This first set of pictures was made at St. Marks in May. The tan vegetation was later identified as algae on the bases of the grass.
We thought it was very odd-looking.
This is the same spot later in September. The tall, rosy-stemmed shrubs are giant pigweed or southern water hemp (Amaranthus australis).
They grow quite large (10 feet and more) and are numerous in the salt marshes at St. Marks.
In looking up this plant, it appears to have had some name changes through the centuries, reflecting botanists' efforts to better categorize into families and genuses and species, etc. This last name was only acquired in 1955.