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The largest seaweed, giant kelp (Macrocystis) is the fastest growing and most prolific of all plants found on earth. Growing from the seafloor and extending along the ocean surface in lush canopies, giant kelp provides an extensive vertical habitat in a largely two-dimensional seascape. It is the foundation for one of the most species-rich, productive, and widely distributed ecological communities in the world.Schiel and Foster's scholarly review and synthesis take the reader from Darwin's early observations to contemporary research, providing a historical perspective for the modern understanding of giant kelp evolution, biogeography, biology, and physiology.The authors furnish a comprehensive discussion of kelp species and forest ecology worldwide, with considerations of human uses and abuses, management and conservation, and the current and likely future impacts of global change.This volume promises to be the definitive treatise and reference on giant kelp and its forests for many years, and it will appeal to marine scientists and others who want a better appreciation and understanding of these wondrous forests of the sea.

In this contribution, we propose fishery driven predator release as the cause for the largest grazing event ever observed in the NE Atlantic. Based on the evolving appreciation of limits to population connectivity, published and previously unpublished data, we discuss whether overfishing caused a grazer bloom of the sea urchin (Strongylocentrotus droebachiensis) resulting in overgrazing of more than 2000 km² kelp (Laminaria hyperborea) forest along Norwegian and Russian coasts during the 1970 s. We show that coastal fisheries likely depleted predatory coastal fish stocks through modernization of fishing methods and fleet. These fish were important predators on urchins and the reduction coincided with the urchin bloom. From this circumstantial evidence, we hypothesize that coastal predatory fish were important in regulating sea urchins, and that a local population dynamics perspective is necessary in management of coastal ecosystems.

Kelp beds provide significant ecosystem services and socioeconomic benefits globally, and prominently in coastal zones of the California Current. Their distributions and abundance, however, vary greatly over space and time. Here, we describe long-term patterns of Giant Kelp ( Macrocystis pyrifera ) sea surface canopy area off the coast of San Diego County from 1983 through 2019 along with recent patterns of water column nitrate (NO 3 - ) exposure inferred from in situ temperature data in 2014 and 2015 at sites spanning 30 km of the coastline near San Diego California, USA. Site-specific patterns of kelp persistence and resilience were associated with ocean and climate dynamics, with total sea surface kelp canopy area varying approximately 33-fold over the almost 4 decades (min 0.34 km 2 in 1984; max 11.25 km 2 in 2008, median 4.79 km 2 ). Site-normalized canopy areas showed that recent kelp persistence since 2014 was greater at Point Loma and La Jolla, the largest kelp beds off California, than at the much smaller kelp bed off Cardiff. NO 3 - exposure was estimated from an 11-month time series of in situ water column temperature collected in 2014 and 2015 at 4 kelp beds, using a relationship between temperature and NO 3 - concentration previously established for the region. The vertical position of the 14.5°C isotherm, an indicator of the main thermocline and nutricline, varied across the entire water column at semidiurnal to seasonal frequencies. We use a novel means of quantifying estimated water column NO 3 - exposure integrated through time (mol-days m -2 ) adapted from degree days approaches commonly used to characterize thermal exposures. Water column integrated NO 3 - exposure binned by quarters of the time series showed strong seasonal differences with highest exposure in Mar - May 2015, lowest exposure in Sep - Dec 2014, with consistently highest exposure off Point Loma. The water column integrated NO 3 - signal was filtered to provide estimates of the contribution to total nitrate exposure from high frequency variability (ƒ >= 1 cycle 30 hr -1 ) associated predominantly with internal waves, and low frequency variability driven predominantly by seasonal upwelling. While seasonal upwelling accounted for > 90% of NO 3 - exposure across the full year, during warm periods when seasonal upwelling was reduced or absent and NO 3 - exposure was low overall, the proportion due to internal waves increased markedly to 84 to 100% of the site-specific total exposure. The high frequency variability associated with internal waves may supply critical nutrient availability during anomalously warm periods. Overall, these analyses support a hypothesis that differences in NO 3 - exposure among sites due to seasonal upwelling and higher frequency internal wave forcing contribute to spatial patterns in Giant Kelp persistence in southern California. The study period includes anomalously warm surface conditions and the marine heatwave associated with the “Pacific Warm Blob” superimposed on the seasonal thermal signal and corresponding to the onset of a multi-year decline in kelp canopy area and marked differences in kelp persistence among sites. Our analysis suggests that, particularly during periods of warm surface conditions, variation in NO 3 - exposure associated with processes occurring at higher frequencies, including internal waves can be a significant source of NO 3 - exposure to kelp beds in this region. The patterns described here also offer a view of the potential roles of seasonal and higher frequency nutrient dynamics for Giant Kelp persistence in southern California under continuing ocean surface warming and increasing frequency and intensity of marine heatwaves.

