Atlantic salmon living in the brackish Baltic Sea have lower muscle pigmentation than populations elsewhere. The pigment in question is the antioxidant and vitamin A precursor astaxanthin, which is synthesized by crustaceans from algal carotenoids. Baltic salmon feed nearly exclusively on the clupeids sprat and herring. To evaluate astaxanthin availability to salmon we assessed astaxanthin levels and isomeric composition in their prey fish. We also analyzed astaxanthin dynamics in the dominant piscivorous fish in the Baltic Sea, the Atlantic cod. The geometrical E-(trans-) and Z-(cis-) isomers were distributed selectively in fish tissues, with highest E : Z ratios in salmon gonads (82 : 18) and lowest in herring gonads (24 : 76). Sprat and herring are not ideal prey with respect to their high whole-body concentrations of Z-isomers, which have low bioavailability for salmon and cod. These Z-isomers predominantly accumulate in the clupeid gonads. A crucial mechanism for the transport of astaxanthin from clupeids to piscivores is the direct transfer of crustacean astaxanthin (mainly all-E) from the clupeid stomachs. Low stomach astaxanthin content in clupeids decreases total astaxanthin transfer to higher trophic levels. In autumn, herring stomachs (including contents) had 12.5 times lower astaxanthin concentrations than sprat stomachs, and herring had 2.8 times less whole-body all-E-astaxanthin (by weight) than sprat. These results confirm recent reports of starvation in the Baltic herring, which may further decrease astaxanthin levels in the Baltic salmon. Cod did not have lower astaxanthin levels than their Atlantic counterpart, which may be attributed to their lower need for astaxanthin and higher food diversity.

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.