ALBERT

Notes: Toxicity experiments indicated that changes in the fauna following the use of paraquat for weed control were likely to be indirect effects caused by the death and destruction of angiosperms. Generally, invertebrates living amongst the emergent vegetation in the margins of the reservoir and on the sediments were unaffected though the density of the benthic ostracod Cypridopsis vidua increased rapidly after the death of angiosperms. Of the planktonic invertebrates, those which were generally closely associated with the macrophytes were eliminated or survived at lower densities. Many of the invertebrates intimately associated with the angiosperms (e.g. Lepidoptera, Tricho-ptera. Gastropoda) were lost completely or colonized the replacement growth of Chara globularis at reduced densities: these effects were extended to the year after treatment. A few species colonizing Chara were able to establish densities comparable with those found on the angiosperms in the pre-herbicide period but in some cases the time of peak density was delayed.Data from fish gut analyses indicated that there was a change in the diet of the eel following the death of the angiosperms and that this was largely a reflection of the loss, or reduction in density, of many invertebrates associated with the angiosperms.

Notes: SUMMARY. 1. A soft-water stream iti upland Wales was dosed with sulphuric acid and aluminiutn sulphate at two successive points to create sitnultaneous episodes of low pH, and low pH with increased aluminiutn. Chemical atid biological responses were measured before, during and after the episode and were compared with a reference zone.2. The pH fell frotn ∼7.0 to 4.28 (±0.18 SD) and 5.02 (±0.10) respectively in the acid and aluminium zones. Corresponding aluminium concentrations during the episode were 0.052 g Al m−3 (±0.008) and 0.347 g Al nr3 (±0.047), the former not differing significantly from the reference zone. The concentration of cadmium rose to 0.002- 0.011 g Cd m−3in both treated areas, but the concentrations of other metals were unchanged.3. In situ toxicity tests were performed with macroinvertebrates and fish. Chironomus riparius. Hydropsyche angustipennis and Dinocras cephalotes suffered no mortality. Ecdyonurus venosus, Baetis rhodani and Gammarus pulex showed up to 25% mortality in both treatment zones and further mortalities occurred after the episode. Brown trout Salmo trutta and salmon Valmo salar s howed 7–10% mortality in the acid zone, but 50–87% in the aluminium zone, where salmon had a significantly shorter LT50than trout.4. The drift of Simuliidae increased during treatment in both acid and aluminium zones. Drift densities of Dixa puherula, Protonemura meyeri, Ephemeralla ignita and Dicranota sp. increased in the aluminium zone. The most pronounced response was by Baetis rhodani in the aluminium zone where drift density increased by ×8.4 during the episode.5. Baetis rhodani was the only taxon to show a significant decline in benthic density during the treatment, and then only in the aluminium zone. Drift could account for most of the losses.6. The depth distribution of invertebrates in the substratum differed between zones following treatment. More individuals were present at the surface of the reference zone (1287 m−2±747) than at the surface of the other zones (〈400 m−2); however, densities at greater depths were similar. These patterns probably reflected differences prior to the treatments.

Notes: SUMMARY. From a survey of the fauna at sixty sites in permanent drainage channels of the Gwent Levels during the spring of 1976 and at some of these sites (thirty-three) during the autumn, speeies and site associations were identified using classification and ordination techniques. Sites in some groups had identifiahly similar environmental characteristics and grouping seemed particularly related to the vegetational stage in the hydrosere, water How and the incidence of salt contamination from the Severn Estuary. Characteristics which were common within particular species groups and whieh, in part, explained their distributions were: adaptation to low oxygen concentrations, food preferences and dispersal mechanisms.Whilst there was some temporal stability in site groups between the spring and autumn surveys, such associations were not idenlical. The oxygen status of channels appeared a major factor determining faunal distribution.Twelve sites, representing a range of channel types, were sampled six times in a 12-month period. Far more species were colleeled within the water column and on plants than in the benthos. Several oligochaetes, e.g. Aulodrilus pluriseta, Dero digitata, Limnodrilus claparedeanus, were not recorded at most sites during the summer probably because of the SUStained lack of oxygen at these sites.Three contiguous reaches of channel were maintained during the period March—October 1977 with different plant cover (no Lemnu; complete Lemna cover: control with partial Lemna cover). The oxygen status of the reaches was different, that with complete Lemna cover being continuously anaerobic near the channel bed for several months, the control reach which had partial Lemna cover being anaerobic for a shorter period and that with no Lemna being only anaerobic at night. Whilst the distribution of macroinvertebrate species was prineipally related to this oxygen status, the control reach whieh was strueturally the most complex, having both submerged and floating plants, contained about twiee as many species as the other reaches during the summer (colleeted in the water column and on plants): benthic samples in this and the reach with complete Lemna cover contained only between zero and two species during the period May- September.

