The sea-surface microlayer of puget sound: Part I. Toxic effects on fish eggs and larvae

The sea surface is an important habitat for the developmental stages (eggs and larvae) of many fish and invertebrates; it is also a concentration point for anthropogenic contaminants entering the sea. Studies were conducted to determine the extent to which the sea surface of Puget Sound was toxic to the early life history stages of fish. Three urban bays with suspected contamination, a rural reference bay, and a Central Sound site were compared. Surface-dwelling eggs and organisms (zooneuston) were collected with a surface-skimming neuston net and their densities enumerated. Sand sole (Psettichthys melanostictus) embryos were exposed in the field and laboratory to the sea-surface microlayer. To develop a useful year-round approach to monitoring sea-surface toxicity, larval development of anchovies, kelp bass, and sea urchins was also evaluated as an indication of sea-surface microlayer toxicity.During the spawning season (February and March), urban boys in Puget Sound had lower concentrations of sand sole eggs and neustonic organisms on the sea surface than did the rural bayor Central Sound reference sites. Compared to the reference sites, laboratory exposure to surface microlayer samples collected from urban bay sites generally resulted in more chromsomal aberrations in developing sole embryos, reduced hatching success of sole larvae, and reduced growth in trout cell cultures. In situ hatching success of sole eggs was reduced by half or more in urban bays compared to reference sites.Toxicity was associated with visible surface slicks and, in urban bays, increased with increasing surface pressure (dynes cm⁻¹). Results to be reported separately (Part II) indicate that toxicity is strongly correlated with the presence of high concentrations of polycyclic aromatic hydrocarbons and metals in the sea-surface microlayer. The toxicity of SMIC samples was similar when evaluated by sole, anchovy, kelp bass, or sea urchin tests. A sea-surface monitoring program could use sea urchin embryos to evaluate site-specific sea-surface toxicity throughout the year.     

Contaminant concentrations and toxicity of sea-surface microlayer near Los Angeles, California

Sea-surface microlayer samples were collected from six nearshore areas receiving different amounts of anthropogenic inputs. The samples were analyzed for selected trace metals, chlorinated hydrocarbons, and polycyclic aromatic hydrocarbons. The relative toxicities of the samples were determined with fish embryo bioassays.Contaminant concentrations generally increased from offshore to the inshore stations. Contaminant concentrations were several orders of magnitude higher in microlayer samples from the highly industrialized Los Angeles and Long Beach harbors compared to samples from a site 15km offshore. Microlayer samples from the inshore stations were significantly more toxic, and induced significantly more developmental abnormalities and chromosome aberrations, than samples from the offshore stations.     

Aquatic surface microlayer contamination in chesapeake bay

The aquatic surface microlayer (SMIC), 50 μm thick, serves as a concentration point for metal and organic contaminants that have low water solubility or are associated with floatable particles. Also, the eggs and larvae of many fish and shellfish species float on, or come in contact with, the water surface throughout their early development. The objectives of this study were (1) to determine the present degree of aquatic surface microlayer pollution at selected sites in Chesapeake Bay, and (2) to provide a preliminary evaluation of sources contributing to any observed contamination.Twelve stations located in urban bays, major rivers, and the north central bay were sampled three times, each at 5-day intervals during May 1986. Samples of 1.4–4.1 each were collected from the upper 30–60-μm water surface (surface microlayer, SMIC) using a Teflon-coated rotating drum microlayer sampler. One sample of subsurface water was collected in the central bay.At all stations, concentrations of metals, alkanes, and aromatic hydrocarbons in the SMIC were high compared with one bulk-water sample and with typical concentrations in water of Chesapeake Bay and elsewhere. SMIC contamination varied greatly among the three sampling times, but high mean contaminant levels (total polycyclic aromatic hydrocarbons, 1.9–6.2 μg 1⁻¹; Pb, 4.9–24 μg 1⁻¹; Cu, 4–16 μg 1⁻¹; and Zn, 34–59 μg 1⁻¹) were found at the upper Potomac and northern bay sites. Three separate areas were identified on the basis of relative concentrations of different aromatic hydrocarbons in SMIC samples - the northern bay, the Potomac River, and the cleaner southern and eastern portions of the sampling area.Suspected sources of surface contamination include gasoline and diesel fuel combustion, coal combustion, and petroleum product releases. Concentrations of metals and hydrocarbons, at approximately half the stations sampled, are sufficient to pose a threat to the reproductive stages of some fish and shellfish. Sampling and analysis of the surface microlayer provides a sensitive tool for source identification and monitoring of potentially harmful aquatic pollution.     

