Ah! I understand that. Is that because the sargassum is a very fine material?

The Seaweed that we get on our beaches here is not littered with such small things, they are large pieces until they start breaking down.

I did a little more research and remembered all the things I learned and forgot at the beaches, school, and the aquarium. What we have washed up is part of several kelp forests. So these are large species of seaweed. Perhaps something like Macrocystis.

I will be sure to take some pictures of these next time I am on the beach and send them to you.

Right... so reduced oxygen I guess... is because this sargassum floats on the top of the water like scum, preventing natural diffusion of oxygen on the water surface because it covers it like a thick blanket.

If there was a way to cheaply agitate the water and move the sargassum away then it would help aerate the water as well. Like a fish tank? They blow bubbles in a fish tank to introduce oxygen and save those organisms.

Perhaps you will know a better way or find someone who does.