Does collembolan grazing influence nitrogen fixation by cyanobacteria in the high Arctic?

A field experiment was performed to test whether cyanobacterial nitrogen fi xation rates increase or decrease when exposed to low and moderate microart hropod grazing intensities. The densities of naturally occurring Collembola , i.e, Hypogastrura viatica, were manipulated on a salt marsh covered with cyanobacteria in the high Arctic, Svalbard, Nitrogen-fixation rates in graz ed cyanobacterial crust were measured and used as an indirect measurement o f cyanobacterial biomass on three dates during one summer. After 30 days, a second order polynomial regression gave a good fit to the data, indicating an increase in nitrogen-fixation rates at low/intermediate grazing pressur es and a decrease at high grazing pressures. Thus, grazing collembolans may influence the nitrogen-fixation rates in an arctic salt-marsh community. A lthough based on a small set of data, the study indicates a compensatory fi xation at low grazing pressures followed by a reduction at high grazing pre ssures.