Shore Slope Research Articles

DISCUSSION OF MEGACLAST TRANSPORT AND EROSION OF ROCKY SHORELINES: A COMMENT ON WILLIAMS (2010)

DISCUSSION OF MEGACLAST TRANSPORT AND EROSION OF ROCKY SHORELINES: A COMMENT ON WILLIAMS (2010) JASPER KNIGHT A recent paper by Williams (2010) described very glacial sediment and/or preglacial weathered debris large boulders (megaclasts) from the Aran Islands, (Moles and Moles 2002). This variable sediment Galway Bay, and argued that wave overtopping of cover strongly influenced patterns of postglacial high cliffs caused clast detachment and landward weathering. Although the extent of glaciation on transport by wave bores. Here I highlight limita- the Aran Islands is unclear (McCarron 2007), tions of Williams's arguments, and show that clast glacially-influenced preconditioning of rock surfaces detachment and transport involves complex pro- through weathering is extremely significant but was cesses and conditions not considered in his paper. not considered by Williams (2010). Pre-detachment conditioning of rocky shorelines Processes of clast detachment It is important to consider clast properties (size, Williams (2010) was concerned with identifying shape, strength, lithology, degree of weathering) the wave height required for clast transport, but prior to considering processes of clast detachment did not first consider processes by which clasts and transport. This is because pre-detachment are detached from rockhead. Limestone weather history strongly influences clast susceptibility to ing decreases rock strength, widens joints and detachment (Naylor and Stephenson 2010), the force makes clasts more easily detachable. Recent studies required for detachment (Nott 2003), and processes show a close relationship between weathering, rock of clast transport (Goto et al. 2009). Williams strength, and likelihood of clast detachment (e.g. does not consider pre-detachment history, but it Hall et al. 2008; Naylor and Stephenson 2010). is significant for the Aran Islands because of its Williams (2010) assumes that all areas of the underlying Lower Carboniferous limestone. Weath- upper cliff face are potential sites for clast detach ering and karstification has led to joint widening ment. However, recent studies show that rocky and block déstabilisation, evidenced by surface shoreline erosion is highly selective, strongly litho grikes, karren and dolines and subsurface caves, logically controlled, and that clasts are detached The Quaternary history of the region is also sig- preferentially from the intertidal zone (Naylor and nificant. Frozen ground during glaciations allowed Stephenson 2010; Knight and Burningham 2011). meltwater erosion to cut channels and oversteepen Williams (2010) applied Nott's (2003) equation for slopes. Permafrost and meltwater erosion were joint-bounded blocks in calculating the significant both active in western Ireland during the late Mid- wave height required for clast detachment. However, landian glaciation (Orme 1967; Lewis 1985). Differ- Nott (2003) was concerned with sloping shore ential glacial erosion also led to exposure of some platforms rather than cliffs, and his equations have bedrock surfaces whilst others were protected by now been superseded (Nandasena et al. 2011). Irish Journal of Earth Sciences 29 (2011), 23-24 ■© 2011 Royal Irish Academy doi: 10.3318/IJES.2011.29.23 23 24 Irish Journal of Earth Sciences (2011) Wave conditions required for clast movement Williams (2010) argued that a landward-moving bore formed as a result of cliff overtopping by white- (broken) or green-water (unbroken) waves. Bores can only form where waves are topographi cally funnelled across a shore platform or cliff-top (Hall et al. 2008). No topography was presented by Williams, hence it is unclear if bores are likely or not. Williams' paper's title explicitly stated it would illustrate the storm versus tsunami wave debate, but failed to do that. Scheffers et al. (2010) presented radiocarbon ages from organics below megaclasts on the Aran Islands and County Clare. They showed clasts were emplaced episodically throughout the last 3,000 years and, although there may be correspondence with periods of high stormi ness (Hansom and Hall 2009), a tsunamigenic origin cannot be rejected. Knight and Burningham (2011) mapped the distribution of upper shoreface boulders in County Donegal and showed that, although clasts can be detached under contempor ary wave conditions, there is a mismatch between the size of contemporary clasts and those that form ridges on the shore platform. This suggests that a spectrum of past and present wave sizes and there fore wave-generating mechanisms are responsible for clast detachment and transport. JASPER KNIGHT School of Geography...

