  We investigate correlations between the location of galaxies in dense
environments and their degree of star-formation activity. Using photometric
redshifts and spectral classifications from the unique 17-band COMBO-17 survey
we are able to precisely isolate galaxies from the Abell 901/902 supercluster
within a thin redshift slice around z=0.16. We compare the detailed photometric
properties of the supercluster galaxies with the underlying dark matter density
field as revealed by weak gravitational lensing. We find strong evidence for
segregation by type, with the highest density regions populated almost
exclusively by galaxies classified according to their rest-frame U-V colours as
quiescent. We also observe a threshold surface mass density from lensing,
\kappa\sim 0.05 (corresponding to a physical density \Sigma = 2.5x10^(14)h
M_\sun Mpc^(-2)), above which star-formation activity is rapidly suppressed.
This abrupt transformation affects primarily the faint end of the star-forming
galaxy population and occurs at a local surface number density corresponding to
roughly 400h^2 Mpc^(-2) to a limit of M*_V+6. When only galaxies brighter than
M*+1 are considered the trends with environment remain, but are more gradual
and extend beyond 2h^(-1) Mpc radius.
