Impacts of seagrass habitat architecture on bivalve settlement

We investigated the effects of differing spatial scales of seagrass habitat architecture on the composition and abundance of settling bivalves in a sub-tropical seagrass community. The density of newly settled bivalves was generally greater atThalassia testudinum grass bed edge (<1 m) compared to interior portions of the bed (>10 m). Deviation from this generalized pattern occurred when high densities of newly settled tulip mussels (Modiolus americanus) were recorded from the interior of the meadow, associated with aggregations of adult mussels. Bivalve settling densities appear to reflect settlement shadows of passively delivered larvae, bedload transport of newly settled individuals from unvegetated regions, as well as gregarious settlement among adult conspecifics. We also investigated the impact of seagrass patch shape and size on settlement by using artificial seagrass units (ASU) in separate short-term and long-term experiments. We found a positive relationship between ASU perimeter and bivalve abundance, suggesting that larval encounter rates with seagrass habitat may determine initial settlement patterns. Using ASUs we also investigated the relative role seagrass epiphytes play in determining the density of settling bivalves. Results showed greater settling densities where epiphytic secondary structure was elevated compared to controls, and bivalve density was significantly greater when ASUs were fouled with a natural community of epiphytes, suggesting that both microstructure and biofilms positively influenced bivalve settlement. We conclude that structural components of seagrass habitats increase bivalve settlement at multiple spatial scales, including epiphytic micro-structure, small-scale patch shape and size, and large-scale within habitat differences.     

三峡库区移民迁建工程地质问题与对策

三峡工程库区地处渝东、鄂西山区地带,丘陵占２１．７％,山地占７４％,平坝仅占４．３％,而这些平坝中最低的一部分又将被水库淹没。库区人口多,长期过度开垦,水土流失严重,后备土地资源缺少,地质环境容量极为有限,人地关系紧张,移民安置难度大,移民总人口达１２０万人。在１９９３年至１９９７年一期移民中,共安置了８．２万移民,在２００３年将需搬迁安置约５０万人,三期移民在２００９年前必须完成。因此,二期、三期移民工作比一期任务更为艰巨、时间更为紧迫。如果说,三峡工程成败在移民的话,那么移民的成败在某种程度上取决于地质。本文对库区移民工程建设中的工程地质,特别是地质灾害及其防治问题进行了研究。     

Post-settlement alteration of spatial patterns of soft shell clam (<Emphasis Type="Italic">Mya arenaria</Emphasis>) recruits

To examine the roles of settlement and early post-settlement processes in patterns of recruitment of the soft shell clamMya arenaria, abundance of juvenileMya at three intertidal sites in Barnstable Harbor, Massachusetts, was monitored over two settlement seasons. Two peaks of settlement occurred in 1998 (July and September) and one peak was recorded in June 1999, indicating that a late season settlement event is not a consistent feature at this site. Abundance of recent settlers (i.e., early recruits, < 1 week past settlement) varied significantly among the tidal flats (sites) that were separated by hundreds of meters to kilometers, but not among plots meters apart. Differences in abundance of settlers likely resulted in these differences in early-recruit abundance among sites. Settlement was greatest at the site with the greatest variability in flow speed. Sites also differed to some extent in their sediment characteristics and macrofaunal assemblages, which may influence larval substrate choice. Between-site differences in abundance ofMya decreased after settlement. The rate of decline of abundance varied among cohorts and sites. Comparison of abundance of recent settlers (up to 145,000 m⁻²) to that of juveniles > 3-mm shell length at the end of the settlement season (up to 60 m⁻²) indicated large losses of individuals during the early post-settlement period. This study demonstrates that spatial patterns inMya abundance can change substantially during the early post-settlement period, and that high mortality rates can result in cohorts contributing little to the population size even when rates of settlement are high.     

