34 THE RELATION OF PLANTS TO TIDE-LEVELS 
The species of alge which frequently occur as epiphytes on Zostera, on 
Ulva, or occasionally on other alge, are the following: Spirulina tenuwissuma, 
Cocconeis scutellum, Melosira borre, M. nummuloides, Navicula greviller, N. 
kennedyi, Synedra affinis, Cladophora (expansa?), Enteromorpha clathrata, 
Ceramium rubrum, and C. strictum. (See plates vir and Ix.) 
Among these epiphytic forms there is no general predominance of any one 
species, though because of their size and color the Ceramiwms may be more 
prominent. We may therefore discuss the species enumerated in alphabetical 
order within each class, beginning with the simplest. 
SCHIZOPHYCEZ. 
Spirulina tenuissima: This alga, as we shall see later, is widely spread from 
mean low water up to the 7-foot level, but it is in the present belt, as an 
epiphyte on Zostera, Ulva, and sometimes on Hnteromorpha clathrata, that it 
is most luxuriantly developed. On the Zostera this alga sometimes forms 
dense yellowish-green patches, sparkling with gas-bubbles and often many 
square decimeters in extent. For example, in 1910, patches of this sort 
were thickly sprinkled over hundreds of square meters of bottom from 1,300 
to 1,500 north by 950 to 1,050 east, covering one-third of the Zostera plants 
and matting scores of their leaves together. Similar, though usually smaller, 
patches of Sprrulina have been seen adhering to the large sheets of Ulva or on 
tangles of Hnteromorpha clathrata in the southeastern parts of the harbor. 
Still smaller patches occur occasionally on stakes or buoys in the middle of the 
harbor. These patches of Spirulina are 5 to 10 mm. thick and practically 
pure, showing but few other organisms within the mass, such as filaments of 
an Oscillatoria or of some other epiphyte of Zostera buried by the growth of 
the Spirulina. These dense growths of Spirulina are confined, in this harbor, 
to levels within a foot or less of mean low water. At higher levels, up to its 
upper limit at about 7 feet, Spirulina occurs sparingly mixed in mats or felts 
with numerous other Cyanophycesx, none of which seem to flourish near mean 
low water, where Spirulina does best. The lower limit of Spirulina in this 
harbor is about 1 foot below mean low water, and it is apparently conditioned 
by the presence in somewhat quiet water, which gets warm at low tide, of a 
substratum such as Zostera or Ulva over which it may spread. In its occupa- 
tion of substrata at higher levels it is restricted probably by the danger of 
desiccation, except in shaded areas or where it is protected by mats of other 
algw. Probably at higher levels also, in some localities, it is kept lower than 
usual because of lack of a suitable substratum. 
BACILLARIALES. 
The epiphytic Diatomee of the bottom of the harbor include the most abun- 
dant and widely distributed epiphytes of this belt. Cocconets scutellum, e. g., is 
found, often in great numbers, on nearly every plant growing in the Inner Har- 
bor. It grows not only on Zostera, but even more abundantly on Ulva and on 
both attached and free plants of other larger alge. It is often especially abun- 
dant on the epiphytic Ceramiums. The distribution of this diatom has not been 
studied in great detail, but it apparently occurs on all living substrata through- 
out the harbor, except near fresh water. In vertical distribution Cocconets is 
found from —2 feet to +1.5 feet. 