Crossing natural and data set boundaries: coastal terrain modelling in the South-West Finnish Archipelago

Geographical information systems (GIS) are important tools in coastal research and management. Coastal GIS applications involve special challenges, because the coastal environment is a complex transitional system between the terrestrial and marine realms. Also acquisition methods and responsibilities for spatial data (and thus their properties) change at the shoreline. This article explores the consequences of this land-sea divide for coastal terrain modelling. We study how methods designed for terrestrial environments can be used to create integrated raster coastal terrain models (CTMs) from coarse elevation and depth data. We focus on shore slopes, because many particularities of coastal terrain and the data which describe it as well as the resulting problems are concentrated in the shore zone. Based on shorelines, terrestrial contours, depth contours and depth points, we used the ANUDEM algorithm to interpolate CTMs at different spatial resolutions, with and without drainage enforcement, for two test areas in a highly complex archipelago coast. Slope aspect and gradient rasters were derived from the CTMs using Horn's algorithm. Values were assigned from the slope rasters to thousands of points along the test areas' shorelines in different ways. Shore slope gradients and aspects were also calculated directly from the shorelines and contours. These modelled data were compared to each other and to field-measured shore profiles using a combination of qualitative and quantitative methods. As far as the coarse source data permitted, the interpolation and slope calculations delivered good results at fine spatial resolutions. Vector-based slope calculations were very sensitive to quality problems of the source data. Fine-resolution raster data were consequently found most suitable for describing shore slopes from coarse coastal terrain data. Terrestrial and marine parts of the CTMs were subject to different errors, and modelling methods and parameters had different consequences there. Thus, methods designed for terrestrial applications can be successfully used for coastal terrain modelling, but the choice of methods and parameters and the interpretation of modelling results require special attention to the differences of terrestrial and marine topography and data.

Habitat level determinants of emergent macrophyte occurrence, extension and change in two large boreal lakes in Finland

Historical (1947–1950) and present day (1998–2000) aerial photograph data of aquatic vegetation were combined with soil, land use and habitat morphology variables in two lakes, oligotrophic L. Näsijärvi (256 km 2) and eutrophic L. Pyhäjärvi (122 km 2), with regulated water levels in southern Finland. Multivariate GLM analyses were used to examine the determining factors of occurrence and horizontal extension of vegetation, occurrence of common reed ( Phragmites australis) and water horsetail ( Equisetum fluviatile) and the vegetation change in 474 sites. Despite differences in geography and water level regulation between the lakes, congruent trends were identified. The vegetation occurrence in both lakes had a positive response to clay soils and incoming nutrients and a negative response to housing and shore slope. Moraine soils, housing and shore slope had a negative effect on horizontal extension of vegetation in both lakes. The occurrence of P. australis was controlled by a negative response to moraine and the occurrence of E. fluviatile to slope in both lakes. The vegetation change in both lakes was characterized by a negative response to housing and slope.

The deep-lake deposit in the Upper Triassic Yanchang Formation in Ordos Basin, China and its significance for oil-gas accumulation

The deep-lake facies of the Yanchang Formation represents a large outflowing lake basin in the Ordos area. Its deposition can be divided into four stages: lake genetic and expanding stage, peak stage, inversion stage and dying stage. All the stages are obviously consistent with the evolution of depositional environment and the paleoclimate in the region. The study indicates that the lake basin has evolution fluctuations from highstand to lowstand for four times in its evolution history, and the deposition center of the lake has not obviously moved, staying along the Huachi-Yijun belt. The deep lake sedimentary system mainly consists of deep water deltas and turbidite fans during the entire evolution course of the lake basin in the Late Triassic. The former mainly developed on the slope of steep shore of the delta in the early period of the deep-water expansion and gradually experienced a big shift from deep-water deltas to shallow-water platform delta. And the latter appeared almost in all the above stages and had two types of turbidite fans, slope-moving turbidite fans and slump turbidite fans. The slope-moving turbidite fans have relatively complete facies belts overlapping one another vertically and consist of the slope channel of inter fans, the turbidite channel, inter turbidite channel and turbidite channel front of middle fans and outer fans (or lakebottom plain). However, the slide-moving turbidity fans are formed in the deep lake with their microfacies difficult to be distinguished, and only the center microfacies and edge microfacies can be determined. The two types of the turbidity fans are similarly distributing in the near-root-slope and far-root-slope regions. The deep-lake deposition governs the distribution of the hydrocarbon and reservoir, while the slope-moving turbidite fans are excellent reservoirs for oil-gas exploration due to their great thickness, widespread distribution and accumulation properties.

