Benthonic Algae— Scagel 
531 
0 2 (mg./L.) 
o 5 10 15 
FIG. 47. Distribution of oxygen with depth at vari- 
ous stations in Queen Charlotte Strait and adjacent 
regions in May, 1956. 
may be continuously exposed for as long as 4 
months during the summer period. Those or- 
ganisms that can tolerate such conditions are rare 
and in this zone one finds chiefly Littorina, which 
is capable of moving sufficiently to extend into 
lower less extreme zones when necessary. In the 
region below (HLHW to MHHW), continuous 
exposure to the air may last for periods ranging 
from a few days to almost 2 weeks, and these 
periods occur at least twice a month, but the 
rest of the time this zone is submerged at least 
once a day. In the region below (MHHW to 
MLHW), continuous exposure to the air is 
rarely for more than a few days at a time, and 
for a few months during the winter the region 
is submerged at least once a day. Although the 
upper limit of a number of the organisms ( Fig. 
50) in this region of the intertidal zone between 
13.1 and 17.4 ft. varies to some extent with the 
organism, it is apparent that in this general 
region an upper boundary is probably deter- 
mined directly or indirectly by the degree of 
exposure to climatic conditions. The precise tide 
level, if there is one, at which this boundary 
occurs is not clear from the data available. The 
effect of surf in the exposed environment may 
also cause some variation in the upper limit of 
the vertical distribution. Since the greatest 
change in degree of exposure within this upper 
region (13.1-17.4 ft.) occurs between MLHW 
and MHHW, a critical level is suggested at this 
point for a considerable number of conspicuous 
organisms. 
All portions of the intertidal zone below 
MLHW are submerged at least once a day for 
various periods. The greatest change in condi- 
tions of submergence (Fig. 45) in the region 
of the middle intertidal zone (8.2-13.1 ft.) 
occurs between HHLW and LLHW, and it is 
at this point where another critical level is sug- 
gested, in some instances as the upper limit and 
in others as near the lower limit of the vertical 
distribution of certain organisms (Fig. 50). 
In the lower intertidal zone (0-8.2 ft.) the 
most extreme change in conditions of exposure 
(Fig. 44) and submergence (Fig. 45) occurs 
between MLLW and MHLW, and this is again 
reflected both at the upper and lower limits in 
the vertical distribution of certain organisms 
(Fig. 50). Another critical level (Fig. 50) 
occurs in the region between LLLW and LHLW, 
where another region of marked change (Fig. 
45 ) exists in conditions of submergence. 
Although the upper limits in most instances 
appear to be relatively sharp, there is less con- 
sistency in this respect concerning lower limits. 
This suggests that other factors, perhaps com- 
petition for space or predation, may be responsi- 
ble for limiting distribution, particularly down- 
ward in some instances. 
DISCUSSION 
Knowing as we do from experimental work 
the responses of some organisms to environ- 
mental factors, it seems reasonable to anticipate 
