that the Ammen population is a low irradiance ecotype of L. 

 digitata , or if a distinct species, one which has evolved 

 adaptations to cope with low irradiance levels. Ecotypes of 

 Laminaria species have previously been documented both for 

 nitrogen (Espinoza and Chapman, 1983) and irradiance (Gerard, 

 1988) . The adaptive significance of the differences in the 

 photosynthetic characteristics between Ammen and coastal Laminaria 

 is evident from a consideration of the predicted net 

 photosynthesis achieved at different light levels (Fig. 12) . By 

 virtue of their lower respiration rate and compensation point, 

 Ammen plants would achieve greater rates of net photosynthesis 

 (and hence growth) than the coastal group at low photon flux 

 densities. In contrast, coastal L. digitata would grow faster 

 than Ammen plants at photon flux densities above 1 umole 

 photons. m " 2 .s" 1 . The predicted responses are supported by the 

 growth data, which indicate that the coastal plants did grow 

 fastest in the high irradiance, 1 m coastal site, whereas Ammen 

 plants grew faster in the lower light environments of the 15 m and 

 30 m sites (Fig. 1) . Because of the reduction in photosynthesis 

 and stimulation of respiration associated with wounding (Gerard, 

 1988) , the calculated photon flux densities reguired to achieve 

 net photosynthesis (Fig. 12) are probably higher than those 

 reguired in nature by intact plants. However, assuming the two 

 groups of plants exhibit similar responses to wounding, this does 

 not affect the interpretation of the data. 



Somewhat surprisingly, coastal L. digitata plants were more 

 efficient at utilizing low light levels than the Ammen population 

 (Fig. 4) . The increase in alpha did not however compensate for 

 the increased respiration rates of the coastal plants. The 

 increase in alpha of the coastal plants was probably due to the 

 higher chlorophyll contents of these plants (Fig. 9) . There was a 

 strong correlation between alpha and content of chlorophyll a for 

 both plant groups (correlation coefficent 0.975), suggesting that 

 much of the additional chlorophyll was in the form of antenna 

 pigments in the photosynthetic units. The lower respiration rates 

 characteristic of the Ammen population suggests that these plants 

 invest less energy in cell maintenance and biosynthesis than the 

 coastal L. digitata . This would explain the lower chlorophyll a 

 contents and enzyme activities of the Ammen population (Figs. 8, 

 9, and 16), all of which would enable these plants to conserve 

 energy reguired for the biosynthesis of these molecules, but which 

 restrict P m a x and hence growth at high light levels. 



The initial biochemical composition of the two plant groups 

 suggest that the coastal L. digitata population was nitrogen 

 limited (absence of cell nitrate and presence of large mannitol 

 pool; Figs. 13 and 15) whereas the Ammen plants were not (Davison 

 et al., 1984). The reduction in cellular amino-N content (Fig. 

 14) and enzyme activities (Figs. 8 and 16) together with a slight 

 increase in mannitol pool (Fig. 13) indicates that nitrogen 

 limitation became more severe for the coastal plants during the 

 period that they were transplanted at the 1 and 15 m site (Wheeler 

 and Weidner, 1983; Davison et al., 1984). The Ammen plants at 



41 



