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Climate change has profound effects on the distribution of kelp forests in the Arctic and sub‐Arctic. However, studies on the responses of kelps to climate change, particularly along the sub‐Arctic regions of the Alaska coast, are limited. Eualaria fistulosa is a foundational kelp species in the Aleutian Islands, with an east–west distribution that extends from Japan to southern southwest Alaska. In this study, we utilized a species distribution model (SDM) to explore changes in the future habitat suitability of E. fistulosa under contrasting Shared Socioeconomic Pathway (SSP) scenarios. Our model exhibited relatively high predictive performance, validating our SDM predictions. Notably, the SDM results indicate that minimum sea surface temperature, annual range in sea surface temperatures, and annual mean current velocities are the three most important predictor variables determining E. fistulosa's distribution. Furthermore, the projected geographic distribution of Eualaria is generally consistent with its observed occurrence records. However, under high emission scenarios (SSP5‐8.5), E. fistulosa is predicted to contract its distribution range by 9.0% by 2100, with widespread disappearance along the southeast Alaskan coast and limited northward migration to Kamchatka Krai in Russia and Bristol Bay in Alaska. These findings contribute valuable insights for conservation strategies via addressing climate‐induced alterations in sub‐Arctic kelp distribution. [ABSTRACT FROM AUTHOR]

Dispersal of reproductive propagules determines recruitment patterns and connectivity among populations and can influence how populations respond to major disturbance events. Dispersal distributions can depend on propagule release strategies. For instance, the bull kelp, Nereocystis luetkeana, can release propagules (spores) from two heights in the water column ("bimodal release"): at the water surface, directly from the reproductive tissues (sori) on the kelp's blades, and near the seafloor after the sori abscise and sink through the water column. N. luetkeana is a foundation species that occurs from central California to Alaska and is experiencing unprecedented levels of population declines near its southern range limit. We know little of the kelp's dispersal distributions, which could influence population recovery and restoration. Here, we quantify how bimodal spore release heights affect dispersal outcomes based on a numerical model specifically designed for N. luetkeana. The model incorporates oceanographic conditions typical of the species' coastal range and kelp biological traits. With bimodal release heights, 34% of spores are predicted to settle within 10 m of the parental alga and 60% are predicted to disperse beyond 100 m. As an annual species, bimodal release heights can facilitate the local regeneration of adults within a source kelp forest while also supporting connectivity among multiple forests within broader bull kelp metapopulations. To leverage this pattern of bimodal spore dispersal in bull kelp restoration management, directing resources toward strategically located focal populations that can seed other ones could amplify the scale of recovery. [ABSTRACT FROM AUTHOR]

Understory assemblages associated with canopy‐forming species such as trees, kelps, and rockweeds should respond strongly to climate stressors due to strong canopy‐understory interactions. Climate change can directly and indirectly modify these assemblages, particularly during more stressful seasons and climate scenarios. However, fully understanding the seasonal impacts of different climate conditions on canopy‐reliant assemblages is difficult due to a continued emphasis on studying single‐species responses to a single future climate scenario during a single season. To examine these emergent effects, we used mesocosm experiments to expose seaweed assemblages associated with the canopy‐forming golden rockweed, Silvetia compressa, to elevated temperature and pCO2 conditions reflecting two projected greenhouse emission scenarios (RCP 2.6 [low] & RCP 4.5 [moderate]). Assemblages were grown in the presence and absence of Silvetia, and in two seasons. Relative to ambient conditions, predicted climate scenarios generally suppressed Silvetia biomass and photosynthetic efficiency. However, these effects varied seasonally—both future scenarios reduced Silvetia biomass in summer, but only the moderate scenario did so in winter. These reductions shifted the assemblage, with more extreme shifts occurring in summer. Contrarily, future scenarios did not shift assemblages within Silvetia Absent treatments, suggesting that climate primarily affected assemblages indirectly through changes in Silvetia. Mesocosm experiments were coupled with a field Silvetia removal experiment to simulate the effects of climate‐mediated Silvetia loss on natural assemblages. Consistent with the mesocosm experiment, Silvetia loss resulted in season‐specific assemblage shifts, with weaker effects observed in winter. Together, our study supports the hypotheses that climate‐mediated changes to canopy‐forming species can indirectly affect the associated assemblage, and that these effects vary seasonally. Such seasonality is important to consider as it may provide periods of recovery when conditions are less stressful, especially if we can reduce the severity of future climate scenarios. [ABSTRACT FROM AUTHOR]

