Theory and empirical studies predict that increasing primary productivity should affect higher trophic levels positively. Didymosphenia geminata is a freshwater diatom that grows attached to the streambed via exopolysaccharide filaments extruded from individual cells. Recently, this alga has formed dense mats covering kilometers of rocky-bottomed rivers throughout the world. Here I tested whether Didymosphenia affects the stream insect community and whether its effects propogate up the foodweb to fish. I used experimental removals of Didymosphenia within one stream and comparisons between streams with and without Didymosphenia blooms, to investigate its effects on stream insect community composition, biomass, density, body size, and energetic content. To test whether the mechanisms causing shifts in insect composition were due to direct or indirect effects of Didymosphenia filaments, I performed growth rate and predation experiments using a filament mimic. To test how changes in insect composition and the refuge provided by filaments affected fish foraging, I measured consumption rates in treatments with substrates lacking or covered with Didymosphenia crossed with insect communities representative of those in streams with and without Didymosphenia blooms. To test the effects of Didymosphenia on fish growth, diet, condition, and abundance, I used mark-recapture techniques in streams with and without blooms.
Results/Conclusions
Results from the observational study and experiments showed that blooms of Didymosphenia caused major shifts in stream insect density and composition but not biomass. Chironomids and a caddisfly increased 10-fold and 3-fold respectively, large-bodied mayflies that are preferred fish prey decreased 5-fold, and one predatory stonefly increased 2-fold. Experiments revealed that the primary mechanism explaining the increase in chironomids was the refuge from invertebrate predators provided by Didymosphenia. In contrast, experiments showed that the decrease in large-bodied mayflies was due to Didymosphenia filaments increasing their susceptibility to invertebrate predators. Experiments revealed that trout ate fewer prey from communities resembling those found in streams with Didymosphenia blooms. Additionally, both the mean body mass and the total amount of energy from insect prey available to trout were lower in streams with Didymosphenia blooms. Hence, trout growth rates were 44 percent lower in streams with Didymosphenia blooms and could not be explained by other factors, such as temperature or density. These results show negative tritrophic effects of algal blooms on fish via structural effects of diatom filaments that shifted prey community composition to smaller bodied, less nutritious species while total prey biomass remained unchanged.