Controls on the Spatial Dimensions of Wetted Hydrologic Margins of Two Antarctic Lakes

Polar deserts are characterized by limited water availability across much of the terrestrial landscape. In the McMurdo Dry Valleys, Antarctica, soil active layers are wetted only briefly after infrequent snowfall (<10 cm yr−1). Adjacent to glacial meltwater streams and lakes, however, sediments and soils appear visibly wetted throughout the summer. We hypothesized that this apparent wicking of water from aquatic environments is controlled by capillary action determined by the physical properties of near‐shore sediments including slope, particle size distribution, and depth to permafrost. We performed synoptic sampling of near‐shore sediments around two lakes in Taylor Valley, East Antarctica. We measured the water content, active layer depth, particle size distribution, and topography to determine the relative importance of shore slope, pore size distribution, and depth of the active layer on the spatial dimension of wetted hydrologic margins. The horizontal dimension (distance from the lake edge) of wetted margins (HW) and slopes ranged from 1.1 to 32.9 m and 1 to 61% and active layer depth ranged from 5.5 to >120 cm. Our results indicate that particle size distributions of near‐shore sediments are homogeneous; slope had a greater effect on the dimensions of wetted margins, explaining 73% of the variance in HW These findings show that the spatial dimensions of wetted zones are controlled by a suite of physical parameters that may be easily measured using remotely sensed data and a limited number of samples. Such information is useful for constraining landscape‐scale models of nutrient redistribution because intermittently wetted soils are zones of enhanced biogeochemical transformation and transport in desert ecosystems.

The effect of seismic events on sliding deformations of the Bratsk water reservoir shores

Research has shown that the creation of the Bratsk water reservoir considerably changed the seismic conditions in this area of the Siberian Platform. Seismic records revealed a significant increase in the energy classes of earthquakes with epicenters in the north of the reservoir, in trap intrusion zone. The change in engineering-geologic conditions and frequent seismic events affected the rate of development of gravitational deformations on the shore slopes.

New Records of Reptiles (Reptilia, Squamata) at Azov Seashore of Taman’ Peninsula (Krasnodar Kray, Russia)

For the first time at Azov seashore of Taman’ peninsula (settlement Golubitskaya and west of Peresyp’ village) there were found populations of meadow lizard Darevskia praticola pontica (Lantz et Cyren, 1919). The lizards live in the lower parts of a high shore slope with a phryganoid vegetation. The density of their population can reach 1 individual per square meter (1 individual per 1 m 2 ), and in the places of their concentration — 5 and more individuals per 1 m 2 . In second place, in the upper parts of the shore slope near village Peresyp’ the glass lizard, Pseudopus apodus (Pallas, 1775) was also found. New records of these species are separated for 30 km to the east and 70 km to the south correspondingly from previously known localities. The runner, Elaphe sauromates Pallas, 1814, which has not been found earlier in the Eastern Azov area, is not rare in the Akhtanizov estuary region.

