Ecology and Management of the Bull Kelp, Nereocystis Luetkeana
A Synthesis with Recommendations for Future Research
By Dr. Yuri Springer, Dr. Cynthia Hays, Dr. Mark Carr, Ms. Megan Mackey with assistance from Ms. Jennifer Bloeser
Introduction
Why the interest in ecology and management of the bull kelp?
Species of large brown algae of the order Laminariales, referred to as kelps, are a conspicuous component of coastal rocky reef habitats in temperate oceans throughout the world. Along the west coast of North America, two genera, the giant kelp, Macrocystis spp. (hereafter Macrocystis), and the bull kelp, Nereocystis luetkeana (hereafter Nereocystis), form extensive forests in shallow (< 30m depth) rocky habitats. Because of their fast growth rate and large stature, these algae are thought to contribute markedly to both the productivity of shallow coastal marine ecosystems and as habitat for a diversity of fishes and invertebrates (Foster and Schiel 1985; Graham 2004; Graham et al. in press). Both of these fundamental ecosystem functions of kelps are realized not only by those species that reside in kelp forests throughout their lives, but also by species that use these habitats as foraging grounds (e.g., shore birds, sea otters) and nurseries (particularly for fishes), for the enhanced growth and survival provided them by the productivity and structural refuge created by the algae (see review by Carr and Syms (2006). Many of these species have been strongly impacted by overfishing and are the focus of conservation efforts (e.g., rockfishes, sea otters). In addition, algae represent important biological links between marine ecosystems. The biomass and nutrients they produce, in the forms of detritus or entire plants, are exported by storms to sandy beaches and submarine canyons where they fuel food webs in the absence of other sources of primary production (Harrold et al. 1998; Kim 1992; Vetter 1995). Floating kelp rafts also may serve as habitat for larval and juvenile fishes, effectively transporting them among spatially isolated local populations of adults (Hobday 2000; Kingsford 1992; Kokita and Omori 1998). Because of the ecosystem functions provided by kelps, the consequences of human impacts to these forests are not limited to the direct effects on kelp populations, but influence indirectly the many species that depend on the presence of these forests. In addition to the functional roles that kelp play in coastal marine ecosystems, they contribute valuable ecosystem services. Not only do they contribute to the productivity of coastal fisheries through the functions they perform for coastal fishes, but the algae themselves are extracted for human consumption, pharmaceutical industries, and as food for the commercial mariculture of abalone. The physical barrier created by forests that line the shore also dampens ocean waves, thereby reducing coastal erosion. They are also of great social and cultural importance because of the myriad of species these forests harbor that foster many human activities (e.g., commercial and recreational fishing, scuba diving, bird watching, kayaking.) Separately, and in combination, these consumptive and non-consumptive activities can translate into socioeconomic values of importance to local coastal communities.
Due to their close proximity to shore, kelp forests are subjected to deleterious anthropogenic impacts that can impair the functions and services they produce. In addition to the direct extraction of kelps, they can be exposed to coastal pollution in the form of nutrient discharge from urban and agricultural sources and thermal pollution from the discharge of cooling waters associated with coastal power plants. Increases in turbidity and rates of sedimentation associated with all of these activities impair photosynthesis (i.e. growth and survival of adult plants) and smother reproductive stages and spores, preventing reproduction and germination. In addition to these localized and regional threats, kelp forests are vulnerable to climate change. The existence and tremendous productivity of these forests rely on the upwelling of deep offshore nutrient-rich waters. This upwelling process is driven by coastal winds that move surface waters offshore, driving their replacement by the deeper nutrient-rich waters. As atmospheric conditions change in response to large-scale climate trends, the timing, location and intensity of coastal winds alter the distribution and magnitude of upwelling, thereby changing the environmental conditions required to sustain kelp forests. Large storms associated with El Niño events are major causes of mortality and the loss of entire kelp forests (Tegner and Dayton 1987). The frequency, duration, and strength of El Niño events have increased in recent years, and these changes may be a direct consequence of concurrent regional climate changes (Trenberth and Hoar 1996).
The direct and indirect impacts of kelp extraction depend very much on the species and means by which it is removed. Historically, extraction has been focused on the giant kelp, Macrocystis, primarily by the pharmaceutical industry. Large specially designed harvesting vessels were used to remove large swaths across forests. Removal is limited to the upper 2 m of the forest canopy. The direct impact on the forests is considered minimal because the canopy is often replaced rapidly by the growth of fronds from the base of the plants. Moreover, the alga is perennial and the reproductive portions of the algae are located at the base of the plant and remain intact during and subsequent to the removal of the canopy. Thus, the algae are able to reproduce and forests persist in the face of large-scale mechanical extraction. However, the indirect effects to the fishes and invertebrates that use the forest canopy as nursery habitat and the many species that require the flux of kelp blades from the canopy to the reef habitat below to fuel a detritus-based food web (akin to litter fall in terrestrial forests), have not been rigorously investigated.
The extraction of bull kelp, Nereocystis luetkeana, is a more recent development, fueled by the demands of abalone mariculture and human consumption. Though relatively smaller in volume and geographic extent, the harvest of bull kelp is problematic. Extraction is primarily by hand from a boat and, like giant kelp, limited to the upper 2 m of the forest. However, the source of buoyancy that keeps the alga upright, along with the reproductive organs of bull kelp, are located at the top of the alga and are thereby removed when the upper portion of the plant is cut off. Because bull kelp is an annual species, removal of the upper portion of plants prior to reproduction can potentially preclude the production of subsequent generations. Because the spores of bull kelp are thought to move very short distances (10’s of meters) on average, local impairment of reproduction might terminate the persistence of a forest. Local recruitment could be subsidize by input of spores from other populations delivered either by drifting reproductive sporophytes or abscised sori. Additionally, the presence of dormant spores produced by previous generations of bull kelp could potentially reseed local populations that have been depleted by harvesting. Because there are few data on spores dispersal potential and dormancy durations, however, these mechanisms of local “rescue” cannot at present be incorporated into management plans in a quantitative manner.
Approach, scope of synthesis, and products
Ecosystem-based management of coastal marine resources is based, in part, on scientific understanding of the broad (i.e. ecosystem-wide) consequences of human uses of the coastal environment, including resource extraction and degradation of habitats. To wisely manage these 2 resources, a clear understanding of the potential impacts of human activities on the resource and the ecosystem is essential. To contribute to this understanding, this report synthesizes the state of knowledge of (1) the ecology of the bull kelp and its role in coastal ecosystems, (2) the past and present human uses of and impacts on this species and, by extension, the coastal ecosystem, and (3) the past and present approaches to managing this resource. We use this synthesis to identify gaps in our knowledge required to make sound management decisions, and recommend priority research needs to inform management of the human activities that impinge on this species and its ecosystem functions and services. The scope of this review spans studies and management programs from Alaska to central California, and includes both data published in peer-reviewed, scientific journals and non-peer-reviewed data (e.g., produced by governmental agencies and NGOs).
This report includes three components: the written summary included here, an electronic database of all the material used to generate this synthesis, and a compilation of most of this published and gray literature in the form of PDF documents. The literature database was compiled in EndNote (version X), one of the most popular and readily available electronic bibliographic databases. The collection of PDF documents will be available as linked attachments through the Endnote library file as well as on the web at both Dr. Carr’s website at UC Santa Cruz and the Pacific Marine Conservation Council website.
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