  We report radial velocities (RVs), projected equatorial velocities (v sin i)
and CaII triplet (CaT) chromospheric activity indices for 237 late-K to mid-M
candidate members of the young open cluster NGC 2516. These stars have rotation
periods between 0.1 and 15 days. Intermediate resolution spectra were obtained
using the Giraffe spectrograph at the Very Large Telescope. Membership was
confirmed on the basis of RVs for 210 targets. We see a marked increase in the
fraction of rapidly rotators as we move to cooler spectral types. About 20 per
cent of M0-M1 stars have v sin i >15km/s, increasing to 90 per cent of M4
stars. Activity indices derived from the first two lines of the CaT (8498A and
8542A) show differing dependencies on rotation period and mass for stars lying
above and below the fully convective boundary. Higher mass stars, of spectral
type K3-M2.5, show chromospheric activity which increases with decreasing
Rossby number (the ratio of period to convective turnover time), saturating for
Rossby numbers <0.1. For cooler stars, which are probably fully convective and
almost all of which have Rossby numbers <0.1, there is a clear decrease in
chromospheric activity as (V-I)_0 increases, amounting to a fall of about a
factor of 2-3 between spectral types M2.5 and M4. This decrease in activity
levels at low Rossby numbers is not seen in X-ray observations of the coronae
of cluster M-dwarfs or of active field M-dwarfs. There is no evidence for
supersaturation of chromospheric activity for stars of any spectral type at
Rossby numbers <0.01. We suggest that the fall in the limiting level of
chromospheric emission beyond spectral type M3 in NGC~2516 is, like the
simultaneous increase in rotation rates in field stars, associated with a
change in the global magnetic topology as stars approach the fully convective
boundary and not due to any decrease in dynamo-generated magnetic flux.