Effects of tides on a sloping shore: groundwater dynamics and propagation of the tidal wave

The influences of tides on a coastal environment with a sloping shore are investigated by means of field observations and groundwater flow modelling. The Belgian western coastal plain consists of a wide shore, dunes and polders where diurnal tides with large amplitude occur. The effects of tides on the groundwater flow are studied using the MOCDENS3D code. First, MOCDENS3D is validated to accurately simulate the propagation, attenuation and lag of a tidal wave in an aquifer. Then groundwater flow and influences of tides are modelled for a cross-section along the French–Belgian border. This gives an exhaustive insight into the spatial and temporal varying groundwater flow and propagation of the tidal wave in the aquifer. Simulation shows that there are two interfering flow cycles. The first is a shallow tidally fluctuating flow cycle on the shore due to the interaction of the gently sloping shore and the tidally oscillating sea level. The second is a deeper flow cycle from the dunes towards the sea. Further, it is indicated that the propagation and attenuation of the tidal wave follows a complex pattern with lateral as well as vertical components. The interaction between tides and shore topography also influences the salinity distribution.

Rocky coast geomorphology and erosional processes: A case study along the Murgia coastline South of Bari, Apulia — SE Italy

Many sites along the Apulian coast (SE Italy) are composed of weathered and fractured carbonate rocks affected by intense erosion and the development of slope instabilities. Here, coastal morphology was mostly produced by Quaternary tectonics, the severe and selective action of modern erosion by sea waves, and locally, was strongly conditioned by human influences. In addition, due to the carbonate nature of rock masses, the role exerted by karst processes has to be considered. This paper provides a critical review of the common eroding processes and factors affecting coastal morphology in a typical Mediterranean coastal carbonate environment. A stretch of the Murgia coastline south of Bari (between San Vito and Polignano a Mare) made up of cliffs and shore platforms interrupted by small pocket beaches was considered. For qualitative and quantitative assessment of shoreline recession, the coastal stretch was monitored by means of field surveys between 1997 and 2003. Furthermore, a comparison of vertical aerial photographs covering the period 1966–1997 (scales 1:33,000, 1:10,000 and 1:5000), topographic maps (scales 1:5000, year 1987, 1:2000, year 1982), and archival terrestrial photographs taken since the 1930s was also employed. In some zones, characterised by a gently sloping shore platform, a significant shoreline retreat was calculated at about 20 m in the last 100 years (0.2 myr − 1 ), which was caused, above all, by human activity. Here, in fact, the presence of small ancient coastal quarries and artificial salinas is widespread; they are located where soft and porous calcarenites crop out. The cliffs are characterized by different types of landslides, with various complex mechanisms of rock slope failure. Cliff recession is episodic and discontinuous in time and space, and occurs in response to single large sea storms, such as that which occurred in January 1999. Between 1997 and 2003, the cliff retreat rates vary within a range of 0.01–0.1 myr − 1 and, in general, decrease where the coast is composed of fine-grained limestones and increase where medium-grained biocalcarenites crop out. Examples of erosion hazard indicators and mass movement typologies are illustrated.

Occurrence of salt water above fresh water in dynamic equilibrium in a coastal groundwater flow system near De Panne, Belgium

A salt water lens is found above fresh water under the shore between Dunkerque (France) and Nieuwpoort (Belgium). This inverse density distribution is in a dynamic equilibrium. It develops due to the infiltration of salt water on the back shore during high tide. Under this salt water lens, water infiltrated in the adjacent dune area flows towards the sea and discharges at the seabed. This water quality distribution differs from the classic salt water wedge under fresh water described in the literature. Here, the evolution to this water quality distribution is simulated with a density dependent numerical model. A large tidal range, shore morphology and a permeable groundwater reservoir are the main conditions for the observed water quality distribution. By altering these conditions, intermediate water quality distributions between the classic salt water wedge and the one discussed here develop. Based on these simulations, it is expected that similar kinds of inverse density distribution could be present in a number of coastal areas, which have tides, a gently sloping shore and a permeable substratum.

Modeling the Effect of Tidal Wetting and Drying on Shore Platform Development

Abstract A simple mathematical model was used to test the hypothesis that the gradient and other aspects of the morphology of shore platforms in argillaceous rocks are determined by the number of wetting and drying cycles experienced at different elevations within the intertidal zone. Three model variants were considered: downwearing by wetting and drying on a pre-existing wave-cut surface; downwearing occurring simultaneously with backwearing by waves at the high tidal level; and downwearing combined with simultaneous backwearing by waves throughout the intertidal zone. Wetting and drying and tidal-duration distributions were obtained for four areas, representing tidal ranges from 1 m to 9.3 m. The amount of downwearing was made proportional to the wetting and drying frequency, and it therefore varied through space and time in response to changes in platform elevation. The model suggested that wetting and drying can produce very gently sloping shore platforms, especially in microtidal environments. Altho...

