Climate Impacts on Sea Turtle Breeding Phenology in Greece and Associated Foraging Habitats in the Wider Mediterranean Region

Abstract

Sea turtles are vulnerable to climate change impacts in both their terrestrial (nesting beach) and oceanic habitats. From 1982 to 2012, air and sea surface temperatures at major high use foraging and nesting regions (n = 5) of loggerhead turtles (Caretta caretta) nesting in Greece have steadily increased. Here, we update the established relationships between sea surface temperature and nesting data from Zakynthos (latitude: 37.7°N), a major nesting beach, while also expanding these analyses to include precipitation and air temperature and additional nesting data from two other key beaches in Greece: Kyparissia Bay (latitude: 37.3°N) and Rethymno, Crete (latitude: 35.4°N). We confirmed that nesting phenology at Zakynthos has continued to be impacted by breeding season temperature; however, temperature has no consistent relationship with nest numbers, which are declining on Zakynthos and Crete but increasing at Kyparissia. Then using statistically downscaled outputs of 14 climate models assessed by the Intergovernmental Panel on Climate Change (IPCC), we projected future shifts in nesting for these populations. Based on the climate models, we projected that temperature at the key foraging and breeding sites (Adriatic Sea, Aegean Sea, Crete, Gulf of Gabès and Zakynthos/Kyparissia Bay; overall latitudinal range: 33.0°-45.8°N) for loggerhead turtles nesting in Greece will rise by 3-5°C by 2100. Our calculations indicate that the projected rise in air and ocean temperature at Zakynthos could cause the nesting season in this major rookery to shift to an earlier date by as much as 50-74 days by 2100. Although an earlier onset of the nesting season may provide minor relief for nest success as temperatures rise, the overall climatic changes to the various important habitats will most likely have an overall negative impact on this population.

Fig 1. Eastern Mediterranean Sea with indication of the 5 high use sites for loggerheads.

Fig 2. Climate model projections of mating period SST and T_a and corresponding projections of the day of first female emergence through the 21^st century.

(A) Means of the projected changes in SST from 13 climate models (RCP 8.5) during the mating season for Crete and Zakynthos/Kyparissia. (B) Projections of the day of first female emergence through 2100 based on climate model estimations of the increase in SST during the mating season at Zakynthos Island. (C) Means of the projected changes in T_a from 14 atmospheric models (RCP 8.5) during the mating season for Crete and Zakynthos/Kyparissia. (D) Projections of the day of first female emergence through 2100 based on atmospheric model estimations on the increase in T_a during the mating season at Zakynthos Island.

Fig 3. Observed and future projections of mean annual SST for the five high use regions.

(A) Mean annual SST at the 5 high use areas for loggerheads in the Mediterranean. Solid lines are the linear trend lines (Adriatic R² = 0.311; Aegean R² = 0.560; Crete R² = 0.674; Gabès R² = 0.474; Zak/Kyp R² = 0.393). (B) Means of the projected changes in annual SST from 13 climate models (RCP 8.5) for the 5 high use sites.

Fig 4. Relationship between number of nests per season and foraging site SST two years prior.

(A) Relationship between number of nests per season at Zakynthos (1984–2009) and the mean annual SST at the 5 foraging sites 2 years prior. Solid line is the linear trend line (R² = 0.190). (B) Relationship between number of nests per season at Rethymno (1990–2004) and the mean annual SST at the foraging sites (Gulf of Gabès, Aegean Sea and Crete) 2 years prior. Solid line is the linear trend line (R² = 0.572).

Fig 5. Means of the projected changes in precipitation rates from fourteen atmospheric models (RCP 8.5) during the mating season for Crete and Zakynthos/Kyparissia.