The role of macrophytes as a refuge and food source for the estuarine isopod Exosphaeroma hylocoetes ( )

The role of submerged macrophytes as refugia from fish predation and as possible food sources for the estuarine isopod Exosphaeroma hylocoetes ( Barnard, K.H., 1940) was investigated. Laboratory experiments tested the effectiveness of artificial vegetation, replicating submerged vegetation, in enabling isopods to elude selected fish predators Rhabdosargus holubi, Glossogobius callidus, Monodactylus falciformis and Clinus cottoides. Isopods preferentially hid in the vegetation (>90%), even in absence of fish. The predatory fish had varying success in finding isopods within the vegetation. Isopod mortality ranged from 2% (R. holubi) to a maximum of 87% (C. cottoides) within vegetation, depending on the fish predator present. Stable isotope and fatty acid analyses ruled out the submerged macrophyte Ruppia maritima and inundated fringing grasses as direct food sources, but highlighted the epiphytic biota (mainly diatoms) found on the submerged vegetation and sediments as more likely food sources. These findings are consistent with gut content analyses. The results suggest that the close association of E. hylocoetes with R. maritima is the result of the vegetation providing the isopod with a refuge against fish predation as well as areas of increased food availability.     

An evaluation of progress in implementing ecosystem-based management of fisheries in 33 countries

The performance of 33 countries was evaluated for ecosystem-based management (EBM) of fisheries in three fields (principles, criteria and implementation) using quantitative ordination including uncertainty. No country rated overall as ‘good’, only four countries were ‘adequate’, while over half received ‘fail’ grades. A few developing countries performed better than many developed nations. Two case studies test the method. In Indonesia, Raja Ampat and Papua, rated similar to the national evaluation, but better performance might follow successful implementation of a planned EBM initiative. A workshop in Australia rated regional fisheries managed by New South Wales 20% lower for EBM than federally managed fisheries.     

Do boat licences have a role in fisheries managed through individual quotas? Experience in Australian fisheries

Some economists have questioned the need for retaining boat licences in fisheries managed through rights-based systems on the assumption that licences act as an impediment to autonomous adjustment. In this paper, the role of boat licences in fisheries managed through rights-based instruments is explored through examining their use in several fisheries in Australian and elsewhere. The study concludes that any impediment to autonomous adjustment that is produced by boat licences is likely to be small relative to other factors. Further, boat licences play an important role in offsetting imperfections in the rights-based management systems that exist in most countries.     

Measurement of wave-by-wave bed-levels in the swash zone

A technique is described to observe and quantify wave-by-wave bed-level changes in the swash zone. The ultrasonic instrument system is non-contact with the beach face surface being measured and the sensors remain outside of the fluid flows causing sediment movement. Sensor resolution combined with the electronic noise inherent within a digital network data-logging system results in a (conservative) measurement accuracy of ± 1 mm, equating to a couple of sand grain diameters in height. Illustrative field results demonstrate the practical use of the instrumentation, and a simple data pre-processing method to separate swashes and intervening bed-level ‘events’ is discussed. These example data reveal rather complex fluctuations of the bed observed over time periods of minutes to hours. Rather strikingly, gross bed-level changes per wave are revealed to be up to many orders of magnitude larger than the observed net rate of beach face evolution. It is outlined how observations of successive bed-level changes at multiple locations within a dense grid, combined with a consideration of sediment continuity, will now enable the total net sediment transported per uprush–backwash to be quantified.     

Photo‐feature: World of Geology

World of Geology is a book that was born from the back pages of Geology Today: a celebration of the richness of the diversity of geological sites worldwide, and a fitting tribute to the impact of a magazine that is marking its thirty‐fifth year in 2019. This photo‐feature revisits some of the spectacular images that have faetured on its back pages over the years     

St Francis: the world's worst dam site

Lying just north of Los Angeles, the total collapse of the St Francis Dam in 1928 has been described as America's worst civil engineering disaster of the twentieth century. Although poorly remembered among today's residents of Los Angeles, the event is better known to engineering geologists. The dam's failure was largely due to the geology of its site, which was inappropriate to the point of disastrous, in more ways than one.     

Two Approaches of Textural Classification for the Location of the Flows of Lava on the Volcanic Site of the Mountain Cameroon

Abstract

This study presents two approaches of textural classification on a synthetic aperture radar (SAR) ERS‐2 image, with the aim of the location of the flows of lava and of the serviced zones around the volcanic site of the Mountain Cameroon. The first approach is a method of progressive supervised classification, where a single class is extracted at the same moment, by using 7 parameters of texture stemming from the run length method. Three classes of texture were extracted by this method and the images from the three classes were merged by the red green blue coloured composition to produce the final map. For purposes of comparison, a not supervised classification was applied to the same image. The applied not supervised classification uses parameters of texture stemming from the co‐occurrence matrix method, and it lays on the detection of the peaks of a histogram. The results obtained by the two methods are coherent, and the validation of the results was made by observations during a recent mission on the site of study.     

