The nuisance alga Gonyostomum semen (Raphidophyceae) has expanded in the Nordic countries during the last decades and can dominate lake phytoplankton communities almost completely. A possible explanation to its dominance could be limited grazing by zooplankton. We investigated the potential grazing pressure on G. semen using an experimental approach supported by field data. We determined the grazing rate by cladocerans, calanoid copepods, and Chaoborus larvae to determine which were able to feed on G. semen. Only the large cladoceran Daphnia magna was able to feed successfully on G. semen. The large cell size of G. semen was likely a limiting factor for the filtering apparatus of smaller cladocerans. The copepod Eudiaptomus gracilis did not graze on G. semen, although the mechanism behind this selective feeding is still unknown. In addition to the experimental study, we quantified the zooplankton and phytoplankton communities in 40 lakes to determine the composition and abundance of the zooplankton communities co-occurring with G. semen, suggesting that large cladoceran species were not present in lakes where G. semen occurred. Hence, the growth of G. semen is not significantly controlled by grazing in natural systems, which likely facilitates bloom formation and invasion success of G. semen.
The invasive and nuisance microalga Gonyostomum semen has recently expanded its geographical distribution and increased its biomass in Sweden and Finland. Temperatures, total organic carbon (TOC), water colour and phosphorous are main factors suggested as drivers for its success. Already in the 1980s there were indications of the same patterns also in Norway, and in this study we further examine these observations based on the hypotheses that (1) G. semen has continued its increase in abundance in Norway since then, (2) after settling in a lake, G. semen may increase its biomass, provided a simultaneous change in climatic factors. We use existing data from Norwegian monitoring databases for the study and additional water samples to investigate distribution. G. semen was detected by microscopy and quantitative PCR, while trends over time in G. semen biomass and possible explanatory variables were analysed by simple regression. We show that G. semen has increased its distribution in Norway since the 1980s, geographically and to new lakes. In eight of the nine lakes studied, the proportion of G. semen biomass in lake phytoplankton has increased with time. These changes seem to be explained only to some degree by temperature, TOC, water colour and phosphorous.
-We used flume experiments to study the effects of a temporary increase in stream flow on macroinvertebrates, leaf litter breakdown and soft-bodied benthic algae; both as a single stressor and eventually in combination with an increase in nutrient supply. In order to understand how well the flumes reflected the nearby stream ecosystem, we compared species composition of macroinvertebrates and benthic algae between the flumes and the nearby stream from which the flumes were supplied with water. As single stressors, nutrient addition and an increased flow velocity from 1.3 to 2.8 cm s À 1 lead to an increase in the biomass of benthic algae, likely reflecting an improved transfer of nutrients into algal patches. However, the combined effect of flow and nutrient addition was smaller than the sum of both individual effects, likely because an increased biomass also was more susceptible to scouring. We found differences in macroinvertebrate and benthic algal taxon identity and abundance between stream and flumes. Since biodiversity is assumed to stabilize ecological functioning in response to disturbances and variation, we conclude that care should be taken in applying results from small scale experiments to stream ecosystems. Nous avons utilisé des expériences en canaux pour étudier les effets d'une augmentation temporaire du débit sur les macroinvertébrés, la dégradation de la litière foliaire et les algues benthiques à corps mou, à la fois comme stresseur unique et éventuellement en combinaison avec une augmentation de l'apport en éléments nutritifs. Afin de comprendre dans quelle mesure les canaux reflétaient bien l'écosystème du cours d'eau voisin, nous avons comparé la composition spécifique des macroinvertébrés et des algues benthiques entre les canaux et le cours d'eau voisin qui alimentait les canaux en eau. En tant que stresseurs uniques, l'ajout de nutriments et une vitesse d'écoulement accrue de 1,3 à 2,8 cm s À 1 entraînent une augmentation de la biomasse des algues benthiques, ce qui reflète probablement un meilleur transfert des nutriments dans le périphyton algal. Toutefois, l'effet combiné du débit et de l'adjonction de nutriments était plus faible que la somme des deux effets individuels, probablement parce qu'une biomasse accrue était aussi plus susceptible de se décoller. Nous avons trouvé des différences dans l'identité et l'abondance des taxons de macroinvertébrés et des taxons d'algues benthiques entre le cours d'eau et les canaux. Puisque la biodiversité est supposée stabiliser le fonctionnement écologique en réponse aux perturbations et aux variations, nous concluons qu'il faut faire preuve de prudence dans l'application aux écosystèmes des cours d'eau des résultats d'expériences à petite échelle.
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