3. Material and methods
In this study, water and surface sediment samples were collected at five sites along a depth transect in the Koljo¨ fjord (Fig. 1).
Sampling was done monthly from August 1993 to December 1994, thus spanning a period of hypoxic conditions which evolved via almost anoxic conditions, and oxic conditions to hypoxic. Apart from sediment sampling, hydrographic data including salinity, temperature and oxygen content, were also measured at each site during every sampling occasion. Monthly values of chlorophyll-a concentrations in the surface water and at 10 m depth in the Koljo¨ fjord are also available (Go¨teborgs och Bohusla¨ns Vattenva°rdsfo¨rbund, 1993–1994).
Sediment samples were collected with a multiple corer (Mark III-400), described by Barnett et al. (1984) and later modified by Barnett.
Sediment colour was determined using the Munsell soil colour chart. The macrofauna were surveyed at every site
and on every sampling occasion.
The organic-carbon and total-nitrogen contents of the sediment were determined using a Carlo Erba NA 1500 analyser.