Context dependency in periphyton and invertebrates: interactions between velocity and nutrients in the Australian wet-dry tropics.

The interactive effect of river velocity and nutrient addition on paver periphyton and benthic invertebrates was examined in a 2-factorial experiment conducted in a 7th order river in the Australian wet-dry tropics during base flow conditions. The velocity factor was represented by 5 categories (0, 0.27, 0.48, 0.78 and 0.97 m/s) and nutrient addition by control and treatment. Nutrients were added by slow release capsules contained in mesh bags. Overall periphyton biomass was lowest at the highest velocity site and responded positively to nutrient addition only at the 0 m/s and 0.78 m/s sites. AFDM varied significantly between the different velocity sites but did not vary relative to nutrient treatments. Invertebrate communities in the high velocity sites differ greatly from all other sites regardless of nutrient treatment but at 0 m/s sites the community was more diverse with nutrient addition. These results highlight the importance of considering context dependency when examining periphyton biomass and invertebrate community structure