The photograph includes male and female gametophytes, and embryonic sporophytes of the giant kelp Macrocystis pyrifera, but resemble those of Ecklonia radiata. To date, most studies on kelps have focused on their macroscopic sporophyte life stage because the microscopic gametophytes and embryonic sporophytes (Figure 1) are difficult to study in the field, which has led to uncertainties about what we know of their roles in kelp forest ecology. Most studies of urchin grazing have focused on their impacts on large kelp sporophytes, and few studies have examined the effects of grazing on kelp microscopic life stages. [Extracted from the article]

Foundation species have strong, positive effects on local community structure; increasing biodiversity and species abundances by providing food and habitat. On coastal temperate and subpolar rocky reefs, canopy-forming kelps form three-dimensional habitats that support numerous fish, invertebrate, and algal species. Throughout the Aleutian Archipelago, unregulated sea urchin grazing has largely removed the foundation canopy-forming kelp, Eualaria fistulosa, and most subcanopy algae. Consequently, most nearshore rocky reefs have shifted from kelp to sea urchin (Strongylocentrotus spp.) dominated habitats. These latter habitats are either urchin barrens devoid of all fleshy macroalgae, or transition forests devoid of all fleshy macroalgae algae but E. fistulosa. These three distinct communities (kelp and transition forests, and urchin barrens) were used to test the influence of E. fistulosa and sea urchins on the associated communities. Contrary to initial expectations, in transition forest habitats where E. fistulosa is the lone macroalga, no differences in community structure or in the size structure of benthic invertebrates were seen relative to urchin barrens. In kelp forests, where E. fistulosa coexisted with subcanopy macroalgae and urchins were less abundant, faunal communities were more species rich with higher abundance, biomass, and percent cover of numerous filter feeders. These findings stress not only the strong negative impact which urchins can exert on the kelp forest communities, but also the context-dependent nature of foundation species. [ABSTRACT FROM AUTHOR]

Kelps have a major role in marine and atmospheric iodine cycling in the coastal zone of temperate regions, with potential wide-ranging impacts on ozone destruction in the coastal marine boundary layer. However, little is known about the impact of kelp forests on iodine speciation in coastal sea water. To address this, we examined iodide and iodate concentrations in seawater in and around a giant kelp forest near San Diego, CA, USA, and a nearby site that was not influenced by kelp biology. Our data shows that while both iodide and iodate concentrations remained unchanged during the year at the nearby site, these concentrations changed significantly in and around the kelp forest, and were strongly related to changes in kelp canopy biomass. In particular, iodide reached its highest concentration and iodate reached its lowest concentration during the summer when the kelp canopies were near their maximum, while the opposite pattern was observed during the winter and spring when the kelp canopies were near their minimum. Further, comparisons of these changes with corresponding changes in seawater temperature and wind speed indicated that these relationships were relatively small compared to those with changes in kelp biomass. Together, our data show a strong relationship between kelp biomass and iodine metabolism. [ABSTRACT FROM AUTHOR]

Recent studies have indicated that long-distance dispersal by kelp zoospores may play an important role in the colonization of newly exposed rocky habitats and in the recovery of recently disturbed kelp forests. This may be facilitated by the vertical transport of zoospores into the shallower portions of the water column where they are exposed to greater alongshore currents that increase their dispersal potential. However, this vertical transport can also expose them to elevated irradiances and enhanced grazing by zooplankton, both of which negatively impact zoospore survival and settlement. In this study, we used plankton tows to show that zooplankton (mysids) were at least seven times more abundant in the surface waters than near the benthos along the edge of a large kelp forest at the time of our spring sampling. We then used feeding experiments and epifluorescence microscopy to verify that these mysids grazed on kelp zoospores. Finally, we conducted laboratory experiments to show that grazing by these mysids over a 12 h period reduced kelp zoospore settlement by at least 50% relative to treatments without grazing. Together with previous studies that have revealed the impacts of high irradiance on zoospore survival and settlement, our study indicates that the vertical transport of kelp zoospores into the shallower portions of the water can also expose them to significantly increased mortality from mysid grazing. Thus, if these patterns are consistent over broader temporal and geographic scales, vertical transport may not be a viable method for sustained long-distance zoospore dispersal. [ABSTRACT FROM AUTHOR]

