3 types of filtration; biological, chemical and mechanical.
If you are using LR
then the bacteria will colonize there and take care of the biological filtration. Your skimmer (if I remember correctly you are running one) and CUC (clean up crew) will take care of the mechanical and a filter sock if you choose to use one. Chemical can be taken care of by adding those to the flow in the sump when you need it.
That being said lots of people, like me, also run a HOB
or canister as well but it's not needed.
In reality, even w/o a sump you could just run a LR
system with a HOB
skimmer and be very successful