Biogenic habitat structure of seaweeds change along a latitudinal gradient in ocean temperature

Abstract

Global warming is affecting all major ecosystems, including temperate reefs where\ncanopy-forming seaweeds provide biogenic habitat. In contrast to the rapidly growing\nrecognition of how climate affects the performance and distribution of individuals and\npopulations, relatively little is known about possible links between climate and biogenic\nhabitat structure. We examined the relationship between several ocean temperature\ncharacteristics, expressed on time-scales of days, months and years, on habitat patch\ncharacteristics on 24 subtidal temperate reefs along a latitudinal gradient (Western\nAustralia; ca 34 to 27º S). Significant climate related variation in habitat structure was\nobserved, even though the landscape cover of kelp and fucalean canopies did not change\nacross the climate gradient: monospecific patches of kelp became increasingly\ndominant in warmer climates, at the expense of mixed kelp-fucalean canopies. The\ndecline in mixed canopies was associated with an increase in the abundance of\nSargassum spp., replacing a more diverse canopy assemblage of Scytothalia doryocarpa\nand several other large fucoids. There were no observed differences in the proportion of\nopen gaps or gap characteristics. These habitat changes were closely related to patterns\nin minimum temperatures and temperature thresholds (days > 20 °C), presumably\nbecause temperate algae require cool periods for successful reproduction and\nrecruitment (even if the adults can survive warmer temperatures). Although the\nobserved habitat variation may appear subtle, similar structural differences have been\nlinked to a range of effects on canopy-associated organisms through the provision of\nhabitat and ecosystem engineering. Consequently, our study suggests that the magnitude\nof projected temperature increase is likely to cause changes in habitat structure and\nthereby indirectly affect numerous habitat-dependent plants and animals