Staurosira is an araphid genus - thus probably planktonic (?). Staurosira construens is a freshwater species.
Staurosira, species undetermined, found in diatomite deposit at Three Lakes, core TL5, at 3.6 to 3.7 m depth.
Staurosira construens found in core TL1 at 5.95 and construens (?) 5.99 m depth.
See Staurosira on Algaebase or
Diatoms.org. For construens see here. Not found on the Natural History Museum of Wales website.