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This chapter discusses the demography and metapopulation ecology of giant kelp, and the processes that underlie connectivity of kelp beds or patches, including spore production and dispersal. There is a considerable size dependency in the demographics of Macrocystis across its life stages. Adults generally decline in numbers over 3–4 orders of magnitude from when they are visible recruits, with longevity depending on sites and conditions. During the initial phases of microsporophyte production, numbers decline by 4–6 orders of magnitude, and there is also massive mortality during the period from spore release to microsporophyte production. Environmental variability in biotic and abiotic disturbances was the primary driver of extinction and recolonization. The great majority of recruitment failures was short term, and connectivity via spores was common. However, many patches are near the limits of spore dispersal from other patches, which highlights their vulnerability to prolonged extinction should unfavorable conditions persist, such as climate change.

This chapter examines giant kelp communities. Areas with giant kelp include a multitude of other species. However, because giant kelp is usually dominant, is the most visually obvious species, and commonly has by far the greatest biomass, such areas are called giant kelp communities. They are also called giant kelp “forests” or kelp “beds.” The abundance of giant kelp varies considerably across areas, and adults may become temporarily absent for many reasons, such as the population being annual, or being removed by grazers, storms, or other episodic oceanographic events. Given that Macrocystis occurs in both hemispheres across many different biogeographical provinces, there is clearly no single community of giant kelp across the regions it occupies.

This chapter discusses the anthropogenic effects on water quality and benthic habitats that negatively affect giant kelp growth and reproduction. These include activities that increase sedimentation, reduce light, and increase turbidity and temperature, causing the decline of Macrocystis. The most important effects of sewer effluent are on microscopic stages and small juvenile sporophytes whose survival and reproduction is inhibited by light reduction, scour, and burial as well as by toxic materials sorbed to particulate organic matter. Excess nutrients that reduce benthic light by stimulating phytoplankton growth may also encourage the growth of algal turfs that directly and indirectly inhibit recruitment of kelp and various large fucoids. Turf-sediment matrices have also been implicated in preventing recolonization of native algal species in Tasmania kelp communities. Ammonia can be toxic, and domestic sewage can contain toxic metals and organic compounds that may be increased if the discharge also contains industrial wastes.

This chapter discusses detached giant kelp communities. Kelp populations are exceptionally productive and an estimated 80% of the productivity ends up as detached detritus. Detritus from Macrocystis, moved by currents and wind, can be an important source of habitat, food, and nutrients for other communities and components within kelp forests. Much of this remains buoyant by floats on the blades and ends up either onshore or else drifting in currents and wind offshore where it may lose buoyancy and sink. Floating masses of giant kelp, often referred to as “rafts” or “paddles,” can be composed of a mix of their original associates from the kelp forest and new pelagic colonizers. The initial floating giant kelp community is composed of giant kelp and other plants, including some without floats that are attached to the holdfast or wrapped up in the raft. It also includes animals living in the holdfast and on the fronds.

This introductory chapter provides an overview of the emergence of the study of giant kelp (Macrocystis) forests. In 1839, Charles Darwin published the first observations on the ecology of giant kelp forests, and made the first analogy between this community and terrestrial forests. However, it was around a hundred years after Darwin's observations in South America that the study of giant kelp forest ecology began. H. L. Andrews' pioneering research on the fauna of giant kelp holdfasts in 1945 included some underwater observations made during surface-supplied, hard-hat diving. The considerable amount of research and monitoring in the 69 years since Andrews' time has provided a more comprehensive view of the biology and ecology of giant kelp. Common aims of research have been to understand the environmental drivers underlying the great spatial and temporal variation in kelp forests, and the role of food web (trophic) dynamics in these fluctuations.

