SANIBEL-CAPTIVA CONSERVATION FOUNDATION

The Sanibel-Captiva Conservation Foundation reported that following the first-ever renourishment of all of Sanibel’s beaches this year with sand trucked in from a sand mine in Moore Haven to offset impacts from Hurricane Ian, many beachgoers have questioned whether or not sea turtle nesting will be affected.

As part of a study that the SCCF started in 2021, the sea turtle team is analyzing how the sand’s characteristics relate to hatchling emergence success through loggers installed at nests on Sanibel and Captiva. In June of 2021, the sea turtle team launched a project investigating how variation in sand grain size, color, compaction and bulk density influences temperature, moisture and water flow through the nest chamber.

“We are also evaluating subsequent impacts on embryonic development and hatchling production,” Coastal Wildlife Director Kelly Sloan said.

Since 2021, 111 nests have been outfitted with temperature, moisture and water level loggers on Sanibel and Captiva and over 1,000 unhatched eggs have been staged to identify when development ended.

“Sand samples are taken at each site, along with beach profile metrics such as elevation and slope,” she said.

Preliminary analyses from data collected in 2021-22 suggest that there were significantly different sand grain sizes on Sanibel and Captiva, and the varying sediment characteristics may impact the rate of water flow and temperature within the nest chamber, possibly resulting in decreased emergence success. The elevation gain associated with renourishment projects may also be an important variable affecting hatch success.

Sand placement projects have been occurring regularly on Captiva between 1961 and 2021. Sand for the projects is typically dredged from an offshore borrow site and piped onto the beach.

Notably, the average annual hatch success on Captiva was found to be generally about 20% lower than non-nourished stretches of Sanibel (2014-20) when nests with any external losses are removed (i.e. depredation, tidal washovers, et cetera). The 20% reduction in hatchlings represents a potential obstacle to loggerhead population recovery.

“This year, we will also be able to compare previous years’ data to determine any impacts or benefits from the renourishment project that occurred on Sanibel,” Sloan said.

WHY BEACHES ARE RENOURISHED

Along the coast of Florida, beach erosion due to storms, hurricanes and sea level rise is becoming increasingly common. Natural processes have been radically changed as humans develop the coast, and interruptions in sediment transport on naturally dynamic beaches prevent normal sand migration patterns.

One of the responses to shrinking shorelines is beach renourishment. Beach renourishment is considered a “soft” measure that can help replace lost sand and is a common response to erosion.

The SCCF reported that while the addition of sand to an erosional beach can benefit sea turtles by creating nesting habitat, questions have been raised concerning the direct and indirect effects of beach renourishment on nesting turtles.

“For example, changes in the physical properties of non-native sand may alter critical characteristics of the nest microclimate, such as sand temperature, moisture, gas exchange and porosity,” she said. “These shifts in incubation environments raise real concerns about incubation time, clutch viability, hatchling fitness, and hatchling sex ratios.”

The results from studies assessing how the hydric, which determines the amount of water available in the sand, and the physical properties of the sand on natural and renourished beaches have produced varied results over space and time, suggesting the effects may be somewhat site-specific.