Histoplasma ulcer of the tongue Hiley, P; Heilbrunn, C; Fields, J
JAMA : the journal of the American Medical Association,
1967-Jun-19, Letnik:
200, Številka:
12
Journal Article
Free surface oscillations and tides of Lakes Michigan and Superior Mortimer, Clifford Hiley; Fee, E. J.; Rao, D. B. ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical and physical sciences,
01/1976, Letnik:
281, Številka:
1299
Journal Article
Recenzirano
From records of water levels at nineteen shoreline stations on Lakes Michigan, Huron and Superior (figure 1), we have prepared power spectra from 95 stationdata sets and 128 spectra of interstation ...coherence and phase difference. Those spectra have been used to . (1) identify the first five free gravitational, barotropic modes (surface seiches) of the three basins; (ii) estimate the corresponding seiche frequencies, Lake Huron table 2, Lake Michigan tables 3 and 4, Lake Superior table 7; (iii) determine, for some modes, the phase progression around the basin imposed by the Earth’s rotation; and (iv) speculate on the structure of other oscillations, including diurnal and semidiurnal tides. , Because the number of recording stations was limited, the phase progression of individual modes could only be determined with confidence for the first and second in Lake Michigan (figure 13), for the first, second, third, and eighth mode in Lake Superior (figures 22 and 32 b)and for the semidiurnal tide in both basins (figure 31). Except for the Superior semidiurnal tide, which progresses clockwise, all the modes illustrated in figures 13 and 22 and the Lake Michigan semidiurnal tide conform to a positive amphidromic pattern - counterclockwise progression. Possible reasons for the difference in tidal behaviour in the two basins are discussed in §4 and by Hamblin (1976). There is very close agreement between the observed frequency and the phase progression of the first three and eighth Superior modes and results from the two dimensional computations of Platzman (1972) and Rao & Schwab (1976). Because some of the level recorders were not protected from local harbour oscillations in the period range below 2 h, and because some of the data sets listed in tables 1 and 6 were available only in the form of hourly readings, spectra from some stations exhibited contamination by aliasing. Section 2 (b) is devoted to a discussion of: (i) the nature of this spectral contamination (see figure 4); (ii) its extent in our examples; and (iii) attempts to minimize its influence through identification of the principal aliases and exploitation of the discovery that useful information can still be extracted from interstation coherence and phase spectra, even if the power spectra from one or both stations of the pair are badly aliased. With aliases identified or absent, the remaining spectral and interstation coherence peaks correspond to free modes (and tides). In Lake Michigan the first three modes are the most strongly excited and are clearly identified as longitudinal seiches (§§ 2 (c-f),2(i)). A transverse (E-W) seiche is also strongly excited, probably in the form of a negative amphidrome, in the south-central reach of the basin (for example T1 in figure 6), but the structure and identity of oscillations corresponding to spectral peaks at higher frequencies cannot yet be resolved. For Green Bay, a 192 km (120 mile) long gulf opening into Lake Michigan, a remarkable double resonance is described in §2(g). The Bay responds as a viscously damped system driven by two forcing oscillations - the semidiurnal tide and the first mode of the main Michigan basin - at respective frequencies 1.93 and 2.67 cycles per day (c/d), one on each side of the natural frequency of the Bay-Lake system, 2.2 c/d (figures 9 and 10). In the Superior basin, topographically more complex than Michigan, the first three longitudinal modes are also the most conspicuous, but some modes above the third are also strongly excited. O f these, the fourth, fifth, and eighth modes can be identified through comparison with Rao & Schwab’s (1976) numerical determinations. The most striking feature of the eighth mode, often strongly excited, is a transverse (N-S) oscillation of the eastern half of the basin as a negative amphidrome (figure 32 b). In spite of prior removal of a linear trend from the input data, the spectra exhibit a steep rise in power as the low-frequency end is approached, where interpretation is therefore difficult. However, examination of the frequency range below 4 c/d, in §§2(h) and 3 (e) and in figure 11, establishes the following points: (i) for reasons discussed in the text, the semidiurnal tidal peak covers a narrower frequency range than peaks corresponding to the seiche modes; (ii) there is minor but persistent evidence of a co-oscillation of the main Michigan basin and Green Bay; (iii) diurnal oscillations arising from tidal and meterorological forcing, §4, are generated more strongly in the Superior than in the Michigan basin; (iv) no spectral peaks are unambiguously identified as surface manifestations of internal waves known to be present, for example in the near-inertial frequency range 1.3—1.4 c/d; and (v) there is a small but significant rise in power near 0.35 c/d in spectra from both basins. Possible but not yet verified explanations of this rise are: meteorological forcing; excitation of a rotational mode (Rao & Schwab 1976); or both. For Lake Michigan a possible further explanation is provided by excitation of the lowest gravitational mode of the combined Michigan-Huron basin, seen in the currents of the connecting straits (figure 12).