Kelp forests represent some of the most diverse and productive habitats on Earth, and provide a range of ecosystem goods and services on which human populations depend. As the distribution and ecophysiology of kelp species is strongly influenced by temperature, recent warming trends in many regions have been linked with concurrent changes in kelp populations, communities and ecosystems. Over the past decade, the number of reports of ocean warming impacts on kelp forests has risen sharply. Here, I synthesise recent studies to highlight general patterns and trends. While kelp responses to climate change vary greatly between ocean basins, regions and species, there is compelling evidence to show that ocean warming poses an unequivocal threat to the persistence and integrity of kelp forest ecosystems in coming decades.

The movement of trophic resources between and among ecosystems, referred to as cross‐ecosystem subsidies, is a common phenomenon. In the marine environment, both adjacent and distant ecosystems are connected by oceanographic forces that transport nutrients, organisms, and other materials. Kelp forest ecosystems are one example of an open marine system that both exports and receives trophic subsidies. Though rocky reefs are rich in kelp‐based energy produced internally, kelp forest organisms also rely on phytoplankton, and the influx of holoplankton and meroplankton from adjacent open ocean habitats. In this paper, we seek to clarify the identity of holoplanktonic and meroplanktonic subsidies, quantify their energetic and nutrient contributions to the kelp forest, and further explore the impacts of these subsidies for individual consumers and for kelp forest communities. We reviewed six individual subsidy organisms that are commonly advected to kelp forests on the West coast of North America, and show that these organisms from the pelagic ocean represent important resource pulses for kelp forest consumers. In addition, we summarize the characteristics of subsidies, consumers, and recipient ecosystems that provide insight into the dynamics of subsidy influx and impacts to recipient systems. Finally, we provide suggestions as scientists move forward with efforts to quantify the impacts of cross‐ecosystem subsidies. Trophic subsidies are a major force shaping both marine and terrestrial communities and ecosystems. Quantitative information about these subsidies and their impacts on food webs will not only improve our understanding of these ecosystems, but also improve food web models, and predictions of ecosystem response to change.

Warming waters and changing ocean currents are increasing the supply of tropical fish larvae to temperature regions where they are exposed to novel habitats, namely temperate macroalgae and barren reefs. Here, we use underwater surveys on the temperate reefs of south-eastern (SE) Australia and western Japan (~33.5°N and S, respectively) to investigate how temperate macroalgal and non-macroalgal habitats influence recruitment success of a range of tropical fishes. We show that temperate macroalgae strongly affected recruitment of many tropical fish species in both regions and across three recruitment seasons in SE Australia. Densities and richness of recruiting tropical fishes, primarily planktivores and herbivores, were over seven times greater in non-macroalgal than macroalgal reef habitat. Species and trophic diversity ( K -dominance) were also greater in non-macroalgal habitat. Temperate macroalgal cover was a stronger predictor of tropical fish assemblages than temperate fish assemblages, reef rugosities or wave exposure. Tropical fish richness, diversity and density were greater on barren reef than on reef dominated by turfing algae. One common species, the neon damselfish ( Pomacentrus coelestis ), chose non-macroalgal habitat over temperate macroalgae for settlement in an aquarium experiment. This study highlights that temperate macroalgae may partly account for spatial variation in recruitment success of many tropical fishes into higher latitudes. Hence, habitat composition of temperate reefs may need to be considered to accurately predict the geographic responses of many tropical fishes to climate change.

