Ecology of Giant Kelp

The distribution of Giant Kelp is greatly influenced by physical factors, particularly the influence of nutrients, currents (which provide cool, nutrient rich waters), water temperature and the frequency of storm events. Specifically, Giant Kelp require a hard surface for attachment, high nutrient concentration, moderate water motion, and cool, clear and clean ocean water for growth. World-wide, Giant Kelp (Macrocystis pyrifera) is found in the cooler waters of North America (Alaska to Baja California), south Africa southern Australia, and the west coast of South America.

Studies in southern California have shown that recruitment of sporophytes is greatest during periods of low temperatures and high nutrients, called recruitment windows. These conditions are evident during spring upwelling when light is also high because of canopy thinning and the creation of 'gaps in the canopy' by winter storms. New sporophytes grow from the substrate to the surface where they may form very dense surface canopies. Kelp canopies can reduce the amount of light reaching the substrate to less than 1% of surface irradiance.

Kelp forests undergo regular seasonal growth, with peak growth in spring and regular dieback of plants during the winter months. Individual Giant Kelp plants can live up to seven years, however they may be shorter lived and removed in environments of high water motion associated with winter storms. Kelp forests are also subject to dynamic changes induced by herbivory. In California, grazing by sea urchins is another major source of kelp mortality. When urchins reach high population numbers they are known to completely remove Macrocystis from an area, resulting in an urchin barren.

The Californian sea otter has been shown to have a significant influence in maintaining kelp forest communities, primarily by eating urchins. Although much is known about the growth and survivorship of adult kelp sporophytes, relatively little is known about the ecology of their microscopic gametophytic stages. These stages are probably highly vulnerable to grazing and sedimentation, such as that from sewage discharges. Studies on microscopic stages of Giant Kelp suggest they are also sensitive to the toxins associated with municipal and industrial waste discharges.