This chapter examines the facilitative and competitive interactions in giant kelp communities. Broadly defined, facilitation denotes positive interactions between species where at least one species benefits and no harm is done to the other. The clearest examples of facilitation in giant kelp forests are the numerous invertebrates that use the fronds and holdfasts of giant kelp as a place to live, a refuge from predators, and perhaps an enhanced food supply in the form of plankton and the small epiphytes on fronds or detritus in holdfasts. On the other hand, competition, a negative interaction, occurs within and between species when individuals require the same resource that is insufficient for all and therefore compromises growth, reproduction, and survival. Within kelp forest patches, the array of species may be partially dictated by intraspecific competition within kelp species. The reduction in understory algae beneath thick canopies suggests that the giant kelp outcompete understory species for light.

This chapter discusses the structure, function, and abiotic requirements of giant kelp, providing a general guide to interpreting variation in Macrocystis populations in nature. In growth rates, kelps are most like bamboos, which are large grasses, and in ecological importance they are analogous to forest trees. Iodine is especially abundant in plants relative to seawater and may function as an antioxidant and an antimicrobial agent; the surface canopy of giant kelp is exposed to airborne particles and aerosols, and can rapidly take up and concentrate iodine as well as other radionuclides. With regard to abiotic requirements for growth and reproduction of giant kelp, these include water temperature, salinity, and light requirements.

This chapter presents an overview of climate changes, and assesses the potential effects on giant kelp and associated organisms. The earth's climate is changing at an unprecedented rate, and many predictions are critical about the consequent changes on ecosystems. In regard to the ocean, the major physical variables of change are likely to be temperature, wave forces, sea level, and pH from increasing atmospheric CO₂. The potential changes to kelp forest ecosystems are far less certain and largely assumed, due not only to the unpredicted and complex ways that climate variables may act on species and their interactions, but also in how they may act in a more direct way on giant kelp itself with its complex requirements and physiological-ecological responses across its life stages.

This chapter provides an overview of Macrocystis, commonly called giant kelp, but also known as giant bladder kelp, string kelp (Australia), huiro (Chile), and sargasso gigante (Mexico). Macrocystis is a genus of brown algae, a group characterized by its containing the accessory photosynthetic pigment fucoxanthin that gives them their characteristic color. “Kelp” originally referred to the calcined ashes resulting from burning large brown algae. It is sometimes used as the common name for all large brown algae, but particularly species in the order Laminariales. Macrocystis and its putative species have undergone considerable taxonomic revision since originally described in 1771 by Linnaeus, who included it with other brown algae under the name, Fucus pyriferus. More recent investigators examined plants as they grew in the field, and used holdfast morphology as the primary character to distinguish species. This resulted in three commonly recognized species: M. pyrifera, M. integrifolia, and M. angustifolia.

This chapter concludes that giant kelp forests have achieved the status of “rain forests” of temperate seas in being the most widely recognized inhabitant of temperate reefs and known for the high diversity they support. Giant kelp forests are largely thriving, and have remained so despite the numerous stressors on coastal ecosystems throughout the past few centuries. With ever-increasing numbers of nature tourists, scuba divers, recreational fishers, and wide-reaching nature documentaries, both the natural beauty of giant kelp forests and the key role they play in the provision of “services” are much better appreciated globally. Moreover, within the scientific literature, giant kelp forests have been a focal point for discussions about regime shifts and alternate states of coastal ecosystems, and they have contributed to the emerging ecological theory and management models related to spatial planning.

This chapter identifies the uses of giant kelp as well as forest-associated species, including those comprising important recreational and commercial fisheries. These include food and fertilizer, chemical production, biofuel and carbon sequestration, aquaculture, and other recreational uses, such as sport fishing and commercial fishing. Consumption is especially common in Japan and China where there is a long tradition of eating kelp, especially Laminaria and Undaria. There is also a long history of seaweed use as food for terrestrial animals. Giant kelp and other seaweeds are also marketed in several areas of the world for fertilizer; however, a more important benefit may be as a soil conditioner. Moreover, the conversion of algae into biofuel results in recycling CO₂ rather than releasing more of it from fossil fuel or sequestering it for long periods of time such as in wood or peat.