Plankton distribution and the impact of copepod grazing on primary production in Fram Strait, Greenland Sea

Two hydrobiological transects across the East Greenland Shelf and the open waters of Fram Strait in summer were chosen to illustrate the distribution and production of phyto- and zooplankton in relation to water masses and ice cover. The parameters used were temperature and salinity, inorganic nutrients, chlorophyll a, primary production, phytoplankton species composition, abundance of the dominant herbivorous copepods Calanus finmarchicus, C. glacialis, C. hyperboreus, Metridia longa and egg production of C. finmarchicus and C. glacialis. Grazing impact of copepodites and adults of these four species was modelled for each station by using egg production rates as an index of growth. Seasonal development of plankton communities was closely associated with the extent of the ice cover, hydrographic conditions and the water masses typical of the different hydrographic domains. Four regions were identified from their biological activities and physical environment: The Northeast Water polynya on the East Greenland Shelf, with a springbloom of diatoms and active reproduction of herbivorous copepods. The pack ice region, dominated by small flagellates and negligible grazing activities. The marginal ice zone, with high variability and strong gradients of autotroph production related to eddies and ice tongues, an active microbial loop and low egg production. The open water, with high station-to-station variability of most of the parameters, probably related to hydrographic mesoscale activities. Here, Phaeocystis pouchetii was a prominent species in the phytoplankton communities. Its presence may at least partly be responsible for the generally low egg production in the open waters. Grazing impact on primary production was always small, due to low zooplankton biomass in the polynya and due to low ingestion in the remaining regions.     

Dynamic system-optimal traffic assignment using a state space model

We propose a new mathematical formulation for the problem of optimal traffic assignment in dynamic networks with multiple origins and destinations. This problem is motivated by route guidance issues that arise in an Intelligent Vehicle-Highway Systems (IVHS) environment. We assume that the network is subject to known time-varying demands for travel between its origins and destinations during a given time horizon. The objective is to assign the vehicles to links over time so as to minimize the total travel time experienced by all the vehicles using the network. We model the traffic network over the time horizon as a discrete-time dynamical system. The system state at each time instant is defined in a way that, without loss of optimality, avoids complete microscopic detail by grouping vehicles into platoons irrespective of origin node and time of entry to network. Moreover, the formulation contains no explicit path enumeration. The state transition function can model link travel times by either impedance functions, link outflow functions, or by a combination of both. Two versions (with different boundary conditions) of the problem of optimal traffic assignment are studied in the context of this model. These optimization problems are optimal control problems for nonlinear discrete-time dynamical systems, and thus they are amenable to algorithmic solutions based on dynamic programming. The computational challenges associated with the exact solution of these problems are discussed and some heuristics are proposed.     

Preview Estimation and Control for (Semi-) Active Suspensions

An active suspension with preview is tested for rounded pulses and a stochastic road surface, and is compared to a passive suspension. The spectacular performance improvement obtained for a step function as road surface is not achieved but the improvement is still significant. The frequency response of the active suspension is determined for comparison with some suspension systems found in literature

An observer to reconstruct the preview information is presented. No model of the road surface is needed. From simulations, it appears that the observer reconstructs both deterministic and stochastic road surfaces satisfactory. However, the influence of measurement noise is not reduced sufficiently.     

The exclusive economic zone: criteria and machinery for the resolution of international conflicts between different users of the EEZ

It is highly probable that one of the results of the Third United Nations Conference on the Law of the Sea will be the recognition of a new jurisdictional zone—the exclusive economic zone (EEZ). This 200-mile zone will be used for a variety of potentially conflicting purposes by both the coastal state and non-coastal states and it is therefore important to ensure that the new convention emerging from the conference should establish criteria and make provision for machinery for the resolution of the international conflicts between different users which will undoubtedly arise. This paper contains a critical analysis of the latest draft articles under consideration at the Conference and considers the criteria by reference to which disputes might be resolved.     

Fracture toughness of steel plate from MV Kowloon Bridge

Chemical analysis, tensile, Charpy, DW NDT and fracture mechanics toughness data are presented for a grade A steel plate from the Kowloon Bridge, a sister ship of Derbyshire. The plate is shown to have poor toughness under dynamic loading conditions. The results leave open the possibility that brittle fracture could have contributed to a structural failure in the Derbyshire even at the high temperature of +30°C which is reported to have existed at the time of the ship's disappearance. The results are felt to support the proposition that present classification society rules need tightening to reduce the risk of brittle fracture in ships.     

