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Three diatom species, Stephanodiscus hantzschii (Ehr.) Grun., Asterionella formosa Hass. and Fragilaria crotonensis Kitt. Hass. were isolated from Lake Maarsseveen where they are dominant and show a successional sequence. The physiological responses of each species to temperature and limitation by silicon and phosphorus were determined over the temperature range of 5° to 20° C using short-term batch culture methods. Stephanodiscus hantzschii had higher maximum growth rates than the other two species at all temperatures, and the maximum growth rates of all species increased with increasing temperature. Temperature affected not only maximum growth rates but also half-saturation constants (K[SUBs]) and the minimum cell quotas. S. hantzschii had low silicon requirements for growth under Si-limiting conditions, and A. formosa and F. crotonensis had higher and nearly identical silicon requirements. The K[SUBs] values for silicon for S. hantzschii were essentially constant from 5°to 20°C but varied greatly for the other two species. A. formosa had the lowest requirements for growth under phosphorus limitation, F. crotonensis was intermediate and S. hantzschii had the highest growth requirements for phosphorus. The K[SUBs] values for phosphorus were constant over the temperature range for both A. formosa and F. crotonensis and were much higher and variable for S. hantzschii. Nutrient competition experiments were performed in continuous cultures at four temperatures and various Si:P ratios. The results generally, but not always, confirmed the predictions based on the Monod relationships for each species. Results not in agreement with predictions were usually because of similar physiological properties of A. formosa and F. crotonensis or because of decreased loss rates for F. crotonensis due to wall growth. In cultures with all three species phosphorus-limited (Si:P > 75), A. formosa often dominated as predicted, although F. crotonensis was sometimes the most abundant species. As predicted, S. hantzschii never dominated at high Si:P ratios. At intermediate Si:P ratios when A. formosa and F. crotonensis were both Si-limited and S. hantzschii P-limited, all three species coexisted because A. formosa and F. crotonensis have almost identical silicon requirements, although sometimes F. crotonensis was more abundant than predicted. At 10°C the results agreed best with the predictions; A. formosa dominated at high Si:P ratios and S. hantzschii dominated as predicted at low Si:P ratios when all three species were Si-limited. [ABSTRACT FROM AUTHOR]

The effect of temperature on the silicon limited growth and nutrient kinetics of Stephanodiscus minutus Grun was examined using batch and semicontinuous culture methods. Short-term batch culture methods gave maximum growth rates which were essentially constant over the temperature range of 10° to 20° C (μm = 0.71-0.80 d[SUP-1]). The half-saturation constant for growth (K[SUBa]) was significantly lowest at 10° C (K[SUBs] =0.31 μM Si; 0.22-0.41), and higher and both 15° C(K[SUBs] = 1.03 μM Si;0.68-1.47) and 20°C (K[SUBs] = 0.88 μM Si; 0.60-1.22). Two method were used to evaluate the semicontinuous experiments. The Droop relationship showed that the minimum cell quota was about 1.50 × 10[SUP-7] μmol Si cell[SUP-1], but there was much overlap in the results at all three temperatures. The Monod growth relationship for the semicontinuous experiments gave estimates of K[SUBs] which were lowest at 15°C (K[SUBs] = 0.12 μM Si), and higher at 10°C (K[SUBs] = 0.68 μM Si) and 20°C (K[SUBs] = 1.24 μM Si), although 95% confidence intervals overlapped. The maximum growth rate estimates for the semicontinuous experiments were similar at 10° and 15°, and higher at 20° C, but the number of points used in making the calculations makes the results less reliable than those from batch cultures. Generally , there were no consistent significant differences in the silicon limited growth of S. minutus over the temperature range studied. Our values of K[SUBs] for S. minutus are the lowest recorded for a freshwater diatom, and are consistent with the distribution of this species in nature. Generally, this species becomes abundant in areas with high phosphorus loading and very low silicon levels (low Si;P loading rates) Stephanodiscus species are also fossil indicators of eutrophication in north temperate lakes. [ABSTRACT FROM AUTHOR]

Growth rates of two clones of the freshwater planktonic diatom Asterionella formosa Hass. were measured under conditions in which external silicon concentrations controlled growth. Clone AfOH2 from Lake Ohrid, Yugoslavia, had a higher maximum growth rate (μ[SUBmax] = 1.11 doublings/day) and apparent half-saturation constant (K[SUBsi] + Si[SUBo] = 1.93 μM Si) than clone L262 from Lake Windermere, England (μ[SUBmax] = 0.61 doublings/day; K[SUBsi] + Si[SUBo] = 1.09 μM Si). K[SUBlim], the silicon concentrations at μ = 0.9 μ[SUBmax], is 13.8 μM Si for clone AfOH2 and 6.5 μM Si for clone L262. These values agree well with published field observations showing A. formosa populations decreasing below 0.5 mg/l SiO[SUB2] (= 8.4 μM Si). Calculations of yield gave a range of 0.5-1.5 μM Si/10[SUP6] cells for clone AfOH2 and 0.6-1.9 μM Si/10[SUP6] cells for clone L262. [ABSTRACT FROM AUTHOR]

Complex Interactions in Lake Communities This book results from a workshop initiated by the National Science Foundation Ecology Program 'to rethink research on community ecology of lakes' and 'to set [...]