Quantitative evaluation of the time-line reduction performance in high definition marine magnetic surveys

The effectiveness of the time reduction procedure is the critical problem for the informative contents evaluation in the high definition (HD) geomagnetic marine surveys. The use of magnetogradiometers is a good solution for the time-reduction in the regional offshore studies but often, for technical reasons, this is not a practicable method in the very detailed coastal, canal or harbour surveys where the use of coastal base stations may be preferable. On the other hand the uncertainty of the full transferability of the coastal magnetograms to large areas of marine surveys can disrupt the time-reduced data sets by a geomagnetic residual time component. The phenomenon is related to the distance from the coastal observatory and to the homogeneity of the local magnetic characteristics of the crust. The maximum applicability distance of the time-line correction (TL) is qualitatively evaluated and is shown to be inconsistent with geomagnetic marine surveys in high definition. In this work we show a quantitative method to evaluate the stability of the coastal observatory magnetograms over the nearby marine area, together with the numerical degree of precision of the correction. The method is based on a double survey of the same profile (timer track: TT) at two different times. The surveys produce two different row data sets where the difference is related only to the geomagnetic time variations. Using the coastal observatory magnetograms we time-reduce the two data sets: if the coastal observatory magnetograms are fully coherent in the whole survey area the difference between the two reduced data sets will be zero. However, if the time variations measured in the observatory are inadequate in amplitude or phase to model the corresponding time variations in the surveyed profile (TT), the discrepancy between the time reduced data sets will not be zero. Similar TTs starting with various course from the base station permit the surveyed area to be split into sectors with variable degree of time coherence and to assign a degree of precision to the time-reduced survey.     

Distributions of Calanus spp. and other mesozooplankton in the Labrador Sea in relation to hydrography in spring and summer (1995-2000)

We collected mesozooplankton samples in the upper 100 m in spring or early summer each year between 1995 and 2000 along a section from Hamilton Bank (Labrador) to Cape Desolation (Greenland), and along additional sections in spring 1997 and early summer 1995. The North Atlantic waters of the central basin were characterised by the presence of the copepods Calanus finmarchicus, Euchaeta norvegica and Scolecithrocella minor and euphausiids. Calanus glacialis, Calanus hyperboreus and Pseudocalanus spp. were associated with the Arctic waters over the shelves. Amongst the other enumerated groups larvaceans were concentrated over the shelves and around the margins. Amphipods, pteropods and the copepods Oithona spp. and Oncaea spp. showed no definable relationships with water masses or bathymetry, while the diel migrant ostracods and chaetognaths were confined to deep water. Metrida longa, also a strong diel migrant, and Microcalanus spp., a mainly deep water species and possible diel migrant, were both sometimes quite abundant on the shelves as well as in the central basin, consistent with their likely Arctic origins.Analysis of community structure along the section across the Labrador Sea indicated that stations could be grouped into five different zones corresponding to: the Labrador Shelf; the Labrador Slope; the western and central Labrador Sea; the eastern Labrador Sea and Greenland Slope; and, the Greenland Shelf. The boundaries between zones varied spatially between years, but community composition was relatively consistent within a given zone and a given season (spring versus early summer). The relationship between community composition and water masses was not entirely straightforward. For example, Labrador Shelf water was generally confined to the shelf, but in spring 2000 when it also dominated the adjacent slope zone, the community in the Labrador Slope zone was similar to those found in other years. Conversely, in spring 1997, when Arctic organisms were unusually abundant in the Labrador Slope zone, there was no increased contribution of shelf water. In addition, North Atlantic organisms were often found on the shelves when no slope or central basin water was present.Although other organisms were sometimes very abundant, the mesozooplankton preserved dry weight biomass was dominated everywhere by the three species of Calanus, which together always accounted for ≥70%. One species, C. finmarchicus, comprised >60% of the total mesozooplankton biomass and >80% of the abundance of large copepods in spring and summer throughout the central Labrador Sea. In western and central regions of the central basin average C. finmarchicus biomass was ca 4 g dry weight m⁻² and average abundance, ca 17?000 m⁻² over both seasons. Highest levels (ca 7 g dry weight m⁻², >100?000 m⁻²) occurred in the northern Labrador Sea in spring and in eastern and southwest regions in early summer. C. hyperboreus contributed ca 20% of the total mesozooplankton biomass in the central basin in spring and <5% in early summer, while C. glacialis accounted for <1%. Over the shelves, C. hyperboreus contributed a maximum of 54% and 3.6 g dry weight m⁻², and C. glacialis, a maximum of 29% and 1 g dry weight m⁻², to the total mesozooplankton biomass.     

Distribution gradients of ichthyoplankton in an East African mangrove creek

As part of an investigation into the role of East African mangrove creeks as fish spawning and nursery areas, nocturnal ichthyoplankton sampling was carried out at five stations in Tudor Creek, Mombasa, Kenya, from September to November 1985. Throughout the creek, the salinity was close to that of seawater at about 35‰, but from the mouth to the upper creek there were gradients of increasing temperature and turbidity, and decreasing depth and dissolved oxygen concentration. Pelagic eggs and larvae of fish considered to be creek residents and creek non-residents were distinguished. Gradients of decreasing egg and larval abundance (particularly of non-residents), and of decreasing diversity of recognized types, were apparent from the mouth towards the upper creek. Such a distribution suggests that the mangrove system is not an important spawning or development area for the earliest life-history stages of non-resident species. The existence of the gradients also has implications both for sampling at fixed stations in tidal waters, and for investigating the existence of circatidal and semilunar spawning cycles in fish.