Seaweeds are ecologically important primary producers, competitors, and ecosystem engineers that play a central role in coastal habitats ranging from kelp forests to coral reefs. Although seaweeds are known to be vulnerable to physical and chemical changes in the marine environment, the impacts of ongoing and future anthropogenic climate change in seaweed-dominated ecosystems remain poorly understood. In this review, we describe the ways in which changes in the environment directly affect seaweeds in terms of their physiology, growth, reproduction, and survival. We consider the extent to which seaweed species may be able to respond to these changes via adaptation or migration. We also examine the extensive reshuffling of communities that is occurring as the ecological balance between competing species changes, and as top-down control by herbivores becomes stronger or weaker. Finally, we delve into some of the ecosystem-level responses to these changes, including changes in primary productivity, diversity, and resilience. Although there are several key areas in which ecological insight is lacking, we suggest that reasonable climate-related hypotheses can be developed and tested based on current information. By strategically prioritizing research in the areas of complex environmental variation, multiple stressor effects, evolutionary adaptation, and population, community, and ecosystem-level responses, we can rapidly build upon our current understanding of seaweed biology and climate change ecology to more effectively conserve and manage coastal ecosystems.

ABSTRACT. As oil transportation worldwide continues to increase, many communities are at risk of oil spill disasters and must anticipate and prepare for them. Factors that influence oil spill consequences are myriad and range from the biophysical to the social. We provide a summary literature review and overview framework to help communities systematically consider the factors and linkages that would influence consequences of a potential oil spill. The focus is on spills from oil tanker accidents. Drawing primarily on empirical studies of previous oil spill disasters, we focused on several main domains of interest: the oil spill itself, disaster management, the physical marine environment, marine biology, human health, economy, and policy. Key variables that influence the severity of consequences are identified, and significant interactions between variables are delineated. The framework can be used to clarify the complexity of oil spill impacts, identify lessons that may be transferable from other oil spill disasters, develop scenarios for planning, and inform risk analysis and policy debates in localities that are seeking to understand and reduce their vulnerability to potential spill disasters. As a case study, the framework is used to consider potential oil spills and consequences in Vancouver, Canada. Major increases in oil tanker traffic are anticipated in this region, creating urgent new demands for risk information, disaster management planning, and policy responses. The case study identifies particular conditions that distinguish the Vancouver context from other historic events; in particular, proximity to a densely populated urban area, the type of oil being transported, financial compensation schemes, and local economic structure. Drawing lessons from other oil spill disasters is important but should be undertaken with recognition of these key differences. Some types of impacts that have been relatively inconsequential in previous events may be very significant in a Vancouver case.

While changes in the abundance of keystone predators can have cascading effects resulting in regime shifts, the role of mesopredators in these processes remains underexplored. We conducted annual surveys of rocky reef communities that varied in the recovery of a keystone predator (sea otter, ) and the mass mortality of a mesopredator (sunflower sea star,) due to an infectious wasting disease. By fitting a population model to empirical data, we show that sea otters had the greatest impact on the mortality of large sea urchins, but that decline corresponded to a 311% increase in medium urchins and a 30% decline in kelp densities. Our results reveal that predator complementarity in size-selective prey consumption strengthens top-down control on urchins, affecting the resilience of alternative reef states by reinforcing the resilience of kelp forests and eroding the resilience of urchin barrens. We reveal previously underappreciated species interactions within a 'classic' trophic cascade and regime shift, highlighting the critical role of middle-level predators in mediating rocky reef state transitions.

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The giant kelp genus Macrocystis C. Agardh (Laminariales, Phaeophyceae) is one of the world's most ecologically and economically important seaweed taxa, yet its taxonomy remains uncertain. Although the genus currently contains four accepted species based on variable holdfast and blade morphology [M. pyrifera (L.) C. Agardh, M. integrifolia Bory, M. angustifolia Bory, and M. laevis C. H. Hay], numerous recent studies on Macrocystis interfertility, genetic relatedness, and morphological plasticity all suggest that the genus is monospecific. We reviewed this evidence and present an explanation for the extreme phenotypic plasticity that results in morphological variability within Macrocystis, driven by the effects of environmental factors on early development of macroscopic sporophytes. We propose that the genus be collapsed back to a single species, with nomenclatural priority given to M. pyrifera.

Over the last two decades, many studies on functional morphology have suggested that material properties of seaweed tissues may influence their fitness. Because hydrodynamic forces are likely the largest source of mortality for seaweeds in high wave energy environments, tissues with material properties that behave favorably in these environments are likely to be selected for. However, it is very difficult to disentangle the effects of materials properties on seaweed performance because size, shape, and habitat also influence mechanical and hydrodynamic performance. In this study, anatomical and material properties of 16 species of foliose red macroalgae were determined, and their effects on hydrodynamic performance were measured in laboratory experiments holding size and shape constant. We determined that increased blade thickness (primarily caused by the addition of medullary tissue) results in higher flexural stiffness (EI), which inhibits the seaweed's ability to reconfigure in flowing water and thereby increases drag. However, this increase is concurrent with an increase in the force required to break tissue, possibly offsetting any risk of failure. Additionally, while increased nonpigmented medullary cells may pose a higher metabolic cost to the seaweed, decreased reconfiguration causes thicker tissues to expose more photosynthetic surface area incident to ambient light in flowing water, potentially ameliorating the metabolic cost of producing these cells. Material properties can result in differential performance of morphologically similar species. Future studies on ecomechanics of seaweeds in wave-swept coastal habitats should consider the interaction of multiple trade-offs.

Summary 1.Organisms' ability to withstand the physical forces of their environment is a key determinant of their success. Mechanical performance of organisms is often associated with the properties of the tissues that compose them. In mechanically stressful habitats, intraspecific variation in tissue properties may result in differential survivorship and enable natural selection to act on material performance. 2. We tested the hypothesis that tissue mechanical properties affect survivorship (a fitness component) of the perennial kelp, Egregia menziesii, in a mechanically stressful, wave-swept intertidal habitat. 3. We measured intraspecific variation in strength and flexibility in 38 E. menziesii and tracked their survivorship in the field over the winter storm season to determine whether variation in mechanical properties led to differential survivorship. 4. Significant interindividual variation was found in most mechanical properties, including strength and flexibility. Individuals with increased flexibility and decreased strength were more likely to survive the duration of our study, although this effect was more pronounced in individuals with smaller holdfasts. Increased frond strength was also associated with a reduction in self-thinning, potentially explaining the observed increase in whole-plant mortality with increasing frond strength. 5.Results from this study demonstrate that variation in tissue mechanical properties among conspecifics can influence survivorship and this may have important evolutionary implications.

We propose that divergent patterns in the distribution of material properties along blades may have different consequences for the performance of kelps and red algae. The positioning of younger tissues at the blade base for kelps may enable these species to attain larger body sizes in wave-swept habitats.