Settlement and Recruitment Patterns of the Soft-Shell Clam, <Emphasis Type="Italic">Mya arenaria</Emphasis>, on the Northern Shore of the Bay of Fundy,Canada

To examine the roles of settlement and early postsettlement processes in the recruitment of the soft-shell clam, Mya arenaria, abundance of recent settlers and juveniles was monitored over two field seasons at four locations on the northern shore of the Bay of Fundy in New Brunswick, Canada. Results showed great spatial and temporal variability in patterns of settlement. M. arenaria appeared to become highly vulnerable to postsettlement processes at a shell length of approximately 2 mm. Postsettlement processes drastically altered patterns of settlement less than 1 year after they were established. Results suggest that local factors at specific sites within the Bay of Fundy, such as hydrodynamics, larval behavior, and early postsettlement events, likely control the abundance of juvenile clams. Additionally, postsettlement events are extremely important in shaping M. arenaria populations in this area. Very few mature adult clams greater than 50 mm in shell length were found at any sampling sites, and no relationship was found between abundance of setters and density of juveniles and adults.     

Settlement patterns of brachyuran megalopae in a Louisiana estuary

Artificial substrate collectors were used to sample settled brachyuran megalopae in Terrebonne Bay, Louisiana, northern Gulf of Mexico. Three taxa,Callinectes sapidus, Uca spp., andRhithropanopeus harrisii, were the most common settlers. Settlement occurred almost year-round, although settlement densities were highest in late summer and early fall. In 1990, all three taxa had several simultaneous settlement peaks; in 1991, one simultaneous settlement peak occurred. FewerC. sapidus andUca spp. settled in 1991 than in 1990, but settlement abundance ofR. harrissii was similar between years. Local weather events, such as heavy precipitation, may have influenced the lower abundances ofC. sapidus andUca spp. in 1991. The megalopae ofC. sapidus andUca spp., which re-invade the estuary from higher salinity offshore waters where larval development occurs, may have been more affected by the lower 1991 estuarine salinity values than the larvae ofR. harrissii, which are retained within the estuary throughout larval development. Settlement ofC. sapidus andUca spp. was correlated with maximum tidal height, tidal amplitude, salinity, temperature, and lunar phase. In 1990, settlement ofC. sapidus was correlated with lunar declination cycles; settlement peaks occurred during equatorial minimum amplitude tides. The settlement ofR. harrisii was associated with tidal amplitude and lunar phase. Associations between environmental variables and settlement of megalopae were not consistent between the two years sampled. The opposing or reinforcing effects of various environmental variables on settlement, e.g., tidal height and rainfall, are superimposed on long-term hydrologic cycles so that dramatically altered cycles of settlement occur among years.     

Cambrian wave‐dominated tidal‐flat deposits,central Wisconsin,USA

In central Wisconsin, Cambrian strata of the Elk Mound Group record deposition on open‐coast, wave‐dominated tidal flats. Mature, medium‐grained quartz arenite is dominated by parallel‐bedding with upper‐flow regime parallel‐lamination, deposited during high‐energy storms that also produced three‐dimensional bedforms on the flats. Abundant wave ripples were produced as storms waned or during fair weather, in water depths ≤2 m. Indicators of variably shallow water (washout structures and stranded cnidarian medusae) and subaerial exposure (adhesion marks, rain‐drop impressions and desiccation cracks, including cracked medusae) are abundant. Parallel‐bedded facies preserve a Cruziana ichnofacies, similar to other Cambrian tidal‐flat deposits. Flats were dissected by small, mainly straight channels, the floors of which were grazed intensely by molluscs. Most channels were ephemeral but some developed low levées, point bars and cut‐banks, probably reflecting stabilization by abundant microbial mats and biofilms. Channels were filled with trough cross‐bedding that is interpreted to have been produced mainly during storm runoff. The strata resemble deposits of open‐coast, wave‐dominated tidal flats on the east coast of India and west coast of Korea. Ancient wave‐dominated and open‐coast tidal flats documented to date appear to have been limited to mud‐rich strata with ‘classic’ tidal indicators such as flaser bedding and tidal bundles. The Cambrian (Miaolingian to early Furongian) Elk Mound Group demonstrates that sandy, wave‐dominated tidal flats also can be recognized in the stratigraphic record.     

