Recovery of marine benthic habitats and fauna in a Swedish fjord following improved oxygen conditions

The Gullmarsfjord on the Swedish west coast has a sill at 40 m and a maximum depth of 118 m. The stagnant bottom water is usually renewed with oxygen-rich water each spring. In 1997 this did not occur, and the fauna was eliminated at depths below about 100 m and severely reduced between 80 and 100 m depth. In spring 1998, the whole fjord was re-oxygenated and the succession of the benthic fauna was studied at 5 stations over a 2 yr period. Simultaneously, the benthic habitat quality (BHQ) was assessed by analysing sediment redox conditions and faunal burrow structures in sediment profile images. Succession of the benthic fauna is described as increases in number of species, abundance and biomass. Conspicuous colonisers below 100 m depth were the polychaete Capitella capitata and the heart urchins Echinocardium cordatum and E. flavescens. Return to pre-oxygen stressed conditions was slowest at the deepest stations. By using a multivariate technique (multi-dimensional scaling) we show that the benthic communities at all depths more or less returned to the same faunal composition as during pre-disturbed conditions. The pioneering and mature benthic faunal Successional Stages I and III were predictable but not the intermediate Stage 11, The path of recovery of the benthic community succession did not retrace the path of degradation. The sedimentary habitats and redox conditions were similarly restored over the 2 yr period as analysed by the BHQ index.