Three numerical models are formulated for long-wave motion in a 180 km long gulf (Green Bay, Wisconsin) that opens into Lake Michigan. These models are used to investigate the response of the Bay to ...wind forcing and excitation by disturbances entering from the main Lake basin. Model simulations of water movements in the Bay have been done for two periods of 4 and 8 days respectively during 1969. Observed fluctuations in water level during these periods have been compared with the corresponding variations predicted by the models. Agreements and disagreements are discussed. These illuminate properties of the Bay’s motion and raise some further questions.
The annual cycle of phytoplankton production in the North Basin of Windermere has been a major object of study by the Freshwater Biological Association for 30 years. The year 1947 provided the first ...opportunity for a combined attempt—concentrated on the water column near the deepest point—to describe in detail and to interpret the annual cycle of temperature, algal cell numbers, and selected chemical variables, with the aim of presenting an integrated picture of openwater conditions, to which other less detailed or more specialized studies may be referred. Temperatures were measured and samples were taken for phytoplankton counts and for chemical analyses (dissolved and total silica, alkalinity, oxygen, nitrate, and phosphate) at weekly or sometimes more frequent intervals from 2 January 1947 to 12 January 1948 and at the following or sometimes more frequent depth intervals; every metre from the surface to 6 m, every 2 m to 12 m and every 5 m from 15 to 60 m. Although marked by an abnormally cold winter and hot summer, the annual temperature cycle (figure 1) followed a normal course. After the ice had disappeared in mid-March isothermal conditions prevailed until the beginning of May. Thermal stratification became established by the end of that month; the main thermocline lay near 9 or 10 m during most of the summer, and occasional temporary thermoclines were formed and destroyed. With autumnal cooling and storms, thermocline depth increased until isothermal conditions were re-established in early December. A parallel study of changes in temperature distribution in the whole basin (the subject of another paper—Mortimer 1952) disclosed a picture of wind-induced displacements of isotherms, followed by internal seiche motion with a dominant uninodal period near 14 h. The influence of these movements on events in the selected water column is discussed. As the column lay near the seiche uninode, conditions in it did not diverge widely from average conditions in the open water. The layer of greatest vertical density gradient (pycnocline) is shown stippled in figure 2. Identical stippling superimposed on later figures illustrates the strong correlation between density stratification and the development of chemical and biological discontinuities in the water column. Suppression of turbulent mixing and of associated friction gave the pycnocline the properties of a slippery interface; and the epilimnion, driven by wind or impelled by seiches, could therefore slide relatively freely without much mixing with layers below. As epilimnion depth coincided with that of the photic layer for much of the season, phytoplankton growth was largely confined to the epilimnion; and replenishment of nutrient salts from below was impeded. While turbulence in the epilimnion was sufficient to keep diatom cells in suspension, this was not so in the pycnocline, where they could sink passively through with little chance of return. The diatom Asterionella is the dominant member of Windermere phytoplankton. A general account of the seasonal cycle in the North Basin is given for the period 1932-61. The cycle of events in 1947, which followed the normal course for the period 1932-61, is described in detail with the aid of diagrams showing the distribution of live cells (figure 3), total cells (figure 4), dead cells (figure 5), and number of cells per colony (figure 6) in depth and time. The crop was low in midwinter, started to increase in early spring, and reached maximum numbers (over 5 million cells per litre in the 0 to 8 m water column) in early June. As the population increased there was a corresponding fall in concentration of silicate in the water (figure 7) to a level at which there was not enough remaining to support one more division of the standing crop. When, in early June, silicate supply could no longer meet the demands of diatom growth, there was a heavy mortality and a catastrophic decline in cell numbers in the epilimnion, until, by late August, there was less than one live