<P> In addition, many kelp species have pneumatocysts, or gas - filled bladders, usually located at the base of fronds near the stipe . These structures provide the necessary buoyancy for kelp to maintain an upright position in the water column . </P> <P> The environmental factors necessary for kelp to survive include hard substrate (usually rock or sand), high nutrients (e.g., nitrogen, phosphorus), and light (minimum annual irradiance dose> 50 E m). Especially productive kelp forests tend to be associated with areas of significant oceanographic upwelling, a process that delivers cool, nutrient - rich water from depth to the ocean's mixed surface layer . Water flow and turbulence facilitate nutrient assimilation across kelp fronds throughout the water column . Water clarity affects the depth to which sufficient light can be transmitted . In ideal conditions, giant kelp (Macrocystis spp .) can grow as much as 30 - 60 cm vertically per day . Some species, such as Nereocystis, are annuals, while others such as Eisenia are perennials, living for more than 20 years . In perennial kelp forests, maximum growth rates occur during upwelling months (typically spring and summer) and die - backs correspond to reduced nutrient availability, shorter photoperiods, and increased storm frequency . </P> <P> Kelps are primarily associated with temperate and arctic waters worldwide . Of the more dominant genera, Laminaria is mainly associated with both sides of the Atlantic Ocean and the coasts of China and Japan; Ecklonia is found in Australia, New Zealand, and South Africa; and Macrocystis occurs throughout the northeastern and southeastern Pacific Ocean, Southern Ocean archipelagos, and in patches around Australia, New Zealand, and South Africa . The region with the greatest diversity of kelps (> 20 species) is the northeastern Pacific, from north of San Francisco, California, to the Aleutian Islands, Alaska . </P> <P> Although kelp forests are unknown in tropical surface waters, a few species of Laminaria have been known to occur exclusively in tropical deep waters . This general absence of kelp from the tropics is believed to be mostly due to insufficient nutrient levels associated with warm, oligotrophic waters . One recent study spatially overlaid the requisite physical parameters for kelp with mean oceanographic conditions has produced a model predicting the existence of subsurface kelps throughout the tropics worldwide to depths of 200 m . For a hotspot in the Galapagos Islands, the local model was improved with fine - scale data and tested; the research team found thriving kelp forests in all eight of their sampled sites, all of which had been predicted by the model, thus validated their approach . This suggests that their global model might actually be fairly accurate, and if so, kelp forests would be prolific in tropical subsurface waters worldwide . The importance of this contribution has been rapidly acknowledged within the scientific community and has prompted an entirely new trajectory of kelp forest research, particularly emphasizing the potential for a spatial refuge from climate change also the explanations to evolutionary patterns of kelps worldwide . </P>

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