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531.
532.
Ephemeral active regions attain maximum development within 1 day or less of their initial appearance and are typically observed for 1–2 days. They appear mostly as small bipolar regions having a typical dimension of about 30000 km and a maximum total flux of the order of 1020 Mx. The ephemeral regions generally do not produce sunspots and flares, though they are identified in H as small active centers.Our observations indicate that the ephemeral regions are frequently generated both near large active centers and in extensive quiet areas of the Sun. The location of emerging ephemeral regions does not appear to be associated with the distribution of the existing network fields. As many as 100 ephemeral regions may form per day. On the average, as much flux may erupt in the form of small ephemeral regions as erupts in larger active centers.The latitude distribution of ephemeral regions appears to be much wider than that of sunspots and major active centers. Their frequency of occurrence does not appear to follow the sunspot cycle. 相似文献
533.
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535.
Variability of dense water formation in the Ross Sea 总被引:1,自引:0,他引:1
This paper presents results from a model study of the interannual variability of high salinity shelf water (HSSW) properties
in the Ross Sea. Salinity and potential temperature of HSSW formed in the western Ross Sea show oscillatory behaviour at periods
of 5–6 and 9 years superimposed on long-term fluctuations. While the shorter oscillations are induced by wind variability,
variability on the scale of decades appears to be related to air temperature fluctuations. At least part of the strong decrease
of HSSW salinities deduced from observations for the period 1963–2000 is shown to be an aliasing artefact due to an undersampling
of the periodic signal. While sea ice formation is responsible for the yearly salinity increase that triggers the formation
of HSSW, interannual variability of net freezing rates hardly affects changes in the properties of the resulting water mass.
Instead, results from model experiments indicate that the interannual variability of dense water characteristics is predominantly
controlled by variations in the shelf inflow through a sub-surface salinity and a deep temperature signal. The origin of the
variability of inflow characteristics to the Ross Sea continental shelf can be traced into the Amundsen and Bellingshausen
Seas. The temperature anomalies are induced at the continental shelf break in the western Bellingshausen Sea by fluctuations
of the meridional transport of circumpolar deep water with the eastern cell of the Ross Gyre. In the Amundsen Sea, upwelling
due to a persistently cyclonic wind field carries the signal into the surface mixed layer, leading to fluctuations of the
vertical heat flux, anomalies of brine release near the sea ice edge, and consequently to a sub-surface salinity anomaly.
With the westward flowing coastal current, both the sub-surface salinity and deep temperature signals are advected onto the
Ross Sea continental shelf. Convection carries the signal of salinity variability into the deep ocean, where it interacts
with modified circumpolar deep water upwelled onto the continental shelf as the second source water mass of HSSW. Sea ice
formation on the Ross Sea continental shelf thus drives the vertical propagation of the signal rather than determining the
signal itself. 相似文献
536.
Variability of dissolved Fe2+ and ∑H2S concentrations in porewaters were studied, using cathodic stripping voltammetry (CSV) microelectrodes, in sediments overlain
by hypoxic waters in the summer from the southeastern region of Corpus Christi Bay, Texas (CCB) and the Mississippi River
Bight (MRB), Louisiana. These measurements were complimented by sediment microcosm studies where oxygen concentrations in
the overlying water were manipulated. Sulfate reduction rates, benthic oxygen demand, total reduced sulfide, porewater sulfate,
and total organic carbon were also determined. Fe2+ and ∑H2S were the major dissolved redox-reactive dissolved species in these sediments. During hypoxic conditions, an upward migration
of porewater Fe2+ and ∑H2S occurred, with Fe2+ reaching much higher maximum concentrations than ∑H2S. Statistically significant (p < 0.05) differences in both Fe2+ and ∑H2S occurred between sediments at the CCB and MRB study sites. Although both sites were Fe-dominated, reactive and dissolved
iron were higher while ∑H2S was lower at the MRB site. However, there were no statistically significant (p > 0.05) difference in regard to ∑H2S between microcosm and field monitoring studies. Results indicated that, for Fe2+ and ∑H2S, relatively large and rapid changes occurred in both the concentrations and distributions of these important porewater constituents
in response to relatively short-term changes in overlying water oxygen content. Model calculations indicated that conditions
in the sediments can be responsible for the induction of hypoxic conditions in the formation of hypoxic overlying waters in
<6 days at CCB and ~20 days at MRB. 相似文献
537.
