholder commodity crop production and the wider institutional environment in which household choices and decisions are made. We argue that the significant lifestyle changes and labour intensive farming methods required for the effective control of Cocoa Pod Borer are incompatible with existing smallholder farming systems, values and livelihoods. To adopt a high input cropping system requires more than a technical fix and some training; it also requires abandoning a ‘way of life’ that provides status, identity and a moral order, and which is therefore highly resistant to change. The paper highlights the enduring influence and significance of local, culturally-specific beliefs and socio-economic values and their influence on how individuals and communities make adaptation decisions. 相似文献
13.
Mark A. Brzezinski Jeffrey W. KrauseMatthew J. Church David M. KarlBinglin Li Janice L. JonesBrett Updyke 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(10):988-1001
Silica cycling in the upper 175 m of the North Pacific Subtropical Gyre was examined over a two year period (January 2008-December 2009) at the Hawaii Ocean Time-series (HOT) station ALOHA. Silicic acid concentrations in surface waters ranged from 0.6 to 1.6 ??M, exhibiting no clear seasonal trends. Biogenic silica concentrations and silica production rates increased by an order of magnitude each summer following stratification of the upper 50 m reaching values of 157 nmol Si L−1 and 81 nmol Si L−1 d−1, in 2008 and 2009, respectively. Sea surface height anomalies together with analyses of variability in isothermal surfaces at 150-175 m indicated that the summer periods of elevated biogenic silica were associated with anticyclonic mesoscale features during both years. Lithogenic silica concentrations increased in the spring during the known period of maximum atmospheric dust concentrations with maximum values of 36 nmol Si L−1 in the upper 10 m. Dust deposition would enhance levels of dissolved iron in surface waters, but there was no response of diatom biomass or silica production to increases in near-surface ocean lithogenic silica concentrations suggesting iron sufficiency of diatom silica production rates.Low ambient silicic acid concentrations restricted silica production rates to an average of 43% of maximum potential rates. Si sufficiency only occurred during the summer period when diatom biomass was elevated suggesting that bloom diatoms are adapted to exploit low silicic acid concentrations. Annual silica production at HOT is estimated to be 63 mmol Si m−2 a−1 with summer blooms contributing 29% of the annual total. Diatoms are estimated to account for 3-7% of total phytoplankton primary productivity, but 9-20% of organic carbon export confirming past suggestions that diatoms are relatively minor contributors to primary productivity and autotrophic biomass, but important contributors to new and export production in oligotrophic open-ocean ecosystems.Annual silica production at HOT is nearly 4-fold lower than estimates at the Bermuda Atlantic Time-series Study (BATS) site in the Sargasso Sea from the 1990s, but annual silica export at the base of the euphotic zone is similar between the two gyres indicating very different balances between silica production and its loss in surface waters. On a relative basis, BATS is a more productive system with respect to silica, where biogenic silica is recycled with high efficiency in surface waters; in contrast the NPSG is a lower productivity system with respect to silica, but where lower recycling efficiency leads to a much higher fraction of new silica production. The two gyres show contrasting long-term trends in diatom biomass as biogenic silica concentrations at HOT have been increasing since 1997, but they have been decreasing at BATS suggesting very different forcing of decadal trends in the contribution of diatoms in carbon cycling between these gyres. Combining the data from both gyres indicates that globally subtropical gyres produce 13 Tmol Si a−1, which is only 51% of previous estimates reducing the contribution of subtropical gyres to 5-7% of global annual marine silica production. 相似文献
14.
