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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 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 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 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 examines the abiotic environment of giant kelp. Macrocystis requires a hard substratum for settlement and attachment, water temperatures between about 4°C and 20°C, sea-bottom light intensities equivalent to 1% or greater than sea-surface irradiance, nitrate concentrations, oceanic salinities, and protection from extreme water motion. Macrocystis occupies much of the Pacific coast of California and Baja California, Mexico. It may be restricted by waters that are too warm or too low in nutrients, or severe water motion. Giant kelp forests flourish and are particularly well developed on outer coasts between depths of around 5 m and 20 m, beyond which sea-bottom light is often decreased for effective recruitment and growth. Macrocystis is usually absent from estuaries and far inside of protected bays because of a shortage of rocky substrata, increased sedimentation, and reduced light. Reduced salinity can also restrict Macrocystis in bays and other areas with large freshwater inputs.

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 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.