There are several methods to estimate the water content in macroalgae. I added a link to one. It usually varies depending of the algae. Morphology, as well as composition, influence water content. Crustose coralline algae (Rhodophyta or red algae belonging to the Corallinaceae family, which also contains articulated coralline algae, both very calcified) may have a relatively low water content, when compared to other less calcified, fleshy algae, such as uncalcified green algae (Chlorophyta - there are, however, calcified green algae as well, as Halimeda). 

http://www.scielo.br/scielo.php?pid=S0100-40422008000400030&script=sci_arttext

In these calculations of dry weight (DW):fresh weight (FW) ratios, it is always important to be consistent in your determination of a DW (in our case, a minimum of 72 hours at 60 C) and a FW (generally more tricky: shaken, blotted with paper, spanned a fix number of rotations with a salad spinner, etc. but be consistent). Generally, it can be around 1:10 (90 % water) to 1:8. Consistency is key; the most source of different values is a variable determination of what is a FW. The case mentioned above (coralline algae) is quite different from most seaweeds.

Determination of Moisture Content of Expressed Plant Tissue Fluids

Ross Aiken Gortner and Walter F. Hoffman

Botanical Gazette

Vol. 74, No. 3 (Nov., 1922), pp. 308-313

Published by: The University of Chicago Press

Stable URL: http://www.jstor.org/stable/2470425

Page Count: 6

Water content of seaweed varies species to species for example carraginophytes Sarconeema contained around 92% water and Gelidium pusillum around 60%. So seaweed water content should be estimated individually. The total water content can be determined by drying the fresh sample at 105 C  till constant weight  (its usually 4 hrs).  Fresh weight - dry weight  = Loss of water i.e water content

The moisture content can be very variable. However, there was a standard value based on comprehensive review adapted by experts of International Atomic Energy Agency in their Technical Report 422, that is 82% moisture.

Dear Saif, Many thanks for your reply. Could you please provide me the link of the report you mentioned? Thanks, Vaibhav

I would highly recommend to measure with your particular species for your studies. This is highly species specific.

If there is generally not much discussion about what is a dry weight (DW; generally a minimum of 72 hours at 60 C), determining what constitutes a fresh weight (FW) is often more problematic because there is wet seaweed and wet seaweed (still soaked with water, or blotted, or?)! It is important to standardize what you call a FW for your study and be consistent.

Personally, I use a salad spinner and a number of spins I maintain constant. This is very reliable (and cheap!).

Having a reliable FW, makes your FW/DW ratio also reliable.

I agree with Dr Chopin, I use the same method as him.

Good luck

Rezzoum

Plenty of literature exists on water content of seaweeds. It is not clear from the question whether author is seeking for data, methodology of water quantification or the reasons for variation in water content. Many a times the tissue construction and chemical composition of thallus determines the water holding capacity of live seaweed. For example, coenocytic forms such as Boergesenia, Valonia etc. contain total body weight as water with dissolved solids- many a times equivalent to seawater.