On temperate rocky reefs, overexploitation of high‐trophic‐level omnivores can result in the decimation of kelp forest habitats by releasing sea urchin population networks from top‐down control. However, the local dynamics of the resulting trophic cascades are context‐dependent. Here, we investigate the community‐wide patterns associated with both diffuse stressors and localized protection of high‐trophic‐level omnivores in kelp forest ecosystems by comparing communities among marine reserves and fished areas in two contrasting regions in terms of fishing intensity and land‐based stressors, Fiordland and the Marlborough Sounds, New Zealand. We find higher densities of the potential sea urchin predators, red rock lobsters (Jasus edwardsii) and banded wrasse (Notolabrus fucicola), in the Fiordland region, and larger effect sizes of fishing on the exploited fish communities in the Marlborough Sounds. Patterns in sea urchin density were consistent with the idea that high‐trophic‐level species, such as large fish and rock lobsters, regulate sea urchin population density, with lower densities of Evechinus chloroticus observed inside marine reserves, in both regions. Nevertheless, densities of E. chloroticus were generally high (>3 m2) in the Marlborough Sounds, likely above a grazing threshold in both fished and reserve sites. The proportion of habitat where sea urchins were absent was 29% in Marlborough Sounds and 90% in Fiordland. Consequently, we observe 49% barren habitat in Fiordland vs. 70%, and a larger effect of fishing on kelp community structure, in the Marlborough Sounds, where fishing effect sizes and land‐based stressors were more severe. We propose that a combination of diffuse stressors including regional overexploitation of important sea urchin predators, sedimentation, and warming of coastal waters likely contributed to regional differences in the responses of trophic interaction chains to localized reductions in fishing within marine reserves. The present study highlights how physiologically stressed and modified kelp forest ecosystems are more susceptible to detrimental phase shifts at a regional spatial scale.

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.

The factors that determine why ecosystems exhibit abrupt shifts in state are of paramount importance for management, conservation, and restoration efforts. Kelp forests are emblematic of such abruptly shifting ecosystems, transitioning from kelp-dominated to urchin-dominated states around the world with increasing frequency, yet the underlying processes and mechanisms that control their dynamics remain unclear. Here, we analyze four decades of data from biannual monitoring around San Nicolas Island, CA, to show that substrate complexity controls both the number of possible (alternative) states and the velocity with which shifts between states occur. The superposition of community dynamics with reconstructions of system stability landscapes reveals that shifts between alternative states at low-complexity sites reflect abrupt, high-velocity events initiated by pulse perturbations that rapidly propel species across dynamically unstable state-space. In contrast, high-complexity sites exhibit a single state of resilient kelp-urchin coexistence. Our analyses suggest that substrate complexity influences both top-down and bottom-up regulatory processes in kelp forests, highlight its influence on kelp-forest stability at both large (island-wide) and small (<10 m) spatial scales, and could be valuable for holistic kelp-forest management.

Foundation species structure communities, promote biodiversity, and stabilize ecosystem processes by creating locally stable environmental conditions. Despite their critical importance, the role of foundation species in stabilizing natural communities has seldom been quantified. In theory, the stability of a foundation species should promote community stability by enhancing species richness, altering the population fluctuations of individual species, or both. Here we tested the hypothesis that the stability of a marine foundation species, the giant kelp Macrocystis pyrifera, increased the stability of the aggregate biomass of a phylogenetically diverse assemblage of understory algae and sessile invertebrates that compete for space beneath the giant kelp canopy. To achieve this goal, we analyzed an 18-yr time series of the biomass of giant kelp and its associated benthic community collected from 32 plots distributed among nine shallow reefs in the Santa Barbara Channel, USA. We showed that the stability of understory algae and sessile invertebrates was positively and indirectly related to the stability of giant kelp, which primarily resulted from giant kelp’s direct positive association with species richness. The stability of all community types was positively related to species richness via increased species stability and species asynchrony. The stabilizing effects of richness were three to four times stronger when algae and invertebrates were considered separately rather than in combination. Our finding that diversity–stability relationships were stronger in communities consisting of species with similar resource requirements suggests that competition for shared resources rather than differential responses to environmental conditions played a more important role in stabilizing the community. Increasing threats to structure-forming foundation species worldwide necessitates a detailed understanding of how they influence their associated community. This study is among the first to show that dampened temporal fluctuations in the biomass of a foundation species is an important determinant of the stability of the complex communities it supports.