Nitrogen (N) is one of the primary nutrients required to build biomass and is therefore in high demand in aquatic ecosystems. Estuaries, however, are frequently inundated with high concentrations of anthropogenic nitrogen, which can lead to substantially degraded water quality. Understanding drivers of biogeochemical N cycling rates and the microbial communities responsible for these processes is critical for understanding how estuaries are responding to human development. Estuaries are notoriously complex ecosystems: not only do individual estuaries by definition encompass gradients of salinity and other changing environmental conditions, but differences in physical parameters (e.g., bathymetry, hydrodynamics, tidal flushing) lead to a tremendous amount of variability in estuarine processes between ecosystems, as well. Here, we review the current knowledge of N cycling processes in estuaries carried out by bacteria and archaea, including both biogeochemical rate measurements and molecular characterizations of N cycling microbial communities. Particular attention is focused on identifying key environmental factors associated with distinct biogeochemical or microbial regimes across numerous estuaries. Additionally, we describe novel metabolisms or organisms that have recently been discovered but have not yet been fully explored in estuaries to date. While the majority of research has been conducted in the benthos, we also describe data from estuarine water columns. Understanding both the common patterns and the differences between estuaries has important implications for how these critical ecosystems respond to changing environmental conditions. 相似文献
The dynamics of the Peru–Chile upwelling system (PCUS) are primarily driven by alongshore wind stress and curl, like in other eastern boundary upwelling systems. Previous studies have suggested that upwelling-favorable winds would increase under climate change, due to an enhancement of the thermally-driven cross-shore pressure gradient. Using an atmospheric model on a stretched grid with increased horizontal resolution in the PCUS, a dynamical downscaling of climate scenarios from a global coupled general circulation model (CGCM) is performed to investigate the processes leading to sea-surface wind changes. Downscaled winds associated with present climate show reasonably good agreement with climatological observations. Downscaled winds under climate change show a strengthening off central Chile south of 35°S (at 30°S–35°S) in austral summer (winter) and a weakening elsewhere. An alongshore momentum balance shows that the wind slowdown (strengthening) off Peru and northern Chile (off central Chile) is associated with a decrease (an increase) in the alongshore pressure gradient. Whereas the strengthening off Chile is likely due to the poleward displacement and intensification of the South Pacific Anticyclone, the slowdown off Peru may be associated with increased precipitation over the tropics and associated convective anomalies, as suggested by a vorticity budget analysis. On the other hand, an increase in the land–sea temperature difference is not found to drive similar changes in the cross-shore pressure gradient. Results from another atmospheric model with distinct CGCM forcing and climate scenarios suggest that projected wind changes off Peru are sensitive to concurrent changes in sea surface temperature and rainfall. 相似文献
Estimates of the moments of inertia of Titan, as separately deduced from its gravitational field and spin pole orientation, are quite different. This discrepancy can be resolved if Titan is either not precessing as a rigid body (e.g. if the shell is decoupled from the interior by an ocean), or if the spin pole is not fully damped (e.g. due to atmospheric excitation). By the end of the Cassini mission, continued monitoring of the changing spin pole orientation, by Cassini radar observations, will determine which effect dominates. 相似文献
This paper discusses connections between the internationalisation of education, and in particular the growth in international students, and processes of urban transformation. The research is centred in Auckland, New Zealand, a city where the number of international students has grown rapidly over the last decade leading to significant impacts on the urban form and experience of the city’s CBD. This includes growth in educational services such as language schools and other private training establishments, new residential geographies characterised by low-cost and low-quality high-rise developments, and new ethnic economies of food, service and entertainment businesses that explicitly target international students. The paper draws on research with South Korean international students and a range of secondary materials to interrogate the connections between student mobilities and changing urban form. In doing so the paper contributes to emerging scholarship on student geographies and the role of students as urban agents through the inclusion of an international dimension that has largely been absent in the extant literature. The findings illustrate that while international students themselves clearly play a significant role in the transformation of urban spaces their influence cannot easily be separated from the contribution of a range of other actors including educational businesses, property developers, transnational migrants and local and national state actors. 相似文献
As anyone travelling on the roads of southeast England between 2006 and 2008 has probably been aware, major improvements have recently been made at the junction of the M25 and A2 motorways, south of the main Dartford crossing of the River Thames ( Fig. 1 ). The roadworks, funded by the UK Highways Agency, with the main contractors Jacobs Babtie and Costain, were accompanied by archaeological investigations carried out by Oxford Archaeology between 2003 and 2006. The archaeological programme had a major Palaeolithic/Pleistocene element under my direction ( Fig. 2 ) that has produced evidence of Neanderthal occupation early in the last glaciation (the Devensian, which lasted from 115 000 to 10 000 bp ), during a period when Britain had until now, been thought to have been entirely deserted. Figure 1 Open in figure viewer PowerPoint Site location and areas of investigation. 相似文献
