Old survey markers indicate the historical changes of local sea levels

David Scott doesn't want to get into the debate about how much humans are affecting global warming.

But Scott, a geologist and private consultant, is interested in documenting evidence along Charlotte Harbor's shorelines that indicates the rise of sea levels since the 1930s and 1940s.

Global positioning instruments and other new technologies have turned traditional survey markers into artifacts. However, Scott believes coastal survey markers along the Charlotte Harbor estuary serve well as historical place markers of local sea levels.

Scott first discovered the survey markers as part of a study documenting the damage and die-off Hurricane Charley caused to mangroves along Charlotte Harbor.

Working with Terry A. Tattar, a biologist and a retired professor with the University of Massachusetts microbiology department, and Betty Staugler, an agent for the University of Florida's Sea Grant program in Charlotte County, Scott assisted in a study documenting the location of the storm-damaged and dying mangroves.

The West Coast Inland Navigation District funded the first study, and Staugler said they are now working to find funding for a second phase of the study that would replant mangroves along the damaged shorelines.

Early in the study, rather than surveying and mapping the area from scratch, Scott decided to use existing survey coastal markers, which hadn't been used in 40 or more years. He started noticing that the markers were closer to the water than they should be -- and some were underwater.

Marking a change

When discussing vertical and horizontal geodetic measurements, Scott compared them to the impacts of placing an inch of water into a glass or atop a piece of paper that's been curled up on its edges.

The inch of water in a glass would appear to be more dramatic because of the vertical rise. The same inch of water would spread out horizontally across the paper and wouldn't appear as visually dramatic. Charlotte Harbor holds water like the piece of paper, or a very wide and shallow soup bowl.

"If you measured the angle of the downward slope, it would be a 0.3 degree of a vertical angle," he said. "If no water was there, you wouldn't know whether you were walking on flat ground or not."

Scott also described the survey markers as "horizontal" markers meant only to gauge longitude and latitude positions.

"We know what the vertical change in sea levels are because we have archival tidal data information, and the (National Oceanic and Atmospheric Administration) keeps very accurate records," Scott said, noting that St. Petersburg is one of the oldest tide-gauging stations in the nation, beginning in early 1900s.

"What these (survey) markers are showing us -- because Charlotte Harbor is so flat, only 15 or 20 feet deep in the middle -- is that the west shoreline is migrating to the west, and the east shore line is migrating to the east, and the north shoreline, Hog Island, is migrating to the north."

Besides indicating the harbor is widening, Scott said the markers indicate more Gulf of Mexico water is flowing into the harbor, and the additional water is a result of rising sea levels.

Erosion is not a factor. Like other estuaries, Scott said, the Charlotte Harbor estuary is protected from significant coastal erosional forces, like the long-shore currents affecting the Gulf shoreline. The wide, shallow flats on both shorelines of Charlotte Harbor also show no signs of significant depletion or accretion associated with erosion.

Originally, the surveyors placed the markers at or above the high-tide line. Records also show subsequent surveyors documenting where the shoreline was relative to the markers.

"Many of them are now underwater," Scott said. "Basically, the shorelines have moved 30 feet inland on both sides of the harbor."

One marker north of Pirates Harbor was discovered beneath a fallen mangrove and is underwater even when the harbor is halfway between high and low tides.

Another marker on the Cape Haze peninsula was 22 feet west of the high-tide line in the 1943, but Scott's research showed it was found in 1954 only 35 feet from the high-water line, and when Scott found the marker in 2005, it was only 11 feet from the high-water line.

Mangrove markers

The mangroves are also indicating the changes in sea level.

The red mangroves are those which people normally associate with mangroves; the trees with the arcing roots in the water, earning them the nickname "walking trees."

Further back on a shoreline, the black and white mangroves are likely to take root.

The black mangrove thrives in slightly higher elevations of upland habitat from the red mangrove. The white mangrove usually occupies even higher elevations than either the red or black mangroves. Unlike its red or black counterparts, the white mangrove has no visible aerial root systems.

The mangroves, Tattar said, are also indicating that the water is rising since the different varieties of mangroves appear to be taking root further inland.

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