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31.
Farooq Azam Grieg F. Steward David C. Smith Hugh W. Ducklow 《Journal of Earth System Science》1994,103(2):341-351
In the Arabian Sea, temporal contiguity of highly oligotrophic and eutrophic periods, along with high water temperatures,
may result in unique features of bacteriaorganic matter coupling, nutrient cycling and sedimentation, which are unlike those
in the classical oligotrophic and eutrophic waters. Bacteria-phytoplankton interactions are suggested to influence phytoplankton
aggregation and its timing. It is also hypothesized that, within aggregates, hydrolytic ectoenzyme activity, together with
condensation reactions between the hydrolysis products, produce molecular species which are not readily degraded by pelagic
bacteria. Accumulation of a reservoir of such slow-to-degrade dissolved organic carbon (DOC) is proposed to be a carbon flux
and energy buffer, which moderates the response of bacteria to the dramatic variations in primary production in the Arabian
Sea. Use of the slow-to-degrade DOC pool during the intermonsoon could temporarily render the Arabian Sea net-heterotrophic
and a source of CO2 to the atmosphere. Stored DOC is also suggested to balance the observed deficit between mesopelagic carbon demand and the
sinking particulate organic carbon supply. Knowledge of the significance of bacteria in carbon storage and cycling in the
Arabian Sea is needed to understand the response of the ocean’s biogeochemical state to strong physical forcing and climate
change. 相似文献
32.
Kimberly E. Medley Mark J. McDonnell Steward T. A. Pickett 《The Professional geographer》1995,47(2):159-168
Human activities and forest-landscape structure are examined along a belt transect that extends 140 km from New York City to northwestern Connecticut. The study quantifies urban structures (population density, land use. transportation) and processes (population growth, urban land development) in the transect, and tests for relationships with the distribution, sizes, and shapes of forests. Our results identify distinct characteristics of urban and suburban environments based on the landscape structure of native forest vegetation. These findings are applicable to the management of natural areas along a gradient of urbanization. 相似文献
33.
Landscape evolution models provide a way to determine erosion rates and landscape stability over times scales from tens to thousands of years. The SIBERIA and CAESAR landscape evolution models both have the capability to simulate catchment–wide erosion and deposition over these time scales. They are both cellular, operate over a digital elevation model of the landscape, and represent fluvial and slope processes. However, they were initially developed to solve research questions at different time and space scales and subsequently the perspective, detail and process representation vary considerably between the models. Notably, CAESAR simulates individual events with a greater emphasis on fluvial processes whereas SIBERIA averages erosion rates across annual time scales. This paper describes how both models are applied to Tin Camp Creek, Northern Territory, Australia, where soil erosion rates have been closely monitored over the last 10 years. Results simulating 10 000 years of erosion are similar, yet also pick up subtle differences that indicate the relative strengths and weaknesses of the two models. The results from both the SIBERIA and CAESAR models compare well with independent field data determined for the site over different time scales. Representative hillslope cross‐sections are very similar between the models. Geomorphologically there was little difference between the modelled catchments after 1000 years but significant differences were revealed at longer simulation times. Importantly, both models show that they are sensitive to input parameters and that hydrology and erosion parameter derivation has long‐term implications for sediment transport prediction. Therefore selection of input parameters is critical. This study also provides a good example of how different models may be better suited to different applications or research questions. Copyright © 2010 John Wiley & Sons, Ltd and Commonwealth of Australia 相似文献
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A flow pattern is characterized by aquifer features and the number, type, and distribution of stagnation points (locations where the discharge is zero). This article identifies a condition for transition of flow patterns in two-dimensional groundwater flow obeying Darcy’s law by examining changes in stagnation points, using the Taylor series expansion of the discharge vector. It is found that the three standard types of stagnation points (minimums, maximums, and saddle points) are completely characterized by the first-order term containing the discharge gradient tensor. However, when the determinant of the tensor becomes zero, stagnation points of other types characterized by higher-order terms come into existence. In this article, we call these zero-determinant stagnation points as critical stagnation points; they may emerge suddenly, split to a set of new stagnation points, or disappear from the flow, resulting in transitions of flow patterns. Examples of both transient and steady flows are used to illustrate the usefulness and significance of critical stagnation points. 相似文献