Primary Productivity of Phytoplankton in Loch Leven, Kinross

Photosynthetic 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 1.59g 02/m2.h and 0.4 to 21.0 g 02/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 depletion 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.

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