Effects of elevated temperature and nutrient deprivation on semi-naturally structured coastal assemblage.
Supervisor: Francisco Arenas (CIIMAR)
Evidence of consequences of climate changes on species distribution and ecosystem services are emerging, and climate driver stressors are expected to intensify in the following years. In North Portugal, the NW Iberian upwelling system already shows signs of weakening, leading to higher sea surface temperature and lower nutrient input in the region. These local environmental changes have led to alteration on communities structure and are potentially associated with an overall decrease of system productivity. Because the individual and combined effects of these stressors on coastal communities are still not fully understood, it is essential to further investigate patterns of changes in order to implement protection and restoration initiatives.

The present study aimed to test individual and combined effects of elevated temperature and nutrient depletion on naturally structured assemblage, and to characterize these effects on different component of the assemblage. The eco-physiological response of the community and its components included growth, chlorophyll fluorescence and productivity. The combination of elevated temperature and lack of nutrients showed a significant adverse effect on kelp growth, whereas Chondrus crispus growth decreased in the nutrient depleted treatment, regardless of the temperature. Although photosynthetic efficiency of kelp was not affected by treatments, individuals showed a decrease of α associated with an increase in Eopt when exposed to low nutrients concentration and high temperature. Overall, the productivity of the whole community decreased under low nutrients and high temperature treatment and the respiration rate increased with temperature. Moreover, all the components of the community showed increased respiration rate and decreased productivity with temperature, whose effects were exacerbated by the lack of nutrients in the case of kelp.

Our results suggest that under weakening upwelling scenarios, the elevated temperature and low nutrient availability will affect kelp ability to maintain high productivity rates. Since kelp is major contributor to the community productivity, these effects will have a consequences an overall increase of community respiration and a decrease productivity of the whole assemblage reducing the ability of those ecosystems to mitigate the effects of climate change.