Two subspecies are generally recognized: the typical Graptemys nigrinoda nigrinoda Cagle, 1954, and G. n. delticola Folkerts and Mount, 1969.

Previously assessed as Near Threatened (1996), presently available information indicates that Graptemys nigrinoda warrants assessment as Least Concern given its substantial and healthy populations across its fairly substantial range (Blankenship et al. 2008).

Graptemys nigrinoda occurs below the Fall Line in the Alabama, Tombigee, Black Warrior, Coosa, Tallapoosa, and Cahaba rivers of Alabama and northeastern Mississippi.

Surveys up to 1980 (McCoy and Vogt in Blankenship et al. 2008) documented extensive and abundant populations throughout much of the extensive range of the species, particularly in areas not intensively used by commercial shipping. The species apparently is able to utilize large impoundments. Densities of 15–23 basking G. nigrinoda per km of river have been reported at different times (review by Blankenship et al. 2008).

Graptemys nigrinoda was considered the fourth commonest Graptemys by Lindeman (pers. comm 6 Aug 2009) based on extensive basking surveys. It continues to be observed in good numbers, in places representing 60–80% of all basking turtles, and populations appear healthy.

Graptemys nigrinoda inhabits rivers and streams with moderate current, and logs and other basking sites; hatchlings and juveniles apparently prefer adjoining sloughs and bayous. Eggs are laid in nests dug on open expanses of fine sand.

Black-knobbed Sawbacks feed predominantly on freshwater sponges, bryozoans, molluscs and insects, as well as taking some plant material.

Females reach 22 cm carapace length (CL), while males remain much smaller at 12 cm CL maximum. Females reach maturity at about 8–9 years and 17 cm CL, while males mature at about 7 cm and 3–4 years. Females produce 3–4 clutches of on average 5.5 eggs annually. Hatchlings measure 36 mm CL. Generation time has not been calculated.  

Natural history reviews by Blankenship et al. (2009), Ernst and Lovich (2009).

Removal of basking logs and snags to improve navigation, collection for pets, intensive local collection of turtle eggs and adults for consumption in the recent past, wanton shooting of basking turtles, mortality from drowning in fishing gear and boat strike, high nest predation by (subsidized) fish crows, raccoons, armadillos and fire ants, and reduced reproductive success as a result of human disturbance of preferred nesting sites, have all been indicated as at least locally significant impacts on populations (Blankenship et al. 2008, Ernst and Lovich 2009).

Graptemys nigrinoda is listed as a Protected Nongame Species by the State of Alabama, as Endangered by the State of Mississippi, and is included in CITES Appendix III (United States) since 14 June 2006.

A variety of riverside protected areas safeguard nesting sites, and to some extent resident populations. Initiatives under way under the Mobile Bay Aquatic Ecosystem Recovery Plan are likely to significantly benefit Graptemys nigrinoda; no measures specific to the species appear urgently needed, though background biological research and population monitoring are desirable.