Anna Sanchez-Vidal Robert W. Collier Antoni Calafat Joan Fabres Miquel Canals 《Marine Chemistry》2005,93(2-4):105-117
Particulate biogenic barium (bio-Ba) fluxes obtained from three instrumented arrays moored in the Alboran Sea, the westernmost basin in the Mediterranean Sea, are presented in this study. The mooring lines were deployed over almost 1 year, from July 1997 to May 1998, and were equipped with sediment traps at 500–700 m depth, 1000–1200 m depth and 30 m above the seafloor (1000–2200 m). The results obtained support the growing body of evidence that the relationship between particulate bio-Ba and Corg throughout the water column in margin systems is clearly different from this relation in the open ocean. In the Alboran Sea, the annual averaged bio-Ba fluxes range from 0.39 to 1.07 μmol m−2 day−1, with mean concentrations of 1.31–1.69 μmol g−1 and bio-Ba/Corg ratios lower than in the open ocean. The low bio-Ba values obtained also indicate that calculating bio-Ba is extremely sensitive to the detrital Ba/Al ratio of each sample. The lithogenic Ba fraction in the Alboran Sea continental margin area contributes between 24% and 85% of the total Ba. Increased bio-Ba export efficiency was observed after periods of high primary productivity and suggests that the processes limiting the bio-Ba formation in the study area relate to settling dynamics of organic matter aggregates. Furthermore, the ballasting effect of the abundant lithogenic and carbonate particles may limit decomposition of organic matter aggregates and enhance the transfer of particles rich in Corg and relatively poor in bio-Ba to the deep seafloor. Lateral input of freshly sedimented biogenic material, including particulate bio-Ba, has been observed on the lower continental slope in the western Alboran Sea. These observations emphasize that the use of the bio-Ba as a proxy of export productivity from the surface ocean must be used cautiously in highly dynamic environments such as those in the Alboran Sea. 相似文献
15.
Contract farmers in central Honduras have been producers of Asian vegetables for American markets since 1989. Contract farming
holds the potential to provide benefits to small farmers who would otherwise be unable to access export markets but production
relations between firms and farmers are often asymmetric and can be exploitative. Poor communication and legacies of mistrust
between commodity agriculture and small farmers lead to ambiguities in the contracting system that are misinterpreted by both
firms and farmers, creating social tensions between contracting parties. This article will consider how farmers’ navigation
of the contracting system and competition between export firms has led to changes in the system. Through competition for a
limited pool of outgrowers, firms adjust their policies in favor of farmers without explicit attempts of labor organization
on the part of the farmers.
相似文献
| Valerie ImbruceEmail: |
16.
长江流域持续发展的态势、问题及对策研究 总被引：8，自引：2，他引：6
通过研究长江流域经济的持续快速健康发展,上中下游的协调发展,产业结构的优化和产业政策的实施,外向型经济发展,基础产业和基础设施的发展与合理布局,大中小城市协调发展和城乡一体化,自然资源的合理开发利用,生态环境的治理与保护,分析了长江流域持续发展的基本态势,存在的主要问题,并提出对策。 相似文献
17.
Claire Mahaffey Claudia R. Benitez-Nelson Robert R. Bidigare Yoshimi Rii David M. Karl 《Deep Sea Research Part II: Topical Studies in Oceanography》2008,55(10-13):1398
Wind-driven cyclonic eddies are hypothesized to relieve nutrient stress and enhance primary production by the upward displacement of nutrient-rich deep waters into the euphotic zone. In this study, we measured nitrate (NO3−), particulate carbon (PC), particulate nitrogen (PN), their stable isotope compositions (δ15N-NO3−, δ13C-PC and δ15N-PN, respectively), and dissolved organic nitrogen (DON) within Cyclone Opal, a mature wind-driven eddy generated in the lee of the Hawaiian Islands. Sampling occurred in March 2005 as part of the multi-disciplinary E-Flux study, approximately 4–6 weeks after eddy formation. Integrated NO3− concentrations above 110 m were 4.8 times greater inside the eddy (85.8±6.4 mmol N m−2) compared to the surrounding water column (17.8±7.8 mmol N m−2). Using N-isotope derived estimates of NO3− assimilation, we estimated that 213±59 mmol m−2 of NO3− was initially injected into the upper 110 m Cyclone Opal formation, implying that NO3− was assimilated at a rate of 3.75±0.5 mmol N m−2 d−1. This injected NO3− supported 68±19% and 66±9% of the phytoplankton N demand and export production, respectively. N isotope data suggest that 32±6% of the initial NO3− remained unassimilated. Self-shading, inefficiency in the transfer of N from dissolved to particulate export, or depletion of a specific nutrient other than N may have led to a lack of complete NO3− assimilation. Using a salt budget approach, we estimate that dissolved organic nitrogen (DON) concentrations increased from eddy formation (3.8±0.4 mmol N m−2) to the time of sampling (4.0±0.09 mmol N m−2), implying that DON accumulated at rate of 0.83±1.3 mmol N m−2 d−1, and accounted for 22±15% of the injected NO3−. Interestingly, no significant increase in suspended PN and PC, or export production was observed inside Cyclone Opal relative to the surrounding water column. A simple N budget shows that if 22±15% of the injected NO3− was shunted into the DON pool, and 32±6% is unassimilated, then 46±16% of the injected NO3− remains undocumented. Alternative loss processes within the eddy include lateral exchange of injected NO3− along isopycnal surfaces, remineralization of PN at depth, as well as microzooplankton grazing. A 9-day time series within Cyclone Opal revealed a temporal depletion in δ15N-PN, implying a rapid change in the N source. A change in NO3− assimilation, or a shift from NO3− fueled growth to assimilation of a 15N-deplete N source, may be responsible for such observations. 相似文献
18.