Modelling the habitat preferences of preadult and adult fishes on the shoreline of the large, lowland Elbe River

Summary From the years 1997 to 2000, fish assemblages from 15 groyne fields and training walls (a current guiding dyke, protecting the groyne field) of the Middle Elbe River (Germany) were examined by electrofishing. The aim of the study was to detect abiotic environmental key variables which determine preadult and adult fish assemblage in typical riverbank structures using multivariate ordination techniques (canonical correspondence analysis) and univariate methods (logistic regression analysis). Habitat preferences of preadult and adult stages of selected fish species were also estimated. Fish sampling was carried out at 190 river stretches; a further 1615 samples were taken by the point-abundance sampling method. In total, 21 732 preadult and adult individuals belonging to 30 fish species were caught; perch, eel, ide, roach, chub, gudgeon, bleak and white bream were the most frequent. Eurytopic species clearly dominated the fish assemblage with 72% total abundance, followed by rheophilic species (26%). Limnophilic fish species (rudd, crucian carp, stickleback and tench) were relatively rare (2% of total abundance), with frequencies of occurrence between 1 and 10%. The highest species diversities were estimated in structurally diverse training walls and groyne fields with defective groynes, whereas low species diversity was found to occur in poorly structured, strongly silted groyne fields with intact groynes. Additionally, typical seasonal and annual changes of the fish assemblage were observed. From the mesoscalic point of view, the structure of the preadult and adult fish assemblage was affected by both spatial (type of groyne field, river stretch) and temporal factors (year, season). Regarding the microhabitat, hydromorphological parameters (slope, current velocity, water level, predominant and secondary substrate) followed by physicochemical water values [dissolved oxygen (DO), temperature and pH-value] are of significance for the structure of the fish community. Furthermore, hiding places (shelter) are of subordinate but significant importance for the fish assemblage in the poorly structured main channel of the Elbe River. Importance of hydromorphological factors on the microhabitat scale was confirmed by multiple logistic regression in 12 of 14 preference models, conducted for different ontogenetic stages of roach, bream, white bream, bleak, ide, chub, gudgeon, asp, perch and eel. Habitat preferences of adult bleak and preadult roach were first determined by physicochemical water values and seasonal influences. Univariate models were generated to describe specific habitat preferences of different species and age-stages regarding the most important environmental variables (depth, velocity, predominant substrate, slope and shelter). Intraspecific differences were noticed between preadult and adult chub regarding preferences for water velocity, or between preadult and adult perch concerning slope of shore. Interspecific differences were observed, e.g. between adult roach and white bream, with regard to the use of shelters.

Effect of wave exposure on vegetation abundance, richness and depth distribution of shallow water plants in a New Zealand lake

SUMMARY 1. The effects of physical disturbance in terms of wave exposure, shore slope and substrate mobility on the presence, species richness, cover and depth limits of the low‐growing, shallow water macrophyte community (called the low mixed community) were examined at 41 shore sites in Lake Wanaka, South Island, New Zealand.2. Wave exposure at a site was described by the previous year's maximum depth of sediment motion and maximum vertical extension of waves, determined from a computer wave model. Shore slope at each site was recorded from −1 to 0 m depth, and sediment stability was assessed as the cover of small gravel.3. The low mixed community was only present on sites where the previous year's maximum depth of sediment motion was <8 m, maximum wave run‐up was <0.3 m, shore slope <0.12 m m−1, and small gravel cover <78%.4. Species richness, cover and depth limits of the low mixed community decreased with increasing disturbance on the sites. Sixty‐two percentage of the variation in species richness could be explained by physical disturbance variables when all sites were included (N=41). When only sites with a low mixed community were included (N=22), only 18% of the variation was explained. Species richness within sites supporting a low mixed community is thus poorly explained by physical disturbance, whereas presence or absence is better explained. There was no evidence to support the intermediate disturbance hypothesis in this study.5. Disturbance as a result of waves explained 86% of the variation in cover among sites with a low mixed community and accounted for 68 and 58% of the variation in upper and lower depth limit of the low mixed community, respectively.6. The models obtained in this study can be used as predictive models for the low mixed community in New Zealand lakes in relation to natural physical disturbance on the shore. By integrating these results with previous studies on the effect of water level fluctuation, we describe a generalised optimum physical habitat for the low mixed community in New Zealand lakes.