R-band light curve of Comet 9P/Tempel 1 during the Deep Impact event

We present high-speed CCD photometry of Comet 9P/Tempel 1 during the Deep Impact event on 2005 July 4 UT. Approximately 2 h and 50 min of R-band data were acquired at Mount Laguna Observatory with a temporal resolution of 5.5 s. The flux increased by 9% in the first minute after impact. This was followed by a more gradual two-part linear rise, with a change in slope at 9.2 min post-impact, at which time the rate of brightening increased from ∼ to ∼. An analysis of the light curve obtained with the guide camera on the United Kingdom Infrared Telescope and yields very similar results. These findings are mildly in disagreement with the 3-part linear rise found by Fernández et al. (2007) in that we do not find any evidence for a change at 4 min post-impact. We interpret the linear rise phase as due to solar illumination of the edge of an expanding optically thick dust ejecta plume. After approximately 20 min, the light curves begin to flatten out, perhaps coincident with the start of the transition to becoming optically thin. In the large apertures (>10^″) the light curve continues to gradually rise until the end of the observations. In smaller apertures, the light curves reach a peak at approximately 50 min, then decrease back towards the pre-impact flux level. The drop in flux in the smaller apertures may be caused by the ejecta expanding beyond the edge of the photometric aperture, and if so, we can use this timescale to infer an expansion velocity of ∼, consistent with previous published estimates.     

Sea ice,hydrological, and biological processes in the Churchill River estuary region,Hudson Bay

A conceptual scheme for the transition from winter to spring is developed for a small Arctic estuary (Churchill River, Hudson Bay) using hydrological, meteorological and oceanographic data together with models of the landfast ice. Observations within the Churchill River estuary and away from the direct influence of the river plume (Button Bay), between March and May 2005, show that both sea ice (production and melt) and river water influence the region's freshwater budget. In Button Bay, ice production in the flaw lead or polynya of NW Hudson Bay result in salinization through winter until the end of March, followed by a gradual freshening of the water column through April–May. In the Churchill Estuary, conditions varied abruptly throughout winter–spring depending on the physical interaction among river discharge, the seasonal landfast ice, and the rubble zone along the seaward margin of the landfast ice. Until late May, the rubble zone partially impounded river discharge, influencing the surface salinity, stratification, flushing time, and distribution and abundance of nutrients in the estuary. The river discharge, in turn, advanced and enhanced sea ice ablation in the estuary by delivering sensible heat. Weak stratification, the supply of riverine nitrogen and silicate, and a relatively long flushing time (∼ 6 days) in the period preceding melt may have briefly favoured phytoplankton production in the estuary when conditions were still poor in the surrounding coastal environment. However, in late May, the peak flow and breakdown of the ice-rubble zone around the estuary brought abrupt changes, including increased stratification and turbidity, reduced marine and freshwater nutrient supply, a shorter flushing time, and the release of the freshwater pool into the interior ocean. These conditions suppressed phytoplankton productivity while enhancing the inventory of particulate organic matter delivered by the river. The physical and biological changes observed in this study highlight the variability and instability of small frozen estuaries during winter–spring transition, which implies sensitivity to climate change.     

Laboratory simulation of the salt weathering of schist: 1. Weathering of schist blocks in a seasonally wet tropical environment

Data describing sediment generation focusing on the temporal evolution of size gradation are required for the prediction of long‐term landform evolution. This paper presents such data for the salt weathering of a quartz‐chlorite schist obtained from the Ranger Uranium Mine in northern Australia. Rock fragment samples are subjected to three different climate regimes: (1) a dry season climate; (2) a wet season climate (both based on observations at the Ranger site); and (3) an oven‐drying sequence designed to test the sensitivity of the weathering process by exposing the rocks to more extreme temperatures. Two MgSO₄ salt solutions are applied, one being typical of wet season runoff and the other a more concentrated solution. Salt solution is applied daily in the wet season experiments and once only at the beginning of the dry season experiments. Results of the experiments reveal four stages of weathering. The kinetics of each stage are described and related to the formation of sediment of different sizes. Wet season climate conditions are shown to produce greater moisture variability and lead to faster weathering rates. However, salt concentrations in the wet season are typically lower and so when climate is combined with observed salt concentrations, the dry and wet season experiments weather at approximately equal rates. Finally, small variations in rock properties were shown to have a large impact on weathering rates, leading to the conclusion that rock weathering experiments need to be carefully designed if results are to be used to help predict weathering behaviour at the landscape scale. Copyright © 2006 John Wiley & Sons, Ltd.