A new role for planners: Working with community residents in formulating alternative plans for street patterns—Before decision making

Citizen participation at the neighborhood level will become effective only if reliable procedures can be developed for involving residents in the formulation of alternative plans before official decisions are made. Therefore, a research project in transportation planning was conducted in the Pico-Union neighborhood of Los Angeles, California, in order to design and test methods for producing reliable information that could be used by the neighborhood as well as by city transportation planners.The transportation problem of most concern to the residents of Pico-Union was the patterns and uses of their local streets, rather than the usual trip-to-work problem. Two independent methods were designed for determining what street patterns and uses were most preferred by the resident: a home-interview method and a photo-comparison method. The second enabled residents to compare photographic simulations of many possible re-designs of one of their local streets, and to make individual and group decisions about them. Results of the two methods generally agreed. However, one part of the research revealed that significant differences in decisions occur when simple changes are made in the orientation and sequence of the same two photographic simulations.Methods can be developed for obtaining information useful in urban planning at the neighborhood level by involving residents in an iterative learning and decision process. Broadly representative interdisciplinary teams are needed for further research to improve the reliability of such methods and for putting them into practice.     

Tests of assignment accuracy: An interurban case study

This paper is concerned with methods of testing the accuracy of traffic assignments. It focuses on the fact that whereas assignment models are usually based on a behavioural hypothesis about drivers' route choice (e.g. cost or time minimisation) the test of the accuracy of the assignment is the extent to which observed link loadings are reproduced. This inconsistency opens up the doubt that an apparently “accurate” assignment on this basis may be a result of compensating errors. It is difficult to apply the same test to accuracy of route choices as is applied to accuracy of link loadings (e.g. chi square, correlation coefficient) and hence a new measure is devised here which can be applied both to comparisons between observed and predicted route choices and comparisons between observed and predicted loadings. It is, moreover, possible to devise a test of significance for this measure so that one can test whether a predicted assignment is significantly different from what one, might have observed on the basis of chance observation. A case study is carried out to test the proposed method. Traffic flows between 72 origins and destinations on either side of the Pennine Mountains in Britain are assigned to a network using different assignment techniques with varied parameters. In all, one hundred and ninety assignments are carried out and the degree of correspondence between observed and predicted route choices and link loadings is measured. The results tend to confirm that the link loadings criterion is not a very stable criterion and that the route choice correspondence criteria seems to behave in a sensible way. A simulation exercise is carried out which produces the probability distribution of the “route-fit” index for different assumed sample levels. The paper concludes by suggesting avenues for further research.     

Recovery of inter- and intra-personal heterogeneity using mixed logit models

Most applications of discrete choice models in transportation now utilise a random coefficient specification, such as mixed logit, to represent taste heterogeneity. However, little is known about the ability of these models to capture the heterogeneity in finite samples (as opposed to asymptotically). Also, due to the computational intensity of the standard estimation procedures, several alternative, less demanding methods have been proposed, and yet the relative accuracy of these methods has not been investigated. This is especially true in the context of work looking at joint inter-respondent and intra-respondent variation. This paper presents an overview of the various different estimators, gives insights into some of the theoretical properties, and analyses their performance in a large scale study on simulated data. In particular, we specify 31 different forms of heterogeneity, with multiple versions of each dataset, and with results from over 16,000 mixed logit estimation runs. The findings suggest that variation in tastes over consumers is captured by all the methods, including the simpler versions, at least when sample size is sufficiently large. When tastes vary over choice situations for each consumer, as well as over consumers, the ability of the methods to capture and differentiate the two sources of heterogeneity becomes more tenuous. Only the most computationally intensive approach is able to capture adequately the two sources of variation, but at the cost of very high run times. Our results highlight the difficulty of retrieving taste heterogeneity with only cross-sectional data, providing further evidence of the benefits of repeated choice data. Our findings also suggest that the data requirements of random coefficients models may be more substantial than is commonly assumed, further reinforcing concerns about small sample issues.