基于GIS和RS的黔南喀斯特地区人居环境自然适应性评价

运用GIS和RS技术,建立包括地形起伏度、温湿指数、水文指数和植被指数的人居环境自然适宜性评价模型,系统评价黔南喀斯特地区人居环境自然适宜性与限制性。结果表明:①黔南喀斯特地区人居环境以临界适宜和一般适宜为主要特征,人居环境自然适宜程度大致呈现出由山地向丘陵、河谷、平原递增的趋势,其中43%的地区为一般适宜区,57%的地区为临界适宜区。②黔南州人居环境自然条件,包括地形、气候、水文和植被不同程度地对人口分布产生影响,其中气候对人口分布的限制性最强,其次是地形条件。科学评价黔南喀斯特地区人居环境自然适宜性,能够引导该区的功能定位、人口分布,促进人口与环境的协调发展。      

Feeding rates of the sea nettle,Chrysaora quinquecirrha,under laboratory conditions

The scyphomedusa, Chrysaora quinquecirrha, preys on a variety of estuarine organisms. A series of experiments were conducted to measure the feeding rate of adult medusae on zooplankton prey. Artemia was used as a substitute for the endemic and abundant copepod, Acartia tonsa. The feeding rates ranged from three Artemia per hour per ml of medusae volume to 107 per hour per ml in 51 trials. The consumption was also linearly related to the initial prey concentration. Certain results suggested that a toxic factor might be exuded by the medusae, affecting the observed feeding rates. The results verify the possibility that Acartia may be a major prey organism in estuarine environments.     

The impact of yellow shore crabs,<Emphasis Type="Italic">Hemigrapsus oregonensis</Emphasis>, on Early Benthic Phase dungeness crabs,<Emphasis Type="Italic">Cancer magister</Emphasis>, in intertidal oyster shell mitigation habitat

Dungeness crab (Cancer magister) megalopae recruit to northeastern Pacific coastal estuaries, and settle into intertidal and subtidal habitats where they molt into Early Benthic Phase (EBP) crabs, and are dependent on epibenthic structure for shelter from predation. Given the importance of shell refuge to their post-settlement ontogeny, the U.S. Army Corps of Engineers began constructing intertidal plots of oyster shell in the Grays Harbor estuary, Washington, to enhance recruitment and mitigate losses of subtidalC. magister entrained and killed during extensive dredging efforts. When shell habitat was newly constructed, settlement and survival ofC. magister were high, and expectations for the mitigation project were met. During the first several years (1992-1997), plots greater than 1 yr postconstruction were colonized by yellow shore crabs,Hemigrapsus oregonensis (often≥75 crabs m⁻², and abundance of EBPC. magister was reduced to nearly zero. While some predation of settling megalopae byH. oregonensis does occur, the relationship between these species is characterized by density-dependent competitive interactions. Laboratory observations of competition for shell habitat indicate thatH. oregonensis are dominant over EBPC. magister and can evictC. magister from refuge spaces. Field experiments show that high densities of the former cause, the latter to emigrate from shell, and suggest detection and avoidance of areas with high densities ofH. oregonensis by settlingC. magister megalopae. More recently (1998-2001), abundance ofH. oregonensis has declined dramatically within plots of oyster shell, apparently due to recruitment failure, and patterns ofC. magister abundance and production have returned to levels consistent with original expectations of the mitigation project. Both intraspecific and interspecific competition for space are significant factors effecting population, regulation ofC. magister when they are strongly dependent on refuge from predation. Efficacy of constructed oyster shell plots as a mitigation tool may hinge on the population dynamics of a species previously considered of little consequence to the target species.     

Beyond Chinatown,beyond Enclave: Reconceptualizing Contemporary Chinese Settlements in the United States

Profound changes in international geopolitics and national policies, as well as global economic restructuring, and technological and communicational development have occurred since Chinatown first emerged. With the rapid pace of globalizing economy, increasing flows of population, commodities, information, and financial resources across the Pacific Rim – particularly in the last two decades – it is imperative that we reexamine the international migration patterns of Chinese population and reconceptualize the resulting Chinese settlement forms in the globalization era. This article attempts to serve such purposes by briefly reviewing the debate centered around historical and contemporary Chinatowns; discussing the impacts of societal and structural factors on Chinese immigration and settlement; presenting a spectrum of contemporary Chinese settlement forms in the United States, and highlighting some key characteristics. The article ends with brief discussion and conclusion sections that summarize some issues in contemporary Chinese community studies.     

