Lag effects in the impacts of mass coral bleaching on coral reef fish, fisheries, and ecosystems

Recent episodes of coral bleaching have led to wide-scale loss of reef corals and raised concerns over the effectiveness of existing conservation and management efforts. The 1998 bleaching event was most severe in the western Indian Ocean, where coral declined by up to 90% in some locations. Using fisheries-independent data, we assessed the long-term impacts of this event on fishery target species in the Seychelles, the overall size structure of the fish assemblage, and the effectiveness of two marine protected areas (MPAs) in protecting fish communities. The biomass of fished species above the size retained in fish traps changed little between 1994 and 2005, indicating no current effect on fishery yields. Biomass remained higher in MPAs, indicating they were effective in protecting fish stocks. Nevertheless, the size structure of the fish communities, as described with size-spectra analysis, changed in both fished areas and MPAs, with a decline in smaller fish (<30 cm) and an increase in larger fish (>45 cm). We believe this represents a time-lag response to a reduction in reef structural complexity brought about because fishes are being lost through natural mortality and fishing, and are not being replaced by juveniles. This effect is expected to be greater in terms of fisheries productivity and, because congruent patterns are observed for herbivores, suggests that MPAs do not offer coral reefs long-term resilience to bleaching events. Corallivores and planktivores declined strikingly in abundance, particularly in MPAs, and this decline was associated with a similar pattern of decline in their preferred corals. We suggest that climate-mediated disturbances, such as coral bleaching, be at the fore of conservation planning for coral reefs.\u

The removal of solutes from organic solvents using nanofiltration

Transport mechanisms and process limitations are relatively well understood for aqueous nanofiltration systems. Much work has also been done on the use of membranes for the removal of suspended matter from organic solvents. The removal of organic solute compounds from organic solvents using membrane technology has been addressed by very few workers, and little is known of the fundamental transport and separation mechanisms. A dense polydimethylsiloxane (PDMS) composite membrane was used to assess the flux and separation performance of a range of organic solute compounds and organic solvents. Solvent flux was modelled with the Hagen- Poisuelle equation and found to fit the model well, with swelling effects being the most likely cause of some deviations. The effect of solvent type and membrane swelling on solute rejection is discussed

Solvent resistant nanofiltration: developing understanding of transport mechanisms

In recent years the possibility of using polymeric nanofiltration (NF) membranes for non-aqueous separations has been explored. There is, however, significant debate concerning fundamental mechanisms where concepts include solution-diffusion and ‘pore’ flow. This paper presents nanofiltration and swelling data for polyacrylonitrile (PAN)/poly-dimethylsiloxane (PDMS) composite membranes with a range of low and higher polarity solvents, some of which contained solutes in the range 84-612 MW. The influences of parameters such as crossflow rate, applied pressure, solute size and solvent polarity on filtration performance are presented and measures of flux and solute rejection are related to membrane swelling. More comprehensive descriptions of the experimental apparati and results are shown in [1-7]

Nanofiltration - a method for solute removal from fuel simulants

The separation characteristics of a dense polydimethylsiloxane (PDMS) membrane were studied using alkyl and aromatic solvents and low-polarity, sulphur bearing, organometallic (OM) and polynuclear aromatic (PNA) solute compounds. Rejection was found to be dependent on transmembrane pressure, crossflow rate (hydrodynamic conditions), solute size and the degree of swelling induced by the solvent. Rejection increased progressively with pressure whilst a threshold condition was observed above which further increases in crossflow had a negligible influence on rejection. Measurements over the molecular weight range 84-612 g/mol showed the membrane to have a cut-off in the region 350-400 g/mol to all but one of the tested PNA compounds (rubrene). An additional correlation using molecular dimensions instead of molecular weight showed the cutoff size to be in the region of 1-2 nm, with all data falling on a well defined rejection/size curve. Solvent type influenced membrane swelling to an extent dependent on the relative magnitude of the solubility parameters for the solvent and PDMS; similar values led to more swelling, higher fluxes and lower rejections. Results support the concept of viscous solvent flow whilst solute transport could be either predominantly viscous or a combination of viscous and diffusive. With larger molecules a size exclusion mechanism was dominant

