Grant: 19-006R
Project Title: Do sand characteristics of a geotextile container dune core project alter sea turtle nest temperatures and reproductive output?
Project Manager: Sarah Hirsch
Organization: Loggerhead Marinelife Center (Non-Profit Organization)
Grant Amount: $19,635.00
Completion Date: 2021-10-18

Summary: Coastal squeeze is an increasing threat to the sea turtles nesting on Florida beaches. Currently, much of the Florida coastline is considered critically eroding habitat and one quarter of the beaches are stabilized with some form of hard armoring. Geocores, reconstructed dunes with sand-filled geotextile bags buried as the dune core, are an erosion control measure implemented to protect inland infrastructure from the encroaching shoreline. Little research has been conducted on the impacts of these structures on sea turtles. A previous study raised questions about geotube impacts to the incubation environment and how these factors influence reproductive success. This project aims to evaluate sand characteristics (color, moisture content, grain size, and carbonate content) and nest temperatures for nests laid over a geocore in comparison to a control, non-geocore area of beach to assess whether the restoration of the dune provides suitable habitat for incubating loggerhead and green turtle nests. By exploring how sand characteristics and nest temperatures play a role in sea turtle hatching and emergence success, this study will have broad implications for future beach restoration projects and can assist coastal managers in implementing measures that minimize the effects these projects have on sea turtle reproductive success. This is critical in a time where beach nourishment and coastal armoring are the primary approaches currently being implemented to combat coastal erosion.

Results: A total of 20 temperature data loggers were deployed in nests (Cc = 2, Cm = 18). Because reviewers suggested restricting putting data loggers in only those nests that were laid high on the beach (above the sand bags on the geocore and at an approximately similar location at the control site) we were not able to find enough loggerhead nests that met that requirement to deploy all 40 of the purchased data loggers as planned. We are hopeful that we can continue to use these data loggers over the course of the next couple of seasons to continue to get temperature information for nests laid over the geocore structure by both loggerhead and green turtles. From the small sample we obtained from green turtle nests, we did find that average nest temperatures at the geocore (30.95C) were significantly lower than those at the control site (31.73C; t = 3.62, p = 3.99 x 10-3). All sand samples have been collected and analyzed for moisture content, Munsell color, grain size, and carbonate content with the equipment purchased through this grant. A generalized linear model with a binomial family and weight by clutch size were evaluated in a model selection framework using (CRAN). Predictor variables included in the analysis were treatment (i.e., geocore or control), elevation, moisture content, grain size, sand color, and carbonate content. For loggerhead hatching and emergence success, the most informative model included moisture content, elevation, and their interaction (AICc = 1242.1, df = 4, weight = 1 and AICc = 1275.6, df = 4, weight = 1, respectively). For green turtle hatching and emergence success, the most informative model included moisture content, grain size, and their interaction (AICc = 2177.8, df = 4, weight = 1 and AICc = 2228.9, df = 4, weight = 0.89, respectively).