Plastic habitats: Algal biofilms on photic and aphotic plastics

•Plastic substrates are colonised by algal biofilms.•Diatom and algal assemblages on plastics are significantly different in photic and aphotic zones.•Diatom assemblages and algal pigment concentrations on plastics did not vary with polymer type.•Diatom assemblages in biofilms on plastics change over time.•Polymer surface degradation in the absence of UV radiation occurs within six weeks.