Kelps across the portuguese coast: evidence of top-down and bottom-up influences

• Main Authors: Joao N Franco, Thomas Wernberg, Brezo Martínez, Pedro Duarte, David Jacinto, Nuno Vasco-Rodrigues, Fernando Tuya
• Format: Article
• Language: English
• Published: Frontiers Media S.A. 2014-04-01
• Series: Frontiers in Marine Science
• Subjects: Climate Change; Herbivory; Nutrients; temperature; latitudinal gradient; rocky reefs
• Online Access: http://journal.frontiersin.org/Journal/10.3389/conf.fmars.2014.02.00086/full

Kelps (large brown seaweeds) are conspicuous elements of the Portuguese coast, although kelp abundance is declining, especially at central and southern Portugal. While many studies point out increased seawater temperature as the main factor explaining kelp decline, little attention has been given to top-down (predatory) influences. Through in situ experiments, we tested how herbivory affects the distribution and abundance of kelp recruits. We compared the abundance and survivorship of recruits, the intensity of grazing on recruits and the abundance of herbivores between Viana do Castelo (northern Portugal) and Peniche (central Portugal). In addition, through an outdoor mesocosm experiment, we disentangled the independent and interactive effects of both seawater temperature and nutrients on kelp recruits performance (mortality, growth, C, N and carbohydrate contents, and photosynthetic potential and efficiency). Our main results showed that herbivores and herbivory intensity is higher at central compared to northern Portugal and that the growth of juvenile sporophytes was larger under a high-nutrient scenarios, particularly when temperatures were low (12, 15 and 16ºC). Despite the presence and fitness of kelps across southern European waters has been exclusively linked to bottom-up influences, our results highlights the importance of top-down effects for kelp survivorship and performance.