Response of Benthic Fauna and Changing Sediment Redox Profiles over a Hypoxic Gradient

The Koljofjord is an enclosed, stratified fjord on the Swedish west coast with hypoxic/anoxic bottom water during most of the year. In the winter 1999/2000, the water in the entire fjord was re-oxygenated after a period of stagnation, but the following summer oxygen concentrations declined to below 1 ml l-1between 20 and 40 m depths. The objectives of this study were to investigate the structure of benthic communities along a depth gradient of declining oxygen concentrations and the impact of fauna on sediment redox conditions. The vertical distribution of the fauna in the sediment was restricted to the upper few centimetres. Dominant species at most stations were the burrower Capitella capitata and the tube-builder Pseudopolydora antennata. The species found in the fjord are probably not particularly tolerant towards hypoxia, but they have life-history traits that facilitate a rapid colonization following improved oxygen conditions. The depth of the Redox potential discontinuity (RPD) layer, a recognizable division zone between oxidized (sub-oxic) and reduced chemical conditions, is dependent on infaunal activity, e.g. burrows, tubes and feeding voids. Measurement of apparent RPD (aRPD) from sediment profile images (SPIs) compared well to electrode measurement of RPD. We conclude that a digital analysis of aRPD from images has many advantages compared to RPD measurements by electrodes.