The question needs to be more specific. Is it about estimating the mass growth? Or is it about quantifying the mass found on the beach. Or something else? Hope this helps.
Dear Susantha, just in case that your question is still of interest to you: Please have a look at the following very useful article which will certainly help you in your analysis:
An efficient method for estimating seagrass biomass
Article An efficient method for estimating seagrass biomass
The best thing about this article is that it has been posted as public full text right here on RG. Thus you can freely download it as pdf file. In my experience it often pays off to directly search the "Publications" section of RG for relevant literature references.