Physiological consequences of rising water salinity for a declining freshwater turtle

Abstract

Sea-level rise, drought and water diversion can all lead to rapid salinization of freshwater habitats, especially in coastal areas. Increased water salinities can in turn alter the geographic distribution and ecology of freshwater species including turtles. The physiological consequences of salinization for freshwater turtles, however, are poorly known. Here, we compared the osmoregulatory response of two geographically separate populations of the freshwater Western Pond Turtle (Actinemys marmorata)-a species declining across its range in western North America-to three constant salinities: 0.4 ppt, 10 ppt and 15 ppt over 2 weeks. We found that turtles from a coastal estuarine marsh population regulated their plasma osmolality at lower levels than their conspecifics from an inland freshwater creek population 45 km away. Plasma osmolalities were consistently lower in estuarine marsh turtles than the freshwater creek turtles over the entire 2-week exposure to 10 ppt and 15 ppt water. Furthermore, estuarine marsh turtles maintained plasma osmolalities within 1 SD of their mean field osmolalities over the 2-week exposure, whereas freshwater creek turtles exceeded their field values within the first few days after exposure to elevated salinities. However, individuals from both populations exhibited body mass loss in 15 ppt water, with significantly greater loss in estuarine turtles. We speculate that the greater ability to osmoregulate by the estuarine marsh turtles may be explained by their reduced feeding and drinking in elevated salinities that was not exhibited by the freshwater creek population. However, due to mass loss in both populations, physiological and behavioural responses exhibited by estuarine marsh turtles may only be effective adaptations for short-term exposures to elevated salinities, such as those from tides and when traversing saline habitats, and are unlikely to be effective for long-term exposure to elevated salinity as is expected under sea-level rise.

Keywords: Freshwater turtles; osmoregulation; salinity; sea-level rise.

Figure 1

Map of western pond turtle (A. marmorata) collection locations including the Suisun Marsh, Solano County in the SFBE (in California) and the UC Davis Arboretum, Yolo County in the Central Valley (in California).

Figure 2

Body size (straight-line carapace length) for western pond turtles (A. marmorata) collected from Suisun Marsh, Solano County (estuarine marsh) and University of California Davis Arboretum, Yolo County, California (freshwater creek) populations. Box plots represent mean body size of each sex by habitat type with 25% quartiles, min and max values. Estuarine marsh turtles were larger than freshwater creek turtles (P = 0.01), and males were larger than females across both populations (P < 0.001).

Figure 3

Baseline field plasma osmolality values for Western Pond Turtles (A. marmorata) collected from Suisun Marsh, Solano County (estuarine marsh) and University of California Davis Arboretum, Yolo County, California (freshwater creek) populations. Plasma osmolality ranges are parsed by sex and population. Box plots represent mean plasma osmolality of each sex by habitat type with 25% quartiles, min and max values. Estuarine marsh turtles had higher baseline field osmolalities when captured than did the freshwater creek turtles (P < 0.001), and males had higher osmolalities compared to females across both populations (P < 0.001).

Figure 4

Mean plasma osmolality for Western Pond Turtles (A. marmorata) during chronic exposure to varying salinities (Days 0 to 20). Mean plasma osmolality values (total number of solute particles per kilogram) are parsed by population and treatment and presented with standard error (see Table 1 for sample sizes). Mean baseline field osmolality value for each population is denoted by horizontal dotted line, and when turtles were moved to freshwater is denoted by a vertical dotted line.

Figure 5

Mean [Na⁺] for Western Pond Turtles (A. marmorata) during chronic exposure to varying salinities (Days 0 to 20). Mean [Na⁺] (mmol/L) are parsed by population and treatment and presented with standard error (see Table 1 for sample sizes). Mean baseline field [Na⁺] value for each population is denoted by horizontal dotted line, and when turtles were moved to freshwater is denoted by a vertical dotted line.

Figure 6

Mean [K⁺] for Western Pond Turtles (A. marmorata) during chronic exposure to varying salinities (Day 0 to 20). Mean [K⁺] (mmol/L) are parsed by population and treatment and presented with standard error (see Table 1 for sample sizes). Mean baseline field [K⁺] value for each population is denoted by horizontal dotted line, and when turtles were moved to freshwater is denoted by a vertical dotted line.

Figure 7

Mean percent body mass change for Western Pond Turtles (A. marmorata) during chronic exposure to varying salinities (Days 0 to 20). Mean percent body mass change values are parsed by population and treatment and presented with standard error (see Table 1 for sample sizes). When turtles were moved to freshwater is denoted by a vertical dotted line.