The coastal lakes rise with runoff water from the north which contains organic matter called tannins that give it the brown color.
When the lake level rises higher than the gulf, the beach opens up and then the lake water runs out into the gulf turning he gulf brown.
Then, when the lake level is below the gulf, the gulf waters run into the lake.
It's been doing this for millions of years!
It is natural, non-toxic and harmless.
Enjoy he gulf.