Notes: SUMMARY. The macro-invertebrate fauna and substrate were studied in a riffle and pool from a regulated (Elan) and an unregulated (Wye) river in summer. There were some differences in the substrate particle size distribution of the two rivers and between the riffle and the pool on the Wye. There was more detritus in pools than in riffles in both rivers and generally more in the Elan than the Wye. Deposits on the bed of the Elan were rich in iron and manganese.On the Wye. there was a greater density of invertebrates in the riffle than in the pool, but species richness was similar. Most species showed some preferences for either the riffle or the pool. The riffle contained a substantial number of Ephemeroptera, Trichoptera, Plecoptera and Simuliidae (52%) whilst the pool was dominated by chironomids (71%). Compared with the Wye, total invertebrate density and species richness were reduced in the Elan. Some species characteristic of riffles were reduced or absent on the Elan. Invertebrate density on the Elan was similar in the riffle and pool; species richness was greater in the riffle. The distribution and abundance of invertebrates is discussed in relation to such environmental factors as water velocity through the substrate and dissolved oxygen supply in interstitial habitats.

Notes: SUMMARY. 1. Single-station diel oxygen curves were used to monitor the oxygen metabolism of the Ogeechee River, a sixth-order blackwater river in the Coastal Plain of southeastern U.S.A., over a period of 4 years. Ecosystem production (P) and respiration (R) were estimated, and P/R ratios calculated to determine the extent of autotrophy characteristic of this type of river. The potential error in oxygen metabolism caused by photo-oxidation of dissolved organic carbon (DOC) in the water was measured and found to be minor.2. Rates of ecosystem primary production measured were intermediate compared to other rivers, ranging from 0.49 to 13.99g O2m−2 day −1.Primary production rates were highest during the summer when water levels were low. Regression analysis indicated that water depth and light absorption by DOM were significant predictors of primary production in this river. Incident light intensities were not significantly correlated with production rate.3. Respiration rates were unusually high, varying between 3.70 and 11.5 g O2 m−2 day − 1. System respiration also varied seasonally, but less than primary production. Rates were slightly higher in spring and summer.4. With one exception, P/R ratios were considerably lower than l throughout the study period, indicating that the Ogeechee River was highly heterotrophic. PIR ratios ranged from 0.09 to 1.3, and averaged 0.25.5. A carbon budget calculated for this river showed floodplain inputs were 7 times autochthonous production. Organic carbon turnover length was 690 km, considerably longer than has been reported for lower-order rivers.

