Kelp forests (Order Laminariales) form key biogenic habitats in coastal regions of temperate and Arctic seas worldwide, providing ecosystem services valued in the range of billions of dollars annually. Although local evidence suggests that kelp forests are increasingly threatened by a variety of stressors, no comprehensive global analysis of change in kelp abundances currently exists. Here, we build and analyze a global database of kelp time series spanning the past half-century to assess regional and global trends in kelp abundances. We detected a high degree of geographic variation in trends, with regional variability in the direction and magnitude of change far exceeding a small global average decline (instantaneous rate of change = −0.018 y⁻¹). Our analysis identified declines in 38% of ecoregions for which there are data (−0.015 to −0.18 y⁻¹), increases in 27% of ecoregions (0.015 to 0.11 y⁻¹), and no detectable change in 35% of ecoregions. These spatially variable trajectories reflected regional differences in the drivers of change, uncertainty in some regions owing to poor spatial and temporal data coverage, and the dynamic nature of kelp populations. We conclude that although global drivers could be affecting kelp forests at multiple scales, local stressors and regional variation in the effects of these drivers dominate kelp dynamics, in contrast to many other marine and terrestrial foundation species. Significance Kelp forests support diverse and productive ecological communities throughout temperate and arctic regions worldwide, providing numerous ecosystem services to humans. Literature suggests that kelp forests are increasingly threatened by a variety of human impacts, including climate change, overfishing, and direct harvest. We provide the first globally comprehensive analysis of kelp forest change over the past 50 y, identifying a high degree of variation in the magnitude and direction of change across the geographic range of kelps. These results suggest region-specific responses to global change, with local drivers playing an important role in driving patterns of kelp abundance. Increased monitoring aimed at understanding regional kelp forest dynamics is likely to prove most effective for the adaptive management of these important ecosystems.

Gridded SST products developed particularly for offshore regions are increasingly being applied close to the coast for biogeographical applications. The purpose of this paper is to demonstrate the dangers of doing so through a comparison of reprocessed MODIS Terra and Pathfinder v5.2 SSTs, both at 4 km resolution, with instrumental in situ temperatures taken within 400 m from the coast. We report large biases of up to +6°C in places between satellite-derived and in situ climatological temperatures for 87 sites spanning the entire ca . 2 700 km of the South African coastline. Although biases are predominantly warm (i.e. the satellite SSTs being higher), smaller or even cold biases also appear in places, especially along the southern and western coasts of the country. We also demonstrate the presence of gradients in temperature biases along shore-normal transects — generally SSTs extracted close to the shore demonstrate a smaller bias with respect to the in situ temperatures. Contributing towards the magnitude of the biases are factors such as SST data source, proximity to the shore, the presence/absence of upwelling cells or coastal embayments. Despite the generally large biases, from a biogeographical perspective, species distribution retains a correlative relationship with underlying spatial patterns in SST, but in order to arrive at a causal understanding of the determinants of biogeographical patterns we suggest that in shallow, inshore marine habitats, temperature is best measured directly.

Despite their ecological and economic importance, no summary of kelp global biogeography has been produced for almost two decades. The circumscription of the order Laminariales and familial and generic relationships in the group have changed considerably recently, in the light of molecular data. A global summary and geographical analysis of kelp species and their distributions (112 species in 33 genera) is presented. These data are analysed and discussed from the perspective of the new consensus of relationships within the group, and likely evolutionary events. The putative ancestors of the kelps occur and are overwhelmingly most diverse, in the cooler waters of northern Japan. The biogeographical evidence suggests three main lines of subsequent evolution: (a) a diversification producing the four ‘derived’ families Alariaceae, Costariceae, Laminariaceae, and Lessoniaceae and most extant genera in the temperate northern Pacific, probably during the Miocene. (b) The evolution of an Arctic flora which invaded the North Atlantic following the opening of the Bering Strait ca. 5.5 Ma. (c) At least four separate crossings, by different genera, of tropical regions from Northern to Southern Hemisphere (and one in the opposite direction). The recorded impacts of man on these distributions have thus far been minimal, with the notable exceptions of Undaria pinnatifida and species of Saccharina (grown in aquaculture systems for human food). Most genera are monospecific, with many confined to either the western or eastern temperate North Pacific, whereas the distribution of the most species-rich genera (Alaria, Laminaria, Saccharina) includes the Arctic, and they are widespread in the North Atlantic. This rapid species-level evolution is hypothesised to have been promoted by the relatively recent invasion of the Atlantic by these taxa. The crossing of the tropics has occurred in warm-temperate species some of which occur and are sometimes abundant, in deeper water in today’s tropics, refuting the widespread view that kelps are only present in cold-water habitats. Most of these Southern Hemisphere kelps are in the family Lessoniaceae, including the only genus not present in the Northern Hemisphere, Lessonia. The origins of this genus are unclear. Knowledge of the phylogeography of the Ecklonia/Eisenia complex is particularly important in understanding Southern Hemisphere kelp biogeography. A number of groups are undergoing active speciation and, with the lack of a consistent species concept, the species-level taxonomy of the group remains to be clarified. There is a great need for molecular phylogeographical studies to provide evidence for many of the hypotheses presented here based on distributional evidence and the currently accepted taxonomic framework.