Spectral intensity in some Scottish freshwater lochs

Summary In the course of investigations on the ecology of submerged aquatic macrophytes a narrow‐bandwidth spectroradiometer has been used to measure underwater spectral intensity (390–750 nm) in a series of lochs approaching the extremes in optical and chemical properties of Scottish fresh waters. Cosine response and immersion‐effect properties of the collector were determined in the laboratory with a colUmated hght source. Diffuse attenuation coefficients, Ee , ranged from 0·55 in Loch Croispol, a calcareous loch, to 2·9 in Loch Leven, a lowland eutrophic loch. Underwater spectral intensity (1 m) relative to subsurface values show a proportional increase in short‐wave radiation in the blue‐green water of Loch Croispol, while the converse is true of the peaty brown water of Loch Uanagan. Attenuation coefficients were derived over 25 nm wavebands in Lochs Croispol, Leven and Uanagan. The water in the latter two lochs is optically similar although Leven is rich in phytopiankton and has an extinction peak at 675 nm. Uanagan represents the brown‐water type of loch most common in Scotland. Loch Croispol has attenuation coefficients for shorter wave‐lengths at least ten times lower than the other two lochs and only approaches their values at 750 nm. Croispol resembles Crater Lake, Oregon. The colour range at the maximum colonizable depths by rooted macrophytes of brown (Uanagan 4 m) to blue‐green (Croispol 6 m) water is equivalent to 196·0 and 230·6 kJ/Einstein (48·2 and 55·6 kcal/Einstein). Red/far‐red ratios even in plankton‐rich water (Leven) were at least three times the value for sunlight (1·3) and within the total photic zone reached three‐figure proportions. Possible implications for light‐sensitive seeds of aquatic species and for morphogenesis and zonation are discussed.