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

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

Research Highlights
► We correlated seaweed habitat patch-characteristics with ocean temperatures.
► We found significant relationships for several patch characteristics.
► Mixed canopies declined, kelp canopies became more monospecific, and fucoid identity changed.
► Changes were best related to minimum and threshold temperature characteristics.
► Changes were subtle, but likely important given contemporary habitat-diversity relationships.