Eli Rinde1, Camilla With Fagerli1, Hartvig Christie1, Kai Sørensen1
1Norwegian Institute for Water Research, Norway
Diverse rocky shore macroalgal ecosystems, established in 12 large outdoor rocky shore mesocosms (13 m3 each), was used to study the impact of ocean darkening and eutrophication on rockweed structure and functioning. The responses include macroalgae coverage, Fucus serratus associated fauna community structure, and photosynthetic performance of F. serratus and sugar kelp (Saccharina latissima). Full coverage of F. serratus and F. vesiculosus was established at lower and upper tidal level respectively. Five adult sugar kelp plants were added to the lower tidal levels. The two treatments; darkening and eutrophication, was applied alone and in combination. We found a significant impact of darkening and eutrophication on several aspects of the habitat forming macroalgae species, as well as on F. serratus associated fauna community. The impact varied between the upper and lower tidal levels, and between treatments. Darkening and nutrients applied alone, reduced the coverage of the rockweeds, whereas the two stressors combined had no impact on F. serratus coverage. This antagonistic impact of the stressors could be explained by their negative impact on fauna abundance and fauna diversity, and hence a possible reduced grazing pressure when combined. There was a negative effect of the two stressors combined on number of fauna species associated to F. serratus, but not on the abundance of individuals. Nutrients and nutrients combined with darkening significantly increased the presence of filamentous brown algae. Darkening had a negative impact on F. serratus photosynthetic performance. Although sugar kelp photosynthesis was enhanced by nutrients, the nutrient loading seemed to increase the species mortality rate due to grazing. The results indicate that the rockweed species are sensitive to coastal darkening and increased nutrient supply. Furthermore, the stressors reduced faunal abundance and species diversity of the community associated to F. serratus, through synergistic negative impact. The results indicate complex, non-intuitive ecosystem responses that cannot be understood without doing experimental studies in complex ecosystems as done in this study and made possible by the large scale mesocosm facilities.
Biography
Eli Rinde is a marine biologist and works as a senior researcher at Norwegian Institute for Water Research. She loves exploring marine life by diving and snorkling and has more than 30 years of experience from ecological studies of blue forests; including kelp forests, seagrass-, rockweed-, and maerl beds. Main activities include field and mesocosm observations and experiments investigating interactions between key species and the effects of climate change and other human impacts.