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.8°C and 0.5°C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s⁻¹) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s⁻¹. Chlorophyll a biomass was generally low (<1 mg m⁻³) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m⁻³) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m⁻³) associated with the frontal jet. Increased zooplankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north side of South Georgia in December would have been in the central-southern Scotia Sea 2–3 months earlier, probably in sea ice affected regions.     

The Taconite Inlet Lakes Project: a systems approach to paleoclimatic reconstruction

A comprehensive study of meteorological, hydrological, limnological and sedimentological conditions in the watersheds of density-stratified (meromictic) lakes around Taconite Inlet, Northern Ellesmere Island, N.W.T., Canada was carried out from 1990–1992. Lakes C1 and C2 contain seawater trapped by isostatic uplift as the former embayments became isolated from the sea. These lakes, and Lake C3, contain varved sediments which provide an annually resolvable paleoclimatic record. By studing the major systems influencing sedimentation in one of these lakes (Lake C2) a better understanding of the climatic controls on varve formation, and hence on the paleoclimatic signal in the varved sediment record, was obtained. The varves of Lake C2 provide a proxy record of summer temperature for the region.This is the first in a series of papers published in this issue on the Taconite Inlet Lakes Project. These papers were collected by Dr R. S. Bradley.     

Discrete element modelling of the influence of cover strength on basement-involved fault-propagation folding

A discrete element model is used to investigate the influence of sedimentary cover strength on the development of basement-involved fault-propagation folds. We find that uniformly weak cover best promotes the development of classical, trishear-like fault-related folds showing marked anticlinal thinning and synclinal thickening, with cover dips increasing downwards towards the fault tip. Uniformly strong cover results in more rounded fold forms with only minor hinge thickening/thinning and significant basement fault-propagation into the sedimentary cover. Heterogeneous, layered, cover sequences with marked differences in strength promote the development of more complex and variable fold forms, with a close juxtaposition of brittle and macroscopically ductile features, which diverge from the predictions of simple kinematic models. In these structures the upper layers are often poor indicators of deeper structure. In addition, we find that in layered cover sequences fault-propagation into the cover is a complex process and is strongly buffered by the weaker cover units.     

The geological and geodynamic evolution of the eastern Black Sea basin: insights from 2-D and 3-D tectonic modelling

Subsidence mechanisms that may have controlled the evolution of the eastern Black Sea have been studied and simulated using a numerical model that integrates structural, thermal, isostatic and surface processes in both two- (2-D) and three-dimensions (3-D). The model enables the forward modelling of extensional basin evolution followed by deformation due to subsequent extensional and compressional events. Seismic data show that the eastern Black Sea has evolved via a sequence of interrelated tectonic events that began with early Tertiary rifting followed by several phases of compression, mainly confined to the edges of the basin. A large magnitude (approximately 12 km) of regional subsidence also occurred in the central basin throughout the Tertiary. Models that simulate the magnitude of observed fault controlled extension (β=1.13) do not reproduce the total depth of the basin. Similarly, the modelling of compressional deformation around the edges of the basin does little to enhance subsidence in the central basin. A modelling approach that quantifies lithosphere extension according to the amount of observed crustal thinning and thickening across the basin provides the closest match to overall subsidence. The modelling also shows that deep crustal and mantle–lithosphere processes can significantly influence the rate and magnitude of syn- to post-rift subsidence and shows that such mechanisms may have played an important role in forming the anomalously thin syn-rift and thick Miocene–Quaternary sequences observed in the basin. It is also suggested that extension of a 40–45 km thick pre-rift crust is required to generate the observed magnitude of total subsidence when considering a realistic bathymetry.     

