Evaluating Nutrient Reduction, Grazing and Barley Straw as Measures Against Algal Growth

The aim of our study was to experimentally investigate whether it is possible to reduce nuisance growth of filamentous algae in freshwater ecosystems. We used an experimental set-up mimicking a shallow pond system and performed a field investigation in the eutrophic moat of Krapperup castle (Southern Sweden), which exemplifies an extremely impaired ecosystem with ample growth of filamentous green algae. Theindoor experimenttested threetreatments: I) reduced nutrient concentrations, II)invertebrate grazers and III) addition of barley straw, which may constitute measures against filamentous algal growth and thereby improve the quality of the ecosystem services provided bywater bodies.Our results showa decreaseincyanobacteriaand diatomabundancesinallmesocosmsas filamentous algae biomass increased, suggesting that the microalgae suffered from nutrient and light competition with filamentous algae.Atendency forlower filamentous algae final biomass,aswell as coverage,was observedinthetreatmentwherethe concentration of nutrients was reduced. Grazers treatment showed a tendency to inhibit filamentous algae growth on artificial macrophytes towards the end of the experiment, suggesting that snails initially fed on their preferred food source (diatoms), until it was almost depleted and then started to feed on filamentous algae. Interestingly, the barley straw treatment was the only treatment promoting macrophytes growth andenhancing diatom biomass, butthis did not affect filamentous algae biomass.However,the ratio between filamentous algae and macrophyte final biomasses was significantly lower in the straw treatment. In a broader context, it is likely that in a long-term perspective the positive effect of barley straw on macrophyte growth will promote a shift from dominance by filamentous algae to macrophytes asmain primary producer.Moreover, our experiment showsthat barley strawmay be effectivein reducing cyanobacterial growth,whichmayleadtoimprovedwater qualityandtherebyecosystem services, suchassupportingandculturalecosystem services, since cyanobacteriamay produce potenttoxins and pose a serious riskto human and animal health.Altogether, our experimental results have important implications for the challenge of reversing nuisance filamentous algal blooms in highly eutrophic systems.