What caused the Tidy Island salt flats?

The surge from the 1921 Tampa Bay Hurricane caused the salt flats on Tidy Island. It carried sand from the south shore over the island, depositing it between the island and the mainland, as explained here. But salt flats need to be an exact distance above sea level. That is, low enough to flood during spring tides, high enough not to flood during neap tides. (For non-sailors: spring tides happen during new or full moons, neap tides happen during quarter or three-quarter moons. Spring tides are higher than neap tides.) On a salt flat, seawater floods the sand twice each month and, if there is no rain, it drys out between those times.

As a result, evaporation of the water concentrates the salt and causes it to crystalize, turning the sand white. The high concentration of salt keeps vegetation from moving into the salt flat. Only the sand deposited nearest the north shore of Tidy Island ended up at the height relative to sea level needed to form salt flats.

There is no documented proof that Tidy Island was actually washed over by the 1921 hurricane. But a contemporary newspaper report states that the nearby fishing village of Cortez was submerged by eight feet of water, which would certainly have submerged Tidy Island. Also, when the owner of Tidy Island in the ’55 became interested in developing it, he only owned that part of the island delineated by the first survey (1906). In order to gain ownership of the entire island, he obtained affidavits from older residents stating that mangroves had spread from the original island towards the mainland sometime after 1917. This is consistent with a 1921 wash-over creating new shallows followed by spreading mangroves and accretion of new land.

1940 and 1970

This is what Tidy Island looked like in 1940.The Grable survey shows the approximate boundaries of the original Tidy Island. The new salt flats are just north of the old shoreline. Mangroves have colonized the sand further north where the water is deeper.

The salt flats on Tidy Island in this 1970 aerial photo are much less extensive than in 1940.

1982

Twelve years later the salt flats are not greatly different. A 1984 thesis from New College of Florida suggests that a 1941 hurricane formed the salt flats by washing over the island. However, the 1940 aerial photo shows them fully formed and mostly covered by mangroves. The 1921 Tampa Bay Hurricane almost certainly caused them.

On the other hand, it is possible that the 1941 hurricane carried new sand over the island onto the older west salt flat. That part of the island is lower than the rest and more susceptible to washover.

This 2018 photo shows old mangroves smothered by sand, possible deposited from the 1941 hurricane. This is the only part of Tidy Island showing signs of a second washover event.

The salt flats today

Increased sea level today gives the salt flats less time to dry out between spring tides, so the concentration of salt on the surface of the sand is less than before. As a result, black mangroves now cover most of them, as shown by this contemporary aerial photo.

The remains of the salt flats are divided into three parts: west, north, and south.

The one at the west end of the island, shown by this photo, is best preserved because it drains freely to the bay, leaving more time to dry between spring tides. This keeps the surface salinity high and prevents vegetation from moving in.

To the east, there are two more salt flats on the south and north sides of a ditch that runs the entire length of the island. The one to the south is in the poorest condition because it is slow to drain. It also receives a lot of rainwater runoff from the adjacent residential upland.

The one to the north is intermediate in condition. It has several parts that are free of vegetation. Bands of black mangroves separate these areas, spreading further every year.

Pink algae

The extreme northeast corner of the north salt flat has an unusual feature. It has many dead black mangroves, apparently drowned by a gradual change in drainage that trapped a large amount of water in that area. Evaporation must then have increased the salinity of the water, killing the mangroves. It also resulted in a bloom of pink algae. This is Dunaliella salina, an algae that grows in water containing as much as 35% NaCl, more than ten times its concentration on seawater. The color comes from bets-carotene, the same chemical that gives carrots their color.

The mangrove swamp further north belongs to New College of Florida and used for ecological research. Mosquito control ditches dug during ’51-’57 and again in ’64-’75 have severely impacted this area. The digging left hundreds of spoil piles, elevated above the low terrain, which are now infested with invasive species. The College is carrying out an EPA-sponsored research project to study methods for restoring the mangrove habitat.