Urban settlement program and land dispute resolution: The State of Israel versus the Negev Bedouin

Geo-political changes and the drive to higher standards of living induce growing numbers of nomad populations to give-up their special way of life. The Bedouin of the Israeli Negev are no exception. Gradually, they settled down in scattered, unauthorized clusters of tents and shacks in the open lands of the northern Negev. Governments of Israel were unsatisfied with the emerging unplanned spontaneous settlements. Consequently, governmental planning agencies came up with a program to house the Bedouin population in seven new towns. The government's initiative in establishing the towns had two main objectives. First, urban settlements were viewed as conduits for the provision of modern utilities and social services that were very hard to supply and distribute among the many small and scattered spontaneous settlements. Second, Bedouin put forward claims over much of the lands they arguably were using in their former way of life as nomads. Although the claims were not – and could not be – supported by legal documents, the government offered the Bedouin subsidized developed urban land in exchange of their claims over the undeveloped dry desert lands. Currently, more than 60 per cent of the Negev Bedouin population reside in the seven planned urban settlements. Nevertheless, only a small proportion of the land claims has been settled. This paper argues that the attempt to achieve both goals via the urban settlement program was too ambitious. While the goal of providing better services in urban areas has been partially accomplished, the goal of removing the claims over the lands is far from being resolved. The paper argues that the link between the two goals should be reconsidered. This revised version was published online in July 2006 with corrections to the Cover Date.     

Annual stability in the use of coves near inlets as settlement areas for winter flounder (Pseudopleuronectes americanus)

Coves near an ocean inlet in Little Egg Harbor in southern New Jersey may be important settlement areas for winter flounder (Pseudopleuronectes americanus), based on the high abundance of small postlarval individuals. During 1994–1996 we sampled in two coves and adjacent areas of this same estuary to determine if this pattern was annually consistent. Collections in spring and early summer indicated that small, recently-settled winter flounder (10–45 mm TL) were abundant in the coves from May to mid-June in every year with maximum mean densities ranging from 1.5–2.5 ind m⁻² and that these same size individuals were never collected at these densities in other habitats in the same estuary. Densities in the coves declined soon after settlement, usually by late June to early July in each year. In order to determine factors that may influence these high settlement rates we examined aspects of habitat quality by determining habitat-specific growth rates in cages for recently-settled individuals (17.5–38.3 mm TL) both inside and outside the coves during 1995. These rates (3.7–9.1 wt d⁻¹) overlapped those of other estuarine habitats (3.1–10.5 wt d⁻¹) suggesting that habitat quality, based on growth, does not differentiate the coves from other potential settlement habitats. Mark/recapture experiments for recently settled individuals (range 13–70 mm TL) in 1994 and 1996 had low recapture rates (1% in both years) suggesting that dispersal from the cove occurred soon after settlement. This pattern confirms that these coves are used as settlement areas but they are probably not used as nurseries because winter flounder, at least in this system, do not settle and stay and, as a result, use other habitats as primary nursery areas.     

Role of Flood Disturbance in Natural Oyster (<Emphasis Type="Italic">Crassostrea virginica</Emphasis>) Population Maintenance in an Estuary in South Texas,USA

A 2-year period with flood versus drought conditions provided the opportunity to examine the effects of flood disturbance on subtidal eastern oyster Crassostrea virginica biology and population dynamics in a south Texas estuary. Oysters were sampled monthly in 2007 and 2008 to examine the impacts of changing environmental conditions on oyster populations. Oysters were also examined quarterly for the presence of Perkinsus marinus. Filtration rates were calculated as a function of oyster size, temperature, salinity, and total suspended solids. Flood events in 2007 caused temporary reductions in salinity and were associated with reductions in oyster abundance, spat settlement, disease levels (weighted prevalence and percent infection), and filtration rates. Oyster populations had generally recovered within 1 year’s time—the oysters were younger and smaller but were just as abundant as pre-flood levels. The rapid return of oysters to pre-flood abundance levels is attributed in part to the ability of oysters in Gulf coast estuaries to spawn multiple times in a single season and in part to their relatively high growth rates. Although flood disturbance may temporarily reduce or destroy oyster populations, the ability of the Mission–Aransas Estuary to retain freshwater pulses within the system and maintain low salinities that are unfavorable for predators and disease can facilitate oyster population recovery. Episodic flood events appear to play a critical role in promoting long-term oyster population maintenance in the Mission–Aransas Estuary. The response of oysters to changing environmental conditions over the short term provides some insights into the potential long-term effects of changing climate.     

