A Nine-Year Study of the Phytoplankton of the Eutrophic and Non-Stratifying Loch Leven (Kinross, Scotland)

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Abstract
(1) Observations from 1968 to 1976 at mainly weekly intervals on the phytoplankton of the shallow, eutrophic and non-stratifying Loch Leven have revealed marked seasonal and annual differences in species-composition and population densities. However, long term trends were apparent, with decreasing concentrations of chlorophyll a, total dry weight and total volume of phytoplankton, although an increase in the latter two features in 1976 coincided with a massive growth of a pale form of Oscillatoria agardhii Gomont. (2) Trends were apparent also in increasing size of dominant algae and decreasing total numbers of individuals producing the various population maxima. A 3000-fold increase in size has accompanied the change towards large species, e.g. Melosira ambigua (Grun.) Müller, Anabaena flos-aquae Brέb. (ex Born. et Flah.), and Oscillatoria agardhii, which have progressively replaced `nano' forms, e.g. Synechococcus nov. sp., Oscillatoria redekei van Goor and some unicellular centric diatoms. Recent population maxima have rarely exceeded 104 individuals per ml, in sharp contrast to earlier growths commonly of 105-106 individuals per ml. (3) Considerable variation in the percentage chlorophyll a of crop dry weight (0.32-1.96), and the ratios of chlorophyll a to volume (< 1 to > 12 \mu g mm^{-3}) and of dry weight to volume (0.31-1.32 mg mm-3), was associated with the continual changes in species-composition. (4) General features of the algal plankton are discussed in relation to the gross physical and chemical characteristics of the loch, whilst some of the observed trends in plankton quantity and quality are considered in relation to changes in phosphorus loading (a reduction of approximately 1.0 g m-2 P per annum since 1972) and in zooplankton quality (a shift from a Cyclops- to a Daphnia-dominated community since 1971). (5) The need for long-term studies based on close time-interval sampling for information on species and populations is emphasized; one source of misinterpretation of results if species-variation is not taken into account is discussed and illustrated with the use of a simple theoretical model.