Re: Why is the formation of macroalgae like that?

Dear Kathleen
Different benthic plants characteristically occupy different tidal levels, and this zonation is partly 
determined by their different abilities to absorb particular wavelengths of light.  At sea surface, 
complete spectrum of visible light is available, but in the water column the different wavelengths 
are either quickly absorbed or scattered within the water column.  
Green algae (eg. Ulva) is commonly found in the shallow water since their pigments absorb both 
short and long wavelengths.
Brown algae (eg. Kelp, Fucus) also contain green chlorophyll in addition to some accessory pigments 
that mask its green color, they are most abundant in somewhat deeper waters and their main 
pigment is fucoxanthin which is more efficient in capturing the blue-green light.  Some red algae 
(eg. Gigarthina) are subtidal in occurrence and have red pigments (phycoerythrin and phycocyanin) 
and are efficient in absorbing the subsurface light.

Now, there are exceptions over the depth-distribution of the algal types;
For instance, some red algae (eg. Porphyra) may be found in high intertidal zone, and some green 
algae (eg. Ulva) may occupy lower regions.

I hope this answers your question
Cheers
Shashank

Reference:
Biological Oceanography - An Introduction (second edition) 1997 Lalli CM and Parsons TR. The 
Open University, Elsevier Publication.