Effects of temperature and velocity fluctuations on diatom community composition in streams

Water temperature and discharge associated with hydroelectric-dam operation can influence diatom communities. We examined the influence of temperature and fluctuating velocities on diatom communities in artificial streams. We focus on the prevalence of upright taxa, because they have been shown to be preferentially consumed by invertebrates. In the first experiment, water temperatures were maintained at ~10°C and ~15°C (9 replicates each). Temperature strongly influenced diatom community structure; 10°C streams supported higher percentages of live diatoms and, after 15 weeks, more upright diatoms compared to the 15°C treatment. After 25 weeks, we imposed 3 velocity treatments (3 replicates per temperature and velocity treatment): stable (11,000cfs), low fluctuating velocities (~7,000-13,000cfs) and high fluctuating velocities (~11,000-18,000cfs). Streams with stable velocities and 10°C water were dominated by upright diatoms. In a second experiment, we implemented both temperature and velocity treatments at the start of the algal colonization sequence. Again, the proportion of live diatoms was higher in 10°C streams. After 19 weeks, algal communities in streams with large velocity fluctuations contained significantly more upright diatoms regardless of temperature, relative to other treatments. Temperature and velocity are both strong drivers of the presence of upright diatoms but their influence was highly dependent on antecedent conditions.