Changes in depth and time of certain chemical and physical conditions and of the standing crop of Asterionella formosa Hass. In the North Basin of Windermere in 1947

Abstract

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.