Shading and watering as a tool to mitigate the impacts of climate change in sea turtle nests

Abstract

Increasing sand temperatures resulting from climate change may negatively impact sea turtle nests by altering sex ratios and decreasing reproductive output. We analyzed the effect of nest shading and watering on sand temperatures as climate mitigation strategies in a beach hatchery at Playa Grande, Costa Rica. We set up plots and placed thermocouples at depths of 45 cm and 75 cm. Half of the plots were shaded and half were exposed to the sun. Within these exposure treatments, we applied three watering treatments over one month, replicating local climatic conditions experienced in this area. We also examined gravimetric water content of sand by collecting sand samples the day before watering began, the day after watering was complete, and one month after completion. Shading had the largest impact on sand temperature, followed by watering and depth. All watering treatments lowered sand temperature, but the effect varied with depth. Temperatures in plots that received water returned to control levels within 10 days after watering stopped. Water content increased at both depths in the two highest water treatments, and 30 days after the end of water application remained higher than plots with low water. While the impacts of watering on sand temperature dissipate rapidly after the end of application, the impacts on water content are much more lasting. Although less effective at lowering sand temperatures than shading, watering may benefit sea turtle clutches by offsetting negative impacts of low levels of rain in particularly dry areas. Prior to implementing such strategies, the natural conditions at the location of interest (e.g. clutch depth, environmental conditions, and beach characteristics) and natural hatchling sex ratios should be taken into consideration. These results provide insight into the effectiveness of nest shading and watering as climate mitigation techniques and illustrate important points of consideration in the crafting of such strategies.

Fig 1. Sand temperature (°C) through time at olive ridley (A,C) and leatherback (B,D) turtle nest depths.

(A) 45cm exposed. (B) 75cm exposed. (C) 45cm shaded. (D) 75cm shaded. Water treatments were: high (721 mm), average (323 mm) and low (100 mm), corresponding to La Niña, neutral and El Niño conditions in Northwest Costa Rica. Vertical lines mark the beginning and end of watering treatments.

Fig 2. Air and sand temperatures (°C) at 45 and 75 cm depth from data loggers.

Temperatures are shown over a five day period during watering from (A) exposed and (B) shaded plots that received high water treatments (721 mm).