Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:The main parameter in muddy, shallow water environments, where food-supply is much higher than in the deep-sea, is the availability of oxygen [van der Zwaan et al., 1999; Jorissen et al., 2007].
Category: