Ecotone or Ecocline: Ecological Boundaries in Estuaries

Two main ecological boundaries, ecotone and ecocline, have been defined in landscape ecology. At this scale, the estuary represents a boundary between rivers and the sea, but there has been no attempt to fit empirical data for estuaries to these boundary models. An extensive data set from the Thames estuary was analysed using multivariate techniques and species-range analysis, in order to investigate whether the ecocline or the ecotone model was most relevant to this estuary. Data for periods of high and low freshwater flow allowed the impact of large-scale fluctuations implicit in both models to be determined.A continuum of assemblages existed along the salinity gradient from freshwater river to the North Sea, with shifts in the ranges of organisms apparent in response to changes in freshwater flow. This pattern closely fits an ecocline model. However, the estuary differs from previously defined ecoclines in having two overlapping gradients in the major stressor: from river to mid-estuary for freshwater species and from sea to mid-estuary for marine species. We propose, therefore, that the estuary represents a two-ecocline model, with fauna inhabiting the mid-estuary being either freshwater or marine species at the edge of their range, rather than ‘ true estuarine organisms ’. This allows a redefinition of the Remane diagram, with estuarine species removed, and supports previous arguments that brackish-water species do not exist. Such two-ecocline models may also exist in other marine systems, such as rocky shores.     

珠江河口湾伶仃洋的地形

河口湾是河流终止于大陆前缘的半围圈的海域,河水和海水分别从河口湾的两端注入并混合为冲淡水。河口湾通常是由于冰后期海面上涨被海水淹没的河流下游部分。伶仃洋就是珠江的河口湾,为华南最大的河口湾。     

New seismic data in the Mozambique Channel

This paper presents seismic reflection and refraction data from the Mozambique Channel, collected between 1971 and 1973. A deep sedimentary basin (up to 5 km of sediments) opens southwards to the Mozambique Basin, and is bounded to the east by the Davie Ridge and beyond by the marginal plateau of Malagasy. A continuous reflector (C), possibly of Cretaceous age, is identified between layers having seismic interval velocities of 2.4–2.8 km/s and 3.1–3.4 km/s. The deepest sediments have velocities of 4.5–4.9 km/s and overlie a layer with velocity 5.5 km/s, which may be volcanic in the north-east of the Channel.The crust occupying most of the Channel is probably pre-Cretaceous in age, and may be largely continental in nature. This is supported by subdued magnetic anomalies and the possibility of a continuous Karroo sedimentary section across the northern Channel. The oceanic crust of the Mozambique Basin may extend as far north as 24°S, into the western Channel only. The problem of the origin of the Mozambique Channel remains unresolved, although a long sedimentary history indicates that Malagasy may have separated from Mainland Africa prior to Karroo times. The Davie Ridge may possibly represent a relict strike-slip fault, which permitted movement along a north-south line.     

Well log interpretation and seismic character of the cenozoic sequence in the northern Alboran Sea

Seismic stratigraphic and main lithological features of the sedimentary cover overlying the basement of the Alboran Sea were established via the analysis of commercial multichannel seismic surveys, geophysical well logs, and well data. Six seismic stratigraphics units (VI to I), bounded by unconformities, form the marine sediments that range in age from early Miocene to Quaternary. They are dated by extrapolation of commercial drilling results from the northern Alboran Sea. Volcanic activity is recorded within sedimentary sequences of units V to II. Undercompaction features are detected in the two basal units.     

Precise positioning for near-bottom equipment using a relay transponder

The Marine Physical Laboratory of the Scripps Institution of Oceanography has developed an acoustic relay transponder for precise relative positioning of near-bottom instruments and geologic sampling devices. Although specifically designed to position equipment lowered on standard wire ropes without a need to maintain direct electrical contact with the surface ship, the relay transponder may be used to track free vehicles, such as deep submersibles, from the surface. The relay transponder is positioned relative to an array of bottom-anchored acoustic transponders. It is interrogated acoustically from the surface ship; it then sequentially interrogates the bottom transponders which, in turn, reply to the ship. From the measurement of the total travel time (ship to relay transponder to bottom transponder to ship) and assuming, or knowing, the sound velocity of the water, we obtain a relayed range measurement. These relayed ranges, used in conjunction with ship to bottom-transponder ranges, allow us to calculate the position of the relay transponder. A recent application of this technique is described in which several gravity core samples from the crest of the Horizon Guyot were positioned with respect to the detailed bathymetry and the geology within the area. The estimated error in positioning the samples is less than 20 m inside a navigational net extending over 100 km².Contribution of the Scripps Institution of Oceanography, new series.     

Geomorphology of unique reefs on the western Canadian shelf: sponge reefs mapped by multibeam bathymetry

Multibeam imagery of siliceous sponge reefs (Hexactinellida, Hexactinosida) reveals the setting, form, and organization of five reef complexes on the western Canadian continental shelf. The reefs are built by framework skeleton sponges which trap clay-rich sediments resulting in a distinctive pattern of low intensity backscatter from the reefs that colonize more reflective glacial sediments of higher backscatter intensity. Bathymetry and backscatter maps show the distribution and form of reefs in two large complexes in the Queen Charlotte Basin (QCB) covering hundreds of km², and three smaller reef complexes in the Georgia Basin (GB). Ridges up to 7 km long and 21 m in height, together with diversely shaped, coalescing bioherms and biostromes form the principal reef shape in the QCB whereas chains of wave-form, streamlined mounds up to 14 m in height have developed in the GB. Reef initiation is dependent on the distribution of high backscatter-intensity relict glacial surfaces, and the variation in reef complex morphology is probably the result of tidally driven, near seabed currents.     

Double-diffusive convection with a non-linear equation of state: Part I. The accurate conservation of properties in a two-layer system

If the equation of state is nonlinear, a given flux of heat across a double diffusive interface causes different buoyancy fluxes in the upper and lower layers. This results in different convective activity in the two layers and can lead to preferential entrainment across the interface in one direction (i.e. a migration of the interface). In this paper we derive the conservation equations for properties (e.g. heat and a solute) across a double diffusive interface between two well-mixed layers. A nondimensional measure of the entrainment across an interface and the most suitable choice for the buoyancy flux ratio are presented. Some surprising facts emerge. First, even for a linear equation of state and in the absence of direct entrainment across the interface, the flux of water across a finger interface is shown to be important. Second, for the heat-solute system, the heat balance equations for each well-mixed layer contain terms proportional to the heat of solution of the solute and the partial specific enthalpy of pure water in a seawater solution. Third, the rate of change of gravitational potential energy of the two-layer system is shown to have several extra terms in addition to the two commonly quoted major terms.     

Design of a tri-axial thrust anemometer for measurement of atmospheric turbulence over water

The BIO Mark 8 thrust anemometer measures the drag of the wind on a perforated table tennis ball mounted on a vertical beam. The tri-axial displacement of the beam is sensed by eddy current proximity sensors. This anemometer has a flat frequency response from 0 to 10 Hz and can measure wind from 0 to 28 m s⁻¹ at temperature from − 19 to + 28°C. It is designed for remote operation for extended periods of time such as on stable towers at sea.     

Reversing Water Exchange Patterns at the Entrance to a Semiarid Coastal Lagoon

Water velocity and density profiles were obtained in late-spring and late-winter to document reversing mean circulation patterns at the entrance to a semiarid coastal lagoon, the Bay of Guaymas, in the Gulf of California, Mexico. The lagoon is shallow but the bathymetry at its entrance is similar to that of temperate estuaries with an asymmetrically positioned channel flanked by shoals. In late-spring the mean circulation at the entrance to the lagoon was driven by horizontal density gradients that arose from excess evaporation over precipitation in the area as evidenced by water density profiles. The lagoon exported relatively warm (25·8 °C) and salty (36·2) water to the Gulf of California through the channel. This export was consistent with inverse estuarine circulation influenced by bathymetry. In late-winter, the circulation at the entrance of the lagoon was mostly driven by wind stress that blew from the northwest, roughly along the main axis of the lagoon. Relatively cool (16·0) °C) and less salty (35·1) water from the Gulf of California was driven into the lagoon within the channel. Density gradients inside the lagoon seem to have played a secondary role in driving the circulation. The late-winter circulation was then estuarine-like, with outflow in the direction of the wind over the shallow areas and a compensatory inflow appearing in the channel as expected from theory of wind-driven flow over bathymetry. This estuarine-like circulation developed despite the lack of measurable freshwater input to the lagoon and was the opposite to that observed in late-spring. These observations then document a reversal in water exchange patterns from season to season in a semiarid coastal lagoon. The observations also constitute one of the few reported examples of flow over shoals driven in the same direction as the wind stress with a compensatory flow in the channel.