Lyapunov Mode Dynamics in Hard-Disk Systems

The tangent dynamics of the Lyapunov modes and their dynamics as generated numerically - {\it the numerical dynamics} - is considered. We present a new phenomenological description of the numerical dynamical structure that accurately reproduces the experimental data for the quasi-one-dimensional hard-disk system, and shows that the Lyapunov mode numerical dynamics is linear and separate from the rest of the tangent space. Moreover, we propose a new, detailed structure for the Lyapunov mode tangent dynamics, which implies that the Lyapunov modes have well-defined (in)stability in either direction of time. We test this tangent dynamics and its derivative properties numerically with partial success. The phenomenological description involves a time-modal linear combination of all other Lyapunov modes on the same polarization branch and our proposed Lyapunov mode tangent dynamics is based upon the form of the tangent dynamics for the zero modes

Solvent induced swelling of membranes - measurements and influence in nanofiltration

This paper describes improvements to an apparatus for in-situ determinations of swelling where a linear inductive probe and electronic column gauge with an overall resolution of 0.1 μm was used for measurements of seven variants of polyacrylonitrile (PAN)/polydimethylsiloxane (PDMS) composite nanofiltration membranes in a range of alkane, aromatic and alcohol solvents. The unswollen membranes incorporated PDMS layers between 1 and 10 μm nominal thickness and were manufactured with a radiation and/or thermal crosslinking step. The tested membranes exhibited a range of swelling dependent on the degree of crosslinking, the initial PDMS layer thickness and the type of solvent. With no applied pressure the PDMS layer on some radiation crosslinked membranes swelled as much as ~170% of the initial thickness whilst other membranes were restricted to a maximum swelling of ~80%. When a pressure up to 2000 kPa was applied to a membrane then swelling could be reduced to ~20% of the value obtained at zero applied pressure. By vertically stacking up to 3 membrane samples it was possible to determine the swelling of PDMS layers as thin as 1 μm, although higher imposed pressures rendered some results unreliable as the measurement resolution of the apparatus was approached. The results of the swelling experiments are contrasted with crossflow nanofiltration performance in terms of solvent flux and solute rejection

Nanofiltration: a technology for selective solute removal from fuels and solvents

This paper describes the principal features of solvent resistant nanofiltration, and in particular its potential in fuel processing. Experimental data for both fuel simulants and a representative petrol fuel are presented to illustrate the salient features. The solute rejection mechanism for low polarity mixtures was size exclusion with a membrane cutoff in the region of 1-2 nm. The extent of solute rejection was dependent on the degree of membrane crosslinking, the membrane swelling induced by the feed and the applied (filtration) pressure. Nanofiltration experiments with the petrol fuel showed a good correlation with the data obtained for the fuel simulants, both in terms of permeate flux and solute rejection. Provided that higher polarity oxygenates were not present in the fuel, it was possible to remove undesirable polynuclear aromatic and organometallic solutes to an extent that was sufficient to reduce valve deposits (by 64%) and emissions gases (by up to 17%) in engine tests. These improvements significantly better the changes in engine performance that are brought about by the more traditional addition of fuel additives such as detergents. The technology provides a method for removing undesirable solutes from mixtures without the need for excessive energy input

Effect of swelling in non-aqueous nanofiltration with polydimethylsiloxane (PDMS) membranes

Transport mechanisms and process limitations are relatively well understood for aqueous nanofiltration systems. Much work has also been done on the use of membranes for the removal of suspended matter from organic solvents. The removal of organic solute compounds from organic solvents using membrane technology has been addressed by very few workers, and little is known of the fundamental transport and separation mechanisms. A dense polydimethylsiloxane (PDMS) composite membrane was used to assess the flux and separation performance of a range of organic solute compounds and organic solvents. Solvent flux was modelled with the Hagen- Poisuelle equation and found to fit the model well, with swelling effects being the most likely cause of some deviations. The effect of solvent type and membrane swelling on solute rejection will be discussed

Evidence for swelling-induced pore structure in dense PDMS nanofiltration membranes

A dense polydimethylsiloxane (PDMS) membrane was used to assess the flux and separation performance of a range of solutes (e.g. poly-nuclear aromatics and organometallics) and organic solvents (e.g. heptane and xylene). Solvent flux was modelled with the Hagen-Poiseuille equation and found to fit the model well, with the degree of swelling influencing the effective pore size and porosity of the membrane. The rejection mechanism for low-polarity solutes was found to be predominantly size exclusion. The rejection varied with solvent type and rejections were higher in poorer-swelling solvents. For instance, the rejection of 9,10 Diphenylanthracene was 2% in a pure heptane solvent compared with 15% in xylene. It is postulated that dense PDMS membranes exhibit the characteristics of a porous structure when swollen with solvent, and that the degree of swelling impacts on the separation performance of the membrane. A comparison between the Hildebrand solubility parameters for the PDMS membrane and the challenge solvent was found to be a good indicator of flux/rejection performance