been made in central California, but those 

 available indicate levels are generally 

 above 1 um in these colder waters with 

 frequent upwelling (Gerard 1976, Broenkow 

 and Smethie 1978). 



In southern California, giant kelp 

 canopies commonly deteriorate during 

 summer when inorganic nitrogen is low. 

 Reduced nitrogen concentrations may have 

 been responsible for the massive loss of 

 Macrocystis during the warm-water period 

 of the late 1950' s (North 1971b, Jackson 

 1977, North et al. 1982), and plants 

 deteriorated at many locations during the 

 recent (1982-84) "El Nino" (Dean pers. 

 comm. , Dayton and Tegner 1984b). We ob- 

 served late-summer canopy deterioration 

 that may have been due to nutrient 

 limitation at two locations in central 

 California during 1979 and 1982. However, 

 temperature and inorganic nitrogen 

 concentrations are inversely correlated 

 (Jackson 1977), so determining whether 

 inorganic nitrogen or temperature (or a 

 combination of both) is responsible for 

 these phenomena is impossible from 

 correlations alone. 



Evidence that low inorganic nitrogen, 

 not temperature, is limiting under low- 

 nutrient/high- temperature conditions 

 comes from fertilization experiments. 

 Dean and Deysher (1983) found that more 

 sporophytes were produced on fertilized 

 artificial substrata inoculated with giant 

 kelp spores and placed within a kelp 

 forest than on similarly treated but 

 unfertilized controls. Zimmerman (pers. 

 comm.) examined the cause of the summer 

 decline in adult Macrocystis growth at Big 

 Fisherman Cove at Santa Catalina Island. 

 Summer growth was increased when adult 

 plants were fertilized with NaN0 3 . 

 However, growth was not as great in this 

 experiment as the highest natural growth 

 rates at other times of the year, 

 suggesting that in summer, other nutrients 

 and/or temperature may be limiting once 

 the nitrogen requirements of the plants 

 are met. Laboratory studies by Manley and 

 North (1984) suggest phosphorous may be 

 particularly important. North (1983b) 

 added nitrate and phosphate to ambient 

 temperature (18 °- 23 °C) water flowing 

 into a large tank containing adult 

 Macrocystis . These plants maintained 

 healthy canopies and had high tissue 



nitrogen levels, while nearby plants in 

 the natural kelp forest, exposed to 

 similar temperatures but not fertilized, 

 suffered canopy losses and had low tissue 

 nitrogen. 



In addition to temperature (and 

 perhaps other nutrients), physiological 

 processes within giant kelp can further 

 obscure the relationship between nitrogen 

 in the water and plant growth. Seaweeds 

 can store nitrogen when the concentration 

 in the surrounding water is high (luxury 

 consumption), and then use these reserves 

 for growth when the surrounding concen- 

 tration drops (Chapman and Craigie 1977). 

 Macrocystis can accumulate non-structural 

 nitrogen compounds (Wheeler and North 

 1981, Gerard 1982b) including nitrate 

 (Druehl pers. comm.), and then use these 

 reserves to maintain growth for at least 

 two weeks in low nitrogen environments 

 (Gerard 1982b). Thus, both the frequency 

 of environmental sampling for inorganic 

 nitrogen and the presence of tissue 

 reserves can affect the interpretation of 

 growth rate vs. inorganic nitrogen data. 



The utilization of inorganic nitrogen 

 in the water is also affected by water 

 motion (Gerard 1982c, Wheeler 1982). 

 Increased water flow over plants enhances 

 uptake by increasing nutrient transport 

 through the diffusion boundary layer 

 (Neushul 1972, Gerard 1982c). Gerard 

 (1982c) and Wheeler (1982) found that 

 nitrogen uptake by Macrocystis increased 

 with increasing current speed, up to a 

 maximum at ^ 2-4 cm/sec. Current 

 velocities in kelp forests are often lower 

 than this (Wheeler 1980b). However, 

 Gerard (1982c) has shown that water flow 

 caused by wave surge can be equivalent to 

 that of the current speeds above, and 

 pointed out that because the plants are 

 attached to the bottom and each blade is 

 attached to a fixed point on the plant, 

 very small waves can produce flag-like 

 blade movement. This motion, plus small 

 currents and surge, are sufficient to 

 saturate nitrogen uptake even under very 

 calm conditions. 



Little information is 



available on 

 other 



possible nutrient limitations in other 

 kelps in giant kelp forests. Work in 

 eastern Canada (Chapman and Craigie 1977, 

 Gagne et al. 1982) and the Arctic (Chapman 



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