Ecotoxicological studies of the impact of construction materials for shore protection on Lake Baikal

Shore slopes of the majority of reservoirs are subjected to wave abrasion. Mainly ferro-concrete structures for shore protection against destruction were built on Lake Baikal. A plan to use new structures, such as wave extinguishing berms and artificial beaches of non-sorting rock material, is in effect at present. For the practical evaluation of this plan, it was necessary to perform an ecotoxicological assessment.The investigations started in 1988 and were carried out annually. At the first stage of the investigations, toxic potential of construction material was examined. Model experiments conducted on various biota did not reveal any toxicity.In the second stage, in the region of soil embankment, the influence of the construction material on hydrochemical composition of coastal waters and on ichthyofauna was studied. The analyses of water samples revealed that there was no negative influence of embankment on chemical composition of water of the littoral zone. The berm construction did not cause any changes in fish community composition. The construction did not influence the number of benthic littoral sculpins (Family Cottidae) or the reproduction of littoral spawning fish.The data reveal no negative influence of the soil embankment on shallow littoral areas.

Hermit crabs (Crustacea: Decapoda: Anomura), gastropod shells and environmental structure: their relationship in southeastern Brazil

Species composition and the pattern of shell utilization by hermit crabs were studied in five areas of São Sebastião Channel. Habitat characteristics were evaluated and related to the presence and relative abundance of crabs. Nine species of hermit crab were found: Clibanarius antillensis, Clibanarius vittatus, Clibanarius sclopetarius, Calcinus tibicen, Paguristes tortugae, Paguristes erythrops, Petrochirus diogenes, Pagurus criniticornis and Pagurus brevidactylus. Habitat complexity, moisture, rocky shore slope and hydrodynamism were the major characteristics influencing hermit crab presence, relative abundance and distribution in these areas. Shell utilization and availability varied among sites and shell utilization seemed to be dependent on differences in local shell supply (size and abundance of living snails) and preferences of the hermit crabs for certain shell types.

A NUMERICAL MODEL FOR THREE-DIMENSIONAL RUN-UP

A Lagrangian finite difference model for non-dispersive (long) and fully non-linear surface waves is presented. The Lagrangian description enables the inclusion of run-up and back-wash at sloping shores. The numerical procedure has been verified through a series of tests, including systematic grid refinements and comparison with analytical solutions for run-up. Results for a few cases with moderately complex geometries are presented. The run-up distribution on an idealized headland is computed and discussed in view of the involved physical mechanisms. Some cases involve a time-dependent bottom topography, corresponding to a moving slide penetrating the fluid surface. The numerical solution procedure appears to be robust, unless wave breaking is encountered, and grid refinement tests show fast convergence once the scales in the problems are resolved.

