similar to 

 elements in 

 North (1980) 



Macrocystis 



2.5. NUTRIENTS 



Most plants, including seaweeds, are 

 photoautotrophic; the sun provides energy, 

 but the plants require a variety of 

 inorganic and some organic nutrients, such 

 as vitamins, to manufacture the chemicals 

 necessary for growth and reproduction. 

 Subtidal seaweeds must obtain all their 

 nutrients from the water because hold- 

 fasts are attached to solid substrata (no 

 soil) and appear to serve no special 

 nutrient uptake functions. Moreover, 

 except for the occasional frond in the 

 surface canopy, tissues are not exposed to 

 air, and thus all metabolic processes 

 occur in water. 



Few macroalgae have been grown in 

 defined culture media in axenic condi- 

 tions, so we know little about their 

 complete nutrient requirements (DeBoer 

 1981). The assumption is that their 

 inorganic requirements are 

 terrestrial plants (14-21 

 various forms; DeBoer 1981). 

 identified 38 elements in 

 tissue, and Kuwabara and North (1980) , 

 using microscopic stages of M. pyrifera 

 cultured in defined media, found that at 

 least nine elements were essential for 

 growth and reproduction. These, along 

 with carbon and oxygen, are listed in 

 Table 3. DeBoer (1981) suggested that of 

 these, nitrogen, phosphorus, iron, and 

 perhaps manganese and zinc, may possibly 

 limit growth of macroalgae in nature, and 

 North (1980) concluded that copper could 

 also be limiting for Macrocystis . 



Of the possible nutrients that could 

 limit kelp forest algal growth in the 

 field, nitrogen has received the greatest 

 attention, particularly as it may affect 

 Macrocystis growth in southern California 

 (Jackson T977, Wheeler and North 1981, 

 Gerard 1982a, b, c, Zimmerman and Kremer 

 1984). Gerard (1982a) indicated that 

 inorganic nitrogen concentrations in the 

 surrounding water must be in the order of 

 1-2 urn (1pm = 1 ya -a torn/ liter) to support 

 a typical giant kelp growth rate of 4% 

 increase in wet weight per day. Inorganic 

 nitrogen concentrations vary widely in 

 nearshore waters, but are particularly 

 high during upwelling or when there is 

 terrestrial runoff (North et al. 1982). 

 They are low (< lym) in summer and fall in 



Table 3. Known nutrient requirements for 

 Macrocystis pyrifera . 



j*From Kuwabara and North 1980. 



Where forms are not given (--), it is 



assumed that the element is used as a 



free ion. 

 ^Frorn DeBoer 1981. 



May limit Macrocystis growth in deep 



oceanic water (Kuwabara 1982). 

 e Toxic to Macrocystis as free ions in 



deep oceanic sea water (Kuwabara 1982). 



southern California (especially above the 

 thermocline if the water is thermally 

 stratified), and during periods when warm 

 water masses move into the region from the 

 south (Jackson 1977, Wheeler and North 

 1981, North et al. 1982). At Catalina 

 Island in southern California, daily 

 variation in nitrate concentration is 

 frequently as great or greater than mean 

 seasonal variations, and at least a one 

 day per month intensive sampling is needed 

 to characterize nitrate at this site 

 (Zimmerman and Kremer 1984). Fewer 

 measurements of inorganic nitrogen have 



15 