The ecology of hazardousness,the experience of South America

In many parts of South America, the socio-economic condition of the population is such that in many respects the people, because of their vulnerable state, are a disaster waiting to occur. The triggering action of the disaster will be an extreme physical or biophysical event. If any advance is to be made in natural hazard management, then the focus must be upon people first. Halt the process of underdevelopment in South American societies and you have taken a major step forward in natural hazard management. The major concern of this paper is not so much with the competition for first place in hazard proneness in South America, but rather the elaboration of the total ecology of what we commonly designate as disasters, which ordinarily occur at the interface of extreme natural phenomena and vulnerable settlement patterns, and which should be seen, as the extreme situation which is implicit in the everyday condition of the population. Vulnerability is the key concept in this relationship. South American nations vary greatly in their hazard proneness, in the vulnerability of sectors in their societies, in the losses and general repercussions of hazardous events, and in their ability to cope effectively with the post-disaster situation, either alone or with international aid.     

The distribution and growth of urban settlements in Saudi Arabia

This paper examines the distribution characteristics of urban settlements in the Kingdom of Saudi Arabia through major dimensions such as settlement regularly, degree of primacy and polarisation at the national and geographic regional levels. The paper points out that the settlement pattern at the national level has transformed from a primate distribution during the pre-plan period (< 1970) to an intermediary distribution during the period of development planning (1970 >). At the geographical regional level the settlement distribution, however, has shown diverse pattern. While the regional settlement distribution of Western and Eastern Region have shown trends of increasing polarisation, settlements in the Northern and Southern Regions are more log-normal in character. This is explained by the increasing growth of small and intermediary towns in the Northern and Southern Regions. The paper concludes that the national trend of decreasing urban primacy, induced by the decentralised national development strategies, is less pronounced due to the absence of a balanced regional investment policy. Since the government expenditure and settlement population growth rates are not related, slow growing primate cities continue to receive a larger share of public expenditure, enabling them to retain their primacy. This calls for a balanced regional investment policy to fully realise the national strategy of regional balance and multi-polar settlement pattern.     

Assessing the use of global land cover data for guiding large area population distribution modelling

Gridded population distribution data are finding increasing use in a wide range of fields, including resource allocation, disease burden estimation and climate change impact assessment. Land cover information can be used in combination with detailed settlement extents to redistribute aggregated census counts to improve the accuracy of national-scale gridded population data. In East Africa, such analyses have been done using regional land cover data, thus restricting application of the approach to this region. If gridded population data are to be improved across Africa, an alternative, consistent and comparable source of land cover data is required. Here these analyses were repeated for Kenya using four continent-wide land cover datasets combined with detailed settlement extents and accuracies were assessed against detailed census data. The aim was to identify the large area land cover dataset that, combined with detailed settlement extents, produce the most accurate population distribution data. The effectiveness of the population distribution modelling procedures in the absence of high resolution census data was evaluated, as was the extrapolation ability of population densities between different regions. Results showed that the use of the GlobCover dataset refined with detailed settlement extents provided significantly more accurate gridded population data compared to the use of refined AVHRR-derived, MODIS-derived and GLC2000 land cover datasets. This study supports the hypothesis that land cover information is important for improving population distribution model accuracies, particularly in countries where only coarse resolution census data are available. Obtaining high resolution census data must however remain the priority. With its higher spatial resolution and its more recent data acquisition, the GlobCover dataset was found as the most valuable resource to use in combination with detailed settlement extents for the production of gridded population datasets across large areas.     