cell per litre in the 0 to 5 m water column. Dead cells reached a maximum some weeks after the live cell maximum, but processes removing cells from the epilimnion eventually reduced total numbers (and total silica, figure 8) there to very low levels. Some silicate replenishment from inflows later occurred, but the main feature of the post-maximum phase was loss of cells through the pycnocline, because single dead cells, and colonies containing a high proportion of dead cells, sank relatively rapidly (figures 5, 9). During this phase there was some increase of numbers in the upper layers of the hypolimnion. But this was only temporary, and there was no accumulation in the lowest layers of the water column, although a layer of dead and dying cells was found on the mud surface. The relationships between the biological situation—in particular the changes in diatom population, already outlined—and the physico-chemical environment are discussed in the light of fluctuations in the chemical variables listed in the first paragraph of this summary. Changes in the distribution of dissolved and total silica (figures 7, 8) were closely related to diatom growth, and are here used to infer the magnitude of total production of diatoms and rates of loss by sinking (figure 9). Respirational consumption of oxygen in the hypolimnion occurred both at the mud surface and in the free water. The distribution of oxygen concentration in depth at the end of summer stratification (figures 11, 13, and tables 1 to 3) suggests, either that the rate of consumption at the mud surface measured in the laboratory is higher than that occurring in the lake, or that considerable downward migration of oxygen occurred within the hypolimnion. Total respirational oxygen consumption is used, in conjunction with sedimentary and dissolved organic carbon estimates, to infer a rough carbon budget for the lake. It is concluded that the carbon fixed by phytoplanktonic photosynthesis was a small proportion of the total organic carbon entering the lake. Changes in alkalinity (figure 10) and nitrate concentration (figure 14) in the epilimnion reflected seasonal changes in the inflowing water; and there was evidence (particularly from distribution of oxygen saturation, figure 12) of horizontal flow, out over the lake surface, of water warmed in shallow littoral areas. The concentration of phosphate, always near the lower limit of reliable estimation, was generally less than 1 jag/1, in the epilimnion and about 2 jag/1, in the hypolimnion. These small concentrations were, however, sufficient to support the observed maximum diatom crop. No significant contribution to the concentration of dissolved nutrients was derived from the deep sediments, the surface of which remained aerobic throughout the period observed.
The first part of this paper is taken up with an historical survey of the relatively few observations, some detailed and some less so, of internal seiches (internal standing waves) in lakes. After a ...description of the thermo-electric thermometer employed, there follow details and illustrations of the evidence, from temperature observations, for such internal waves in the northern basin of Windermere. Two main phases could be distinguished: (i) motion under wind stress leading to quasi-steady states with some or all of the isotherms tilted; (ii) internal seiche motion which developed after the wind had dropped. These observations confirm the findings of Wedderburn and his collaborators on the Scottish Lochs (1907-15). The results from Windermere are presented, not because any such confirmation is necessary, but in order to secure belated recognition of the fact that Wedderburn's 'temperature seiche' is not an isolated phenomenon, but is an everyday feature of movement in stratified lakes subject to wind action. As this movement is an important and largely unrecognized factor in lake environment, this paper is addressed mainly to limnologists. In its latter part, results of theoretical analyses of a detailed series of observations are presented in non-mathematical form. The applicability of a theory of oscillations in a basin with three layers of differing density (set out in an appendix by M. S. Longuet-Higgins) is tested by comparing theoretical and observed deflexions of selected isotherms from their equilibrium levels, resulting from internal waves after a gale. This theory also enables horizontal components of velocity and displacement to be calculated for each layer. Complicating factors in natural lakes are enumerated, and the influence of internal waves on lake biology and sedimentation is discussed.
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