1. Growth rates of seven species of planktonic algae were determined in culture over a range of temperature from 2 to 35°C. Additional observations on growth and viability were made for 13 species in the temperature range 20-35°C. 2. There was a wide range of growth rates between species at their optimal temperatures, from 1.7 divisions day )1 (Asterionella formosa) to 0.3 divisions day )1 (Ceratium furcoides).3. There were considerable differences between species for growth at low and high temperature. Certain algae, including the diatom A. formosa and the flagellates Cryptomonas marssonii, Dinobryon divergens and Eudorina unicocca var. unicocca, had growth rates of 0.4 divisions day )1 or more at 5°C. The cyanophyte Tychonema (formerly Oscillatoria) bourrellyi, the xanthophyte Tribonema sp., the desmid Staurastrum cingulum and the large dinoflagellate C. furcoides grew poorly or not at all at this temperature. All 21 species tested could grow at 25°C, but many -including most of the diatoms, some cyanophytes, and all the flagellates -failed to grow persistently at 30°C. Only Aphanizomenon flos-aquae survived with moderate increase at 35°C, a lethal temperature for the other species. 4. The applicability was considered of proposed quantitative formulations of the ratetemperature relationship. Simple exponential relationships applied only to very limited lower ranges of temperature. The relationship proposed by Bȇlehrádek was a better fit over a wider temperature range, but still excluded rate-decline at high temperature. 5. The interspecific differences found are of potential significance for restrictions in natural distributions associated with season, altitude (especially above 500 m) and latitude.

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Suminary 7%c growth of Ceratium hirundinella in EsthwaiteWater, a small productive lake, was investigated during 1975; the horizontal distribution of the alga was studied on four occasions over a 3-week period. Measurements of lake temperature, oxygen concentration, and vertical stratification of Ceratium at close intervals of depth, were also made. Lake population estimates from large samples at many stations were compared with small samples at the regular sampling station. On two occasions the means of these samples were significantly different (F<001). On one date, the estimate of Ceratium abundance in the lake from the large sample at many stations was more than twice the value obtained from the sample collected at the regular sampling station. 77ie population of Ceratium was contagiously distributed in all the large samples. On 3 days the horizontal distribution showed an increase in Ceratium numbers towards the windward shore of the lake. Very large accumulations of Ceratium were found in the metalimnion, just above the anaerobic hypolimnion, dttring late afternoon. Possible mechanisms of horizontal pattern development are considered, based on the measttrements of the vertical distribution of Ceratium, and meteorological factors. The ecological consequences for Ceratium of vertical and horizontal movements, and its spatial distribution in the lake, are discussed.