The geography of water conflict and cooperation: internal pressures and international manifestations

Studies on transboundary water conflict and cooperation generally consider interstate relations over shared water resources as distinct from intrastate relations. While connections have been made between international water relations and regional relationships in general, it is conceivable that international water conflict and cooperation may also be influenced by domestic water events and vice versa. This paper seeks to investigate the dynamics of water interactions across geographic scale and their relationship to broader international affairs. The research approach involves the creation of an analytical framework for assessing possible linkages between external and internal interactions over freshwater resources. The framework is applied to three case studies – the Middle East, South Asia and Southern Africa – utilizing 'event data'. To validate the findings from the quantitative analyses, the results are compared with conventional qualitative understandings of water and overall relations in the three regions. The comparison demonstrates not only the efficacy of the analytical framework in general, but also highlights, at least in terms of the specific case studies selected, the disparate water dynamics across geographic regions and the importance of considering water events, both national and international, within larger political and historical contexts.     

Discrete-element modelling of detachment folding

A two‐dimensional, discrete‐element modelling technique is used to investigate the initiation and growth of detachment folds in sedimentary rocks above a weak décollement level. The model depicts the sedimentary rocks as an assemblage of spheres that obey Newton's equations of motion and that interact with elastic forces under the influence of gravity. Faulting or fracturing between neighbouring elements is represented by a transition from repulsive–attractive forces to solely repulsive forces. The sedimentary sequence is mechanically heterogeneous, consisting of intercalated layers of markedly different strengths and thicknesses. The interlayering of weak and strong layers within the sedimentary rocks promotes the localization of flexural flow deformation within the weak layers. Even with simple displacement boundary conditions, and straightforward interlayering of weak and strong layers, the structural geometries that develop are complex, with a combination of box, lift‐off and disharmonic detachment fold styles forming above the décollement. In detail, it is found that the modelled folds grow by both limb rotation and limb lengthening. The combination of these two mechanisms results in uplift patterns above the folds that are difficult, or misleading, to interpret in terms of simple kinematic models. Comparison of modelling results with natural examples and with kinematic models highlights the complexities of structural interpretation in such settings.     

Water uptake of riparian plants in the lower Lhasa River Basin,South Tibetan Plateau using stable water isotopes

Riparian plants can adapt their water uptake strategies based on climatic and hydrological conditions within a river basin. The response of cold-alpine riparian trees to changes in water availability is poorly understood. The Lhasa River is a representative cold-alpine river in South Tibet and an under-studied environment. Therefore, a 96 km section of the lower Lhasa River was selected for a study on the water-use patterns of riparian plants. Plant water, soil water, groundwater and river water were measured at three sites for δ¹⁸O and δ²H values during the warm-wet and cold-dry periods in 2018. Soil profiles differed in isotope values between seasons and with the distance along the river. During the cold-dry period, the upper parts of the soil profiles were significantly affected by evaporation. During the warm-wet period, the soil profile was influenced by precipitation infiltration in the upper reaches of the study area and by various water sources in the lower reaches. Calculations using the IsoSource model indicated that the mature salix and birch trees (Salix cheilophila Schneid. and Betula platyphylla Suk.) accessed water from multiple sources during the cold-dry period, whereas they sourced more than 70% of their requirement from the upper 60–80 cm of the soil profile during the warm-wet period. The model indicated that the immature rose willow tree (Tamarix ramosissima Ledeb) accessed 66% of its water from the surface soil during the cold-dry period, but used the deeper layers during the warm-wet period. The plant type was not the dominant factor driving water uptake patterns in mature plants. Our findings can contribute to strategies for the sustainable development of cold-alpine riparian ecosystems. It is recommended that reducing plantation density and collocating plants with different rooting depths would be conducive to optimal plant growth in this environment.