Effects of warming on the green alga Cladophora glomerata : Ecological and genetic responses

Abstract: Since the beginning of the 20th century, average global surface temperature has increased ~0.7 °C and the current scenarios predict that it will continue to rise additional 2-4 °C during the 21st century. Although many emphasise the need to better understand how warming affect ecosystems on both communities and species level, conclusive data is relatively scarce and our understanding of the effects of warming on community processes and species’ genetic and plastic responses to climate change is still in its infancy.This thesis aims at investigating the ecological and ecophysiological responses to warming of the vegetation in the littoral zone in the Baltic Sea and whether warming affects the genetic composition of the dominant algal species in this zone, the green alga Cladophora glomerata. Warming was studied by comparing properties of the vegetation and the genetic composition of C. glomerata grown in natural communities in unheated sites and heated sites with 3.4-10.3 °C higher water temperature. The studies in this thesis were performed in the Forsmark (60°24'N 18°09'E) and Oskarshamn (57°25'N 16°40'E) areas.The thesis shows that warming changed the community composition in Cladophora-belts, but not the species richness. It also shows that vegetation coverage and height increased with warming. The results also suggest that the vegetation at the heated sites was exposed to lower levels of oxidative stress. The results showed relatively high genetic diversity in the strictly asexual species C. glomerata, but that the genetic diversity decreases with warming. Further, all analyses showed differences between samples from heated and unheated sites and revealed specific patterns for samples from the heated sites. Additonally, C. glomerata at heated sites were more strongly affected by the effect of season than at the unheated sites. An analysis of the genotypic composition of C. glomerata from heated and unheated sites in the two different sub-basins of the Baltic Sea (Forsmark and Oskarshamn) revealed a congruent direction in selection to warming.In all, this new knowledge increases the understanding of how a habitat-forming filamentous alga in a coastal ecosystem may respond to current and future global warming.