Carotenoid level varies to a great extent among different seaweeds. Brown seaweeds are particularly rich in carotenoids especially in fucoxanthin, β-carotene, violaxanthin. Quantitative estimation of Fucoxanthin in brown seaweeds namely Sargassum horneri, Cystoseira hakodatensis and Undaria pinnatifida 1.3mg/g dwt in C. hakodatensis 2.4mg/g dwt in S. horneri and 2.3mg/g dwt in U. pinnatifida. In the case of red seaweeds, zeaxanthin, the major carotenoid. Astaxanthin, found in microalgae, yeast, salmon, trout, krill, shrimp, crayfish, crustaceans, etc. is a keto-carotenoid belonging to a larger group of phytochemicals known as terpenes. However, like other carotenoids, astaxanthin has self-limited absorption orally and such low toxicity by mouth that no toxic syndrome is known. It is an antioxidant and the free-radical terminating effectiveness of each carotenoid is heavily modified by its lipid solubility, and thus varies with the type of system being protected.

Caretinoid concentation is a very varing value from species to species. Environmental condition also inflence on it in one species....

I believe you will need to be more specific than just "carotenoids"; even splitting between carotenes and xanthophylls would not be precise enough. You want to specify which compound(s) among the carotenes and xanthophylls you are really interested in for whatever application(s).

Then, giving an average concentration is pretty difficult and not really meaningful. These concentrations vary highly according to species, seasons and environmental conditions (including water).

I agree with Dear Thierry. Like other secondary metabolites, concentrations of  "carotenoids" vary with respect of species, seasons and agro-climatic and phytogeographic conditions.

Please refer the book chapter 'Pigments and minor compounds in algae' by S. Kraan, in Functional ingredients from algae for foods and nutraceuticals, H. Dominguez (Ed), Woodhead Publishing Series, Technology and Nutrition, Number 256, 205 - 251pp.