Notes: A natural-source electromagnetic sounding of the earth made near a surface conductivity anomaly will resolve different features of the underlying conductivity structure than a sounding in a more uniform region. the surface-conductivity anomaly deflects horizontal electric currents induced by an external source into a vertical plane converting transverse-electric (TE) mode currents into the transverse (TM) mode. the resulting current distribution involves both vertical current flow and spatial variations with shorter wavelengths than the external field, providing increased resolution of resistive layers and of the conductivity structure at shallow depths. We exmine the sensitivity of the converted-mode response for the vertical-gradient sounding (VGS) method in order to plan electromagnetic soundings in a narrow ocean strait such as the Strait of Georgia between Vancouver Island and the Canadian mainland.An integral-equation method is used to model the current system induced by a mode converter, consisting of a known conductivity structure, such as a body of ocean water. It is shown that the depth of penetration of the secondary current distribution produced by the mode converter depends on both the horizontal scale of the feature and the distance from its edge. Within this depth range the current system is strongly perturbed by the existence of either conductive or resistive layers. the sensitivity of the VGS response (the ratio of the horizontal magnetic field at the base and surface of the mode converter) is examined using forward modelling of layered conductivity structures. the response is found to be dependent on both the TE and TM current systems. For a narrow ocean strait such as the Strait of Georgia, a measurement of the converted-mode VGS response along a line of sites on the floor of the strait, will provide resolution of conductive and resistive layers in the upper 10 km. the appropriate frequency range over which the VGS response should be measured in the strait is 10−2 Hz to 10 Hz.In our investigation of mode conversion we examine both the frequency- and time-domain response. Snap shots showing the current system evolving in the earth after a step or impulse illustrate the interaction of the EM signals with resistive and conductive layers. We show that the time-domain response can be used in a ‘geometrical sounding’analogous to seismic refraction to determine the conductivity structure. Finally we examine the limitations on the accuracy of the frequency and time-domain VGS response imposed by natural signal levels and instrument sensitivity.

Notes: Total field magnetic values recorded during a survey by RRS Charles Darwin off Ghana yielded large track-crossover errors of up to 120 nT (RMS value of 58.7 nT), which masked the weak magnetic anomalies in this equatorial region. the heading effect of the ship's magnetic field and strong diurnal variation in the Earth's field are likely causes of the errors. A heading effect experiment shows differences of up to 30 nT for Charles Darwin on different headings, which have been corrected for. the diurnal variation has been calculated by using the magnetic field observations themselves, because observatories are either too distant or were inoperative at the time of the survey. A method that uses the anomalies corrected for heading effect and differences at track crossovers was found to produce an acceptable curve, with an amplitude of 120 nT and a shape similar to that of equatorial observatories. Fully corrected anomalies have crossover errors of up to only 40 nT with an RMS value of 17.5 nT. These anomalies reveal a linear magnetic anomaly low along the continental slope off Ghana.

Notes: In a typical marine controlled-source electromagnetic (CSEM) experiment, one or more self-contained receivers capable of measuring small variations in the electromagnetic field are located in an array on the sea-floor. A remote horizontal electric dipole is towed close to the sea-floor by a surface vessel. Variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers. The variations contain information about the distribution of electrical conductivity of the subsurface rocks. The electrical conductivity is related to such critical physical parameters as porosity, temperature and fluid content.Published studies of the response of the sea-floor to electromagnetic excitation have not included any systematic variation in the sea-floor conductivity with horizontal direction. A horizontally isotropic oceanic lithosphere is not an appropriate assumption particularly in the vicinity of the ridge. Here, for example in the near-surface layers, basalts are extruded onto the sea-floor within the very narrow, fissured, neo-volcanic zone. The fissures may be filled with sea water and sediment which are better conductors than typical basalt. Alternatively, as time progresses, they may become sealed with hydrothermal mineral deposits which can be less conductive than the host rocks. In both instances, the electrical conductivity of the composite material in the direction oriented along the fisures parallel to the ridge will be greater than that in the transverse direction. Measures of anisotropy on sea-floor of varying age are an indication of the history of the tectonic activity at the ridge.The electromagnetic effects of lateral anisotropy are examined through the study of the response of a simple double half-space model. An upper half-space representing sea water is connected to a lower half-space representing anisotropic crustal material. A general theory describing the excitation of this model by any transient electromagnetic source is developed and the step-on responses for systems composed of a horizontal electric dipole transmitter and horizontal magnetic and electric field receivers are computed.The results of previous work indicate that following a change in current in the transmitter, two distinct changes in horizontal electric field strength separated in time are observed at the receiver. The first is caused by the diffusion of the electromagnetic field through the relatively resistive sea-floor, the second is caused by diffusion through the relatively conductive sea water. The times at which the two events occur are robust measures of the isotropic conductivity of the sea-floor and the sea water respectively.The results of this study show that shape and time of the initial change in the electric field varies systematically with both the average conductivity and the anisotropy of the sea-floor. For some orientations of the transmitter and the receiver, the effect of increasing the anisotropy is similar to the effect of increasing the average conductivity. The initial event is delayed. However, there are orientations for which increasing the anisotropy actually results in an earlier arrival. Natural Science and Engineering Research Council in Canada.

