kelp vs seaweed

kelp

noun
  • Any of several large brown algae seaweeds (order Laminariales). 

  • The calcined ashes of seaweed, formerly used in glass and iodine manufacture. 

verb
  • To gather kelp. 

seaweed

noun
  • Any of numerous marine plants and algae, such as a kelp. 

  • Any of various fresh water plants and algae. 

How often have the words kelp and seaweed occurred in a corpus of books? (source: Google Ngram Viewer )