Vertical Distribution of the Nematode Fauna in a Coastal Sediment Influenced by Seasonal Hypoxia in the Bottom Water

Abstract

The vertical distribution of a nematode assemblage in a muddy, sheltered bay, Kilviken, at the Swedish west coast was studied monthly for one year. Measurements of redox potentials in the sediment as well as salinity, temperature and oxygen concentration of boltom water were conducted. The sediment was almost devoid of macrofauna. The oxygen concentration of bottom water was lowest in August, i.e. 0.2 ml O₂ 1⁻¹ (2.5% atm. saturation). The nematode fauna consisted of 23 species. Abundances reached 8.9 × 10⁶ ind. m⁻² in June and dropped to 1.7 × 10⁶ ind. m⁻² in August. This difference was ascribed to mortality of the surface fauna due to oxygen deficiency and sulphide poisoning induced by the hypoxic bottom water. The subsurface fauna reacted to the oxygen deficiency and H₂S stress by migrating upwards and forming a more narrow and shallow depth distribution than observed during other months of the sampling period. Sahatieria pulchra (G. Schneider, 1906) dominated the nematode assemblage, inhabited the redox potential discontinuity (RPD) layer and deeper sediment horizons, and was the only metazoan in the sediment in September when the RPD-layer reached the sediment surface.