Notes: New oceanic crust is being formed along active segments of the global mid-ocean ridge (MOR) system. The presence of an axial magma chamber and associated zones of partial melt and hydrothermal activity located up to several kilometres beneath the seafloor is central to almost all recently proposed theories of crustal formation. Seismic images of the top few kilometres beneath the fast spreading East Pacific Rise (EPR) have already been obtained. Reflection profiles place strong constraints on the geometry of the axial magma chamber but refraction data provide only coarse estimates of the subsurface temperature, distribution of partial melt and porosity, parameters required to distinguish between the proposed petrological models. Electrical conductivity is directly related to all these critical parameters and therefore electromagnetic experiments should be designed to help characterize the ridge environment.We determine the magnetic field B(t) on the seafloor caused by a sudden change in current in a 2-D electric dipole aligned perpendicular to the strike of the ridge. The finite element technique is used to solve the governing differential equation numerically in the Laplace s-domain. The transformation to the time domain is by the Gaver-Stehfest method. We show that the extraction of two basic parameters from the response curve Ḃ(t) can provide sufficient information to identify the more important features of the petrology. The parameters are the response amplitude Ḃmax, which is the maximum derivative of δtB(t), and the diffusion time γT, the time at which this maximum occurs. The behaviour of γ as a function of distance from the source is analogous to that of first arrival time in refraction seismology. The value of γ is a weighted integral of the conductivity along the most resistive path between the source and the receiver.A highly conductive, partially molten magma chamber beneath the ridge axis slows the rate of diffusion of electromagnetic fields across the ridge, increasing γ but also reducing Ḃmax at sites on the side of the ridge opposite the transmitter. A melt lens ponding as a thin layer on top of the chamber increases Ḃmax at the ridge crest and increases γ at sites on the far side. Hydrothermal fluid circulation in the uppermost 2 km of the crust reduces Ḃmax everywhere across the ridge but increases γ only at sites within 0–3 km of the ridge crest. Electromagnetic energy in this case can reach the more distant points via paths which by-pass the fluids.Inferences made from the results of 2-D modelling indicate that a practical experiment would require a 104 A m horizontal electric dipole (HED) transmitter located 5 km off-axis and receivers with a sensitivity of at least 1 pT s−1 over a time window up to 10 s.

Notes: We describe the conception, design, construction and testing of a towed electromagnetic system capable of mapping the near-surface electrical conductivity of the sea-floor. The transmitter and receiver coils are arranged coaxially, and dragged along the sea-floor. The transmitter coil is 2 m in length and 1 m in diameter, and contains 100 turns of wire. It is energized from the surface by a constant voltage, whose polarity is reversed every 5 ms. The resulting transient magnetic field is detected in the receiver coil. Received signals are amplified and sent back to the surface for processing and analysis.Following a transition in the transmitter current, two distinct transients are observed at the receiver. These events correspond to electromagnetic energy which has diffused through the sea-water and less conductive sea-floor, respectively. The onset, amplitude and decay of the first transient are primarily a function of the conductivity structure of the sea-floor.A successful survey with the system was carried out in shallow coastal waters east of Vancouver Island. The survey yielded 20 conductivity measurements along three lines. The data are stacked 512-fold, and the shape and amplitude of the resulting noise-reduced signal are compared with theoretical signals using a generalized linear inversion process. The shape, amplitude, and delay time of the received signal are indicative of the conductivity of the bottom sediments. The resulting model is a layer of mud of conductivity 1.2 S m-1 and variable thickness overlying rock or sediment with a conductivity of about 0.1 S m-1. The model is consistent with seismic log profiles obtained during the survey, and with conductivity values expected for surficial, marine sediments.