The Maltese Islands : climate, vegetation and landscape

The Maltese Islands, situated in the central Mediterranean, occupy an area of only some 316 km2. The climate is typically Mediterranean: the average annual rainfall is c. 530 mm of which some 85% falls during the period October to March; the mean monthly temperature range is 12--26 °C, and the islands are very windy and sunny. Although small, the Maltese Islands have a considerable diversity of landscapes and ecosystems which are representative of the range and variety of those of the Mediterranean region. The islands are composed mainly of limestones, the soils are young and are very similar to the parent rocks, and there are no mountains, streams or lakes, but only minor springs; the main geomorphological features are karstic limestone plateaux, hillsides covered with clay taluses, gently rolling limestone plains, valleys which drain runoff during the wet season, steep sea-cliffs on the south-western coasts, and gently sloping rocky shores to the Northeast. The main vegetational assemblages are maquis, garigue and steppe; minor ones include patches of woodland, coastal wetlands, sand dunes, freshwater, and rupestral communities; the latter are the most scientifically important in view of the large number of endemic species they support. Human impact is significant. Some 38% of the land area is cultivated, c. 15% is built up, and the rest is countryside. The present landscape is a result of the interaction of geology and climate, coupled with the intense human exploitation of the environment over many thousands of years, which has altered the original condition of the vegetation cover, principally through the diversion of vast tracts of land to cultivation, the construction of terraces, water catchment devices, irrigation channels and drainage ditches, the grazing of animals on uncultivated land, and the development of land for buildings and industry. The scantiness of the soil, combined with the erratic rainfall and the periodic disturbance of the vegetation cover, has resulted in extensive erosion. As a consequence it is now difficult for the original vegetation to reassert itself, affecting the landscape drastically and permanently. Much of the original native flora has been lost or marginalised and the present day non-urban landscape is now dominated by vegetation consisting mainly of ruderal and introduced species. As the population increases, and human pressure on the environment mounts, such trends are likely to continue and it is only very recently that some important initiatives have been taken to manage the environment and halt the deterioration of the landscape.

THE ABUNDANCE AND FOOD CONSUMPTION OF PISCIVOROUS BIRDS ON LAKE KARIBA, ZIMBABWE-ZAMBIA

Summary Hustler, K. & Marshall, B.E. 1996. The abundance and food consumption of piscivorous birds on Lake Kariba, Zimbabwe-Zambia. Ostrich 67:23-32. About 40 species of piscivorous birds have been recorded on Lake Kariba (Zimbabwe-Zambia) but only 26 were observed during this study, carried out in 1986–87. The Reed Cormorant Phalacrocorax africanus was the most numerous and made up 51.4% of all the birds seen, followed by the Whitewinged Black Tern Chlidonias leucoptera (26.7%), and the Darter Anhinga melanogaster (10.8%). Several species roosted or bred in the drowned trees which project above the lake surface but gently sloping shores were the most important feeding grounds. Relatively few birds were seen on steeply sloping shores or in open water. There was a distinct pattern of seasonal abundance, with birds being most numerous in the rainy season (November-April). The relative abundance of some fish species also was greatest during this period. There was a positive relationship between the numbers o...

Intrusion of Turbid Water in Stratified Lakes Driven by Resuspension of Sediments on Sloping Boundaries

The thermal ‘staircase’, a layer of constant temperature, was observed in seasonal thermoclines following the resuspension of cohesive sediments resuspension on the shore slope of Lake Biwa. When a moderate onshore wind blew, the bed materials were resuspended up to the mid thermocline with height of 2.5 m above the bottom, and settled down forming the staircase above the intrusion. As the consequence, the thermocline was divided into two layers by the staircase with a thickness of 0.7 m. To investigate the resuspension and settling process due to surface and internal waves in stratified lakes, laboratory experiments were conducted using a tank with a partial slope at one side. When the internal waves generated as the barotropic waves in the tank, the resuspension phenomena occurred frequently. Both surface and internal waves are essential for the resuspension process. In the stratified fluid, the distance and duration of suspended sediment transport are very long compared to that in the non-stratified one due to occurring of the intrusion into the stratified layer.

Tracking juvenile cod ( Gadus morhua L.) in northern Norway using acoustic tags

Juvenile cod ( Gadus morhua L.) were caught as 0-group larvae in July 1992 and held in captivity in a sea cage in a fjord in Finnmark county. In July 1993 five 1-year-old individuals (21–27 cm) were tagged externally with a small ultrasonic transmitter and released into the fjord. The movements of the cod were followed for 4–93 h. The juveniles inhabited the shore slope at a depth between 10 and 30 m, and did not enter deeper parts of the fjord. No consistent pattern of movement was observed. Cruising speeds of 0.2–0.3 body-length s −1 were recorded. The maximum recorded distance from the release site was 2350 m. The juveniles were identified from the number of vertebrae, as Arcto-Norwegian cod. Results are discussed in relation to recruitment of cod to the fjords in northern Norway.