Laboratory studies have demonstrated that the microscopic stages of kelps can rapidly resume development from a delayed state. Like terrestrial seeds or aquatic resting eggs, banks of delayed kelp stages may supplement population recovery after periods of stress, playing an important role for kelp populations that experience adult sporophyte absences due to seasonal or interannual disturbances. We found that removing the microscopic stages from natural rock substratum could prevent the appearance of juvenile kelp sporophytes for three months and the establishment of a diverse kelp assemblage for over four months within a southern California kelp forest. Juveniles were observed within one month in plots where microscopic stages were left intact, which may confer an advantage for the resulting sporophytes as they attain larger sizes before later recruiting neighbors. Microsatellite diversity was high (expected heterozygosity HE ≈ 0.9) for juveniles and adults within our sites. Using a microsatellite-based parentage analysis for the dominant kelp, Macrocystis pyrifera, we estimated that a portion of the new M. pyrifera sporophyte recruits had originated from their parents at least seven months after their parents had disappeared. Similar delay durations have been demonstrated in recent laboratory studies. Additionally, our results suggest that zoospore dispersal distances >50 m may be supported by including additional microsatellite loci in the analysis. We propose a mixed-age and, potentially, a mixed-origin bank of M. pyrifera gametophytes promotes maximal genetic diversity in recovering populations and reduces population genetic subdivision and self-fertilization rates for intact populations by promoting the survival of zoospores dispersed >10 m and during inhospitable environmental conditions. [ABSTRACT FROM AUTHOR]

Elevated irradiance has a profound effect on the successful dispersal and establishment of kelp zoospores, affecting their physiology and viability. The research to date, however, has been on zoospores localized near the benthos, with little attention on the importance of vertical transportation and subsequent exposure to increased irradiance. Therefore, we wanted to investigate the effects of exposure to high irradiance on the reproductive planktonic life-history stages of kelps Macrocystis pyrifera (L.) C. Agardh and Pterygophora californica Rupr. Zoospores of both species were exposed to different irradiances (75, 275, 575, 1,025 μmol photons · m−2 · s−1) over varying durations (1, 2, 4, 8, 12 h) and subsequently monitored for settlement competency, gametophyte development, and reproductive viability. Settlement success for M. pyrifera was uniform throughout all irradiance × time treatments, while settlement for P. californica decreased with increasing exposure time but not irradiance, although settlement was generally reduced at the highest irradiance level. Following zoospore settlement, germ tube development was visible in the gametophytes of both species within 1 week, although a significant decline of germ tube density in P. californica was observed with increasing irradiance. Similarly, a decrease in germ tube development with increasing exposure was observed across all irradiance levels for M. pyrifera, but irradiance itself was not significant. Further development into embryonic sporophytes was remarkably similar to gametophyte development, suggesting that the effect of exposure of kelp zoospores to high irradiance on subsequent sporophyte production is mediated through gametophyte development as well as zoospore survival. [ABSTRACT FROM AUTHOR]