Gordon Waitt 《The Australian geographer》1994,25(1):36-49
This article investigates the spatial dimension of exports of automobiles from the Republic of Korea (ROK), the market for which has been dominated since 1986 by the United States (US). Two theories are reviewed which have been advanced to explain the pattern and nature of trade, comparative and competitive advantage. This paper examines these theories and assesses their explanatory utility in the light of the evidence from the ROK. The ROK trade patterns in the automobile industry are used to assess these theories and it is suggested that the results are applicable to other locations and industrial sectors. 相似文献
19.
The classification and influencing factors of modern marine productivity were reviewed at the beginning. We discussed the pros and cons of different paleoproductivity proxies. Based on these discussions, we collected paleoproductivity reconstructions in tropical marine from previous studies and focus on the glacial-interglacial features, periodicity and forcing mechanisms of tropical marine productivity. We found that the productivity in most tropical sites decreased from MIS 2 to MIS 1. The productivity was not always higher in glacial: The glacial-interglacial pattern of productivity turned at MIS 22 in western Pacific. There were remarkable differences between tropical productivity and high latitude productivity. The precession and obliquity bands were more significant in tropical productivity and ~30 ka cycles caused by the superimposing of different orbital cycles were common in tropical. The coccolith based productivity seemed to have a quasiperiod of 400 ka and more researches are needed to discover the relationship between productivity and global 13C in this band. 相似文献
20.
Thomas R. Anderson Vladimir A. Ryabchenko Michael J.R. Fasham Victor A. Gorchakov 《Deep Sea Research Part I: Oceanographic Research Papers》2007,54(12):2082-2119
A three-dimensional hydrodynamic-ecosystem model was used to examine the factors determining the spatio-temporal distribution of denitrification in the Arabian Sea. The ecosystem model includes carbon and nitrogen as currencies, cycling of organic matter via detritus and dissolved organic matter, and both remineralization and denitrification as sinks for material exported below the euphotic zone. Model results captured the marked seasonality in plankton dynamics of the region, with characteristic blooms of chlorophyll in the coastal upwelling regions and central Arabian Sea during the southwest monsoon, and also in the northern Arabian Sea during the northeast monsoon as the mixed layer shoals. Predicted denitrification was 26.2 Tg N yr−1,the greatest seasonal contribution being during the northeast monsoon when primary production is co-located with the zone of anoxia. Detritus was the primary organic substrate consumed in denitrification (97%), with a small (3%) contribution by dissolved organic matter. Denitrification in the oxygen minimum zone was predicted to be fuelled almost entirely by organic matter supplied by particles sinking vertically from the euphotic zone above (0.73 mmol N m−2 d−1) rather than from lateral transport of organic matter from elsewhere in the Arabian Sea (less than 0.01 mmol N m−2 d−1). Analysis of the carbon budget in the zone of denitrification (north of 10°N and east of 55°E) indicates that the modelled vertical export flux of detritus, which is similar in magnitude to estimates from field data based on the 234Th method, is sufficient to account for measured bacterial production below the euphotic zone in the Arabian Sea. 相似文献