We found that while the number of species in a community tends to remain stable over time, changes in the composition of species are more common. We also found that assemblages of diatoms (single-celled plants), invertebrates (e.g. insect larvae) and fish at the same sites each show different patterns over time. This suggests that tracking species identities as well as species richness is important, and that we cannot necessarily rely on monitoring one taxa to learn about an ecosystem as a whole.