Morphological variation is common in kelps (Phaeophyceae, Laminariales) and often results from differences in environmental conditions. Stipe hollowing, in particular, occurs in several kelp species worldwide but to date has been investigated for only a few species and primarily at local scales. Here we describe the patterns of stipe hollowing both within and among populations of Eisenia arborea across 800 km of its distribution along the west coast of North America. Our results indicate that there are large latitudinal differences among populations in both the occurrence of stipe hollowing (i.e. frequency of hollow stipes within a population) and hollowing extent (i.e. length of the hollow cavity relative to stipe length) in E. arborea sporophytes. Stipe length varied among our study locations, with the most southern population exhibiting the tallest stipes, the most northern population exhibiting the shortest stipes, and the three central populations exhibiting intermediate stipe lengths. The occurrence of stipe hollowing and hollowing extent also varied along a latitudinal gradient, with the more southern populations exhibiting both increased frequencies of hollowing and greater hollowing extents. In fact, although hollowing occurred in almost all (97%) of the sporophytes examined at the most southern location, it was absent in the sporophytes examined at the most northern location. Finally, although stipe hollowing was more common in larger stipes across all locations because of the overwhelming effect of location on the occurrence of stipe hollowing, stipe length alone is not a suitable predictor when considered across this species range. [ABSTRACT FROM AUTHOR]

Recent discussions on scaling issues in ecology have emphasized that processes acting at a wide range of spatial and temporal scales influence ecosystems and thus there is no appropriate single scale at which ecological processes should be studied. This may be particularly true for environmental disturbances (e.g. El Niño) that occur over large geographic areas and encompass a wide range of scales relevant to ecosystem function. However, it may be possible to identify the scale(s) at which ecosystems are most strongly impacted by disturbances, and thus provide a measure by which their impacts can be most clearly described, by assessing scale-dependent changes in the patterns of variability in species abundance and distribution. This, in turn, may yield significant insight into the relative importance of the various forcing factors responsible for generating these impacts. The 1997–98 El Niño was one of the strongest El Niños ever recorded. I examined how this event impacted giant kelp populations in the northeast Pacific Ocean at 90 sites ranging from central Baja California, Mexico to central California, USA. These sites spanned the geographic range of giant kelp in the Northeast Pacific and were surveyed just before, immediately following, several months after, more than 1 year after, and nearly 2 years after the El Niño. I used a hierarchical sample design to compare these impacts at five spatial scales spanning six orders of magnitude, from a few meters to more than 1,000 km. Variance Components Analyses revealed that the El Niño shifted control over giant kelp abundance from factors acting at the scale of a few meters (local control) to factors operating over hundreds to thousands of kilometers (regional control). Moreover, El Niño resulted in the near-complete loss of all giant kelp throughout one-half of the species’ range in the northeast Pacific Ocean. Giant kelp recovery following El Niño was far more complex and variable at multiple spatial scales, presumably driven by numerous factors acting at those scales. Recovery returned local control of giant kelp populations within 6 months in southern California, and within 2 years in Baja California. [ABSTRACT FROM AUTHOR]

Kelp (Laminaria digitata) uses iodide as a unique inorganic antioxidant to protect its surface and apoplastic space against reactive oxygen species such as ozone, hydrogen peroxide and superoxide, with implications for atmospheric and marine chemistry as well as regional climatic processes. If kelp is covered by seawater, this results in iodide leaching into surrounding sea water. In this study, the influence of the kelps Laminaria digitata , L. hyperborea , L. ochroleuca and Saccharina latissima on iodine speciation chemistry was explored at two sites in Oban (Argyll, Scotland) and Roscoff (Brittany, France) based on diver-operated in situ sampling. Seawater samples were subsequently analysed voltammetrically, accompanied by determination of extractable iodine concentrations in the tissues of the thalli surveyed by ICP-MS. The main result is that iodide concentrations in the vicinity of kelp thalli are strongly enhanced, especially at low tide, while iodate concentrations are decreased in comparison to open coastal water and open ocean concentrations. Using diver-operated in situ sampling with seawater samples subsequently analysed voltammetrically, this study finds that iodide levels in the vicinity of kelp thalli are strongly enhanced, especially at low tide, while iodate levels are decreased in comparison to open coastal water and open ocean concentrations. [Display omitted] • This study uses a new diver-operated in situ syringe sampling technique. • Samples analysed voltammetrically. • Iodide levels around kelp thalli are strongly enhanced, especially at low tide. • In contrast, iodate levels are decreased in comparison to open waters. [ABSTRACT FROM AUTHOR]