SynopsisPhotosynthetic productivity of phytoplankton in Loch Leven was studied over a 4-year period (1968–71), using the oxygen light and dark bottle technique. Marked seasonal changes in hourly and daily rates of gross photosynthetic productivity are described within the range 0·02 to 1·59 g O2/m2.h and 0·4 to 21·0 g O2/m2.day respectively. Hourly rates are shown to be relatively insensitive to variations in surface light intensity, whereas daily rates are influenced to a considerable extent by the duration of incident radiation (daylength).The phytoplankton itself exerts a dominant influence on underwater light penetration, accounting for ca 75 per cent of light extinction at highest crop densities. This self-shading effect contributes to the poor correlation observed between crop density and areal gross productivity. The chlorophyll a content per unit area in the euphotic zone often approached its estimated theoretical limit of 430 mg/m2.In general, increase in photosynthetic capacity (per unit content of chlorophyll a) accompanied increase in water temperature. During certain periods an inverse relationship between photosynthetic capacity and population density was evident. Reduction in photosynthetic capacity is attributed, in part, to the high pH values (> 9·5) with concomitant CO2-depIetion associated with dense phytoplankton crops.Estimates of net photosynthetic productivity were frequently zero or negative, even over periods when algal populations were increasing and dissolved oxygen and pH values were above their respective air-equilibrium values. Underestimation of gross photosynthesis due to photochemical oxidation, photorespiration or the use of stationary bottles could not account for this apparent anomaly. The most probable sources of error in the estimates of net photosynthetic productivity are discussed.
Seasonal changes in incident irradiance and underwater light penetration at Loch Leven from 1968Leven from to 1971 are discussed in relation to the photosynthetic behaviour and crop density of phytoplankton.Light extinction was highest in the blue and lowest in the orange spectral regions, a pattern typical of other turbid waters. Euphotic depth varied between 1-2 and 7 4 m and was on average c.three times the Secchi disc transparency. Underwater light extinction depended chiefly on phytoplankton crop density (estimated as chlorophyll a). Despite the shallowness and wind-exposed situation of the loch there was no evidence of appreciable light extinction due to sediment disturbance. Possible causes of variability in the relationship between the minimum vertical extinction coefficient {kmin) and the concentration of chlorophyll a are discussed.The value of kg, the increment in kmin per unit increment in algal concentration, was estimated from field data as 00086 In units per mg chl ajm^ and from laboratory spectroradiometer data as 0-0079 In units per mg chl ajm^. These k» values imply theoretical upper limits for the amount of chlorophyll a in the euphotic zone (Lnmax) of 430 and 468 mg chl ajm"^, respectively. Observed euphotic chlorophyll a contents (Sn) were sometimes cio.se to these upper limits.Typical photosynthesis I depth profiles are described. Profile area is shown to be related to the logarithm of the ratio between surface-penetrating irradiance do') and the irradiance (h) defining the onset of light-saturation of photosynthesis. Standardized profiles, plotted on a common scale of ^optical depth'. Correspondence: Dr M. E. Bindloss, Institute of Terrestrial Ecology, 12 Hope Terrace, Edinburgh EH9 2AS, Scotland. are used to illustrate the relatively minor infiuence of variations In Io' and h on hourly rates of photosynthesis per unit area. The saturation parameter {h) generally increased as photosynthetic capacity (Pmax) increased; the temperature-dependence of h is explained by the temperature-dependence of the enzyme-controlled (dark) reactions of photosynthesis, which control P max-A spring peak in the ratio between surface penetrating irradiance (L') and h is interpreted as a result of a lag in the seasonal increase in water temperature with increa.se in surface irradiance. The gradient (K') of the linear light-limited region of the photosynthesis-irradiance curve showed little variation and had an average value of0 31 mg Ozlmg chl a.h per 1 Wjm^ (PAR).Interactions between mixed depth, underwater light extinction and phytoplankton productivity are discussed; comparisons are made with other shallow, optically deep lakes.
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