The Santa Fe River in northern Florida: effect of habitat heterogeneity on turtle populations

Abstract

The Santa Fe River (SFR) in northern Florida and its springs provide a unique ecosystem for a wealth of flora and fauna, and support a unique freshwater turtle assemblage. We conducted a 6-year mark-recapture study of ten turtle species to assess how habitat heterogeneity among sites affects riverine turtle populations. Because the SFR ecosystem has not been well described and the major emphasis of this paper is the effect of habitat variation on turtle populations, we provide thorough descriptions of the SFR basin and specific habitats in which we sampled turtles. The SFR originates as a tannin-stained blackwater river, but receives substantial input of clear, alkaline, thermally stable water from numerous artesian springs in its lower reaches. We used mark-recapture and demographic data to evaluate differences in turtle assemblages and population structure on a spatial scale. We compared turtle assemblages between a 5 km reach of blackwater river habitat and a 9 km reach of spring-influenced river habitat. We found the same ten species in both habitats. Hand capture while snorkeling suggested similar relative abundance of species in both river habitats, but baited hoop trap captures suggested that Chelydra serpentina (Snapping Turtle) and Sternotherus minor (Loggerhead Musk Turtle) are proportionately more abundant in the three spring-influenced habitats we sampled. The total density of all turtle species combined appears to be three to four times greater in the spring-influenced river reach than in the blackwater river reach. Examination of population structure of the three most abundant species (Pseudemys suwanniensis [Suwannee Cooter], S. minor, and Trachemys scripta [Yellow-bellied Slider]) in the river and adjacent spring habitats revealed that some springs feeding the SFR may function as nursery habitats. Results demonstrate the importance of habitat diversity (beta diversity) to the riverine turtle assemblage. If we had sampled turtles only in one section of the river or only in spring habitats in the lower SFR, our perceptions of assemblages and population structure would have been vastly different. The fate of the turtle populations in the SFR depends on the quantity and quality of water discharging from its springs. If the long-term trend of declining spring flows continues, we predict that SFR turtle populations will be detrimentally affected in multiple ways such as loss of nursery habitat and reduced recruitment. Less dilution of dark tannic water flowing down from the upper SFR may cause shifts in assemblage and population structure, as well as in population densities. Given the uncertain future of ecological conditions in the SFR, we advocate continued long-term monitoring of this unique turtle assemblage.