Abstract
In lakes, trophic change and climate change shift the relationship between nutrients and physical factors, like temperature and photoperiod, and interactions between these factors should affect the growth of phytoplankton species differently. We therefore determined the relationship between P-limited specific growth rates and P-quota (biovolume basis) of Stephanodiscus minutulus and Nitzschia acicularis (diatoms) at or near light saturation in axenic, semi-continuous culture at 10, 15 and 20 °C and at 6, 9 and 12 h d−1 photoperiod. Photoperiod treatments were performed at constant daily light exposure to allow comparison. Under these conditions, we also performed competition experiments and estimated relative P-uptake rates of the species. Temperature strongly affected P-limited growth rates and relative P uptake rates, whereas photoperiod only affected maximum growth rates. S. minutulus used internal P more efficiently than N. acicularis. N. acicularis was the superior competitor for P due to a higher relative uptake rate and its superiority increased with increasing temperature and photoperiod. S. minutulus conformed to the Droop relationship but N. acicularis did not. A model with a temperature-dependent normalised half-saturation coefficient adequately described the factor interactions of both species. The temperature dependence of the quota model reflected each species’ specific adaptation to its ecological niche. The results demonstrate that increases in temperature or photoperiod can partially compensate for a decrease in P-quota under moderately limiting conditions, like during spring in temperate lakes. Thus warming may counteract de-eutrophication to some degree and a relative shift in growth factors can influence the phytoplankton species composition.
Highlights
Interactions between nutrients and physical factors like temperature and light are important for phytoplankton growth
The dominance of filamentous cyanobacteria during spring in a shallow lake depended on the combined effects of winter temperature and the P:Si ratio [2] and the timing of the phytoplankton bloom was synergistically affected by water temperature and phosphorus supply [6]
N. acicularis had a higher maximum specific growth rate than S. minutulus at 10–20 uC and 12 h d21 photoperiod but growth rates were similar under shorter photoperiods
Summary
Interactions between nutrients and physical factors like temperature and light are important for phytoplankton growth. Turbidity on the other hand increases absorbed radiation and influences the water temperature and thermal structure [3,4], and water temperature affects nutrient cycling processes [5]. The dominance of filamentous cyanobacteria during spring in a shallow lake depended on the combined effects of winter temperature and the P:Si ratio [2] and the timing of the phytoplankton bloom was synergistically affected by water temperature and phosphorus supply [6]. The occurrence of several Aulacoseira (formerly Melosira) species could be differentiated along an irradiance/photoperiod – phosphorus gradient, suggesting trade-offs between these factors [7]. Understanding these effects may depend on how well we understand the physiological response of individual species
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