Gomphonema mexicanum has a higher stria density, more narrow apices and a lower L:W ratio.
Gomphonema ventricosum has a broader axial area, especially at the central area, and higher stria striae at mid-valve.
Gomphonema manubium is more robust, with a much broader axial area and higher areola density.
Gomphonema submehleri has a higher stria density and narrower valves.
Gomphonema gibba has a narrower headpole and less coarse stria. Note also, that similar but undescribed species, are present in the southeastern states.