The light climate of Loch Leven, a shallow Scottish lake, in relation to primary production of phytoplankton

Bindloss, M.E.. 1976 The light climate of Loch Leven, a shallow Scottish lake, in relation to primary production of phytoplankton. Freshwater Biology, 6 (6). 501-518.

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Seasonal changes in incident irradiance and underwater light penetration at Loch Leven from 1968 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 (k min) and the concentration of chlorophyll a are discussed. The value of ks, the increment in kmin per unit increment in algal concentration, was estimated from field data as 0·0086 In units per mg chl a/m2 and from laboratory spectroradiometer data as 0·0079 In units per mg chl a/m2. These ks values imply theoretical upper limits for the amount of chlorophyll a in the euphotic zone (Σn max) of 430 and 468 mg chl a/m2, respectively. Observed euphotic chlorophyll a contents (Σn) were sometimes close to these upper limits. Typical photosynthesis/depth profiles are described. Profile area is shown to be related to the logarithm of the ratio between surface-penetrating irradiance (Io') and the irradiance (Ik) defining the onset of light-saturation of photosynthesis. Standardized profiles, plotted on a common scale of 'optical depth', are used to illustrate the relatively minor influence of variations in Io' and Ik on hourly rates of photosynthesis per unit area. The saturation parameter (Ik) generally increased as photosynthetic capacity (Pmax) increased; the temperature-dependence of Ik is explained by the temperature-dependence of the enzyme-controlled (dark) reactions of photosynthesis, which control Pmax. A spring peak in the ratio between surface penetrating irradiance (Io') and Ik is interpreted as a result of a lag in the seasonal increase in water temperature with increase 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 of 0·31 mg O2/mg chl a.h per 1 W/m2 (PAR). Interactions between mixed depth, underwater light extinction and phytoplankton productivity are discussed; comparisons are made with other shallow, optically deep lakes.

Item Type: Publication - Article
Digital Object Identifier (DOI):
Programmes: CEH Programmes pre-2009 publications > Other
UKCEH and CEH Sections/Science Areas: _ Pre-2000 sections
ISSN: 0046-5070
Additional Keywords: Loch Leven
NORA Subject Terms: Botany
Ecology and Environment
Date made live: 23 Jul 2010 12:21 +0 (UTC)

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