Experiments with lake phytoplankton in large enclosures

The use of large plastic enclosures, or 'Lund tubes', in lakes as a semi-natural basin for experiments, approximating to lakes within lakes, is discussed. The advantages and disadvantages of the tubes were studied, and the results of investigations into the validity of studies using such tubes and the effects of fertilisers on phytoplankton are presented.

Transient flows in enclosures: A generalised approach for modelling the effects of geometry, heat gains and wind

Transient flows in a confined ventilated space induced by a buoyancy source of time-varying strength and an external wind are examined. The space considered has varying cross-sectional area with height. A generalised theoretical model is proposed to investigate the flow dynamics following the activation of an external wind and an internal source of buoyancy. To investigate the effect of geometry, we vary the angle of the wall inclination of a particular geometry in which a point source of constant buoyancy is activated in the absence of wind. Counter-intuitively the ventilation is worse and lower airflow rates are established for geometries of increasing cross-sectional areas with height. We investigate the effect of the source buoyancy strength by comparing two cases: (1) when the buoyancy input is constant and (2) when the buoyancy input gradually increases over time so that after a finite time the total buoyancy inputs for (1) and (2) are identical. The rate at which the source heat gains are introduced has a significant role on the flow behaviour as we find that, in case (2), a warmer layer and a more pronounced overshoot are obtained than in case (1). The effect of assisting and opposing wind on the transient ventilation of an enclosure of constant cross-sectional area with height and constant heat gains is examined. A Froude number Fr is used to define the relative strengths of the buoyancy-induced and wind-induced velocities and five different transient states and their associated critical Fr are identified. © 2010 Elsevier Ltd.

EXPERIMENTS ON AQUATIC ECOSYSTEM REGIME SHIFT IN ENCLOSURES NEAR LAKE DIANCHI, CHINA

To discover how a lake converts from a turbid state to clean state, and what drives this process, we constructed controlled enclosure ecosystems and used the ecological remediation method to force ecosystems to convert from the turbid state to the clean state. Our results show that the driving forces include temperature., macrophyte, silver carp and mussel, which form a combined force to drive the controlled ecosystem to switch. There is a threshold existing in treated enclosure ecosystem during the conversion from turbid to clean state. When TP <0.09 mg.L-1, Chl-a <0.036 mg.L-1, transparency >62 cm, TN <2.15 mg.L-1, CODMn <13.7 mg.L-1, tubidity <10, and the number of algal cells <10(6) cells.L-1, the treated ecosystem changes sharply from turbid to clean state. The conversion process can be divided into three phases: turbid state, clean-turbid transitional state as well as clean state, and described with the power function Y = a*X-b (where Y is water parameter, X is time, a and b are constants), which indicates that the shift in the enclosure ecosystem from turbid to clean state is discontinuous.

Application of static and dynamic enclosures for determining dimethyl sulfide and carbonyl sulfide exchange in Sphagnum peatlands: Implications for the magnitude and direction of flux

A static enclosure method was applied to determine the exchange of dimethyl sulfide (DMS) and carbonyl sulfide (OCS) between the surface of Sphagnum peatlands and the atmosphere. Measurements were performed concurrently with dynamic (flow through) enclosure measurements with sulfur-free air used as sweep gas. This latter technique has been used to acquire the majority of available data on the exchange of S gases between the atmosphere and the continental surfaces and has been criticized because it is thought to overestimate the true flux of gases by disrupting natural S gas gradients. DMS emission rates determined by both methods were not statistically different between 4 and >400 nmol m−2 h−1, indicating that previous data on emissions of at least DMS are probably valid. However, the increase in DMS in static enclosures was not linear, indicating the potential for a negative feedback of enclosure DMS concentrations on efflux. The dynamic enclosure method measured positive OCS flux rates (emission) at all sites, while data using static enclosures indicated that OCS was consumed from the atmosphere at these same sites at rates of 3.7 to 55 nmol m−2 h−1. Measurements using both enclosure techniques at a site devoid of vegetation showed that peat was a source of both DMS and OCS. However, the rate of OCS efflux from decomposing peat was more than counterbalanced by OCS consumption by vegetation, including Sphagnum mosses, and net OCS uptake occurred at all sites. We propose that all wetlands are net sinks for OCS.

Predicting HCl concentrations in fire enclosures using an HCl decay model coupled to a CFD-based fire field model

The amount of atmospheric hydrogen chloride (HCl) within fire enclosures produced from the combustion of chloride-based materials tends to decay as the fire effluent is transported through the enclosure due to mixing with fresh air and absorption by solids. This paper describes an HCl decay model, typically used in zone models, which has been modified and applied to a computational fluid dynamics (CFD)-based fire field model. While the modified model still makes use of some empirical formulations to represent the deposition mechanisms, these have been reduced from the original three to two through the use of the CFD framework. Furthermore, the effect of HCl flow to the wall surfaces on the time to reach equilibrium between HCl in the boundary layer and on wall surfaces is addressed by the modified model. Simulation results using the modified HCl decay model are compared with data from three experiments. The model is found to be able to reproduce the experimental trends and the predicted HCl levels are in good agreement with measured values

Bad news for satisfied tenants: On the social construction of social rented housing as dreadful enclosures

As long as the social rented sector - which comprised 6% of the housing stock - housed traditional families and the allocation procedures were rather loose, little commotion came about. A combination of changes in family structures (leading to the in stream of ethnic minorities), economic changes (leading to the in stream of poor people), and the strengthening of allocation procedures towards those most in need, did change perceptions. Marginalisation and ghettoisation became during the 1990s the buzzwords when talking and writing about social rented housing. In this article, we will explain the background of the scapegoat trends and the possible consequences for social tenants in particular and for the social rental housing in general.