Response of a stream diatom community to top predator manipulations

I investigated the effects top predator manipulations have on the community structure of benthic diatoms in a small, third-ordered stream ecosystem. Following the concept of trophic cascades in food webs, changes in top predator abundance should, via herbivore elimination, influence community metrics and species composition of stream periphyton. Using in situ stream enclosures, I varied the density of two common stream predators, sculpin and a perlid stonefly larva. Over three weeks, in three seasons, I sampled both invertebrate prey and diatoms from tiles placed in each enclosure. Diatoms did respond to predator manipulations. Changes in diversity, species richness, evenness, and dominance varied with treatment and between seasons. Cluster analysis showed pronounced assemblage dissimilarities before and after predator manipulations. However, species composition did not change between treatments, though rankings of the ten most abundant species did shift. Top predators can alter the community composition and structure of stream benthic algae indirectly through grazer suppression and directly by disruption due to foraging activity. This study demonstrates that top predators can impact nonadjacent trophic levels and that predator type and abundance should be considered when comparing diatom communities temporally and spatially.