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1. A number of planktonic cyanobacteria species form resting stages that survive in the sediments of lakes. The significance of this life history strategy to the ecology of new planktonic populations was investigated in Esthwaite Water, a mesotrophic lake in the English Lake District. 2. A simple trapping technique was used to quantify vertical movements of five species of buoyant gas‐vacuolate cyanobacteria from close to the sediments, along a depth transect. 3. ‘Recruitment’ from the sediments was found to be widespread amongst the cyanobacteria species associated with the summer phytoplankton community. 4. Estimates of the vertical upward fluxes of cyanobacteria based upon trap catches could not account for observed increases in the planktonic populations suggesting that ‘recruitment’ was not a significant source of biomass. 5. Vertical upward movements of Anabaena solitaria were recorded prior to this species becoming established in the plankton suggesting that benthic populations might be a source of cells for initial pelagic growth of populations of this species. 6. Low numbers of vegetative filaments of Anabaena flos‐aquae, Aphanizomenon flos‐aquae and Oscillatoria agardhii were observed in the plankton through the winter. These small overwintering populations appeared to be the primary source of inocula for the large summer populations of these species.

1. The pattern of fluctuations in the total biomass and species composition of phytoplankton in the shallow, eutrophic Loch Leven exhibits considerable inter-annual and within-year variability. Nevertheless, studies over the 18-year period reviewed here (l%8-85) show that many of the observed changes can be explained in terms of light regimes and the concentrations and fluxes of nutrients. On occasion, the incidence of fungal parasitism and of protozoan, rotiferan and crustacean grazing is also important.2. Changes in annual mean algal biomass from very high levels in the late 1960s and early 1970s to somewhat lower levels in the late 1970s. followed by occasional high peaks in the present decade, are attributed to shifts in phosphorus loading-particularly from a P-rich industrial source.3. In spite of complex and erratic sequences of algal species, seasonal patterns in the size distribution of the phytoplankton assemblage have been identified. However, these also changed. During the 4 years prior to \971, Daphnia hyalina was not recorded in the plankton, and small algae have been abundant only in late winter or early spring.4. The potential importance of the weather is highlighted as one of the possible causes of the irregular appearance of algal species. The effect of thecharacteristically variable, oceanicclimateofNorthemBritainon this large (13.3 km^). shallow (z = 3.9 m) loch is thought to be of particular importance.5. The influence of the weather on phytoplankton sequences is explored by comparing records of monthly flushing rate values with time-series data on aspects of the aquatic environment and plankton populations. The preliminary assessment suggests that variation in flushing rate (p) has a considerable effect on temperature regimes and the supplies and in-loch dynamics of nutrients; through such changes, p controls major features of phytoplankton succession such as the temporal abundance of diatoms, as well as detailed sequences of events relating to the development and CoTTCspoDdeace Dr A. E. Bailcy-Watis. c/o ITE. Bush Estate Penicuik. Midlothian EH26 OQB, Scotland. 85 86 A. E. Bailey-H'atts et al.

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1. Total phosphorus (TP) cycles were examined in seventeen lakes from the Scottish lowlands and Northern Ireland, which varied in annual mean TP from 17 to 192 pg P 1-*. 2. It is clear from the data that the annual range of TP concentrations increases as the annual maximum TP increases. Although the annual minimum TP increases somewhat with increasing annual maxima, even enriched lakes show a pronounced TP minimum. As a result, the annual maximum values are highly correlated with the ranges and means, but less so with the annual minima. 3. The data are discussed in relation to a conceptual sine wave model and it is suggested that enrichment modulates the amplitude of the sine wave. 4. Mechanisms conditioning the maximum and minimum values are reviewed.

The species composition of planktonic diatom crops in Loch Leven, Kinross, Scotland varied cotisiderably over the years studied with centric forms predominating. Dense populations were often produced atid rates ofittcrea.se and decrease were high.Relations with cUssolred silica did not appear always to be simple; instances occurred during which both diatoms and silica were increasing or decreasing. Moreover., certain rates of increase in dissolved and frustule silica exceeded the rates at which silica flows into the loch even when the inflows appeared to be the only available dissolved silica source.

A study was made on a freshwater planktonic population of centric diatoms, mainly Stephanodiscus rotula (Kütz.) Hendey, in Loch Leven, Kinross, Scotland. The observations were analyzed in relation to changes in water temperature and chemistry, parasitism by a chytridiaceous fungus, grazing by a recently described protozoan and competition from another alga (Synechococcux sp.). Each of these factors is thought to have affected the recorded rise and fall in diatom population density, in particular, silica limitation, fungal parasitism and protozoan grazing. The protozoan involved (Asterocaelum algophilum Canter) forms digestion cysts, the predominantly diatomaceous contents of which suggested very selective feeding but laboratory experiments with dual cultures did not confirm this. They indicated that both the size and quality of algae affected their suitability as food. Feeding activities of Asterocaelum seemed to be more or less confined to a solid surface; grazing rates in cultures shaken to maintain the animals and algae in suspension were low relative to those observed in unshaken cultures. This suggests that much of the grazing occurred on diatoms that had sunk onto the sediments.