Deep Bottom Water Research Articles

Water circulation and nutrients in the North-west Irish Sea

The results of hydrographic observations made in the Irish Sea to the west of the Isle of Man during 1954–1961 and 1965–1968 are presented and discussed. The picture that emerges suggests that an appreciable part of the main south-north flow through the Irish Sea passes to the west of the Isle of Man. The core of high salinity water (as defined by the isohalines) can be associated with the deep trough, especially in the early part of the year, but more often than not is biased towards the Isle of Man. The salinity pattern in June 1955 may be interpreted in support of Williamson's (1956) suggestion that the flow through the Irish Sea is irregular. There is evidence that Atlantic water, although considerably diluted by fresh water run-off, penetrates southwards off the Irish coast. In winter hydrographic conditions tend to exhibit vertical uniformity, but bottom water of higher salinity which is also distinguishable by small temperature differences may sometimes be present. Occasionally there may be a very marked vertical discontinuity (as in March 1966) which is not only distinguishable by temperature and salinity differences but also by an inverse concentration of nutrient salts. Such an inverse relationship becomes completely obscured as a result of phytoplanktonic activity in the spring. During summer, surface warming brings about marked temperature layering which persists and consolidates in the region characterized by weak tidal currents. In particular, the area to the south west of the Isle of Man becomes very stable, although perhaps not completely stagnant as suggested by Simpson (1971). It is still possible for the denser bottom water to flow gradually northwards and replacement to take place at intermediate depths, below the surface mixed layer. Associated with the temperature layering is a well defined pattern of nutrient, dissolved oxygen and chlorophyll a distribution. Nutrient depletion occurs in the upper 30 m of the water column and may occasionally extend to greater depths. In particular, nitrate virtually disappears during the summer and would thus seem to be the limiting nutrient. Coinciding with the fall in nutrients is a rise in oxygen saturation. The highest chlorophyll a concentrations, however, tended not to coincide with the highest oxygen saturations, and may thus be indicative of dead or dying phytoplankton accumulating beneath the surface mixed layer. In the deep bottom water of the trough nutrient concentrations remain high and low oxygen saturations suggest that this is due to regenerative processes. Nitrite differs from other nutrients in that once the surface mixed layer has become established, a well defined maximum tends to occur at some depth beneath it. Following the breakdown of temperature layering the autumnal overturn distributes nutrients evenly throughout the water column. Nitrite can be particularly high at this time but declines rapidly as nitrate and other nutrients increase.

Determination of sediment properties from first shear-mode Rayleigh waves recorded on the ocean bottom

First shear-mode Rayleigh waves from five shallow focus earthquakes with epicenters in the mid-Atlantic ridge have been recorded by a hydrophone located on the ocean bottom in deep water (5.7 km) 260 km northeast of Antigua, West Indies, and by short-period seismographs at Bermuda. The surface wave train, in most cases, is a sinusoidal signal of long duration and nearly constant period; only the steep part of the dispersion curve is recorded. The predominant surface wave period is between 7.1 and 7.8 seconds for propagation paths to Bermuda and between 5.7 and 6.5 seconds for paths to the hydrophone. The steep parts of the dispersion curves for Rayleigh waves of the first shear mode are controlled primarily by the properties of the unconsolidated sediment layer. Theoretical dispersion curves have been computed as a function of sediment thickness and shear velocity gradient, and the results have been compared with the observed dispersion data. These comparisons give estimates of average sediment thicknesses that range between 0.5 and 0.9 km for the parts of the North Atlantic considered in this study. For the assumed sediment shear velocity profile, average sediment thicknesses derived from profiler results in the same region are consistently smaller than thicknesses derived from dispersion results. The difference may be explained by assuming that dispersion takes place principally in the thickest pockets of sediments along the path. Alternatively, the dispersion results can be made to agree with profiler results by assuming extremely low sediment shear velocities. The required average shear velocity for a layer of sediment 150 meters thick is found to be approximately 75 m/sec.

Measurement of Background Radiation in Lakes

Measurements of background radiation were made in five lakes–two Wisconsin, two New Hampshire, and one antarctic. These lakes were characterized by a small amount of ionizing radiation in mid—water and increasing quantities near the air and sediment boundaries. Moreover, the radioactivity of the sediments in the Wisconsin and New Hampshire lakes usually increased toward the shore line. Count—rate values for the sediments near the shore line were up to ten times higher than the values recorded near the bottom in deep water. A similar pattern of radioactivity distribution in the sediments was not apparent in Lake Vanda, a perennially ice—covered antarctic lake. Various hypotheses were suggested to explain the origin of the anomalous distribution of ionizing radiation in these lakes and its possible role as an environmental factor.

Standing Crops of Fishes in Three Small Lakes Compared with C14Estimates of Net Primary Productivity

The standing crops of fishes which inhabited three small lakes were estimated immediately after rotenone treatment by combinations of procedures which included collections of all fish along the shorelines, from the surface waters offshore, and from the bottoms in shallow water; collections and counts from random sampling areas on shore and in shallow water; and collections and counts from randomly selected transect lines on bottom in deep water by SCUBA divers. Estimates of the net primary productivities of the lakes were made by the C14 method. When the lakes are ranked according to their estimated standing crops, the order of ranking is confirmed by the estimated net primary productivity of the most productive lake, but is in conflict with the productivity estimates from the intermediate and least productive lakes. Analyses of the structures of the fish populations of the lakes before reclamation and evaluations of techniques are also presented.

Current‐measurements in the Georges Bank Canyons

Among the many speculations concerning the origin of submarine canyons, bottom‐currents of various types have been appealed to frequently. This is one theory to which it is possible to apply some quantitative data, and to form an estimate as to the importance which may be attached to this means of producing submarine canyons, both now and in past ages. I refer of course to normal currents due to differences in densities caused by temperature and salinity, and to those which have a tidal origin, and not to the type recently proposed by Daly. Consequently during the summer of 1936 measurements of the bottom‐currents were made in three of the submarine canyons of Georges Bank from the research vessel Atlantis of the Woods Hole Oceanographic Institution. Because of the physical difficulties of obtaining readings on the bottom in deep water, in a region where tidal currents at the surface are strong, the results are far from giving a complete picture. Nevertheless, some idea may be obtained of the magnitude of the velocities which might be expected.