The periphyton colonisation surfaces are hardened ashless filter papers placed over the top
of the reservoir jars and held in place by push-fit plastic rings. A thick plastic lid fits over the
entire box. Twenty holes cut in the lid fit exactly over the colonisation papers so that the
stream current flows over a flat, unbroken surface. The lid is secured by tightening nuts onto
four long bolts welded to the base of the box. On the surface of the lid, five 20 mm high
longitudinal partitions maintain parallel flow and prevent diffusion of different treatments
onto neighbouring rows. The trays and internal fittings are not easy to fabricate and detailed
design notes are not given here. Samplers can be purchased or hired from NIWA Instrument
Systems, P.O. Box 8602, Christchurch (phone 03 348 8987; e-mail g.elley@niwa.cri.nz).