Mads Solgaard Thomsen Karen J. McGlathery Anna Christina Tyler 《Estuaries and Coasts》2006,29(3):465-473
We determined the distribution of macroalgae in Hog Island Bay, a shallow coastal lagoon in Virginia, USA, seasonally at 12
sites from 1998 to 2000 and at 3 representative sites from 2000 to 2002. We analyzed macroalgal biomass, taxonomic richness,
and abundance of two non-native species, the cryptic invaderGracilaria vermiculophylla and the conspicuousCodium fragile, with respect to season, location (mainland, mid lagoon, barrier island sites), and elevation (intertidal, subtidal). Taxonomic
richness, total algal biomass, and nonnative biomass peaked in the summer months when temperature and light availability were
highest. A few stress tolerant and ephemeral algae dominated the algal assemblage.G. vermiculophylla constituted 74% of the entire algal biomass, was the most abundant alga in all seasons, locations, and elevation levels,
and was positively correlated with taxonomic richness and abundance of filamentous species.Ulva curvata, Bryopsis plumosa, andC. fragile accounted for an additional 16% of the algal biomass. There are distinct habitats in Hog Island Bay that can be classified
into low diversity-low biomass regions near the mainland and barrier islands and high diversity-high biomass regions in the
open mid lagoon, where abundant shells for attachment and intermediate levels of water column nutrients and turbidity likely
create better growth conditions. Taxonomic richness and biomass were higher in subtidal than intertidal zones, presumably
due to lower desiccation stress. This study provides an example of how a single invasive species can dominate an entire assemblage,
both in terms of biomass (being most abundant in all seasons, locations, and tidal levels) and species richness (correlating
positively with epiphytic filamentous taxa). By adding hard-substratum structural complexity to a relatively homogenous soft-substratum
system,G. vermiculophylla increases substratum availability for attachment and entanglement of other algal species and enhances local diversity. Without
widespread and abundantG. vermiculophylla, taxa likePolysiphonia, Ceramium, Bryopsis, Ectocarpus, andChampia would likely be much less common. This study also highlights the importance of using DNA analysis of voucher specimens in
monitoring programs to accurately identify cryptic invaders. 相似文献
538.
Tim McCreanor Liane Penney Victoria Jensen Karen Witten Robin Kearns Helen Moewaka Barnes 《New Zealand geographer》2006,62(3):196-207
Abstract: This paper examines the culturally specific experiences of belonging within Oruāmo/Beachhaven, a suburb in North Shore City. In-depth interviews with 32 caregivers of young children expose the fact that the ethnic groups represented – Māori, Samoan and Pākehā– vary in their uses and understandings of, as well as feelings for, residential neighbourhood. Examination of the themes of natural environment, social relations, continuity of residence, facilities, organisations and place transformation reveal that, while representatives of all groups had views on these matters, there was a variable degree of engagement with and priority afforded to these concerns. 相似文献
539.
Kathleen M. Swanson Judith Z. Drexler David H. Schoellhamer Karen M. Thorne Mike L. Casazza Cory T. Overton John C. Callaway John Y. Takekawa 《Estuaries and Coasts》2014,37(2):476-492
Salt marsh faunas are constrained by specific habitat requirements for marsh elevation relative to sea level and tidal range. As sea level rises, changes in relative elevation of the marsh plain will have differing impacts on the availability of habitat for marsh obligate species. The Wetland Accretion Rate Model for Ecosystem Resilience (WARMER) is a 1-D model of elevation that incorporates both biological and physical processes of vertical marsh accretion. Here, we use WARMER to evaluate changes in marsh surface elevation and the impact of these elevation changes on marsh habitat for specific species of concern. Model results were compared to elevation-based habitat criteria developed for marsh vegetation, the endangered California clapper rail (Rallus longirostris obsoletus), and the endangered salt marsh harvest mouse (Reithrodontomys raviventris) to determine the response of marsh habitat for each species to predicted >1-m sea-level rise by 2100. Feedback between vertical accretion mechanisms and elevation reduced the effect of initial elevation in the modeled scenarios. Elevation decreased nonlinearly with larger changes in elevation during the latter half of the century when the rate of sea-level rise increased. Model scenarios indicated that changes in elevation will degrade habitat quality within salt marshes in the San Francisco Estuary, and degradation will accelerate in the latter half of the century as the rate of sea-level rise accelerates. A sensitivity analysis of the model results showed that inorganic sediment accumulation and the rate of sea-level rise had the greatest influence over salt marsh sustainability. 相似文献
540.
The use of chemical dispersants to combat oil spills at sea: A review of practice and research needs in Europe 总被引:1,自引:0,他引:1
In order to better understand the practice of dispersant use, a review has been undertaken of marine oil spills over a 10 year period (1995-2005), looking in particular at variations between different regions and oil-types. This viewpoint presents and analyses the review data and examines a range of dispersant use policies. The paper also discusses the need for a reasoned approach to dispersant use and introduces past cases and studies to highlight lessons learned over the past ten years, focussing on dispersant effectiveness and monitoring; toxicity and environmental effects; the use of dispersants in low salinity waters; response planning and future research needs. 相似文献