Possibilities of agricultural settlements in the northern coastal area of Kenya —an analysis of relevant climatic factors

Kenya is one of those African countries with a very high population pressure. In cultivatable areas we find up to 650 rural inhabitants per sq km. The search for new possibilities of agricultural settlement is an indispensable task. Many sciences are involved, also the geography of climates. Probably the largest potential settlement area from the climatic point of view is the Northern coastal zone. It has been settled in medieval times much more densely than today. War-like invasions of nomads have decreased the population since the 16th century. The average density of the cultivatable strip has been less than 5 inhabitants per sqkm in the last census year 1969. New settlement schemes have been developed since 1970, but they face the difficulty to choose the right varieties of crops. Careful climatic investigations are necessary for the further selection of areas and for the agricultural extension service. Here, only a first approach has been made. The main emphasis was laid on the effective rainfall in the so-called agrohumid period of the year and its probability. A new system of climatic classification was applied to give a frame for further, more specialised studies.     

Larval settlement of the Atlantic rangia,Rangia cuneata (Bivalvia: Mactridae)

Rangia cuneata larvae were given the opportunity to settle on substrate types that differed in grain size, organic content, and bacterial abundance. Larvae settled in very fine sand (63–125 μm) and fine and medium sand (125–500 μm) more than in silt and clay (<63 μm) or on a hard substratum (empty polystyrene petri dishes). Proportionally more larvae settled onto untreated sediment from the adult habitat than onto the same sediment autoclaved, or treated with either H₂O₂ to remove organic material, or treated with antibiotics to reduce bacterial abundance. However, it appears that competent larvae do not delay settlement, even in the absence of “attractive” substrates.     

Assessment of frozen-ground conditions for engineering geology along the Qinghai–Tibet highway and railway, China

The Qinghai–Tibet Highway and Railway (the Corridor) across the Qinghai–Tibet Plateau traverses 670 km of permafrost and seasonally frozen-ground in the interior of the Plateau, which is sensitive to climatic and anthropogenic environmental changes. The frozen-ground conditions for engineering geology along the Corridor is complicated by the variability in the near-surface lithology, and the mosaic presence of warm permafrost and talik in a periglacial environment. Differential settlement is the major frost-effect problem encountered over permafrost areas. The traditional classification of frozen ground based on the areal distribution of permafrost is too generalized for engineering purposes and a more refined classification is necessary for engineering design and construction. A proposed classification of 51 zones, sub-zones, and sections of frozen ground has been widely adopted for the design and construction of foundations in the portion of the Corridor studied. The mean annual ground temperature (MAGT), near-surface soil types and moisture content, and active faults and topography are most commonly the primary controlling factors in this classification. However, other factors, such as local microreliefs, drainage conditions, and snow and vegetation covers also exert important influences on the features of frozen ground. About 60% of the total length of the Corridor studied possesses reasonably good frozen-ground conditions, which do not need special mitigative measures for frost hazards. However, other sections, such as warm and ice-rich or -saturated permafrost, particularly in the sections in wetlands, ground improvement measures such as elevated land bridges and passive or proactive cooling techniques need to be applied to ensure the long-term stability of thermally unstable, thick permafrost subsoils, and/or refill with non-frost-susceptible soils. Due to the long-history of the construction and management of the Corridor by various government departments, adverse impacts of construction and operation on the permafrost environment have been resulted. It is recommended that an integrated, executable plan for the routing of major construction projects within this transportation corridor be established and long-term monitoring networks installed for evaluating and mitigating the impact from anthropogenic and climatic changes in frozen-ground conditions.     

Changing patterns of population distribution in Zimbabwe

This paper critically evaluates the causes and consequences of changes in population distribution in Zimbabwe during the colonial period and since independence in 1980. Five main aspects of population geography are examined. Firstly, by way on background on contemporary patterns of settlement and land use, the history of tenure policies is outlined. Secondly, the distribution of the African population as revealed in the 1982 census is described and major changes between the census years of 1962, 1969 and 1982 are discussed. Thirdly, changing patterns of settlement and land use within the peasant farming areas (Communal Lands) are examined in the context of increasing population pressures. Fourthly, trends in the growth of the main urban centres and the process of urbanisation of the African population are described. Fifthly, post-independence development policies directed at effecting changes in the distribution of population are discussed with particular reference to the land resettlement programme. The present distribution of population is seen as the product of interacting political, economic and environmental factors, knowledge of which is essential in devising appropriate development strategies to achieve a more equable balance of population and resources.